Technology for Wastewater Treatment at a Pencil Factory
Abstract Generated abstract
This paper examines wastewater generated during ammonia and steam treatment of pencil slats at the Tomsk pencil factory, characterized by high ammonia content, strong color and odor, suspended solids, and a large organic load. Laboratory tests compared reagent treatments and found that slaked lime was the only tested reagent that appreciably precipitated organic substances, although even large lime dosages removed only about 60 percent and temperature had little beneficial effect. Because the wastewater contains chemically diverse organics from wood extraction and paraffin processing, the authors propose a combined treatment scheme: paraffin recovery, lime coagulation and settling with sludge regeneration, followed by potassium permanganate oxidation for decolorization before partial reuse and discharge.
Full Text
WASTEWATER TREATMENT TECHNOLOGY FOR A PENCIL FACTORY
S. A. Babenko, O. I. Popova, S. I. Olenev
(Submitted by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)
Wastewater from the Tomsk pencil factory is formed mainly in the process of treating pencil slats in autoclaves with ammonia water and live steam. The process takes place at a temperature of 125–135°C, a pressure of 2 atm, and lasts two hours. Then the condensate is drained, and the slat is treated with live steam under the same conditions for one more hour. The resulting condensate is drained, the slat is placed in a paraffin bath for 1 min, and then held in the autoclave for 15 min. Excess paraffin also enters the common discharge.
The autoclave effluent is a black liquid with a resinous-coniferous odor, which disappears upon dilution 64-fold. The color disappears upon dilution 1024-fold. The transparency of the natural wastewater and of the water after two hours of settling is equal to zero. The chemical composition of the wastewater under study (Table 1) shows a significant content of ammonia and organic substances.
Table 1
Chemical composition of the wastewater of the Tomsk pencil factory
| Substances contaminating the water | Content, mg/l |
|---|---|
| Ammonia | 1520–1700 |
| Nitrites | traces; nitrites not detected |
| Sulfates | 25.6–30.0 |
| Iron | not detected |
| Chlorides | 3.0–6.0 |
| Phenols and cresols | 3.0–5.0 |
| Ether-soluble substances | 850–1700 |
| Suspended solids at 105°C | 1100–1200 |
| Suspended solids, ignited | 35.0–40.0 |
| Dry residue | 13000–19000 |
| Ignited residue | 160–330 |
| BOD\(_5\) | 19000–20000 |
| COD | 17000–18000 |
| pH | 8.5–9.2 |
| Oxidizability | 50.0–60.0 |
Wastewaters similar in composition are subjected to rough treatment by treatment with slaked or chlorinated lime and carbon dioxide gas [1]. Data are available on the precipitation of lignin with aluminum sulfate in the presence of polyacrylamide [2, 3].
Preliminary experiments on treatment of the wastewater under study showed that only slaked lime has the ability to bind organic substances. Chlorinated lime, calcium chloride, inorganic salts, mineral acids, and high-molecular-weight flocculants do not possess any appreciable property of precipitating organic substances even at significant reagent dosages.
Table 2
Content of organic substances in treated wastewater as a function of lime dosage.
Initial organic content 24 g/L, water temperature 20°C
| Ca(OH)\(_2\) dosage, g/L | Content of organic substances, g/L | Percent treatment |
|---|---|---|
| 10 | 13.84 | 42.0 |
| 20 | 13.23 | 43.0 |
| 30 | 12.99 | 45.8 |
| 40 | 12.18 | 48.8 |
| 50 | 12.0 | 50.0 |
| 60 | 11.97 | 50.0 |
| 80 | 9.23 | 61.2 |
| 100 | 9.01 | 62.5 |
The effect of lime dosage and temperature on the completeness of wastewater treatment (Tables 2, 3) showed that increasing the lime dosage tenfold increases the degree of water treatment by only 20% (42% at a lime dosage of 10 g/L and 65.2% at a dosage of 100 g/L). Raising the water temperature from 15 to 80°C at a lime dosage of 40 g/L has practically no effect on the process of treating water to remove organic substances.
The insufficient treatment of wastewater with lime can be explained by the varied composition of the organic substances. It is known that when wood is treated with alkaline solutions, a large portion of extractive and aromatic substances is removed from it; that is, selective dissolution does not occur, and the alkaline extracts prove to be chemically heterogeneous. In coniferous wood, resin acids constitute a significant part of the extractive substances [4].
Hot water dissolves both carbohydrates and lignin. Part of the dissolved substance is apparently a lignocarbohydrate complex; most of the material dissolving in water consists of polysaccharides.
The treatment conditions for the planks—the alkaline medium, duration, and high temperature—create favorable conditions for the transfer into the water of carbohydrates, lignin-like and aromatic substances, oils, and resins. Such an abundance of organic substances diverse in their properties and structure cannot be removed from the water by any single method. Therefore, after alkaline treatment and the removal of about 60% of the organic substances, the wastewater was subjected to further treatment and decolorization with oxidizing agents. Manganese and magnesium oxides, calcium hypochlorite, gaseous chlorine and oxygen, and potassium permanganate were investigated as oxidizing agents. The last proved preferable
Table 3
Content of organic substances in treated water as a function of temperature.
Initial organic content—phenol—24 g/L. Lime dosage—40 g/L
| \(t^\circ\)C | Content of organic substances, g/L | Percent treatment |
|---|---|---|
| 15 | 12.18 | 49.4 |
| 30 | 11.94 | 50.2 |
| 50 | 11.94 | 50.2 |
| 60 | 11.71 | 51.1 |
| 70 | 12.40 | 48.4 |
| 80 | 13.35 | 44.5 |
oxidizing agent; at a consumption rate of 3–4 g/l, complete decolorization of the wastewater occurred.
On the basis of the studies carried out, it is possible to recommend a scheme according to which the wastewater from the autoclave is fed to a paraffin trap, where the paraffin particles are captured and sent to the paraffin bath. The water and lime milk are then thoroughly mixed in a mixer and fed to a settling tank. The sludge from the settling tank is dewatered on a filter press and regenerated in a calcining furnace, after which it is returned to the cycle, while the water and filtrate are treated with potassium permanganate in an oxidizing reactor. Part of the purified water is used to prepare solutions of \(KMnO_4\) and lime, and the remaining part is discharged into the sewer system.
REFERENCES
- A. Petru. Industrial Wastewaters. Moscow, Stroiizdat, 1965.
- Treatment and Utilization of Wastewaters from the Pulp and Paper Industry. Kiev, Publishing House of the Ukrainian NIINTI, 1968.
- Chemistry of Wood. Riga, 1968, No. 1.
- Chemistry of Wood. Moscow, Publishing House “Lesnaya promyshlennost,” 1967.