Engineer N. S. MAKEROV

GAS RECOVERY

FROM SEWAGE SLUDGE

Tomsk
1928

Engineer N. S. Makerov.

Gas Recovery from Sewage Sludge.

I. METHODS OF GAS RECOVERY.

Attempts to recover gas from sewage sludge and to use it as fuel date back to the end of the last century. These attempts proceeded in three directions, pursuing the aim of recovering gas by means of:

1) driving off the gas (entgasung),
2) carburation (vergasung), and
3) digestion (ausfaulung) of the sludge.

The first method consists of dry distillation of the sludge, in other words, heating the sludge without access of air to such a degree that volatile substances are released; i.e., this method is analogous to that used in the recovery of illuminating gas.

Such a method of recovering gas from sewage sludge could not be successful, owing to fuel costs that were excessive in comparison with the value of the gas obtained, costs caused by the high water content of the sludge.

The second method consists of the following.

Air or steam is blown into dried and calcined sludge. When air is blown in, the oxygen of the latter combines with the carbon of the organic matter of the sludge and forms carbon monoxide, which, together with the nitrogen of the air, gives producer gas.

When steam is blown in, the oxygen of the latter combines with the carbon of the organic part of the sludge and forms carbon monoxide, which, together with the hydrogen liberated from the steam, gives water gas.

This method of recovering gas had certain chances of success in the case of treating sewage by settling with preliminary coagulation, when brown-coal powder was used as a constituent part of the coagulant. In this case the sludge, pressed on filter presses and dried in the air, contained a large percentage of carbon (at the expense of the brown-coal powder) and was suitable for recovering gas from it by carburation.

An installation for recovering gas by this method, erected by the Cologne firm Deutz in the Berlin suburb of Oberschöneweide, operated from 1906 to 1908. A similar installation also existed in the German city of Elbing.

However, this method of gas recovery as well, as may be seen, was applied only in isolated cases and is now not used, in connection with the abandonment of coagulation of sewage with brown-coal powder as an uneconomical operation.

The third method—obtaining gas from sewage sludge by digesting the sludge—first carried out on a comparatively large scale in 1896 in the city of Exeter, in England, has by the present time become widely distributed and has fully justified its right to exist.

Gas recovery by this method is not an independent task. The gas is obtained only incidentally, as a by-product in the digestion of sludge—an operation pursuing the aim of proper preliminary treatment of the sludge.

Sludge liberated from sewage, when it is in a fresh state, possesses certain negative properties, such as, in...

from the sanitary standpoint, as well as from the point of view of the further operations required for its treatment, namely:

1) it contains a large percentage of water (88–96%), and, consequently, has considerable volume and weight, which creates great difficulties in its transport;
2) it is difficult to dry;
3) it has an extremely unpleasant odor;
4) it attracts flies, which may serve as carriers of various infectious agents.

Therefore, at the present time it is considered expedient, in order to eliminate these major drawbacks of fresh sludge, to subject the latter to digestion, since digested sludge

1) contains a considerably smaller percentage of water (75–80%), and, consequently, has smaller volume and weight, which greatly facilitates its transport;
2) dries easily;
3) is free of unpleasant odor;
4) does not attract insects.

For this purpose fresh sludge is left for a certain period of time in the chambers of one device or another, where it digests. As a result of the putrefaction process, the organic matter of the sludge is partly mineralized, partly dissolved, and partly converted into gases.

Thus gas formation is only a consequence of the process whose aim is a certain treatment of the sludge; therefore, when there are devices serving for sludge digestion, all measures for gas recovery amount merely to preventing the gas from escaping into the air as it leaves the digestion chambers, and to capturing, collecting, and storing it.

II. TECHNICAL MEASURES FOR GAS RECOVERY BY THE METHOD OF SLUDGE DIGESTION.

The technical measures applied for capturing and collecting the gas that forms in the digestion chambers consist in the installation of:

1) coverings over the digestion chambers that are impermeable to gas, so as not to allow the gas to escape into the air;
2) gas receivers, installed over openings in the coverings, for receiving the gas retained by the coverings;
3) a network of gas pipelines carrying the gases from the individual gas receivers to a main line, which in turn carries the gas to the place of its storage or consumption.

A. Coverings of digestion chambers.

The construction of coverings for digestion chambers depends on the design of the structures they cover. The most complex are the coverings over digestion chambers of the septic-tank type and over those separate digestion chambers which constitute structures of large area. The simplest are the coverings over digestion chambers of small area, and they are carried out especially simply in Emscher wells, where only a small area between the walls of the wells and the walls of the settling chambers has to be covered.

The material most often used for coverings is reinforced concrete; for greater gas impermeability, reinforced-concrete coverings are coated with a tar coating.

With respect to the surface of the sludge water in the digestion chamber, covers may be assigned to three categories:

1) permanent covers located above the water surface;
2) permanent covers located below the water surface;
3) covers floating on the surface of the sludge water.

The arrangement of covers above the water surface, on the one hand, simplifies the construction of the covers; on the other hand, it leads to the formation beneath the cover of a comparatively large gas space, which may cause:

1) leakage of gas through possible leaks in the cover or in the walls of the chamber above the water level, and
2) penetration into the gas space, through the same leaks, of atmospheric air, which may occur when sludge is rapidly discharged from the digestion chamber; the penetration of air into the gas space may cause an explosion (Imhoff).

These fears, however, are not shared by everyone, since the penetration of atmospheric air into the gas space can be prevented by proper care of the installation and by slow discharge of sludge from the digestion chamber (Früss).

Examples of such covers for separate digestion chambers are the covers in the installations of the cities Essen-Frohnhausen, Essen-Nord, and Oberhausen.

The arrangement of covers below the water surface complicates the construction: in separate digestion chambers it requires the formation of slots in the cover (Fig. 1), makes it difficult to drain the chambers, and does not permit them to be backfilled with earth directly over the cover. However, such a construction eliminates the formation of a gas space and prevents the possibility both of gas leakage through leaks and of the penetration of atmospheric air into the chamber.

Examples of this type of cover for separate digestion chambers are provided by the installations in the cities of Essen-Rellinghausen and Stuttgart; for Emscher wells—the installations in the city of Essen-Rellinghausen and at the Berlin irrigation fields of Wassmannsdorf.

Floating covers, like the covers of the type just considered, eliminate the formation of gas spaces and ensure this condition at any level of the sludge water in the digestion chamber. However, such a construction cannot be regarded as simple and easy to execute.

An example of floating covers is furnished by the covers of the digestion chambers of the Birmingham installations, where they consist of floating pontoons.

B. Gas receivers.

For collecting the gas retained by the covers, openings are left in them, and over these openings metal gas receivers are installed, consisting of iron bells of pyramidal, cylindrical, etc., shape. From these gas receivers the gas is carried away through a gas pipeline.

Gas receivers have various designs.

The gas receiver first used in Essen-Rellinghausen in the installation with Emscher wells (Fig. 2) has the following arrangement.

Over an opening measuring \(0.83 \times 0.66\) meters in the concrete cover, which is submerged 1 meter below the water surface, there is installed

a gas receiver of pyramidal form. The base of this gas receiver is located 30 cm below the water surface, which is achieved by installing the gas receiver not directly on the cover, but on walls built above the cover on three sides of the opening. To prevent floating sludge from penetrating into the gas receiver, an inclined grating is installed beneath the gas receiver, with access to it from the side not enclosed by a wall. Through the opening on this side the grating can be cleaned without interrupting the operation of the gas receiver. Suitable materials for the grating are wood or a metal capable of resisting corrosion; iron rods, however, are subject to rapid destruction. From the top of the gas receiver runs an iron pipeline serving to carry off the gas. Since the grating does not fully protect against the penetration of floating sludge into the gas receiver, the gas receiver is made removable.

The depth to which the base of the gas receiver is submerged is determined by the pressure required to force the gas into the network of pipelines carrying off the gas. A submergence depth of 30 cm, as given in the present example, is usually sufficient to create the gas pressure necessary for conducting the gas over a short distance. In the case of conducting the gas over a long distance, the pressure, and correspondingly the depth of submergence of the base of the gas receiver, increases from centimeters up to 60, and in special cases requires proper calculation.

Such an arrangement is at the same time also a precautionary measure against an increase of gas pressure in the gas receiver above a certain limit. Namely: if the gas pipeline is clogged for some reason and does not carry off gas, the pressure in the gas receiver will increase until the water level in the gas receiver drops to the base of the gas receiver and until the gas escapes outward through the side gap, not enclosed by a wall and intended, as indicated above, for cleaning the grating.

The gas receiver of the type described suffers from the drawback that the grating installed beneath it poorly prevents floating sludge from entering it. After a certain interval of time such a gas receiver becomes clogged with sludge, which appears even on the surface of the water surrounding the gas receiver, emerging through the opening intended for access to the grating. Therefore a gas receiver of this design requires rather frequent removal and removal of the floating sludge.

In Fig. 3 a gas receiver of another type is shown—Imhoff’s patent, Fries and Blunk. The essential feature of this gas receiver is the use in it, instead of a grating for retaining floating sludge, of a wooden shield consisting of a frame filled with boards fitted together with tongues. This shield first closes the opening in the cover, measuring 0.50 × 0.60 m, and only then is an iron bell installed above the shield, having openings at the base in its walls, serving for the exit of water when the pressure under the bell increases and for the exit of gas when this pressure reaches extreme limits. To keep the shield from floating up, it is held by a beam inserted into grooves located along the sides of the opening. Such a wooden shield is completely impermeable to floating sludge and at the same time excellently permits gas to pass. Floating sludge retained by the shield and accumulating beneath it can be discharged through an opening in the wall of the chamber, closed by a slide valve. However, according to some data...

(Sierp), in well-operating installations, washing out of the floating sludge takes place, after which the latter, once washed out, sinks to the bottom; thus there is no need to remove the floating layer. One inevitably has to deal with a floating layer only in those cases when the sludge discharged from the sewage contains many different fats and oils of mineral origin.

Instead of wooden shields for retaining the floating sludge, concrete porous slabs are sometimes used. Owing to their own weight, the latter do not float up and do not require special fastening in place. They are more durable than wooden shields. The thickness of the concrete porous slabs is 5 cm. The slab is enclosed in a frame of angle iron, to which, at the corners, round iron rods are attached, joined together at the top and forming a device by which one can take hold when it is necessary to lift the slab in order to expose the opening in the cover that it conceals. The same purpose can be achieved by embedding in the concrete porous slabs, during their manufacture, a pair of staples in each.

Fig. 4 shows another type of gas receiver, used in an installation with Emscher wells on the Berlin irrigation fields of Wassmannsdorf. The gas receiver—an iron well of cylindrical shape with a spherical top—is installed over an opening 0.60 meters in diameter in the cover. To protect the gas receiver from the entry into it of floating sludge, a concrete porous slab is installed beneath it. A special feature of the gas receiver is its installation in an annular trough filled with water and serving as a water seal that prevents atmospheric air from entering through slits in the lower part of the gas receiver. This latter can occur when the water level in the well falls, for example when a large portion of sludge is discharged. The depth of the annular trough is determined by the creation of the necessary gas pressure. Such a device is at the same time a precautionary measure against the rise of gas pressure above a certain limit.

For the discharge of floating sludge from beneath the porous slab, a pipe provided with a gate valve is provided.

A water seal of the described type, in the form of an annular trough, can also be used for gas receivers installed above covers located above the water surface. Of course, in this case the filling with water of this annular trough must be carried out artificially.

B. Gas-pipeline network.

From all the gas receivers of the installation, the gas is carried by means of iron pipes into a common main, which in turn conveys the gas to the gas holder or to the place where the gas is consumed.

On each pipe leading from a gas receiver there must be a cock, so that, if necessary, any gas receiver can be isolated from the common gas network.

To remove water condensing as the gas passes through cold pipelines, the gas pipes are provided with water traps.

For measuring the quantity of gas, it is desirable to install gas meters on the gas pipeline leading from each digestion chamber.

III. COMPOSITION OF THE GAS AND ITS HEATING VALUE.

The gas formed during the digestion of sewage sludge always consists of carbon dioxide, methane, and nitrogen; other components, such as hydrogen, hydrogen sulfide, oxygen, and carbon monoxide, appear in it only accidentally.

Carbon dioxide appears in digestion gas primarily as a product of the vital activity of microorganisms carrying out the decomposition of sludge, during their respiration; it is then also formed as a product of decomposition in various processes. For example, in the alcoholic fermentation of sugar,

\[ C_6H_{12}O_6 = 2C_2H_5OH + 2CO_2, \]

in methane fermentation, for example, from calcium acetate,

\[ (CH_3COO)_2Ca + H_2O = 2CH_4 + CaCO_3 + CO_2 \]

Methane is formed, on the one hand, through methane fermentation; on the other hand, through the direct combination of carbon dioxide with hydrogen, according to the investigations of Söhngen, by the equation:

\[ CO_2 + 4H_2 = CH_4 + 2H_2O \]

Nitrogen can also be formed in various ways. In any case, the main mass of nitrogen appears as a result of the decomposition of protein substances. During the decomposition of protein substances, amino acids split off, and with the further decomposition of the latter, ammonia is split off, e.g.:

\[ CH_2NH_2COOH + H_2O = CH_2OHCOOH + NH_3 \]

This ammonia is subsequently, under the influence of nitrifying bacteria, converted through molecular nitrogen into nitrites and nitrates, while part of the nitrogen probably does not participate in the reaction and escapes in the free state as a component of the gas.

Oxygen, as a component of digestion gas, is a rare occurrence. From the nature of the processes taking place in digestion chambers, it is difficult to suppose that oxygen could be contained in the latter in a free state.

Its presence can be explained either by

1) improper sampling, or
2) diffusion from the surface of the water into the gas.

Hydrogen is formed in various processes, chiefly, in acid fermentation, which is observed especially during the period of “start-up” of digestion chambers or during periods when their normal operation is disturbed. Subsequently the amount of hydrogen decreases and finally it disappears completely.

Thus, it has been found that, in the decomposition of various substances—carbohydrates, proteins, vegetables, meat, cellulose, and others—under the influence of activated sludge, hydrogen is always formed. However, in the gas of digestion chambers it is encountered very rarely; therefore it is concluded that hydrogen is always formed, but is immediately consumed, according to the above-cited Söhngen equation.

This is confirmed by the following experiment.

If through a digestion chamber that gives gas containing no hydrogen, the same gas is passed, but with 20% hydrogen artificially introduced into it beforehand, then after passing through the digestion chamber the gas loses

hydrogen introduced into it. In conducting this experiment it is necessary to strive to ensure that the gas passing through the chamber, containing hydrogen, comes into the closest possible contact with the sludge. This latter is achieved by the use of nozzles for introducing the gas.

Hydrogen sulfide in the gas of well-functioning digestion chambers is not observed. It appears chiefly where already putrefied privy waters enter the installation.

Sierp indicates the following causes for the appearance of hydrogen sulfide:

1) If the incoming water is rich in sulfates, then these sulfates are split into sulfides, and the latter, by neutralizing the organic acids, yield hydrogen sulfide.

2) If, when too large a quantity of sludge is discharged or when too large a quantity of fresh water is introduced, a strong dilution of the sludge takes place in the digestion chamber, accompanied by acid fermentation, then the bound hydrogen sulfide that was present in the sludge in the form of ferrous sulfide is released.

Even an insignificant content of hydrogen sulfide in the gas of digestion chambers gives the gas the unpleasant characteristic odor of hydrogen sulfide.

The content in the gas of digestion chambers of one or another of its constituent elements is subject to considerable fluctuations.

Table I, compiled by us on the basis of data from various authors for different installations, gives an idea of the percentage content of the constituent parts in the gas.

The carbonic acid content ranges from 3.4 to 35 percent: in digestion chambers recently put into operation it is lower and, with time, gradually increases. In digestion chambers with very old digested sludge the content of carbonic acid in the gas may reach 35%.

The methane content ranges from 60 to 90%: in digestion chambers recently put into operation, the methane content is higher than in chambers with old sludge.

Thus, the percentage content of carbonic acid and the percentage content of methane in the gas are in inverse relation: an increase in the percentage content of carbonic acid in the gas is accompanied by a decrease in the percentage content of methane in it, and vice versa.

The increase, over time, of carbonic acid in the digestion gas and the decrease of methane are explained by the following causes:

1) At the beginning of operation of digestion chambers, carbonic acid dissolves in the sludge water; over time, however, as the water becomes saturated with carbonic acid, the latter dissolves in the water in ever smaller quantity and, consequently, is released in greater quantity in the form of gas.

This is also supported by the fact that, when sludge water is boiled, the steam formed initially contains up to 95% carbonic acid.

2) The formation of methane with time, after the already completed strong decomposition of the methane-forming substances, weakens.

3) Groenewege cites the fact of a decrease in the quantity of methane as a consequence of a decrease in the number of methane ferments, explaining the latter by the fact that methane ferments gradually settle to the bottom and are discharged together with the sludge.

4) H. Blunk and F. Sierp suppose that, owing to the secretions of bacteria and the toxic action of these secretions, the activity of methane bacteria weakens and, consequently, the methane content decreases. Then, after the discharge of sludge and the introduction of a new quantity of sewage ...

Table 1. Composition of gas from sludge digestion and its calorific value.

Constituent parts of gas	Unit	Imhoff, by data of 1923	Imhoff, by data of 1925	Sierp: from individual wells	Sierp: mixed from many wells	Sierp: purified	Bender: Wansdorf	Sohler: Stuttgart	Mahr: Kattwig	O. Kammann: Worcester	O. Kammann: Lawrence	O. Kammann: Atlanta	O. Kammann: Panama	O. Kammann: Birmingham	O. Kammann: average for European conditions	Hatfield: Decatur
Methane	percent	65–90	80–85	65–95	75–85	90–98	75	75.5	79–85	75.2	78.9	84.1	60	67	80	70–80
Carbon dioxide	percent	5–35	7–20	5–35	15–20	0–2	19	14	4.8–5.3	5.9	3.4	4.6	14	30	up to 20	20–30
Nitrogen	percent	0–8	0–8	0–5	0–5	0–8	6.9	4.7	10–16	17.4	16.3	3.1	16.9	3	0–8	—
Hydrogen	percent	0–10	—	0–8	—	—	0.1	4.8	—	0.3	—	8	—	—	—	—
Hydrogen sulfide	percent	—	—	0–2.5	—	—	—	—	—	1.4	0.4	—	—	—	—	0.1–1
Oxygen	percent	—	—	—	—	—	—	—	—	—	0.4	0.4	1.1	—	—	—
Calorific value	calories	7000–9000	7000–8000	—	7500–8200	—	6900	8000	—	—	—	6300	5340	—	8000–9000	—

water into the digestion chamber¹, a considerable dilution of the sludge occurs, and methane bacteria continue their activity with greater energy.

The nitrogen content in the gas of digestion chambers is subject to large fluctuations. The reasons for this circumstance have not been found.

The hydrogen content ranges from 0 to 10%.

It has been established that hydrogen usually appears only for a short time at the beginning of the operation of digestion chambers, and then, as the digestion chambers mature, its content in the gas decreases to zero.

The explanation of this phenomenon lies in the above-mentioned equation of Söhngen.

In addition to the constituents considered above, Michael and Dr. Koch also observed the presence of carbon monoxide in the gas of digestion chambers in quantities up to 0.5% (the Chemnitz installation); Walsawh, at the Cole Hall installation (Birmingham), also found carbon monoxide in digestion gas in the amount of 0.55%.

The calorific value of digestion gas depends on its composition and, according to Table I, ranges from 5,340 to 9,000 calories. A higher percentage of methane in the gas and a lower percentage of carbon dioxide determine a greater calorific value.

Table II gives, for comparison, data on the composition and calorific value of several other gases.

Table II.

Composition of several gases and their calorific value

Gas constituents	Oil gas, percent	Coke gas, percent	Carbureted water gas, percent	Water gas, percent	Pintsch gas, percent
Hydrogen (H₂)	32	50	40	48	12.4
Methane (CH₄)	48	36	25	2	45.4
Ethylene (C₂H₄)	26.5	4	8.5	—	35.7
Nitrogen (N₂)	3	2	4	5.5	3
Carbon monoxide (CO)	—	6	10	38	0.6
Oxygen (O₂)	0.5	0.5	0.5	0.5	2
Carbon dioxide (CO₂)	—	1.5	3	6	0.7
Calorific value, calories	7600	5400	5200	2600	13500

IV. QUANTITY OF GAS

The quantity of gas recovered from sewage sludge depends on many conditions.

The following factors influence the productivity of a gas-recovery installation:

1) the temperature of the digestion chambers,
2) mixing of fresh sludge with old sludge,
3) the quantity and character of the organic matter in the fresh sludge,
4) the character of the sewage waters, and
5) the degree of alkalinity of the medium.

Temperature of the digestion chambers. Gas formation does not occur at all at low temperatures. It begins only at 6° C and increases as the temperature is raised up to a certain limit. At temperatures above this limit, gas formation begins to decline.

According to Imhoff, this limiting temperature is 25–30° C.

The dependence of gas formation on temperature is illustrated by Imhoff’s diagram, shown in Fig. 5, which gives the quantity of gas, in liters, recovered over two months from one kilogram of organic matter in fresh sludge at various temperatures.

The dependence of gas formation on temperature could be confirmed by a large number of examples. Only some of them are given below.

Thus, Fig. 6 presents a diagram compiled from the data of experiments at Wansdorf (Berlin), which gives the quantities of gas in cubic meters per 1,000 cubic meters of sewage waters and recovered by month at various sludge temperatures. The sludge temperature, as is seen from the diagram, depends on the temperature of the atmospheric air.

Fig. 7 presents a diagram giving the total quantities of gas, in cubic meters, recovered at the treatment station of the city of Stuttgart by month at various temperatures of the digestion chambers. From the diagram one may also discern the dependence of the temperature of the digestion chambers on the temperature of the sewage waters,^1 and of the temperature of the latter on the temperature of the atmospheric air.

According to Mahr’s data, at the installation in Kettwig, in the summer months more than 150 cu. m of gas per day is recovered, while in winter only 60–70 cu. m per day; moreover, during the thawing of snow the gas yield becomes still lower.

According to Sierp’s data, at an average annual sludge temperature of 12½° C, 3 cubic meters of gas per person per year are recovered (i.e., converted per person per day, 8.2 liters).

According to E. Bendler’s data, in accordance with the experiments at Wansdorf, at an average annual sludge temperature of 13½° C, an average of likewise 8.2 liters per person per day is recovered, with fluctuations from 12.1 liters per person per day in summer, at a temperature of 17° C, to 4.6 liters per person per day in winter, at a temperature of 11° C.

According to O. Kammann’s data, gas recovery is likewise expressed as 3 cu. m per inhabitant per year at a digestion-chamber temperature of 10–12° C.

K. Imhoff gives 8 liters of gas per inhabitant per day, or about 3 cu. m per inhabitant per year.

W. Sohier, for the city of Stuttgart, gives 8–10 liters per person per day.

^1) The greater part of the installation of the city of Stuttgart consists of Emscher wells and Neystadt basins.

The firm “Kremer-Klärgesellschaft,” according to data from the firm’s director Pritsch, when designing its installations uses, for German conditions, a standard of 8 liters of gas per inhabitant per day.

In view of what has been said above concerning the dependence of gas formation on temperature, it may be concluded that the above figures for gas recovery can be increased by artificially raising the temperature of the digestion chambers. This method was applied in Birmingham, where Watson and Martin increased the gas yield to 30 liters per person per day. Imhoff, in the city of Essen-Rellinghausen, by artificially heating a separate digestion chamber by admitting hot water into it, raised the temperature of the sludge from 8–10° C. to 21° C., increasing gas formation several times over.

Mixing the fresh sludge entering the digestion chambers with old sludge. When fresh and old sludge are mixed, the best contact between them is achieved, and, consequently, the transfer of putrefactive bacteria to the fresh sludge and, together with this, an intensification of the processes of putrefaction and gas formation.

By means of artificial mixing of the sludge in a separate digestion chamber of the Essen-Nord Prüss installation, it was possible to double gas formation.

O. Kammann believes that by artificially raising the temperature of the digestion chamber by several degrees and artificially mixing their contents, the gas yield can be increased to 6–8 cubic meters per inhabitant per year.

Quantity and character of the organic matter in fresh sludge. If wastewater, after mechanical sedimentation, is also subjected to biological treatment by means of activated sludge, and the residual activated sludge is fed into the digestion chambers and digested together with the sludge separated from the water during mechanical sedimentation, then in this way the quantity of organic matter in the sediment increases considerably, and along with this the gas yield also increases.

According to Siepr’s data, with such a method, gas recovery can be brought up to 16 liters per inhabitant per day, as against 8 liters obtained with mechanical treatment of wastewater alone.

The influence of the character of the organic matter of the sludge on gas yield is illustrated by the following data, obtained by Clark and Gage, who subjected sludge from various sources to decomposition (Table III).

Table III.

Quantity of gas from sludge of various sources

Sources of sludge	Content in the sludge of organic matter, %	Quantity of gas per unit of organic matter
Tanneries	51	0.00
Wastewater of the city of Lawrence	78	7.45
Digesters	46	9.00
Effluent after continuous oxidizers	45	121.00

Character of wastewater. A great influence on the decomposition of organic sediment, and consequently also on gas formation, is exerted by the pri-

industrial waters, since the latter often contain toxic substances capable of harming the microorganisms of putrefaction. If the toxic substances are a temporary and at the same time insignificant admixture in the wastewater, they may have little effect on gas formation. But a constant inflow of toxic substances, even in insignificant quantities, may lead to a complete cessation of the processes of decomposition and gas formation.

Such waters unfavorable for the processes of gas formation are the wastewater of coke plants with a high content of resins, phenol, rhodan; waters after gas purification; waters from brass factories with a high content of copper salts; waters from tanneries containing arsenic-chromium salts; and all strongly acidic wastewater and wastewater containing free chlorine (Sierp).

Thus, in order to obtain a sludge capable of putrefying and producing gas, if toxic substances are present in the wastewater, the latter should first be rendered harmless.

Since in recent years chlorination of wastewater has become widespread for the purpose of disinfection, as well as for eliminating odor, Professor Gordon M. Fair and his assistant C. L. Carlson at the university in Cambridge (Mass.) carried out laboratory tests intended to determine how preliminary chlorination of wastewater affects the process of sludge putrefaction. The experiments were conducted with two series of sludge samples from the Emmerich wells, each series consisting of two samples: one from wells with preliminary chlorination of the wastewater and another without it. The scale of putrefaction was taken to be the yield of methane gas from the sludge and the concentration of hydrogen ions.

In particular, the results of gas formation revealed in these tests are expressed by the diagram shown in Fig. 8. From this diagram it is apparent that during the first 4–6 weeks gas formation proceeds more rapidly in the putrefaction of sludge from previously chlorinated liquid and more slowly in the putrefaction of sludge from non-chlorinated water; subsequently, however, the reverse occurs: methane gas forms more rapidly in the non-chlorinated samples and more slowly in the chlorinated ones.

This phenomenon may be explained by the following causes.

1) Chlorination may have a coagulating effect and lead to the precipitation of the finest colloidal or dissolved organic substances and, consequently, may promote an increase in the amount of organic matter in the sludge, which in turn entails an initial intensification of gas formation.

2) Chlorination may kill those microorganisms contained in wastewater which lead to acid fermentation, as well as those microorganisms which hinder the activity of methane-producing bacteria; consequently, owing to chlorination, the processes of methane-gas formation are intensified in the initial stage of the putrefaction process.

Considering that, technically, at treatment plants the processes of sludge putrefaction are not carried to complete putrefaction of the organic matter, but only to such a state in which the sludge becomes suitable for its further treatment, increased gas formation precisely in the initial period of the process may have an advantageous effect on gas yield. Consequently, preliminary chlorination of wastewater may increase the gas output.

Degree of alkalinity of the medium. On the basis of data from various investigators, O. Kammann considers that the hydrogen-ion concentration of the sludge water in digestion chambers that is optimal for gas recovery lies within the limits of 7.3 and 7.6 pH.

V. UTILIZATION OF GAS.

Gas recovered from sewage sludge may be used:

1) in the raw state,
2) in the purified state.

A. Utilization of gas in the raw state.

Gas in the raw state may be used:

1) for purposes of heating, lighting, and cooking food;
2) for gas engines;
3) for obtaining steam and electric power;
4) as fuel for refuse-incineration plants.

For purposes of heating, lighting, and cooking food, the gas may be used:

a) for the needs of the purification plant,
b) for sale or delivery to gasworks.

An example of the use of gas for the purification plant’s own needs may be, for instance, the installation of the city of Essen-Rellinghausen, where part of the gas is used to raise the temperature of an individual digestion chamber. At some installations the gas is used in part to heat booths with chlorinating apparatus.

In most cases the gas recovered at purification plants is delivered to gasworks, which use it as illuminating gas.

When gas is transferred to municipal gasworks, it is either introduced into the municipal gas holder, or it enters directly into the municipal gas network, or it first goes to the gasworks. If the municipal gas holder is located near the purification plant, the gas is introduced into it (Essen-Rellinghausen, Kettwig). If a municipal gas network passes near the purification plant, the gas may be introduced directly into it. In this case it is preferable to introduce the gas, for better mixing with the municipal gas, into the main gas main, since when gas is introduced into a secondary line there may be cases in which the consumer receives either only municipal gas or only digestion gas (methane). This latter circumstance (the variable character of the gas) requires readjustment of the burner nozzle and therefore naturally causes constant complaints from consumers. Moreover, methane has a low ignition velocity, which, without the adoption of proper measures (wire gauzes), at a certain gas pressure in the network causes flame blowback. Such an arrangement is therefore not permissible in every household.

Sometimes the gas first goes to the municipal gasworks and from there, already mixed with municipal gas, enters the network. A similar example is provided by the purification plant of the city of Stuttgart, where the digestion gas is supplied to the municipal gasworks located 10 km from the plant, is mixed there with unpurified coal gas, and together with the latter passes through the usual purification processes before entering the municipal network.

If it proves impossible to use the gas for the sewage-treatment plant’s own needs, and it likewise cannot be delivered to gasworks because the latter are too far away, the gas obtained at the treatment installation is compressed to a pressure of 150 atmospheres in steel cylinders and sold outside. In compressed form it is transported over distances and can be used for heating and lighting purposes on individual estates, in laboratories, factories, etc. Owing to its initial cheapness (see below, ch. VII), and also to the fact that compression is carried out by compressors operating on the same gas, gas in compressed form can withstand transport over considerable distances and successfully compete with other kinds of fuel.

In the same compressed form, but after preliminary purification, the gas may be sold for use for other purposes as well (see item B of the present chapter).

Digestion gas, as a fuel for gas engines, is already being utilized at many installations.

Thus, in Brisbane (Australia), since 1914 an engine of 17 h.p. has been operating on digestion gas obtained from an installation serving 13,000 inhabitants.

In Austin (Texas), a sewage pumping station operates on gas from Imhoff tanks.

In Birmingham, at the Cole Hall installation, a 32-horsepower gas motor has been operating since 1921; and at the Saltley installation, in 1926, work was begun on equipping it for the recovery of gas and for the installation of a 150-horsepower gas engine. According to the calculations of the chief engineer of the Birmingham installation, Engineer Whitehead, 1,150,000 horsepower-hours can be obtained annually from the gas of the latter installation.

At the installation in Halle (Germany), gas from Imhoff tanks has, since the end of 1927, been used in part to operate a 12-horsepower gas motor, by which a compressor is set in motion; this compressor serves a mammoth pump for raising 1,000 liters of sludge (with a water content of 90%) per minute to a height of about 7.5 meters. The motor consumes about 500 liters of gas per horsepower-hour.

It is proposed to re-equip the pumping station of the Wassmannsdorf irrigation fields (Berlin) in order to convert it to operation on its own gas from Imhoff tanks, which were put into operation in July 1927.

In Decatur, according to data from 1926, it was proposed to use part of the gas recovered from Imhoff tanks for gas engines at the aeration station, at a consumption of about 400 liters per horsepower-hour.

Digestion gas can be used as a fuel for obtaining steam and electrical energy.

Finally, it has found application as fuel for refuse-incineration stations: thus, in the city of Austin (Texas), part of the digestion gas is used in burning refuse.

B. Utilization of Gas in Purified Form

In certain branches of technology, gas from digestion chambers may find application after appropriate purification, i.e., in a state freed from hydrogen and carbon dioxide. Such gas, already representing high-percentage methane, can be used:

1) for autogenous cutting and welding;

2) In the chemical industry: some American patents provide for obtaining from methane hexamethylenetetramine, a product used in the vulcanization of rubber and in the manufacture of artificial resins (Plaussone’s patent), as well as for obtaining halogen-substituted methanes (methyl chlorides).

VI. GAS PURIFICATION.

In order to obtain methane of high percentage, it is necessary to remove hydrogen and carbon dioxide from the digestion gas.

Hydrogen may be eliminated by the following method, based on the data of Ch. III.

The gas under pressure is passed through a “spent” digestion chamber, feeding it into the lower part of the chamber and spraying it by means of nozzles for the best contact with the sludge. Then the hydrogen of the gas combines with the carbon dioxide contained in the chamber and forms methane.

According to American data, by this method it was possible to bring down to 0% the hydrogen content in a gas that contained it in the amount of 18%. Likewise, according to German data, the hydrogen content in a gas of 18.1% was reduced by this method to 0%.

Carbon dioxide can be removed from the gas by one of the following methods:

1) By passing the gas under pressure through bottles filled with water; in this case the carbon dioxide contained in the gas dissolves in the water, and the gas comes out purified; the water, of course, must periodically (when saturated with carbon dioxide) be let out of the bottles and replaced with fresh water. This method of purification can be combined with the obtaining of compressed gas, when the latter is released in cylinders, if the purification unit (bottles with water) is placed between the compressor and the cylinders intended for the compressed gas.

2) By passing the gas through milk of lime: in this case the free carbon dioxide will be bound into calcium carbonate.

3) By introducing wash water into the digestion chamber: in this case the dissolution of carbon dioxide in the sludge water is intensified and, consequently, a smaller amount of it is obtained in the form of gas.

The carbon dioxide content in the gas can be reduced to 0.5—2%, depending on its content in the raw gas.

As a result of purification, a gas is obtained consisting of methane in the amount of 92—98%, nitrogen 0—8%, and carbon dioxide 0.5—2%.

If hydrogen sulfide is present in the digestion gas as a constituent part, which happens very rarely, then its removal from the gas is accomplished by installing on the gas pipelines a special system of filters (of bog iron ore).

Removal from the gas of water condensing as the gas passes through cold pipelines is carried out through water pots installed on the gas pipelines.

VII. ECONOMIC CALCULATION OF A GAS-RECOVERY INSTALLATION.

For the economic calculation of a gas-recovery installation, the following data are necessary:

1) The cost of reconstructing the installation (if an installation for wastewater treatment already exists) and the cost of equipment for gas recovery; or the difference in the cost of constructing a treatment installation with

equipment for gas recovery, and without such equipment (if the matter concerns the construction of a new treatment station).

2) The cost of transmitting the gas over the distance to the place where it is consumed.

3) The cost of annual operation of the gas-recovery installation (expenditure on labor, the compressor installation, gas purification, etc.).

4) Loan and repayment terms.

5) The quantity of gas that can be obtained in a year.

6) The price at which the gas can be sold.

On the basis of these data one can calculate, on the one hand, the annual expenses for paying interest and repaying the loan, for deductions to the renewal fund, and for operating the installation, and, on the other hand, the annual income from the sale of gas, and thus determine the profitability of the installation.

If the gas-recovery equipment is installed simultaneously with the construction of the treatment station, its additional cost is negligible. If, however, the gas-recovery device is added later, then its cost in that case is, of course, considerably greater and depends on the design of the digestion chambers.

According to Imhoff’s data, the cost of the additional devices and equipment, including gas pipelines on the territory of the treatment station, amounts to only 1% of the construction cost of the installation of Emscher wells.

The selling price of gas at which it is supplied by sewerage treatment stations to gas works, despite its high calorific value, is at present rather low.

In Essen-Rellinghausen, the gas from the sewerage treatment installation belonging to the Ruhr Union is purchased by the gas works belonging to the city at 3½ pfennig per cubic meter; in Kettwig—at 5 pf. per cubic meter.

In Stuttgart it has been proposed to supply gas to the gas works at a price of 7 pf. per cubic meter.

In Zurich—at 5 pf. per cubic m.

These prices are very low and are due to the fact that the gas-producing sewerage installations, having no other outlets for the sale of the gas, are forced to agree to low prices and deliver it to gas works that dictate their own terms. And the works often pay for this gas only ⅔ of the cost price of municipal coal gas.

The profitability of a gas-recovery installation will be incomparably higher if it is in the same hands as the gas works, since the digestion gas will then be priced at the price of the gas supplied by the works to consumers. In the Ruhr region, for example, municipal gas is supplied at 19 pf. per cubic meter.

D. Klemens Delkeskamp (Wiesbaden) values gas from digestion chambers, according to its calorific value in comparison with coal gas and according to the cost of the latter, at 35 pf. per cubic m.

Despite the very low prices at which digestion gas is presently sold by the treatment installations of the Ruhr Union, the income from gas recovery, according to the testimony of Imhoff and Sierp, nevertheless covers no less than half of the operating expenses of the treatment installations.

Below is given the economic calculation for the gas-recovery installation of the city of Stuttgart, consisting of Emscher wells, Neustadt bas—

…tanks and Imhoff tanks, with a total gas yield of 1,000,000 cubic meters per year (from 350,000 inhabitants).

Cost of reconstruction and equipment:

No.	Item	Cost
1)	covering the digestion chambers with reinforced concrete	63,000 marks
2)	gas receivers	3,150 “
3)	gas pipelines to the gas holder	21,000 “
4)	machinery installation for pumping the gas to the gas plant over a distance of 10 km	13,650 “
5)	gas holder	2,100 “
6)	gas purifier	2,100 “
	Total	105,000 marks

Annual operating cost:

No.	Item	Cost
1)	supervision	1,700 marks
2)	packing and lubricating materials	580 “
	Total	2,280 marks

Item	Cost
Interest, 8% on 105,000 marks	8,400 “
Depreciation:
2% of the cost of the covering	1,260 “
10% “ “ of the remainder	4,200 “
Total annual cost	16,140 marks
Revenue from the sale of 1,000,000 cubic m of gas at 7 pf. per cubic m	70,000 marks

Net income per year: 70,000 − 16,140 = 53,860 marks.

Thus the gas-recovery installation will pay for itself in two years and in subsequent years will yield a net profit of more than 60,000 marks per year.

VIII. THE SIGNIFICANCE OF GAS RECOVERY.

The data given in the preceding chapter, concerning the economic advantages of gas recovery, do not exhaust its significance. Gas recovery also has other advantages, namely advantages of a sanitary and operational-technical character:

1) When gas-recovery equipment is present, the digestion gas is retained by the covers, collected by gas receivers, and conveyed by gas pipelines to the place where it is consumed or stored. Therefore it does not enter the air and does not affect the inhabitants. In addition, digestion chambers isolated by covers under no circumstances (acid fermentation) spread odors.

2) When there is a cover completely isolating the digestion chamber from access by atmospheric air, the floating layer formed on the surface of the contents of the digestion chamber is likewise subjected to anaerobic decomposition, then sinks to the bottom of the chamber and is removed together with the sludge. Thus, in this case, maintenance of the installation is considerably simplified in comparison with those devices in which air has access to the surface of the contents of the digestion chambers and where the floating layer must be removed separately¹.

But if the sludge contains a great deal of oil and fat, which are difficult to decompose and hinder the settling of the sludge to the bottom, then even with covers that protect the digestion chambers from the access of air and promote the rotting of the floating layer, the latter requires special removal.

3) Where covers are present for the purpose of obtaining gas, no foaming masses of sludge can escape to the outside, a phenomenon sometimes observed during periods of a particularly “morbid” condition of the Emscher wells.

IX. PRECAUTIONARY MEASURES.

In the absence of hydrogen sulfide in the gas of the digestion chambers, and with the gas deprived of odor, special caution should be observed when cleaning the digestion chambers or carrying out any other work in them, since it is difficult to determine whether there is gas in the chamber or not. To prevent accidents, it is recommended in such cases, when descending into the digestion chambers, to use gas masks.

In some cases artificial perfuming of the digestion gas with mercaptan is resorted to.

SOURCES:

O. Kammann.—Der heutige Stand der Gasgewinnung und—Verwertung aus dem Klärschlamm der Abwasserreinigungsanlagen.

H. Blunk und F. Sierp.—Gewinnung und Verwertung des Gases aus Faulräumen städtischer Kläranlagen.

F. Sierp.—Die Verwertung der Abwässer zur Gasgewinnung.

K. Imhoff.—Die Verwertung des Methans aus Faulräumen.

K. Imhoff.—Fortschritte der Abwasserreinigung.

K. Imhoff.—Die städtische Abwasserreinigung.

Mahr.—Erfahrungen beim Bau und Betriebe der Kläranlage Kettwig.

Heilmann.—Die Verwertung des Faulraumgases.

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

Erich Bendler.—Die Vorreinigungsanlage auf dem Rieselfeld Wassmannsdorf.

W. Sohler.—Die wirtschaftliche Bedeutung der Klärgasgewinnung auf dem Stuttgarter Hauptkläranlage.

Haller.—Studien über Abwasserchlorung.

William D. Hatfield.—Recovery of Gas From the Decatur Imhoff Tanks.

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

To the article by engineer N. S. Meinerov.

Fig. 1.

Labels in the figure: Gas

Fig. 3.

Labels in the figure: Gas receiver; Gas

Fig. 2.

Labels in the figure: Gas receiver; Gas

For the article by Eng. N. S. Manerov.

Gas receiver

Gas

Fig. 4.

Dependence of gas formation on temperature
according to Imhoff

Amount of gas
from 1 kg of organic
matter of fresh
sludge over two months

Right-hand scale: \(0^\circ,\ 5^\circ,\ 10^\circ,\ 15^\circ,\ 20^\circ,\ 25^\circ,\ 30^\circ,\ 35^\circ,\ 37^\circ\)

Bottom scale: \(0,\ 100,\ 200,\ 300,\ 400,\ 500,\ 600,\ 700\) liters of gas

Curve labels: 62; 220; 485; 698; 667

Fig. 5.

to the article by Eng. N. S. Manerova

Gas Quantity

as a function of temperature according to data from experiments at Waßdorf (Berlin)

Cubic meters of gas per 1000 cubic meters of sewage water

Legend:

— · — · — quantity of gas

• • • • air temperature

——— water temperature

Temperature

20° C.
15°
10°
5°
0°
−5°

Months: Jan., Feb., Mar., Apr., May, June, July, Aug., Sept., Oct., Nov., Dec.

Fig. 6.

Dependence of Gas Formation

on preliminary treatment of sewage water with chlorine.

Vertical axis: liters of methane per 1 ton of dry organic matter at 750 mm pressure.

Horizontal axis: duration of the experiment in weeks.

Fig. 8.

Legend:

—— 1st series of tests, chlorinated.

• • • • 1st series of tests, non-chlorinated.

— · — · — 2nd series of tests, chlorinated.

·········· 2nd series of tests, non-chlorinated.

To the article by Eng. N. S. [[unclear: surname]].

Amount of Gas

as a function of temperature

Total amount of gas obtained at the treatment plant of the city of Stuttgart.

Left axis: cubic meters of gas
Right axis: temperature, °C

Months along the horizontal axis: Sept.; Oct.; Nov.; Dec.; Jan.; Feb.; Mar.; Apr.; May; June; July; Aug.; Sept.; Oct.

Fig. 7.

Legend:

— Amount of gas

-- -- -- Air temperature

·········· Temperature of the digestion chamber

— · — · — Sewage-water temperature

1. Special removal of floating sludge in these latter devices is not required in cases where there are arrangements for mixing the sludge and where, consequently, the floating sludge mixes with the sludge settling on the bottom. ↩↩