Engineer N. S. MAKEROV

REMOVAL AND UTILIZATION

OF SEWAGE SLUDGE

Tomsk

1928

Engineer N. S. Makerov.

Removal and Utilization of Sewage Sludge

Preface.

Mechanical treatment of wastewater, pursuing the aim of clarifying it, yields as a result:

1) wastewater freed, to one degree or another, from insoluble impurities, and
2) impurities separated from the wastewater (hereinafter referred to as sludge).

The clarified water is discharged into a water source, while the sludge is removed beyond the limits of the treatment plant, with the adoption of appropriate measures so that it cannot cause any harm to the population.

Operations involving sludge constitute a very important problem, since they must at the same time satisfy the following conditions:

1) sanitary, and
2) economic.

Sludge, depending on the type of structures in which it is separated, has different volume and character.

From this point of view, the following should be distinguished:

1) sludge separated in grit chambers,
2) the mass retained by screens and sieves, and
3) sludge separated in settling structures.

The differences in volume. Dunbar gives the following figures, characterizing, in percentage terms, the quantities of impurities separated by various structures in the dry state:

sludge from grit chambers . . . . . . . . . . . 16%;
substances retained by screens¹ . . . . . . 10%;
suspended substances (i.e., sludge separated in
settling structures) . . . . . . . . . . . . . 74%.

The water content of the sludge of each of these categories is not the same. It is expressed by the following figures:

sludge from grit chambers . . . . . . . . . . 35—40%;
mass retained by screens . . . . . . . . . . . 75—80%;
sludge from settling structures . . . . . . . 80—96%.

The data given above on the amount of dry matter in the sludge and the water content in it make it possible to judge also the volume of the sludge.

Namely: the volume of sludge from settling structures considerably exceeds the volume of sludge of the first two categories.

Below, concrete data are presented confirming this conclusion.

The volume of sludge separated in grit chambers depends to a great extent on the velocity of water flow through the grit chamber and on whether street waters enter the sewerage network or not.

The yield gives the actual amount of sludge from sand traps as 1.5 cu. meters per 100,000 inhabitants.

In Worcester, with an average rate of flow of water into the sand traps of 0.15 meters per second, 0.024 cu. meters of sludge is formed per 1,000 cu. meters of wastewater.

The volume of the mass retained by screens and sieves depends to a considerable degree on the size of the openings in the screens and sieves. According to Kinnicut, Winslow, and Pratt, it ranges between 0.022 and 0.290 cu. meters per 1,000 cu. meters of wastewater.

The volume of sludge separated in installations for settling wastewater depends on various causes set out below (see Ch. I), and in plain settling, according to Allen, is expressed as 0.5–2.0 cu. meters per 1,000 cu. meters of wastewater.

Differences in composition. Sludge from sand traps is predominantly mineral in character, while sludge from settling tanks and the mass retained by screens are chiefly organic in character.

Methods of treatment. Owing to the difference in character, sludges require different methods of treatment.

Sludge from sand traps contains little water, but consists almost exclusively of substances of mineral origin, and therefore requires the simplest methods of handling. Usually this sludge is taken beyond the limits of the treatment station and dumped in low places, being used in this way to eliminate soil irregularities.

The mass retained by screens and sieves already contains a considerable percentage of water and, by its whole character as an organic mass, is predominantly subject to putrefaction. Therefore the measures for its removal are somewhat more complicated.

The usual method consists in transporting this mass beyond the limits of the treatment station and burying it in the ground.

Sometimes the substances retained on the screens are used as fertilizer. In Dresden the mass retained by these Riensch-Wurl screens is dried in the open air in the yard of the treatment station, and is then transported by farmers for use as fertilizer.

With a moisture content not exceeding 75%, the mass may be burned in a destructor together with municipal refuse.

The greatest difficulties are presented by the removal and neutralization of the sludge obtained in installations for settling wastewater: it consists predominantly of substances of organic origin and is therefore subject to putrefaction; it has a very high water content and therefore requires special methods of handling; finally, it is obtained in an extremely large volume, and therefore the costs of its removal are very considerable.

The methods of removal, treatment, and utilization of sludge separated in the settling installations of mechanical treatment stations are set forth in the present article.

I. Character, composition, and quantity of sludge.

Sewage sludge is a rotting mass containing 80–99% water¹), with a specific gravity of 1.003–1.060.

Sludge contains a significant quantity of organic substances.

¹) Here activated sludge is also taken into account, the moisture content of which is 98% [[unclear: end of note]].

According to Frühling, fresh sludge contains \(2/3\) organic substances and \(1/3\) inorganic. Prüss believes that fresh sludge contains 60—70% organic substances, while in rotted sludge the quantity of organic substances decreases by \(1/3\)—\(2/3\).

Some analyses of sludge are given in Table VI.

The quantity of sludge depends:

a) on the character of the sewage, and
b) on the method of its treatment.

Thus, the same waters, when chemically treated and when treated with activated sludge, give the greatest quantities of sludge; when treated in Emscher tanks and similar installations, the least.

According to the data of Kinnicutt, Winslow, and Pratt, depending on the character of the sewage and the method of its treatment, from 1.2 to 8.4 tons of sludge with a water content of 90% can be obtained per 1000 cubic meters of sewage, according to the following table (Table I).

Table I.

Quantity of sludge depending on the character of the sewage and the method of its treatment.

Method of sewage treatment	In England and on the continent	In America
	tons of sludge per 1000 cu. m. of sewage	tons of sludge per 1000 cu. m. of sewage
Chemical treatment	6.0—8.4	3.6—4.8
Plain sedimentation	4.1—5.5	2.4—3.1
Septic tanks	1.9—2.9	1.2—1.7

The larger quantity of sludge per cubic unit of sewage in England and on the continent is explained by the greater concentration of these waters in comparison with the sewage of America.

Allen gives the following table, characterizing sewage sludge obtained with various methods of sewage treatment.

Table II.

Character of sludge under various methods of sewage treatment.

Method of sewage treatment	Volume of sludge in cubic meters per 1000 cu. m. of sewage	Moisture, in %	Dry matter, tons per 1000 cu. m. of sewage
Emscher tanks	0.4—0.8	80—85	0.04
Plain sedimentation	0.8—2.0	88—96	0.14
Chemical treatment	4.0—6.0	86—94	0.34
Activated sludge	4.0—16.0	98—99	0.16

The above tables give the quantity of sludge per cubic unit of sewage, moreover for each type of treatment.

quantities of sludge per unit of wastewater are subject to considerable fluctuations. These fluctuations, as has already been indicated above, are explained by the differing character of wastewater, depending on its degree of concentration, or otherwise on the norm of water consumption per person. The less water is consumed by each inhabitant, the more concentrated the sewage liquid is, and the greater the quantity of sludge per cubic unit of wastewater.

Metcalf and Eddy cite data relating to one city where, with a change in water consumption from 110 liters to 426 liters per inhabitant per day, the amount of sludge changed from 4.27 liters to 0.91 liters per cubic meter of wastewater.

Therefore, when calculating sludge, some relate the latter not to the volume of wastewater, but to the inhabitant: the amount of sludge per inhabitant is more constant and is subject to smaller fluctuations than the amount of sludge per cubic unit of wastewater. Thus, for the example indicated above, the amount of sludge per inhabitant varied only within the limits from 0.29 to 0.41 liters per day.

Thus:

Dunbar considers that each inhabitant accounts for, daily, 0.45 liters of fresh sludge with a water content of 90%.

Imhoff gives, per person per day, 1.8 liters of fresh sludge with a water content of 97.5%, and 0.9 liters of fresh sludge with a water content of 95%. For digested sludge, Imhoff gives a norm per person per day of 0.2 liters with a water content of 80–85%.

Dunbar and Imhoff, consequently, proceed from the position that per person per day there are 45 grams of dry matter contained in the sludge.

Prüss assumes per person per day 1 liter of sludge with a water content of 95%, i.e. proceeds from the supposition that per person per day there are 50 g of dry matter, of which 30–35 g are of organic origin.

The information given above concerning the quantity of sludge can serve only for approximate purposes. For more precise calculations concerning the question of sludge treatment, one should obtain one’s own data. Namely:

1) If there is in the city a treatment station with sedimentation devices, the quantity of sludge can be calculated on the basis of direct measurement.

2) If the city has sewerage, but does not yet have installations for separating suspended substances from wastewater, then an understanding of the quantity of sludge with which it will be necessary to operate can be obtained by carrying out investigations with settling of wastewater in an experimental installation. In an extreme case, one may determine by laboratory means the quantity of suspended substances contained in the wastewater, and then adopt a known percentage of their retention, depending on the type of settling structure, and be guided by data on the operation of similar devices in other cities (quantity of retained substances in dry condition; percentage of moisture).

3) If, however, there is no sewerage in the city and, consequently, it is impossible to carry out the above-mentioned experiments, then, for the necessary calculations concerning the quantity of sludge, one has to be guided by the experimental data of cities having conditions analogous to the proposed ones—

treatment facilities and situated under living conditions identical to those of the city under consideration.

If the first condition exists, i.e., if there are settling structures in the city, then, in order to determine the quantity of sludge obtained at the treatment plant, one resorts, as indicated above, to direct measurement of the quantity of sludge in the settling structures.

Measuring the volume of sludge when the sludge is under water involves certain difficulties, since the boundary between sludge and water, owing to the closeness of the specific weights of sludge and water, is not sharply defined. In sedimentation basins, where the sludge is distributed over the entire length of the basin, direct measurement of sludge under water is usually not carried out. In structures of the well type, however, determination of the sludge surface may be made by one of the following methods:

1) A thin, light iron sheet, suspended horizontally on a cord, is immersed in the standing water of the settling well and slowly lowered until the tension of the cord ceases, which will indicate that the sheet has reached the surface of the sludge. By measuring the length of the immersed part of the cord, i.e., consequently, the thickness of the layer of water above the sludge, one can also determine the thickness of the sludge layer, if the former is subtracted from the depth of the well from the water surface to its bottom.

2) A glass tube, 2.5–5 cm in diameter, open at both ends, is immersed by one end into the well to a depth that is—presumably—below the surface of the sludge. Then the lower end is closed with a stopper attached to a wire that has been previously passed through the tube, and the tube is removed from the well. The position of the sludge surface in the well is determined by measuring along the tube from the water surface in it to the sludge line visible through the glass.

3) Several wide-mouthed bottles are fastened to a vertical rail at a small distance from one another. The bottle stoppers are arranged so that they can be inserted into the necks of the bottles and withdrawn from them by means of a wire that comes out at the upper end of the rail. Wishing to make a measurement, the bottles are closed with stoppers and the rail with the bottles is lowered into the well to any depth; then the stoppers are pulled out of the bottles. After a certain interval of time the bottles are closed with stoppers and the apparatus is removed from the well. The sludge level will then be located between the uppermost bottle containing sludge and the lowermost one that does not contain sludge.

Determination of the sludge surface in the well must also be carried out in order to determine the time for discharging sludge from the well. Discharge is carried out when the sludge surface in the well reaches a definite height.

II. Preliminary Preparation of the Sludge

The question of the fate of sludge may be resolved in two directions. The first solution consists in removing the sludge beyond the limits of the treatment plant and disposing of it in such a way that it can no longer cause any nuisance to the surroundings.

The second solution has in view the utilization of the sludge, owing to the presence in it of certain valuable substances.

Both solutions, when carried out, encounter familiar difficulties. Let us examine them, in order.

First, when the sludge contains a great deal of water, this entails a very large volume and weight of sludge, which makes its transport and, above all, its unloading difficult.

Second, because fresh sludge is capable of quickly becoming putrid, accompanied by the release of offensive odors, its handling and treatment near residential areas are made difficult.

Therefore efforts are made, insofar as possible, to eliminate or reduce these shortcomings even before the sludge is discharged from the structures used for settling sewage waters.

The measures of preliminary preparation are as follows.

1) To lessen the first inconvenience, i.e., the excessive water content in them, settling structures are given a known design with the aim of obtaining a sludge of lower moisture content.

2) To eliminate the second drawback, i.e., the ability either to turn putrid or to become putrid and emit unpleasant odors, artificial digestion of the sludge is resorted to.

The removal of sludge from settling structures with a lower water content is achieved, for example, by the following measures:

1) By constructing silt cylinders, similar to those used in the clarification installations of Kremer. This consists in the fact that the lower funnel-shaped part of the well ends in a comparatively deep cylinder of small diameter (about 1 meter), into which the sludge descends. Owing to this design, the sludge is, first, compacted under the pressure of the large layer of water lying above it and, second, when it is removed from the bottom of this cylinder, it is less disturbed in admixture with the sludge water, which promotes its removal with a lower water content.

According to the data of Kussel, in similar installations of Kremer the water content is from 51.5 to 55%.

2) By using scrapers, for example the Dorr system. The Dorr system installations are cylindrical settling tanks with a diameter of from 1.8 to 68.6 meters, having a bottom with a slope toward the middle. In such a tank there is a system of metal scrapers mounted on an axis placed in the center of the cylinder. This system rotates at a speed of from one to 15 revolutions per hour, and the scrapers, touching the bottom of the tank, move the deposits lying on it toward the center of the tank, whence a pipe leads them out either by gravity or by pumping. The sludge is removed from the tank without disturbing the settling process and without stopping the operation of the structure.

According to the data of Babbit, in installations of the Dorr system the water content is reduced to 95–96% moisture.

Digestion of sludge for eliminating its tendency to putrefy and smell consists in the following: sludge separated from sewage waters is either left in settling structures or transferred to special chambers. After a certain period of time, during which the sludge rots, the sludge is removed from these structures in a state already so decayed that subsequently it no longer putrefies and no longer burdens with odors either those persons who have to deal with it or the residential areas of its treatment.

In further such cases there will be differences between fresh and digested sludge, etc.

THE DIFFERENCE BETWEEN FRESH AND DIGESTED SLUDGE.

One difference between fresh and digested sludge was indicated above. It consists in the fact that digested sludge is not subject to further decomposition and does not possess that unpleasant odor which makes the handling of fresh sludge so difficult. This odor does not permit the treatment of fresh sludge near populated places, since it causes fully justified protests by residents. The treatment of digested sludge, however, can be carried out on the grounds of the treatment plant and near residential quarters of the city.

But besides this advantage, digested sludge possesses a whole series of others, depending on the special character of digested sludge.

In fresh sludge the water is to a considerable degree inside the cells, of a cellular character, which constitute the greater part of the sludge. These cells, as it were, lock up the water. In addition, fresh sludge is rich in colloids, which have the capacity to swell from water and to retain it.

During sludge digestion the cellular envelopes and colloids are destroyed, and the water that had been retained by them up to that time is released.

Therefore digested sludge contains, above all, a considerably smaller quantity of water than fresh sludge: fresh sludge contains on the average more than 90% water, while digested sludge contains on the average only 80%. The reduction in the moisture of sludge is very substantially reflected in its volume and weight.

The change in the weight of sludge when the moisture content is reduced can be calculated by the following formula:

\[ S_1=\frac{S(100-p)}{100-p_1} \]

where:
\(p\)—the initial percentage of moisture.
\(p_1\)—the final percentage of moisture.
\(S\)—the initial weight of the sludge.
\(S_1\)—the final weight of the sludge.

According to this formula, a reduction of sludge moisture from 90% to 80% leads to a reduction in the weight of the same quantity of sludge by 2 times. A reduction of moisture from 95%, which fresh sludge often possesses, to 80% (the average moisture of digested sludge) leads to a reduction in the weight of the sludge by 4 times.¹ Thus these figures very convincingly confirm the significance of the sludge-digestion process.

In operations with sludge it is therefore necessary to try, in this way, to bring it to a definite moisture content.

The determination of sludge moisture is carried out in the following manner.

A carefully mixed, accurately weighed sample of sludge is subjected to evaporation until all the water has been removed from it—this is recognized by the fact that the sample ceases, upon further rubbing, to lose weight. The difference in the weight of the raw—

sludge or dried sludge consists entirely of sludge water. Expressing the moisture content of sludge as a percentage after this therefore gives no difficulty.

Further advantages of digested sludge are as follows.

As a result of the decomposition of cellular membranes and colloids during the process of digestion, they lose their sliminess and viscosity; digested sludge, even with a lower water content, is more liquid, easier to discharge, and easier to pump than fresh sludge.

Then, digested sludge is distinguished by its ability to dry quickly on drained drying beds. This latter property of digested sludge is explained by the following causes:

1) As a result of the destruction of colloids and cellular substances, digested sludge is such in structure that it does not retain water and readily releases it.

2) Digested sludge, when released onto drying beds, is permeated with bubbles of gas, the escape of which from the sludge, when the latter is in the digestion chamber, is impeded, owing to the pressure of the overlying layers of sludge and sludge water. Owing to the presence of gas bubbles in the sludge, the specific gravity of the sludge when it arrives on the drying bed is less than the specific gravity of water, and therefore the sludge floats on the surface of the sludge water contained in it; the latter, however, freely drains away.

Fig. 1.

Figure 1 presents three photographs of a glass vessel containing digested sludge from the Emscher well. The first photograph was taken immediately after the vessel had been filled with sludge discharged from the well. The surface of the sludge in the vessel is marked with a paper label. The second photograph was taken 12 hours after the vessel had been filled with sludge. The sludge floats on the surface of its own water, owing to the pres—

there being no gas bubbles in it. The dried film remains black for several days. The sludge, deprived of gas, settles to the bottom of the vessel.

3) When gas bubbles are removed from the sludge, pores remain in the latter, which also promote the drying of the sludge.

Fresh sludge, since its specific gravity is greater than the specific gravity of water, but upon release onto drying beds settles downward, blocks the pores of the draining material and impedes the escape into the drainage of the sludge water located above the sludge.

The practically smaller volume of digested sludge and its ability to give up its water easily to the drainage make it possible to be limited to considerably smaller areas for its drying.

Finally, digested sludge, while being dried in the air, does not attract insects, whereas fresh sludge is beset by masses of yellow flies (scatophaga stercoria), which may become a cause of infection.

The distinguishing external signs of fresh and digested sludge are very characteristic, although recognizing the degree of sludge digestion is a matter of practice. Fresh sludge is gray or yellowish; digested sludge is black (owing to the content of ferrous sulfide). Fresh sludge has an unpleasant odor; digested sludge has a resinous odor. The sludge water of fresh sludge is turbid and foul-smelling; the sludge water of digested sludge is light-colored and odorless. Fresh sludge is slimy; digested sludge is granular, porridge-like.

When a sludge sample is placed in a glass vessel, its separate constituent parts are clearly distinguishable in fresh sludge: paper, food scraps, hair of all kinds, etc.; in digested sludge, owing to the breakdown of the individual constituent parts, these latter cannot be distinguished.

A sample of sludge spread in a thin layer on a plate or tin sheet, set in a slightly inclined position and left for a quarter of an hour, shows no change in the case of fresh sludge, whereas in the case of digested sludge cracks and rivulets immediately form, owing to the separation of the sludge water from the mass of sludge and its running off.

STRUCTURES FOR SLUDGE DIGESTION¹

Sludge digestion is carried out in structures of two main types:

1) The first type consists of:

a) structures having a common chamber both for the settling of wastewater and for the digestion of sludge (septic tanks).

b) structures with separate chambers for the indicated processes, but connected with one another through connecting channels (Travis and Deywidas tanks; structures of the Emscher-well type).

2) The second type consists of separate digestion chambers:

a) entirely removed from the settling structures (separate digestion chambers proper).

b) having a common wall with the settling structures (plants by Prüss; Neinstadt basins).

Sometimes a combination of the first and second types of structures is used: for preliminary digestion the first type serves (stru—

type of structure of the digestion chambers; for final digestion, the second (separate settling chambers for digestion).

Structures of the first type suffer from the fact that in them, not to mention the septic tanks, the existence of a connection between the digestion and sedimentation chambers may cause deterioration in the quality of the clarified waters. On the other hand, the temperature conditions of operation of these chambers, owing to enrichment of the chambers with wastewater, are extremely favorable.

Structures of the second type, owing to the isolation of the sedimentation chambers from the digestion chambers, are more reliable with respect to the quality of the treated waters. However, the “separate settling chambers for digestion” suffer from the drawback that they do not use heat from wastewater for their heating and, in order to achieve the same effect as structures of the first type, require artificial heating. However, structures of the Imhoff tank type and the Pissa tank, as utilising the heat of wastewater for heating the digestion chambers, are devoid of this defect.*)

CALCULATION OF DIGESTION CHAMBERS.

The calculation of digestion chambers is a rather difficult task, owing to the abundance of the factors that influence their capacity.

Thus, the dimensions of digestion chambers are influenced by the following circumstances:

1. The quantity of fresh sludge and its character;
2. The degree of reduction of the sludge volume during its digestion;
3. The time of digestion;
4. The degree of utilisation of the chamber;
5. The size of the installation.

Some of these factors, in turn, depend on a whole series of causes.

Namely:

The quantity of fresh sludge and its character are influenced by: the quantity of wastewater, the standard of living, the character of the wastewater, the sewerage system, the content of suspended matter in the wastewater, the operation of settling devices, and the water content of the sludge.

The degree of reduction of the volume of sludge during digestion depends on the time of digestion, the temperature of the digestion chamber, the degree of mixing of the sludge in the chamber, the freshness of the sludge, and the design of the digestion chamber.

The time of digestion is influenced by the necessary degree of digestion, temperature, the duration of the cold season of the year, the freshness of the sludge, the degree of mixing of the sludge, etc.

The degree of utilisation of the volume of the digestion chamber depends on the design of the chamber.

To take all these factors into account when calculating the capacity of digestion chambers is impossible; therefore, one must make a certain allowance for the accuracy of the calculation. In this respect, the experience of other cities situated under living conditions identical with those of the city under consideration is of extreme value.

Adjustment for an improper calculation accepted as a basis may, however, be made comparatively painlessly in calculating the volume of the digestion chamber for sediment—

*) Comparisons from other points of view are not made here. Details are given in the author’s article: “Mechanical Treatment of Wastewater in Connection with Sludge Treatment.”

new observations on the experimental installation during its operation by:

1) installing an additional settling chamber for sludge digestion;
2) artificial elevation of the temperature of the built digestion chamber; and
3) artificial mixing of the contents of the chamber.

The last two measures may be used to adjust the volume of separate digestion chambers.

There are several methods for calculating digestion chambers.^1 The complexity of some of them, when they are examined in detail, comes down to the following.

By the volume of fresh sludge entering the digestion chamber, and by the degree of daily reduction of this volume during digestion, the volume that each daily portion of sludge occupies on each day of the digestion period is determined. Since, during the digestion period, new portions of fresh sludge enter the chamber daily, the capacity of the chamber is determined by summing the volumes occupied by each daily portion of sludge, beginning with the first day and ending with the last day of its stay in the chamber.

Of course, when determining the quantity of fresh sludge and the degree of reduction of its volume, as well as when assigning the digestion time, one must remember that all those factors which influence these articles are to be indicated.

The designation of the digestion time for our climatic conditions should be approached with particular caution, owing to the low winter temperatures and the duration of the cold period. From this point of view, serious attention should be paid to the use of measures for thermal insulation of digestion chambers and to the possibility of artificial heating of certain types of separate digestion chambers with utilization as fuel of the gas obtained in digestion (see Fig. VI).

III. Disposal of Raw Sludge.

In Chapter II it was indicated that one of the solutions concerning the fate of sludge is its disposal, i.e. its removal beyond the limits of the treatment station with the sole aim of getting rid of it, but in such a way that it cannot subsequently cause any nuisance to the population.

Sludge may be disposed of:

1) in the raw state; and
2) after a preliminary reduction of its water content.

Disposal of sludge in the raw state, i.e. in the form in which it leaves the settling structures (fresh sludge), or the digestion chambers (digested sludge), is carried out by one of the following methods:

1) by the dilution method; and
2) by burial or in winter.

^1 Methods for calculating digestion chambers are described in the author’s article: “Calculation of Digestion Chambers for Sewage Sludge.”

DILUTION METHOD.

The method of eliminating sewage sludge by dilution consists in carting away or discharging the sludge into a large natural body of water, relying on the self-purifying capacity of the latter.

The use of this method of sludge elimination can be permissible only if the following conditions are met:

1) The body of water into which the sewage sludge is carted or discharged must possess a sufficient reserve of oxygen to prevent the possibility of the sludge rotting in the water.

2) The body of water must have so swift a current that the possibility of solid sludge particles settling on the bottom is excluded.

Such conditions may exist when sludge is carted out to sea and when it is discharged into a large river during flood time.

Examples of cities that cart sludge in its raw state out to sea are London and Manchester (England). For this purpose these cities use special vessels of a particular design. These vessels take on about 1,000 tons of sludge per voyage. When the sludge tanks of the vessels are empty, their bottoms are located at approximately 0.15 meter above the waterline; when the tanks are completely loaded with sludge, the surface of the sludge is also approximately 0.15 m above the waterline. This makes it possible to unload the tanks by gravity. The unloading time is about 17 minutes. The unloading is regulated by means of valves, which are operated from the deck. Unloading is usually carried out while the vessel is under way, moving at a reduced speed of about 10 knots per hour.

No definite provisions exist concerning the necessary degree of dilution of sewage sludge when sludge is discharged into rivers during flood time.

According to Johnson’s data, based on experiments in the city of Columbus (Ohio), the permissible dilution coefficient is 1/800.

In any case, dilution by 1,500–2,000 times should be considered sufficient.

Naturally, when establishing the dilution coefficient, one must take into account the character of the sludge: fresh sludge requires a greater degree of dilution, while digested sludge requires a smaller one.

The method of rendering sludge harmless by dilution is especially advantageously combined with sludge digestion.

Namely:

If the discharge of sludge into a river is permissible only during flood time, it may be advisable to store it until the moment when the condition of the water source (flooding) allows this to be done. Storing sludge means subjecting it to digestion. By the latter, moreover, a reduction in the volume of sludge and a change in its character are achieved, creating the most favorable conditions for the disposal of sludge by dilution.

BURYING SLUDGE IN THE GROUND.

The burying of raw sludge in the ground is carried out in the following manner:

V-shaped trenches about 0.60 meter deep are dug in the field. After 1–2 months, when the earth has become dry and loose, sludge is placed in these trenches, which is then either immediately or after ...

one or two days, they are covered with the earth taken from the trenches, at the very least to a depth of 0.30 meters.

The soil used for this purpose must either possess a natural draining capacity or be provided with artificial drainage.

There are insufficiently definite data on the required size of areas for burial of sludge. Nor are there definite data concerning the time after which a field may again be used for the same purpose or for some other purpose.

Kinnicutt, Winslow and Pratt believe that for 1,000 tons of fresh raw sludge, 2 acres (about 1 hectare) of land are required, considering it possible, with sandy soil, to limit this to one acre (about ½ hectare).

According to data from Watson in Birmingham, for the daily quantity of about 1,170 tons of sludge of 94.5% moisture content, about 1.2 kilometers of trenches, 0.90 m wide at the top and 0.45 m deep, are required daily.

According to some data, the same field may be used again after 1½–2 years, provided that two months after burying the sludge the ground has been plowed and sown, and after harvesting the stubble field is again plowed and left fallow for about a year.

According to other data, however, sludge may remain raw and foul-smelling for a number of years, and the field may prove unsuitable for further use.

This method is not widespread, since it requires large areas of land.

In addition, it cannot be used during the winter months in localities with a severe climate.

IV. Treatment of Sludge for the Purpose of Reducing Its Moisture Content

It has already been pointed out above (Art. II) that a high water content in sludge creates considerable difficulties both in the disposal of sludge and in the utilization of its valuable constituent parts.

For some methods of disposal and utilization of sludge, the high content of water in it leads only to inconveniences and increased cost of transport; for others it is an obstacle that even precludes the possibility of their application.

The inconveniences of transporting raw sludge are due to the liquid state of the sludge, which requires special vehicles with watertight bodies and special devices for handling it (buckets, pumps, etc.). The high cost of transporting raw sludge is caused, in addition, by the large volume and weight of the sludge, requiring a large number of transport units.

Reducing the moisture content of sludge thus eliminates the indicated inconveniences and leads to cheaper transport.

Thus, fresh sludge, the water content of which has been reduced to 75%, and digested sludge, the moisture content of which has been reduced to 60%, lose their liquid character and can be loaded with an ordinary shovel or pitchfork and transported in wagons with ordinary bodies.

The indication of the impossibility of applying—because of the high water content of the sludge—some methods of elimination and utilization of sludge concerns such measures as are based precisely on the dryness of the material, as, for example, the incineration of sludge. With a high moisture content of the latter, direct incineration of sludge is impossible; when fuel is used for this purpose, too much of it would have to be expended solely on the preliminary evaporation of the water from the sludge.

To reduce the moisture content of sludge, the following means exist:

1) Drying of sludge in the open air, carried out by means of

a) sludge beds and
b) drying grounds.

2) Filter presses.
3) Centrifuge.
4) Vacuum filters.
5) Drum dryers.
6) Dixon process.
7) Application of ferrous sulfate and ferric anhydride.
8) Composting.
9) Combination of some of the above-mentioned means.

SLUDGE BEDS.

Sludge beds, called sludge lagoons in England and America, are earthen basins into which fresh raw sludge is admitted either by gravity or by pumping, and in which its digestion and drying take place.

Sludge beds differ from digestion chambers in that sludge is not discharged from the digestion chambers in a liquid state, while sludge that has already been digested enters drying grounds. Sludge beds, therefore, by the nature of the work they perform, constitute a combination of digestion chambers with drying grounds.

The drying of sludge in beds, occurring both through evaporation of water and through the outflow of it through the bottom of the bed into the soil, leads, after a certain period of time, to the transition of fresh raw sludge into an earthy state with a moisture content of up to 75%.

Sludge beds, by the nature of their construction, may be of three types. Namely, they may consist of:

1) natural depressions in the ground;
2) basins artificially excavated in the ground;
3) basins formed by enclosing a given area with earthen embankments.

In the last two cases the depth of the basins or the height of the embankments is determined by the thickness of the layer of sludge placed in the bed.

The soil most suitable for the construction of beds is considered to be one that possesses a natural draining capacity. If the soil has a poor draining capacity, then artificial drainage is sometimes resorted to. However, because of the character of fresh sludge, described in Chapter II, rapid clogging of the upper soil layers occurs, and therefore the presence of artificial drainage proves useless. It is useful, before sludge is introduced into a bed, to plow the soil.

Sometimes the bottom of the bed is constructed with a layer of crushed stone or gravel, with drain pipes laid beneath this layer at a distance of 3–6 meters.

thus bringing the pond closer, in terms of design, to the drying beds discussed below and intended precisely for the drying of digested sludge.

A site for ponds is desirable to be open to the best possible drying by evaporation.

Sludge may be discharged into the pond in the following ways:

a) in a thick layer, \(0.60\)—\(1.20\) m in thickness, and
b) in thin layers, one upon another, the next being applied only when the preceding one has completely dried.

The drying time depends on the following factors:

a) climatic conditions,
b) preparation of the base,
c) degree of moisture of the sludge,
d) thickness of the layer of sludge discharged into the pond.

In general, the drying time ranges from several weeks—in hot weather, with a good base and a thin layer of sludge—to 3—6 months under less favorable conditions.

No definite standards have been established for the area required for the construction of ponds.

According to Babitt’s data, in Reading, Pa. one acre of area for sludge ponds is required for \(0.32\) cubic m. of sludge.

According to Imhoff’s data, for sludge ponds an area is required 10 times greater than that needed for drying beds (see below), i.e., on 1 sq. m. of pond area sludge may be treated in an amount corresponding approximately to 3 inhabitants.

Sludge ponds have the following disadvantages:

1) they require large areas,
2) they spread an intolerable stench,
3) they yield putrefying sludge water, requiring special artificial or natural biological treatment.

Owing to this, the use of sludge ponds may be allowed only on condition that there are vacant areas remote from populated places and at the same time unsuitable for other purposes.

Sludge ponds, however, can sometimes be operated successfully if one of their functions is taken away from them—namely, sludge digestion—i.e., if they are assigned only the task of drying the sludge, thereby bringing them closer in purpose to drying beds and leaving the character of construction and operation proper to sludge ponds.

In this sense, sludge ponds are applicable, for example, for mineral sludge capable of settling and compacting well under sludge water; the latter property of mineral sludge makes it possible, by gradually raising the embanking wall of the pond, to collect in the pond a very considerable thickness of sludge, saving on area. This method was applied in wartime by the Ruhr Association in the city of Oelfach, where the embanking pond wall, consisting of dried sludge, was constructed gradually and the site was raised to a height of 3 meters.

With ordinary municipal sludge, such a method cannot be used; an example of this is a case that occurred in 1912 in Birmingham. There, liquid sludge, pumped into a space between railway embankments, lay for a year; not having undergone compaction, it was again in a liquid state and was extracted from there and placed for drying on sludge drying beds.

Then the sludge areas can be used for the settled sludge in wintertime, in order to free the ordinary drying beds from work during this period of the year.

SLUDGE DRYING BEDS.

Sludge drying beds, called in America sludge drying beds or sludge filters, are installations intended for drying digested sludge and are built according to two schemes:

1) Beds of the first type are arranged with a drained subsoil and with a loading of coarse-grained material (cinders, gravel, crushed stone, etc.).

2) Beds of the second type are small basins with a concrete bottom, in which channels are provided with drains laid in them.

Drying beds of both the one and the other type may be constructed:

a) open, and
b) under glass roofs.

Drying beds of the first type.

Sludge drying beds of the first type (Fig. 2)¹ are most often arranged with an earthen bottom, planned in such a way that it has a slope toward the drainage pipes. The latter are laid in ca—

Fig. 2.

the sludge with the water suitable for the outlet. The drainage pipes—tile pipes with a diameter of from 7.5 to 20 cm—are usually placed at a distance of 1.20–3.00 m from one another.

Over the foundation thus prepared, filtering material is laid in a layer from 0.30 to 0.60 m thick.

The filtering material is taken of various coarseness and is laid in layers in such a way that the layer of the coarsest material (size 5–6.5 cm) is at the bottom, and the layer of the finest at the top.

On the lower layer go gravel, crushed stone, slag, coke, and on the very top—sand.

This sand is gradually removed together with the dried sludge and must be periodically renewed again.

For the convenience of removing the dried sludge, if it is removed by means of carts, wooden tracks are laid along the middle of each bed.

The wooden track is placed in one of the following two ways:

1) level with the surface of the filtering material, so that during loading of the beds the sludge layer is located beneath it;

2) on a special platform, raised so high that the track remains above the sludge at the time when the beds are filled with it. (Fig. 3).

Fig. 3.

If unloading is carried out by machines, then the wooden track is laid along the boundary between the beds and along the sides of the beds, depending on the design of the machines.

The side walls of drying beds are most often made of boards driven into the grooves of posts or fastened to the latter at the side. Less often the walls are built of low earthen banks.

The height of the walls above the filtering surface is determined by the thickness of the layer of sludge admitted.

The total area of the drying beds is determined by calculation (see below). The size of the individual beds (compartments) is governed by considerations of the convenience of unloading them from the dried sludge.

Thus, when laying a rail track down the middle of the beds, the width of each bed most convenient for manual unloading is considered to be 4 meters.

With mechanical unloading, the width of each bed depends on the dimensions and design of the machine.

The length of the bed in this case does not play a role and may be assigned at discretion, depending on local conditions (the topography and plan of the available site) and on the quantity of sludge discharged from the digestion tanks at one time.

Examples of drying beds.

Treatment plant Wassmannsdorf (Berlin).

The treatment plant on the irrigation fields of Wassmannsdorf (Berlin) has 20 drying beds of 400 sq. m each, measuring 16 meters in length and 24.5 meters in width. The width of the beds is determined by the dimensions of the unloading machine.

The total thickness of the filtering material is 0.50 meter. It is laid in four layers:

Layer	Material	Thickness of layer
upper layer	sand	10 cm
second layer	fine gravel	5 cm
third layer	medium gravel	15 cm
lower layer	coarse gravel	20 cm
Total		50 cm

Drains made of perforated ceramic pipes 10 cm in diameter are laid under the filtering material along the beds every 4.75 meters, four drains for each bed.

The sludge is supplied by a trough running between two groups of beds (10 beds in a group). From the trough to each bed run pipes 40 cm in diameter.

Treatment plant in Bergedorf (near Hamburg).

The size of each bed is 12 × 40 meters.

The total thickness of the filtering material is 0.30–0.35 m. The filtering material is laid in two layers:

Layer	Material	Thickness of layer
upper layer	sand	10 cm
lower layer	gravel increasing in coarseness downward	20–25 cm
Total		30–35 cm

The greater thickness is provided above the drains.

Treatment plant in Trei/on, N. J.

The total area of the drying beds is about 8120 sq. m, divided into beds, each measuring 55.5 × 6.1 m.

The filtering material consists of three layers of gravel, sorted by size, and an upper layer of sand. The minimum size of gravel grains is 1½ mm (1/16 in.), the maximum 63½ mm (2½ in.). The layer of sand has a thickness of 0.05 m. The total thickness of the filtering layers is from 0.25 m to 0.35 m. The maximum thickness of the filtering material is above the drainage pipes. The latter are located at a distance of 4 m from one another, placed in trenches and lined with crushed stone.

In the middle of each bed a narrow-gauge track is laid for removing the dried sludge.

The sludge enters the bed by gravity through 20 cm cast-iron pipes onto concrete slabs placed at one end of each bed, and from there spreads over its entire surface.

Treatment plant, Cleveland.

The size of each drying bed is 3 × 6 meters. The beds have a wooden tight floor, laid with a slope toward drains placed along the middle of each bed. Above the floor lies filtering material of the following layers:

Layer	Filtering material	Layer thickness
upper layer	sand	2.5 cm
second layer	gravel	7.5 cm
third layer	crushed stone, size 2.5 cm	10.0 cm
lower layer	crushed stone, size 2.5–5.0 cm	12.5 cm
Total		32.5 cm

To protect the sand from contamination, before each sludge admission to the bed a layer of dry construction refuse 2.5–7.5 cm thick is spread over it. This refuse is removed each time together with the dried sludge.

Treatment plant, Hartford, Wis.

The total area of the drying beds is 12.2 × 24.4 m, divided into four compartments. Each bed has a concrete floor. On the concrete floor earthenware drains are laid at a distance of 0.60 m from one another. Over the concrete floor filtering material is laid in 3 layers with a total thickness of 0.525 m.

Layer	Filtering material	Layer thickness
upper layer	sand	7.5 cm
middle layer	crushed stone, size 6 mm–19 mm (¼ in.–¾ in.)	15.0 cm
lower layer	crushed stone, size 19 mm–51 mm (¾ in.–2 in.)	30.0 cm
Total		52.5 cm

Drying beds of the second type.

Of the drying beds of the second type, part of which is shown in Fig. 4, three beds used in the Emscher operation are presented.

A bed 4 meters wide has a concrete bottom with two longitudinally arranged troughs, each 30 cm deep and 40 cm wide. Drainage pipes are placed in these troughs, covered with filtering material consisting of four layers of slag with grains gradually decreasing upward in size.

Fig. 4.

In the middle of the bed, along the bottom, there is a rail track. The concrete floor rests on a base of boiler slag, with drainage water from this layer discharged into the drainage pipes of the drying bed.

Discharge of Sludge onto Drying Beds

The loading of drying beds with sludge, for the most part because of their natural location below the sludge outlet from the structure, is carried out by gravity through pipes or troughs.

The sludge is discharged in a thin layer.

The layer thickness recommended by Imhoff is 0.20 m.

According to Bach, the layer thickness is 0.30 m.

In practice, the thickness of the discharged layer is subject to considerable variation, and in Table III data are given regarding it for installations in several cities.

Table III.

Thickness of the layer of sludge discharged onto drying beds.

City name	Approximate thickness of the layer of sludge discharged onto the drying bed, in centimeters
Aschaffenburg	18–20
Hildesheim	30
Gerdauen	40–50
Ohrdruf	25–30
Amberg	up to 50
Pr.-Holland	25
Prenzlau	40
Bergedorf	30

Drying Time of Sludge on Open Drying Beds

The drying time of sludge on open drying beds depends on the following factors:

1) the season and the weather,
2) the character of the sludge,
3) the water content in the sludge, and
4) the thickness of the sludge layer.

Thus, a cold season and rainy weather slow drying and, consequently, lengthen its duration; a hot season and dry weather accelerate drying and, in this way, shorten its duration.

Well-digested sludge requires less time for drying than poorly digested sludge. This follows from the properties of sludge described in Chapter II.

Naturally, the more water the sludge contains, the more time, other conditions being equal, is required for its drying; and conversely: the lower the percentage of moisture possessed by the sludge, the less time is necessary for it to dry.

Finally, the thinner the layer of sludge admitted to the drying bed, the sooner it dries; the thicker the layer of sludge, the more slowly it dries.

In the table below (Table IV) the drying time of sludge on drying beds in certain cities is indicated.

Table IV.

Drying time of sludge on drying beds in certain cities.

Name of city	Character of sludge	Drying time in weeks
Gerdauen	City installation with Kremer digestion chambers	2
Amberg	Same	2—3
Pr.-Holland	Same	5
Prenzlau	Same	4—8

When drying on sludge drying beds well-digested sludge, and when loading the beds with layers 0.20 m thick, Imhoff considers it normal, under the conditions of the German climate, to use each bed 12 times during the year.

Sludge Drying Beds under Glass Roofs

In order to make it possible to dry sludge at any season of the year and to prevent precipitation from moistening sludge that has already been partially dried, drying beds are sometimes arranged with glass roofs over them, of the type used for greenhouses.

Examples of such beds are the sludge drying beds of the cities of Cleveland, Canton (Ohio), Alliance (Ohio), and Chicago (Illinois).

In Cleveland, on covered drying beds, sludge from Emscher tanks, containing from 85 to 93% moisture, dries well—

[[unclear: end of previous sentence]] during the time of year. Open beds were loaded 15 times during the course of a year (during the warm months). Covered beds can take sludge 32 times a year, i.e., the productivity of covered drying beds proved to be 2½ times higher than that of open ones.

In the city of Canton, drying beds with glass roofs showed productivity 3 times greater in comparison with open beds.

In Fig. 5 drying beds with glass roofs in the city of Alliance are shown. They cover 1012 sq. m. and cost 16,900 rubles (1917).

Fig. 5.

Calculation of drying beds.

The size of the area required for drying sludge depends on the following factors:

1) the quantity of sludge (or the number of inhabitants),
2) the character of the sludge (well-digested, poorly digested),
3) the moisture content of the sludge,
4) the climate, and
5) the construction of the drying bed (open, with a glass roof).

The influence of the quantity of sludge (or number of inhabitants) on the size of the area required for drying sludge requires no explanation. It is self-evident that, all other conditions being equal, the more sludge that must be dried, the larger the drying bed area required.

The character of the sludge is of extremely great importance in determining the size of drying beds. It has already been indicated above that well-digested sludge has the ability to dry out much more rapidly than poorly digested sludge. And the faster the drying of the sludge proceeds, the smaller, of course, the area that will provide the necessary capacity.

The moisture content of the sludge also has an influence on the size of the drying beds: since the more water the sludge contains, the more time will be required for its drying and, consequently, the smaller the throughput capacity of the beds will be; in this case, a larger size of them will be necessary.

In localities with a warm climate, sludge drying proceeds faster; in localities with a cold climate, more slowly, and therefore drying beds in the former will, other conditions being equal, be of somewhat smaller size than in the latter.

The construction of drying beds has a special influence on the size of the required area in the sense that covered ones possess a significantly greater throughput capacity than open ones, and therefore the former will have smaller dimensions than the latter.

The calculation of the size of drying areas must be preceded by the determination of the following elements:

1) the quantity of sludge that must be dried during the year;
2) the thickness of the layer with which the sludge is discharged onto the drying bed;
3) the number of discharges of sludge onto the drying areas during the year (or the drying time of each portion of sludge).

From these elements the required area of the drying beds is determined as follows.

Let: \(Q\) be the annual quantity of sludge in cubic meters,

\(n\) — the number of discharges per year,

\(h\) — the thickness of the layer of each sludge discharge, in meters.

Then the required area of the drying beds \(\omega\) is expressed by the formula:

\[ \omega = \frac{Q}{n \cdot h} \]

Taking, according to Imhoff, for each inhabitant 0.2 liter per day of digested sludge, 12 discharges per year, and the layer thickness of a discharge equal to 0.20 m, we obtain, according to the preceding formula, the required area \(\omega\) per inhabitant:

\[ \omega = \frac{0.2}{1000} \times \frac{365}{12} \times \frac{1}{0.2} = 0.03 \text{ sq. m.} \]

This standard for drying areas, per inhabitant, is given by Imhoff for German conditions, which permit sludge discharges to be made 12 times during the year.

The firm “Kremer-Klärgesellschaft” builds drying areas for the digested sludge of its installations on the basis of 0.04 sq. m per inhabitant.

The reciprocal of these figures gives the number of inhabitants corresponding to 1 sq. m of drying area.

According to Imhoff’s standard, 1 sq. m of drying areas corresponds to 33 inhabitants; according to the standard of Kremer’s firm, 1 sq. m of area corresponds to 25 persons.

If the size of the digestion tank, either because of imperfections in the construction or because of insufficient volume, does not bring the digestion of the sludge to the proper degree, then the areas must be of larger size; moreover, the worse the sludge is digested and the closer in its qualities it approaches fresh sludge, the larger the areas must be.

According to Babbitt’s data, for sludge from Emscher tanks with 85% moisture content, 32 sq. m of drying beds are required per 1000 persons (accord-

exists (according to Imhoff’s standard), whereas for other types of sludge or for drying beds the ranges vary from 83 to 830 sq. m per 1,000 persons, i.e., more than 2.5—25 times greater than for sludge from Emscher tanks.

Thus, only on the basis of experiments with the sludge for whose drying the beds are being designed can their proper size be determined.

With a prolonged winter, the number of discharges, if sludge drying is carried out only in the summer, is reduced, and the size of the bed must be correspondingly increased.

If, for our conditions, 6 sludge discharges in summer are assumed, then there may be two cases:

1) the winter portions of sludge are stored in digestion chambers and dried in summer—in this case the size of the beds must be doubled, since the beds bear a double load.

2) the winter portions of digested sludge are discharged into special winter sludge ponds, while the drying beds serve only for the summer portions—in this case the size of the drying beds will be ordinary.

For small installations, the size of the drying beds, because of the possibility of insufficient maintenance of them and also of the digestion chamber, is recommended to be doubled.

Calculation of drying beds with glass roofs.

According to the data of Kinnicutt, Winslow, and Pratt, provided that covered drying beds are heated, in order to maintain a temperature of \(T^\circ\) during the three winter months (for the conditions of C.-A.C.Ш.) above the freezing point, with an average thickness of the discharged sludge layer of 0.25 m, on 1 sq. m of covered bed 8.3 cu. ft. of sludge containing 87% water may be dried per year to a condition in which the sludge can be removed with shovels.

According to Imhoff’s standard, 1 sq. m of open bed can receive such sludge 2.4 cu. m per year, i.e., the heated covered bed has a carrying capacity 3½ times greater than the open one.

Data were cited above on covered beds in Cleveland and Canton, in which the carrying capacity of covered beds was higher than that of open beds by 2.5 (Cleveland)—3 (Canton) times.

Thus, the calculation of drying beds with glass roofs may be carried out by the same method and according to the same data as for open beds, with the assumption that the number of discharges per year may be increased 3-fold; in other words, for a covered bed the size may be taken as 3 times smaller than that obtained by calculation for an open one.

Information on the sizes of existing drying beds.

Below are data on the sizes of drying beds existing at the treatment plants of several cities (Table V).

Table V.

Dimensions of drying beds in certain cities.

City name	Type of structures delivering sludge	Number of inhabitants in the city	Size of drying beds in sq. m per inhabitant	Number of inhabitants per 1 sq. m of drying-bed area
Ludenscheid-Friedrichstal	Separate stone digesters	16,000	0.078	13
Steele	Emscher wells	30,000	0.036	28
Hattingen (Spar- und Bauverein)	Emscher wells	2,000	0.075	13
Iserlohn	Emscher wells	17,000	0.053	19
Witten	Emscher wells	22,000	0.068	15
Arnsberg	Emscher wells	5,000	0.026	38
Kettwig	Emscher wells	6,300	0.048	21
Essen-Rellinghausen	Emscher wells and separate stone digesters, part with activated sludge	45,000	0.016	63
Langendreer	Emscher wells	30,000	0.058	17
Gevelsberg	Emscher wells	17,000	0.030	33
Menden	Emscher wells	12,500	0.045	22
Velbert	Emscher wells	20,000	0.030	33
Neviges	Emscher wells	6,000	0.048	21
Netheim	Emscher wells	12,500	0.045	22
Wetter	Emscher wells	4,000	0.036	28
Amberg	Separate stone digesters, Kremer	30,000	0.040	25
Hildesheim	Separate stone digesters, Kremer	60,000	0.040	25
Bergedorf	Emscher wells, sludge to fields	—	0.100	10
Trenton, N. J.	Emscher wells, sludge to fields	150,000	0.050	20
Hartford, Wis.	Separate stone digesters	5,500	0.054	19

The variety in the size of the beds per inhabitant is explained by differences in the quality of the sludge, caused by the influx of industrial waters, as well as by other reasons.

Operation of drying beds.

The sludge placed on drying beds remains on them until it reaches such a condition that it can be handled subsequently with an ordinary shovel.

During drying, and depending on the amount of moisture lost, the sludge changes its character from dough-like to crumbly.

Dried, rotted sludge is for the most part dark-gray in color. It has the smell of vigorous healthy growth, very coarse and light-

it decomposes. In the fracture of a piece of such sludge, small and large voids are visible with which it is full and which are formed by gases that have escaped from the sludge. These gases saturate the sludge when it is in the digestion chamber, and after the sludge is discharged onto the drying bed, the gases volatilize from it and leave pores in it. Dried digested sludge has the smell of garden soil.

Its moisture content is 55–60%.

Investigations of the dry matter in digested sludge, carried out by Spilner and Bink, showed that the amount of organic matter in dried sludge, in comparison with the amount in raw sludge, decreases only slightly. A loss of organic matter of only 0.15–0.95% was found.

Thus, if digested sludge is discharged onto drying beds with a water content of 80% and loses moisture there to 60%, its weight is reduced by a factor of 2; when sludge with a water content of 85% is discharged and dries on the drying beds to the same moisture content of 60%, its weight will decrease almost 2.7 times. The volume of the sludge is reduced to a lesser degree than the weight, because of the looseness of the dried sludge.

The volume of sludge on drying beds decreases approximately 1.5 times.

Below is an example concerning the calculation of the effect obtained from the operation of digestion chambers together with drying beds.

Example. Fresh sludge containing 95% water weighs 100 tons. Determine: 1) how much the sludge will weigh after digestion in the digestion chamber to 80% moisture content?

2) How much will the same digested sludge weigh after drying on drying beds to 60% moisture content?

Solution. 1) Fresh sludge contains 95% water and, consequently, 5% dry matter. Therefore, 100 tons of fresh sludge contain 5 tons of dry matter, including, approximately, 65% organic and 35% inorganic, i.e. organic matter

\[ 5 \cdot \frac{65}{100} = 3.25 \text{ tons} \]

and inorganic matter

\[ 5 \cdot \frac{35}{100} = 1.75 \text{ tons}. \]

Since in the digestion chamber about \(1/3\) of the organic matter decomposes, passing partly into solution and partly into gases, \(2/3\) of the organic matter, or

\[ \frac{2}{3}\cdot 3.25 = 2.17 \text{ tons}, \]

will remain.

In all, the dry matter, mineral and organic together, will remain in the sludge after the latter has been digested:

\[ 1.75 + 2.17 = 3.92 \text{ tons}. \]

With a water content of 80% in the digested sludge, its weight will be:

\[ \frac{3.92 \cdot 100}{20} = 19.6 \text{ tons}. \]

Thus, the weight of digested raw sludge will be 19.6 tons. Consequently, the effect of the operation of the digestion chamber is expressed in a decrease in the weight of the sludge by

\[ 100 - 19.6 = 80.4 \text{ tons}, \text{ i.e. by } 80.4\%. \]

2) neglecting the reduction in the organic mass of the sludge during drying. Thus, according to the above, this is expressed by insignificant figures, i.e., assuming that the sludge dried on drying beds will contain the same 3.92 tons of dry matter, we obtain the weight of the sludge, at a water content in it after drying of 60%:

\[ \frac{3.92 \cdot 100}{40} = 9.8 \text{ tons.} \]

Thus, the weight of all the dried sludge will be 9.8 tons. Consequently, the effect of the drying beds will be expressed by a reduction in the weight of the sludge by

\[ 19.6 - 9.8 = 9.8 \text{ tons, i.e. by } 50\%. \]

The combined effect of the digestion chamber and the drying beds is expressed by a reduction in the weight of the sludge by

\[ 100 - 9.8 = 90.2 \text{ tons, i.e. by } 90.2\%. \]

Clearing the beds of dried sludge.

Removal of dried sludge from the drying beds may be carried out:

1) by hand,
2) by machines, and
3) by a combined method.

Removal of sludge by hand.

Removal of sludge by hand is carried out in the following manner. If the rail track is located on an embankment raised above the surface of the sludge, then a trolley is placed on the track at the beginning of the bed and loaded

Fig. 6.

they are thrown from it into a railway car. And so they gradually move forward toward the end of the bed.

If the railway track is laid level with the filtering material (beds of the first type) or along the bottom (beds of the second type) and, consequently, is located under the sludge, the tip-cart is advanced as far as the wall of the bed and the bed is first cleared at the beginning of the track lying beneath the sludge. Having cleared the end of the track, the tip-cart is moved onto it (part of the wall above the track must, in this case, be removable); thus, gradually clearing the bed and the track of sludge by moving the latter with shovels into the tip-cart, they advance toward the opposite end of the bed.

Figure 6 shows manual removal from the beds of dried sludge.

Removal of sludge by mechanical means

For the removal of dried sludge from beds, machines of the following types are used:

1) cranes,
2) scrapers.

Figure 7 shows one of the machines of the second type. The machine is designed in such a way that it can remove sludge simultaneously from two adjacent beds. Depending on the location of the level of the drying bed in relation to the level of the railway track along which the machine moves, its frames, supporting the endless chains with buckets, may be set higher or lower and, moreover, independently of one another. The machine permits sludge-removal work to be carried out on one side only, while the other half is not operating. The buckets attached to the endless chain have a special shape, allowing them to scoop up the dried sludge from the bed and dump it into an assembly bunker located in the upper part of the portal. The bunker has, in its lower part, a slide gate by which the outlet opening is closed. From the bunker the sludge can be loaded into the bodies of railway cars, which are brought under the discharge opening of the bunker and filled with dried sludge when the bunker slide gate is opened.

Fig. 7.

The machine is driven by an electric motor, a gasoline engine, or an oil engine. It can move both in the longitudinal and in the transverse direction, and in this way it can be used to unload any area; an approximate plan of these is shown in Fig. 8.

Fig. 8.

Such a machine is serviced by only one man, who also loads the cars from the bunker.

The normal capacity of the machine is 25 cubic meters per hour for each battery (i.e., when working on one side). Thus, the total capacity of the machine, when both batteries are in operation, is 50 cubic meters of sludge per hour.

Assuming that the machine is used at \(1/4\) of its full capacity, its annual output, with an 8-hour working day and 300 working days per year, is

\[ \frac{50 \cdot 8 \cdot 300}{4} = 30{,}000 \text{ cubic m} \]

of sludge.

On the basis of this figure, E. Nèllissen gives the following economic comparison of machine and manual labor in unloading drying beds of dried sludge.

Machine work. The cost of the machine is about 13,890 rubles. At 15% for repayment of the loan and payment of interest, the following annual expenses are obtained for the various types of engines:

ITEMS OF EXPENDITURE	Annual expenditure in rubles. Electric motor	Annual expenditure in rubles. Gasoline engine	Annual expenditure in rubles. Petroleum engine
Annual cost of the installation (15% of the cost of the machine)	2083	2083	2083
Wages of one worker	1158	1158	1158
Current and fuel	583	509	389
Oil	23	35	28
Material for cleaning and lubrication	37	46	46
Spare parts	139	139	139
Total per year	4023	3970	3843

With an annual output of the machine of 30,000 cubic meters of sludge, the cost of machine work per 1 cubic meter of sludge with the various forms of energy will be, respectively: 13.4 kopecks, 13.2 kopecks, and 12.8 kopecks.

Manual labor. Assuming that, on average, one worker loads about 1.2 cubic meters of sludge into a trolley per hour, or 9.6 cubic meters per 8-hour working day, then to load 30,000 cubic meters of sludge over the course of 300 working days (per year), a gang of workers will be required:

$$ \frac{30000}{300 \times 9.6} = 11 \text{ persons.} $$

The annual expenditure will be expressed by the following figures:

Item	Amount
Depreciation of equipment, shovels, etc.	463 rub.
Wages of a gang of workers of 11 persons	787 rub. × 11 = 8657 rub.
Salary of an attendant for supervision and accounting	926 rub.
Social charges	338 rub.
Administrative expenses	354 rub.
Total per year	10338 rub.

Per 1 cubic meter of sludge this amounts to:

$$ \frac{10338}{30000} = 34.4 \text{ kopecks.} $$

When comparing the cost of manual work with machine work (using electric power), a saving is obtained with machine work of \(34.4 - 13.4 = 21\) kopecks per 1 cubic meter of sludge, and the total saving for the year is expressed by the sum of \(10338 - 4023 = 6315\) rubles.

This calculation is based on an annual quantity of sludge of 30,000 cubic meters, and thus for a very large city it may be regarded as demonstrative. For smaller cities, however, manual labor may prove more advantageous.

Removal of sludge by a combined method. The combined method of sludge removal consists in the dried sludge being removed from the site by hand with shovels and thrown into the bucket of a crane or onto a belt conveyor. The crane or conveyor then delivers the sludge into a trolley.

The crane is moved along a rail track on which its cars and bypasses run.

The conveyor, mounted on a two-wheeled carriage, is moved directly over the drying bed. With wide beds, two conveyors may be used, one of which transfers the sludge to the other, and the latter discharges it into a car (Fig. 9).

Fig. 9.

Character of the drainage water flowing from sludge beds and its treatment.

Since drainage water, flowing from other sludge beds during the drying of sludge, is discharged into a watercourse, its ultimate quality cannot be a matter of indifference from the sanitary point of view.

In this sense it is necessary to distinguish the drainage water obtained when fresh and digested sludge are dried. After fresh sludge has been dried, the water, as containing a large quantity of organic substances, is subject to pollution, and therefore, where the capacity of the watercourse into which it is discharged is insufficient, it must be subjected to biological treatment.

Drainage water obtained when digested sludge is dried is water already, to one degree or another, digested and possessing a lesser capacity for pollution (especially when the digestion chambers have not been properly operated). In addition, the filtering material of the sludge beds (in beds of the first type) acts as a biological oxidizer: the organic substances retained in the sludge water are absorbed to a certain degree by the filtering material, mineralized by microorganisms, and washed out into the drain water or in subsequent sludges. Thus, the drainage water from digested sludge does not require special treatment. Its quantity itself is also negligible.

For Essen-Nord, for example, according to Spillner and Blunk, the quantity of sludge water is equal to 0.1 liter per 1 cubic meter of wastewater, or 1/10000 of the wastewater flow.

FILTER PRESSES.

There are several systems of sludge filter presses. Filter presses (Fig. 10) of any system essentially consist of a series of cast-iron plates of round or square form, having an area on average of about ¾ square meter. On the surface of the plates there are grooves which, when the plates are set in place, take a vertical direction. The edges of the plates project above both surfaces of the plates by approximately 2.5 cm, with a width of 5–7.5 cm. The surfaces of these edges are free of grooves and are finished smooth so that dense adjoining—

Fig. 10.

the belt, when one plate is pressed against another. Thanks to this arrangement, the plates, pressed one against another by their rims, form chambers about 5 cm wide with their grooved surfaces.

Each plate has an opening in the center 15–20 cm in diameter.

Both surfaces of each plate are covered with panels of canvas or another heavy filtering fabric. In the center of each panel an opening is cut, of a diameter somewhat smaller than the diameter of the plate opening. The edges of the two panels of one and the same plate are sewn together: the outer edges to the outer edges, the inner edges to the inner edges. Consequently, the plate is surrounded by fabric, while the inner opening remains open. Instead of sewing the outer edges of the panels, these edges may be attached to the outer rims of the plates. The plates thus prepared are mounted on frames that move along lateral guides. The number of plates in filter presses ranges on average from 50 to 125. Before the machine is started, the plates are pressed tightly against one another—in small presses by hand, in large ones mechanically. After this, sludge is introduced into the channel of the machine, formed by the central openings of the plates; under a pressure of 3.5–7 atmospheres it fills the chambers between the plates and gives off water through the fabric into the grooves, along which it flows down to the lower part of the filter and from there into the discharge pipe. The pressure is maintained for ¼–½ hour. When working with activated sludge, the pressure is raised to 9 atmospheres and maintained for 1–2 hours. After the proper pressing time has elapsed, pressing is stopped, the plates are moved apart, and the sludge briquettes are dumped from the press chambers into mine cars. The briquettes contain on average 60–70% water. The filtering fabric serves 30–45 days, after which it must be replaced.

Pressing sludge is facilitated by adding lime to it in the amount of 3–7% of the weight of the sludge being pressed; in this case the briquettes are more durable and it is easier to handle them subsequently.

The addition of lime in the form of milk of lime is usually carried out in a special tank into which the sludge enters and where, after the lime has been added, it is subjected to settling for 12–24 hours. The water is removed, and the sludge that has settled, by means of compressed air, is driven through a grate into the presses.

The liquid squeezed out of the sludge is very turbid, is liable to putrefy, and therefore, depending on local conditions, requires one or another treatment. In some cases it enters the sewer that brings wastewater to the treatment plant and, together with it, passes through the treatment processes; in others it is treated separately by a biological method.

The capacity of presses depends to a considerable extent on the character of the sludge; therefore, a definite guarantee of capacity is rarely given by manufacturers.

The treatment of sludge by means of filter presses has the following advantages:

1) The small area required for the sludge-treatment installation—a feature that may prove very important when there is a shortage of space for air drying of fresh sludge or when this place is close to residential districts.

2) Rapidity of operation in comparison with drying in air.

3) Slight odors from the sludge, both during its treatment (owing to the rapidity of the process) and after treatment (because with the water squeezed out, ...

from the sludge, a considerable amount of organic matter subject to putrefaction is removed).

At the same time, this method of sludge treatment has the following very substantial drawback.

This is its high cost, caused by:

1) the high cost of the equipment and of operation, and
2) the necessity of adding lime before pressing.

CENTRIFUGES.

The simplest system of sludge centrifuge consists of a perforated metal drum, covered with filtering cloth and rotating inside a second metal drum. The sludge is placed in the inner drum; as the latter rotates, the sludge is thrown against the perforated walls and gives up its water through the filtering cloth into the outer drum, from which the water is removed through a discharge pipe. The peripheral speed of the inner drum is about 1800 meters per minute.

A drawback of centrifuges of this system is the need to stop the machine each time it is unloaded and filled.

The most advanced centrifuge is the “The Besco-ter-Meer,” which can operate continuously without stopping for loading and unloading sludge.

This machine (Fig. 11) consists of a drum rotating at high speed on a vertical hollow shaft. The drum has six radial, star-shaped chambers, each of which is divided into two compartments by a vertical screen parallel to one side of the chamber. Thus, one of the compartments of each chamber has parallel sides. All six compartments with parallel sides are provided at both ends with openings that are closed by slide gates. Through one opening the compartment can communicate with the hollow shaft, and through the other—with the space surrounding the drum and bounded on the outside by a casing. The other six compartments each have in their lower part an opening, each of which communicates, by means of pipes, with an annular channel located beneath the drum.

The operation of the centrifuge is as follows.

At the beginning of the drum’s rotation, the inner openings that connect the compartments with parallel sides to the hollow shaft are open, while the outer openings are closed. Therefore the sludge, entering through the hollow shaft, enters these compartments and, under the influence of centrifugal force, is compressed in them, giving up water through the screens into the adjoining compartments. From the latter, the water flows into the annular channel and from it passes out through the discharge pipe.

When the sections with parallel sides become completely filled with dewatered sludge, the inner openings close automatically and the outer ones open; the dried sludge mass, under the influence of centrifugal force, is thrown out of the drum, strikes the casing of the machine, and, breaking into small pieces, falls through a hopper into the body of a small wagon or onto a conveyor.

Sludge dried by a centrifuge contains 50–70% water. The liquid flowing out of the centrifuge is highly putrescible and therefore, depending on conditions, is subjected to one or another

treatment: it is either directed to connection with the untreated wastewater and passes together with the latter through all the treatment processes, or is subjected to separate treatment by a biological method.

Fig. 11.

The hourly capacity of such a centrifuge with a motor of 15 horsepower at 900 drum revolutions per minute, according to the manufacturer’s data, is 1.8 tons of sludge containing 60–70 percent moisture.

Treatment of sludge in a centrifuge has the same advantages as treatment of sludge by filter presses.

But, just as in treatment by filter presses, centrifuging of sludge is expensive.

VACUUM FILTERS.

A vacuum filter is a drum rotating about a horizontal axis, the skeleton of which is covered with a filtering cloth supported by a wire mesh. The interior of the cylinder is divided, by walls along its axes, into several compartments into which rarefaction or compressed air may be introduced. During rotation the cylinder is partially immersed in the sludge that is located in a trough installed under the cylinder. In the compartment of the cylinder that is immersed in the sludge, rarefaction of the air is produced, as a result of which the sludge is drawn onto the filtering cloth; and as this compartment emerges from the trough, water is sucked from the sludge as it is pressed against the cloth, thanks to the vacuum. After \(3/4\) of a revolution of the cylinder, i.e. shortly before the next immersion of the compartment in the trough with the sludge, some compression of air is produced in the compartment, as a result of which the dried sludge is blown off the filtering surface and removed by a plate into the sludge receiver.

The speed of rotation of the cylinder is about 0.3 m per minute at the circumference.

The equipment shown in Fig. 12 depicts an Oliver filter having the following dimensions: length 4.3 m, diameter 3.5 m, and 46.0 sq. m of filtering surface. The capacity of such a filter is approximately 0.05 cu. m of sludge per sq. m of filtering surface per minute.

The evaluation given above for filter presses and centrifuges is also applicable to vacuum filters.

Fig. 12.

Rotary Dryers

In certain cases of utilization of sewage sludge, its moisture content must be reduced to 10% or even less. Neither air drying nor mechanical methods of dewatering bring it to such a condition. In these cases it is necessary to resort to artificial drying of the sludge in rotary dryers, whereby, as a preliminary step, the sludge is usually freed of excess water by methods of mechanical drying.

Figure 13 shows one of the rotary dryers. The dryer consists of two concentric rotating cylinders, which are joined together by special tubes placed as indicated in the drawing by the letter A. The left end of the cylinders rotates in a chamber, and the space between the outer cylinder and the chamber serves for the passage of combustion gases from the firebox located in the lower part of this chamber. From this space the gases enter, through tubes A, into the inner cylinder and pass through this cylinder to its right end. Here the gases change direction and, through the annular space between the inner and outer cylinders, move to the left end of the installation, from which they enter the smoke pipe through an exhauster.

Fig. 13.

The feed to the dryer is introduced into the annular space between the two cylinders at the point designated “Feed.”

Since the right end of the cylinders is somewhat lower than the left end, and since blades arranged longitudinally are attached to the outer cylinder, the sludge rises upward as the cylinders rotate and then falls; gradually advancing toward the right end, it is discharged from there in a dried state.

Thus, drying of the sludge is carried out by hot gases both indirectly, through the walls of the cylinders, and directly, by direct passage of the gases through the annular space.

The dryer is about 18.3 meters long and 2 meters in diameter. Its capacity is expressed as the evaporation of about 2720 kg of water per hour. The temperature of the gases in the annular space (in contact with the sludge) is about 370° C. The rotational speed of the cylinders is about 8 revolutions per minute.

Odors from dryers can be avoided by directing the gases through a dust collector and scrubber.

Since, during drying in tray dryers, the sludge cakes into lumps, in order to make its further utilization possible it has to be ground in special mills.

Artificial drying in tray dryers requires extensive equipment and considerable fuel expenditures.

THE DICKSON PROCESS.

The method of sludge dewatering first applied on a large scale by Dickson in Dublin (Ireland) consists in mixing fresh sludge with 0.5% brewer’s yeast and leaving it to ferment for 20—24 hours at a temperature of 32°—34° C. As a result of fermentation the sludge floats up, and the water beneath it is discharged. In this way the water content of the sludge is reduced to approximately 80%.

REDUCTION OF THE WATER CONTENT OF SLUDGE BY MEANS OF SULFURIC ACID AND SULFUROUS ANHYDRIDE.

1) A method of reducing the water content in sludge by means of sulfuric acid, which has not yet passed beyond the experimental stage, has been tested primarily with activated sludge. This process consists in mixing a certain quantity of sulfuric acid into sludge heated to a temperature of 38°—40° C. (sometimes, in addition to sulfuric acid, sodium carbonate is first added). The addition of sulfuric acid releases carbon dioxide from the sludge (or from the sodium carbonate) and causes the sludge to float to the surface. The water beneath the sludge can be discharged. Sludge treated in this way contains about 86% moisture.

2) The process of treating sludge with sulfurous anhydride has likewise not yet found practical application. This process, tested with activated sludge, consists in blowing into it sulfurous anhydride (gas) mixed with steam, which is obtained by burning sulfur. In this way the sludge is changed from a flocculent state into a granular one, thereby reducing the water content of the sludge.

COMPOSTING.

Composting, by which is meant the mixing of raw or dried sludge with dry municipal refuse, is a means of reducing the water content of the sludge, since refuse that is poor in water, especially if it contains ash, absorbs part of the moisture from the sludge.

Manual work in mixing sludge with refuse is difficult and costly; therefore this method has not become widespread.

In Kassel, the mixing of fresh raw sludge with municipal refuse is carried out in a primitive manner. Sludge is lifted from the sedimentation basins by means of a vacuum boiler, from which it flows by gravity into a natural depression in the ground—a sludge pond located near the treatment station. Into this same pond is poured ...

waste delivered by municipal cartage. No artificial mixing of the sludge with waste is carried out.

Recently, in connection with the use of mechanical devices for mixing sludge with waste, interest in the use of the composting method has again appeared.

Thus, according to data from 1927, in Munich it was intended to apply a partial use of raw sludge, mixing it by means of a special mixing apparatus with fine urban refuse obtained in the sorting of household refuse.

Subsequently such a mixture may be used as fertilizer or subjected to incineration in refuse-incineration furnaces (see below).

COMBINATION OF SOME METHODS.

Sometimes the methods described above for reducing the moisture content of sludge are used in combination with one another.

Thus, for example, treatment of sludge by means of filter presses or centrifuges precedes its further dewatering in drying pans; after composting, air drying is used; treatment with chemical substances may precede further drying in air or dewatering by mechanical means, etc.

V. Disposal of Treated Sludge.

The disposal of sludge which, for the purpose of reducing its moisture content, has undergone one or another treatment—the various methods of which were described in the preceding chapter—is carried out by one of the following methods:

1) removal to the sea.
2) burial in the ground.
3) dumping in low-lying places, and
4) incineration.

REMOVAL TO THE SEA.

The removal to the sea of raw, untreated sludge was described in Chapter III.

This same method, under suitable conditions, may also be applied to sludge that has been previously treated in order to reduce its volume and weight and, consequently, to reduce the cost of transport.

Thus, in Providence (U.S.A.), the removal of sludge to the sea is preceded by its treatment on filter presses.

BURIAL OF SLUDGE IN THE GROUND.

Sludge with moisture content reduced by one method or another can be removed with considerably lower costs than raw sludge, into a field and buried there or plowed into the soil. The area of the field required ...

For this purpose, sludge is considerably less suitable than in the case when the removal of raw sludge is carried out by the usual method. Kinnicutt, Winslow, and Pratt believe that the area of the field for burying dry sludge may be at least five times smaller than when burying raw sludge from the same side—owing to the reduction in the volume of sludge during drying. On the other hand—owing to the circumstance that the field may thereby be used again sooner.

DUMPING SLUDGE IN LOW-LYING PLACES

This method consists in the dried sludge being transported to places where there are lowlands and dumped in them.

In this case, when dried fresh sludge is transported, these lowlands must be located in an area sufficiently distant from dwellings and traveled roads, and one that cannot soon be used for building purposes, since the sludge will emit unpleasant odors.

If, however, well-rotted sludge is transported, the dumps may be located near inhabited places.

In Worcester, Mass., sludge from sludge presses is conveyed by carts in wagons, which are taken a short distance by electric locomotive to a deep valley located 1.2 km from the press station. Here the sludge is unloaded from the overpass.

In Bochum (Germany), sludge transported out for filling a valley is dumped there in embankments 3 meters high. The sludge was dumped with fairly steep slopes and was sufficiently dense to support a rail track for wagons.

The Emscher Association makes extensive use of this method of sludge disposal for correcting irregularities in the soil resulting from the development of coal mines.

Thus, since disposal of sludge by filling low-lying places also performs another task—leveling areas that may have practical significance—this type of sludge removal may be classified, rather, as a method of utilization.

INCINERATION OF SLUDGE

Experiments with sludge incineration have shown that it presents no special difficulty to incinerate sludge if the water content in it does not exceed 60%, although usually its incineration is carried out together with municipal refuse or with the admixture of some quantity of fuel.

Thus, in Ealing-on-Thames (England), in 1883, a destructor was built for incinerating municipal refuse and sludge. The destructor installation consisted of seven Fryer chambers, each with a grate area of 1.50 × 1.50 meters. In 1889 a crematorium was attached to this installation, the hot gases from the crematorium passing through the destructor. At first, sludge from nearby raw sludge beds was mixed with refuse; from 1902, sludge in compressed form with a water content of about 60% has been delivered to the destructor chambers, and there it is mixed with dry household refuse in the proportion: 1 part sludge briquettes to 1.5–2 parts refuse. In this way from 2,500 to 3,000 tons of pressed sludge were liquidated per year.

In Worcester, Mass., in 1891, experiments were carried out on the incineration of sludge. In one of these experiments, 45 tons of sludge, containing about 46% water, were burned with a certain quantity of firewood. In another experiment, sludge was burned at a rate of about 2 tons of sludge over 3 hours with a small consumption of fuel.

Extensive experiments on the incineration of sludge were carried out in the town of Huddersfield (England), where for this purpose a two-chamber destructor of the Horsfall system was used, with a floor of fire-brick grates of 3 sq. m in each chamber. In these experiments, pressed sludge was mixed with coke refuse or clinker from gas works, which contained a sufficient quantity of combustible matter. The mixing proportion was as follows: 5 parts sludge to 1 part fuel.

In some cases the clinker obtained in the incineration of sludge was used for various purposes, e.g., as loading material for biological oxidizers, as an aggregate for concrete, etc.

VI. Utilization of Sludge.

As already mentioned above (Ch. II), sewage sludge may in certain cases be utilized, since it contains certain valuable substances.

Thus, sludge may be used:

1) for agricultural purposes, namely:

a) as a direct fertilizer, and

b) as material for the factory preparation of a saleable fertilizer;

2) for the extraction of fat from it; and

3) for the extraction of gas.

USE OF SLUDGE FOR AGRICULTURAL PURPOSES.

The use of sewage sludge for agricultural purposes is conditioned by the content in it of fertilizing substances, namely: nitrogen compounds, phosphoric acid, potassium, calcium, magnesium, and organic substances.

The content of fertilizing substances in sludge fluctuates within considerable limits depending on the character of the sludge, and, moreover, regardless of the method by which the sediment is separated from the wastewater; from this point of view, the most valuable is sludge formed by domestic waters. Industrial waters, depending on the content of various impurities (acids, resins, etc.), reduce the value of sludge as a fertilizing agent and may even render it wholly unsuitable for the indicated purpose. Such, for example, is sludge obtained after the chemical treatment of wastewater as a result of a high content of fat and iron and lime salts.

Table VI gives analyses of sludge of various character for certain cities.

Table VI.

Analyses of sewage sludge from certain cities.

Character of sludge and name of city	Nitrogen	Fat	Volatile substances	Nonvolatile substances
	in percent of dry basis	in percent of dry basis	in percent of dry basis	in percent of dry basis
Sludge from Imhoff tanks
Atlanta	1.37	4.8	34.1	65.9
Fitchburg	1.71	4.6	35.3	64.7
Batavia	2.93	3.1	47.0	53.0
Sludge after chemical treatment
Worcester	1.62	1.2	41.8	58.2
Activated sludge
Milwaukee	5.82	2.2	67.8	32.2
Houston	4.64	7.1	62.9	37.1

In order to determine the fertilizing value of sludge, comparisons were made with several kinds of fertilizers.

Thus, Hough gives a comparison of fresh dried sewage sludge with manure fertilizer (Table VII), while Stutzer gives a comparison of dried sludge with manure fertilizer (Table VIII).

Table VII.

Analyses of fresh sludge and manure fertilizer.

Constituents	Fresh sludge	Manure fertilizer
Water	75.00	75.00
Nitrogen	0.71	0.50
Phosphoric acid	0.41	0.25
Potassium	0.10	0.63
Ash	3.50	4.76
Organic substances	14.65	20.24

Table VIII.

Analyses of digested sludge and manure fertilizer.

Constituents	Digested sludge	Manure fertilizer
Water	46.00	77.00
Nitrogen	1.06	0.54
Phosphoric acid	0.71	0.25
Potassium	—	0.70
Inorganic substances	30.00	6.00
Organic substances	24.00	17.00

Arnhold gives an analysis of sludge, i.e., of sludge deposited at river mouths when they flow into the sea, which has long been used as a fertilizing agent in coastal districts (Table IX).

Table IX.

Analysis of sludge.

Constituents representing value	%
Nitrogen	0.30—0.50
Phosphoric acid	0.26—0.28
Potassium	0.60—0.90
Lime	5.90—7.50
Magnesia	1.60—1.80
Organic substances	8.00—10.00

All these data characterize sewage sludge as a fertilizing substance which, in the quantity of valuable elements contained in it, is not inferior to other types of fertilizers that have quite proved themselves.

The organic substances contained in the sludge, which enrich the fertilized soil with humus, are of considerable value. Humus retains moisture in the soil, preventing both its too rapid seepage into the subsoil and its too rapid evaporation. In this sense, sludge is equally valuable both for heavy clay soils, loosening them, and for sandy soils, enriching them with humus.

In addition, the bacterial richness of sludge plays a great role: thanks to the activity of bacteria, heat is generated in the soil, and the gases appearing as a result of their vital activity promote the loosening of the soil.

Sludge for direct use as fertilizer is employed:

1) in the raw state, and
2) after drying.

In the raw state, sludge is used for fertilizing fields only in exceptional cases, when the agricultural lands are in the immediate vicinity of the treatment stations.

In this case the following methods may be employed:

1) It flows by gravity, or is delivered by means of a pump, from the settling installations or from the digestion chambers into barrels, is carted to the fields, and is spread there. After drying, the sludge is plowed into the soil. Of interest is the arrangement in the city of Oberhausen of a well on columns, from which the sludge can be discharged directly into barrels (Fig. 14).

Fig. 14.

2) The sludge is pumped onto the fields by pumps in a thin layer. Thus, in the city of Hagen i. W., an area of field is covered with sludge twice in 3 years, with a layer of 0.20 m. After drying, the field is plowed. Fig. 15 shows the discharge of sludge onto the pre-drying field; the sludge is delivered by a pump from the installation of the city of Essen–Rellinghausen.

Fig. 15.

is utilized much more often in a dried state.

In this case, sludge with considerably lower costs than raw sludge is transported to the fields and plowed in there. Fig. 16 shows the transport to the field of dried, digested sludge from the Essen-Nord installation.

Only after oven drying, when the sludge has caked into lumps, is the dried sludge preliminarily ground in special mills and scattered on the field in a powder-like state, the latter operation being carried out either by hand or by machine.

Fig. 16.

Comparison of fresh and digested sludge from the standpoint of their fertilizing value.

Fresh and digested sludge, both chemically and physically, show a great difference. Certain properties of both kinds of sludge have already been considered above, and from both the technical and sanitary standpoint all advantages prove to be on the side of digested sludge. In this respect there are no two opinions. As for the comparative value of fresh sludge and digested sludge as a means of fertilization, opinions differ here.

Some prefer to use fresh sludge, others digested sludge.

The former substantiate their views with the following considerations:

1) fresh sludge contains more fertilizing elements, since, owing to digestion, about 40% of its nitrogenous substances are lost.

2) Digested sludge contains more sulfides, especially iron sulfides, which, upon further decomposition in the soil, may harm the finest roots of plants.

The latter cite the following motives in defense of their views:

1) Rotted sludge, in terms of its technical and sanitary qualities, is a more acceptable material to handle than fresh sludge.

2) Fresh sludge, when it is raw, cakes the soil, clogging it with the finest paper and fabric fibers, which impede the access of air to the roots of plants. When it is dried, it forms clods with an undried core and a hard dry crust; these clods do not disintegrate for a long time. In rotted sludge the majority of fibrous substances have already undergone decomposition; rotted raw sludge dries quickly and forms clods that readily crumble owing to their porous structure.

1. Fresh sludge gives more weeds in the field: weed seeds that have entered it with sewage retain their ability to germinate; in rotted sludge these seeds have undergone decomposition and therefore do not sprout.

2. The harm from the presence of sulfides in rotted sludge can be eliminated if the sludge is allowed to lie for some time in the air before it is plowed into the soil: then the sulfides oxidize into harmless sulfates.

Sierp, on fields belonging to the treatment plant of the city of Essen-Rellinghausen, carried out comparative tests of the productivity of land:

1) without fertilization,
2) with fertilization by fresh sludge (together with sediment from the sand trap), and
3) with fertilization by rotted sludge (the rotted sludge, before use, lay for some time in the air).

The experiments were carried out with the planting of the following vegetables: carrots, beans, cabbage, potatoes, and turnips.

The results of Sierp’s experiments are shown by the data of Table X. The figures in this table give, in percent by weight, the surplus (+) or deficit (—) of produce harvested from plots fertilized with sludge, in comparison with the quantity of produce harvested from unfertilized land.

Table X.

Comparative productivity of land without fertilizer and of land fertilized with fresh and rotted sludge.

Crop	Fertilization with fresh sludge	Fertilization with rotted sludge
	Surplus (+) or deficit (—) of produce in % by weight, compared with unfertilized soil	Surplus (+) or deficit (—) of produce in % by weight, compared with unfertilized soil
1. Carrots	+ 16.8	+ 20.8
2. Beans
a) 1st variety	— 3.2	+ 46.8
b) 2nd variety	— 16.3	+ 46.8
3) Cabbage
a) Wirsing	—	+ 63.6
b) Red	+ 7.7	+ 16.8
c) White	+ 24.0	+ 47.7
4. Potatoes	+ 17.6	+ 49.0
5. Turnips	+ 7.0	+ 9.0

Thus, according to the results of his experiments, Sieper emphasizes the advantage of digested sludge over raw sludge from the standpoint of its fertilizing value.

Activated sludge has a very high fertilizing value. It contains 2–4 times more organic nitrogen than sludge of other kinds. This circumstance is due to the following reasons.

1) In activated sludge, the organic substances contained in it do not undergo decomposition.

2) When sewage is treated with activated sludge, the finely suspended substances and colloids, which could not be captured in the sludge by simple settling, pass into the sediment.

However, activated sludge has a considerable drawback in the sense of the high water content in it (97–99%), although removable by some of the methods described above;¹ for example, in Milwaukee secondary settling of activated sludge is used (reducing moisture from 98–99% to 95–97%), followed by pressing (reducing moisture to 70%) and concluding with drying (reducing moisture to 10%).

Data on the selling price of sludge.

Below are reported data on the selling price of sludge, obtainable by the sewerage treatment plants of several cities for fertilizing purposes.

Thus, digested sludge from the Emscher tanks of the city of Krefeld, dried on drying beds, is sold at the following prices:

City	Selling price
Hildesheim	46 kopecks per cubic meter
Gerdauen	1 ruble 85 kopecks per cubic meter
Ohrdruf	93 kopecks per cubic meter
Prenzlau	23 kopecks per cubic meter

Digested sludge from the Emscher wells, dried on drying beds, in the city of Bergedorf is sold at the price of 56 kopecks per cubic meter.

At the treatment installations of the Emscher Association, dried sludge from Emscher wells is sold at the price of 23 kopecks per cartload.

In the city of Cassel, fresh sludge from settling basins, mixed with municipal refuse, after a year of curing, is sold at the price of 23 kopecks per cartload.

Use of sludge as a material for the manufacture of artificial fertilizer.

In general, sewage sludge is an incomplete fertilizer, especially owing to the low content in it of phosphoric acid and potassium.

Thus, according to Hönig’s calculations, from sewage it may be possible to obtain, in round numbers (per hectare), 35 kg of nitrogen, 21/2 kg of phosphoric acid, and 25 kg of potassium.

The required quantity of nutrients for the soil, when cereals are grown per hectare, is, according to Hönig, 31 kg of nitrogen, 50 kg of phosphoric acid, and 100 kg of potassium; according to Dethweiler, 60–100 kg of nitrogen, 80–120

kg of phosphoric acid and 100 kg of potassium. The stated norms differ considerably from one another, which may be explained by differences in the character of the soils.

When sludge is supplemented with the missing quantities of fertilizing elements, it is equivalent to an artificial commercial fertilizer. Extensive experiments in this direction were carried out in Milwaukee, where up to 100 tons of activated sludge were processed daily into fertilizer. In the state of Wisconsin (U.S.A.) extensive experiments were conducted in planting cereal crops in fields fertilized with activated sludge, to which the necessary amount of phosphoric acid and potassium had been added. The trials gave excellent results, in some cases proving better than when artificial commercial fertilizer was used.

Therefore, at present sludge is finding application in plants that manufacture fertilizing substances. These plants use sludge as a base in the manufacture of artificial fertilizer, supplementing it with the elements it lacks.

EXTRACTION OF FAT FROM SLUDGE

The extraction of fat from sludge may pursue a twofold aim:

1) To increase the value of the sludge as a fertilizer. Since fats, being a component part of sludge, are not only not assimilated by plants or bacteria, but also worsen the quality of the soil, burdening and impeding the access of air to the roots of plants.

2) To utilize the extracted fat, which in itself represents a certain value.

Special interest in the process of extracting fat from sewage sludge began to appear from the time when the method, patented in 1915 by the engineer G. W. Miles, for treating sewage waters with sulfuric acid and ferric anhydride was introduced into practice: this treatment, owing to the conversion of soap into fatty acids, considerably increases the fat content of the sludge.

The extraction of fat from sludge requires very complicated and expensive equipment (production of the acid, pressing of the sludge, drying, extraction of the fat, distillation of the fat). It may be conceived of as a commercial undertaking only under the condition of a very high fat content in the sludge, caused by the special character of sewage waters, characteristic only of certain branches of industry (for example, wool-scouring establishments).

As for the sludge of municipal (domestic) sewage waters proper, extracting fat from it is an undertaking that is certainly unprofitable.

EXTRACTION OF GAS FROM SLUDGE¹

The extraction of gas from sludge was originally carried out by the following methods:

1) dry distillation of the sludge, i.e., heating it without access of air, and

2) carburetion, i.e. blowing dried and finely crushed sludge with air (production of generator gas) or with steam (production of water gas).

At the present time these methods, because of the large expenditures on fuel, which are not covered by the value of the product obtained, have been abandoned.

Now, the extraction of gas from sewage sludge is not an independent operation. Gas is obtained during the digestion of sludge—a process whose purpose is the well-known preliminary preparation of sludge. Only measures are taken to collect the gas, consisting in the installation of:

1) gas-tight covers over the digestion chambers,

2) gas receivers taking up the gas retained by the covers,

3) gas pipelines carrying the gas from the gas receivers to a gas holder or to the points of its consumption.

This gas consists everywhere of methane, carbon dioxide, and nitrogen; sometimes, in small quantity, besides these it contains hydrogen, hydrogen sulfide, oxygen, and carbon monoxide. The quantities of the individual elements making up digestion gas differ for different installations.

O. Kammann gives the following average composition of the gas for European installations:

Component	Content
Methane	80%
Carbon dioxide	20%
Nitrogen	0—8%

In general, owing to the high content of methane in digestion gas, this gas is combustible, with a high calorific value, exceeding the calorific value of illuminating gas and reaching in some cases 8000—9000 calories.

The quantity of digestion gas obtained depends on various conditions, namely:

1) the temperature of the digestion chambers,

2) the perfection of the contact of fresh sludge with old,

3) the quantity and character of the organic matter in the fresh sludge,

4) the character of the wastewater, and

5) the degree of alkalinity of the medium.

On average, according to German data, for each inhabitant connected to the sewerage system, 3 cubic meters of gas per year may be obtained.

Digestion gas is utilized:

1) in raw form, and

2) in purified form.

In raw form it is consumed:

a) for heating boilers, lighting, and cooking food;

b) for gas engines;

c) for obtaining steam and electrical energy;

d) as fuel for refuse-incineration stations;

In purified form it may be used:

a) for autogenous cutting and welding,

b) in the chemical industry for obtaining certain substances, for example: hexamethylenetetramine and halogen derivatives of methane.

VII. Data on the Cost of Various Methods of Treatment and Disposal of Sludge

There is limited information concerning the cost of the various methods of treatment and disposal of sludge.

Below are given data reported by Kinnicutt, Winslow, and Pratt, based on the materials of the English Royal Commission, which may give some idea of the comparative cost of the various methods of treatment and disposal of sludge (Table XI).

Table XI.

Cost of certain methods of treatment and disposal of sludge.

Method of treatment and disposal of sludge	Number of examples	Lowest cost of treating sludge containing 90% water, including interest and amortization of capital and all other expenses, in kopecks per ton	Highest cost of treating sludge containing 90% water, including interest and amortization of capital and all other expenses, in kopecks per ton	Average cost of treating sludge containing 90% water, including interest and amortization of capital and all other expenses, in kopecks per ton	Note
Removal to fields	5	5	11	8
Removal to sea	6	16	26	19
Burial in the ground	3	15	27	19
Pressing, group I	10	18	28	23	This group includes cities with a population of 30,000 and more, where the wastewater is subjected to simple settling or chemical treatment and where no especially large quantity of lime is added to the sludge.
Pressing, group II	11	29	48	44	This group includes cities with a population of less than 30,000, as well as small towns where, depending on the character of the sludge, from 5 to 20% lime is added.
Pressing and incineration	1	Excluding interest and amortization of capital: 51	Excluding interest and amortization of capital: 51	69

VIII. Conclusion.

The decision on the fate of the sludge delivered by a treatment plant, i.e., whether to subject it only to liquidation or to utilize it for one purpose or another, as well as the decision on the method of preliminary treatment of the sludge, depends on the character of the sludge, on the size of the plant, on its location, on the availability of the necessary areas, and on other local conditions.

However, while taking as a basis for the solution of these questions considerations of a sanitary and technical nature, it is necessary to make a general conclusion concerning the preparation of the sludge for all further operations, namely: it is desirable that the sludge leave the structures of the treatment plant in a digested condition, in which it possesses qualities making it quite acceptable for further handling. Exceptions may, chiefly, be made for cities situated near the sea, if they are able to send the sludge out to sea, although even in that case digestion of the sludge may prove to be a desirable operation, as one that reduces transport costs.

Along with the general proposition on the desirability of sludge digestion, the method of sludge utilization that is possible in this case is clarified, namely: the extraction of gas from it. This method has particular advantages at large plants.

In general, in deciding the questions raised above, the principal aim should be seen as rendering the sludge harmless and eliminating the harm it causes to the surrounding population. As for the utilization of the sludge, it can be applied only when the proper conditions for it exist, and in any case considerations of a sanitary nature cannot be neglected on account of it. The utilization of sludge should be regarded as a rational method of sludge liquidation that also entails the extraction of the valuable elements contained in the sludge. Sludge utilization should not be regarded as a commercial enterprise, since in this case infringements of sanitary order are always possible.

For reasons of an economic nature, sludge should be removed beyond the limits of the treatment station only after preliminary dewatering on drying sludge beds. Such beds for digested sludge may be located on the territory of the station without any nuisance to the station workers or to the population living nearby. In winter, digested sludge may be discharged not onto drying beds, but into special winter trenches.

Dewatering of sludge by other methods—on filter presses, in centrifuges, by means of vacuum filters, in drying kilns—requires expensive apparatus and large operating costs.

Methods for reducing the moisture content of sludge by means of sulfuric acid and ferrous sulfate have been little studied and so far have not been applied in practice.

Composting, when done by hand, is a heavy operation; for mechanical work, however, expensive equipment is required.

The most inexpensive methods of eliminating digested, dewatered sludge are:

1) dumping in low-lying places, and
2) plowing into the field.

The first of these methods is, at the same time, a form of sludge utilization, since it increases the value of land areas requiring leveling. The second method may also be regarded as a form of sludge utilization if, after removal, the field is used for agricultural purposes.

Special attention should be given to the question of discharging sludge into a river.

When a city is located on the bank of a river whose flow rate increases greatly during high water, it may sometimes prove rational to make use of this period to discharge into the river not only wastewater, without treating it, but also the sludge that had previously been removed from the wastewater.

In such a case, the summer portions of sludge, after being digested in digestion chambers, may be dried on drying beds and removed to the field; the winter portions of sludge may be stored in digestion chambers until the spring high water and then discharged into the river.

Of course, such a method of sludge removal must be applied with due caution, taking into account the interests of the riparian population and observing the requirements that were set out above in describing this method.

Below, Table XII indicates those methods of preparation, treatment, disposal, and utilization of sludge that may be the most suitable under various conditions.

Table XII.

Methods of preparation, treatment, disposal, and utilization of sludge under various local conditions.

	Preparation	Treatment	Disposal	Utilization
City near the sea	—	—	Removal to the sea	—
City near the sea	Digestion	—	Removal to the sea	Gas production
City on the bank of a river1	Digestion	In summer—drying on sludge beds	Removal to the field or to low places	Gas production. Use as fertilizer
City on the bank of a river1	Digestion	In winter—storage in digestion chambers	Discharge into the river at high water	Gas production
In other cases	Digestion	In summer—drying on sludge beds	Removal to the field or to low places	Gas production. Use as fertilizer
In other cases	Digestion	In winter—discharge into ice ponds	Removal to the field or to low places	Gas production. Use as fertilizer

Sources:

H. Bach.—Die Abwasserreinigung.

Dunbar.—Leitfaden für die Abwasserreinigungsfrage.

Hans Fichmüller.—Der Kremer-Kläranlage und die Abwasserfischteiche der Stadt Amberg.

Karl Günther.—Über offene Abwasserkanäle und Schlammtrockenplätze.

Helbing.—Die Emschergenossenschaft in Essen.

O. Herb.—Der Wert des belebten Schlammes als Düngmittel.

Imhoff.—Der Ruhrverband.

Imhoff.—Taschenbuch der Stadtentwässerung.

Imhoff.—Die Schlammbehandlung in Emscherbrunnen.

Keppner.—Die Abwasserbeseitigung in München.

Kusch.—Zweistöckige Absitzbecken oder getrennte Schlammfaulbehälter?

F. Langbein.—Die Abwasservorkläranlage auf dem Bötzberge des Berliner Rieselfeldes Wassmannsdorf als Gaslieferer.

Fritz Nelissen.—Maschinelle Schlammverladung für Kläranlagen.

Prüss.—Die Behandlung des Gewerblichen Abwassers in Emschergebiet.

Sierp.—Über den Dungwert von Faulschlamm und Frischschlamm.

Babbitt.—Sewerage and Sewage Treatment.

Jerry Donohue.—Separate Sludge—Digestion System for Small Town Use.

P. N. Daniels.—The New Sewage Treatment Plant of Trenton, N. J.

Folwell.—Sewerage.

Kinnicutt, Winslow, Pratt.—Sewage Disposal.

Materials from the author’s foreign business trip in the summer of 1927.

1. A river with a flow rate greatly increasing during high water. ↩↩↩↩↩↩↩↩

2. By irrigation. ↩