Sewage Sludge

Use of Sewage Sludge

The growth of urban population and industrial development are accompanied by an increase in the volumes of wastewater and its sludge. In industrially developed countries, on average 19–25 kg of dry sewage sludge is generated per inhabitant per year. In the Russian Federation, the estimated average annual output of dry sludge is 2.5 million t.

At treatment facilities in cities and large settlements, the amount of sludge is 0.5–1.0% of the volume of treated wastewater. Fresh sludge obtained after treatment in primary settling tanks has a moisture content of 92–95%. The sludge is subjected to disinfection and dewatering. Depending on the technology, sewage sludge is subjected to composting, digestion, or thermal drying.

During sludge digestion, disinfection is carried out in methane tanks at a temperature of 56–58°C, into which primary settling tank sludge and activated sludge are fed in a 1:1 ratio. Digested sludge is dried on sludge beds to a moisture content of 60–80%.

In industrially developed countries, up to 32% of sewage sludge is used as organic fertilizer.

In the Russian Federation, only 5–7% of sewage sludge is used as fertilizer, which is due to insufficient introduction of technologies for preparing it for agricultural use.

The main technological stage in preparing sludge for use is disinfection. Currently, treatment facilities use methods of chemical, thermal, and biological disinfection of sewage sludge.

Chemical disinfection of sludge consists in treatment with fungicides. The moisture content of the product is about 60%, which complicates its spreading over the fertilized area. Thermal disinfection produces dried sludge with a moisture content of less than 60%. Disinfection takes place at a temperature of 56–58°C.

Depending on the treatment technology, sludge intended for use as fertilizer may be liquid with a moisture content of 92–97%; partially dried or dewatered with a moisture content of 60–80%; dried with a moisture content of 10–40%.

The optimal time for applying fertilizers based on sewage sludge is autumn, for primary tillage, and also summer, when cultivating fallow and early-vacated fields of crop rotation. During autumn and spring, soluble compounds contained in the sludge, primarily chlorine, are leached from the root zone. On weakly and moderately cultivated soddy-podzolic soils, fertilizers are applied at rates of 10–15 t/ha on a dry matter basis; on cultivated soddy-podzolic sandy loam soils and chernozems, 5–10 t/ha. Sludge application is combined with mineral fertilizer application.

Chemical Composition of Sewage Sludge

The chemical composition of sludge varies greatly and depends on the production technology and composition of the treated wastewater.

During thermal drying, sludge dewatered in centrifuges or vibrating filters to a moisture content of 80% is dried at a temperature of 600–800°C to a moisture content of 40%.

The chemical composition of sewage sludge includes nitrogen, phosphorus, and calcium; it is poor in potassium. The amount of organic matter in raw sludge is up to 75% on a dry matter basis; it consists mainly of proteins, carbohydrates, fats, lignin, and bacteria. The composition includes trace elements: manganese 500–2000 mg/kg dry matter, copper 1000–5000 mg/kg dry matter, and zinc 1200–6000 mg/kg dry matter. The ash composition is specific and determined by the composition of industrial waters.

Sludge contains heavy metals, petroleum products, detergents, and other hazardous compounds. Therefore, their use requires constant monitoring of composition. It is safer to apply sewage sludge on heavy, humus-rich soils. On light and low-humus soils, their application is combined with chemical amelioration.

In terms of heavy metal content, most sludge meets international agroecological requirements.

Sludge intended for use as fertilizer must contain at least 40% organic matter, 1.6% nitrogen, 0.6% phosphorus (P₂O₅), 0.2% potassium (K₂O), and moisture content no more than 82%.

Doses of sewage sludge from sludge beds range from 20 to 50 t/ha depending on chemical composition and content of hazardous substances, as well as crop requirements and soil cultivation level.

It is advisable to use sludge for fertilizing parks, forest nurseries, lawns, and bast fiber crops; for other crops, with permission from sanitary and hygienic control authorities. Sewage sludge is not used for vegetable crops.

Composting of Sewage Sludge

During composting, fresh sludge is dried to a moisture content of 50–55% and mixed with peat in a 3:1 ratio. Then it is piled into heaps, where the temperature reaches 60°C, which leads to the death of nonspore-forming microorganisms, eggs and larvae of helminths and flies.

When municipal solid waste and sawdust are used as fillers, the filler-to-sludge ratio is taken respectively from 0.5:1 to 1–1.5:1 in summer and from 1:1 to 2–3:1 in winter. All types of peat are suitable for composting with sludge. Peat compost is prepared at any time of year. The ratio of sludge to peat depends on the quality of the latter and the time the compost is laid. In winter, for good heating of the compost, the relative peat content is increased to 2–1.5:1. In spring-summer preparation, the ratio is 1.5–1:1. Compost quality improves with the addition of lime at a rate of 15–20 kg/t of compost. The maturation period is 1.5–2 months in summer and 3–4 months in winter. The end of composting is determined by the absence of helminth eggs in compost samples taken at a depth of 0.5 m. When properly prepared, composts without mineral fertilizer additives, at pH 6.7–7.0, contain on a dry matter basis at least 50% organic matter, 1.8–2.0% total nitrogen, 1.0–1.2% total phosphorus, and 0.2–0.5% potassium.

A condition for using sewage sludge and composts as fertilizer is compliance with environmental protection requirements. They are applied on level plots not subject to water erosion, with a groundwater level no higher than 40 cm from the soil surface. When applied on reclaimed areas, protective strips at least 30 m wide are left along main canals.

References

1. Mineev V. G., Sychev V. G., Gamzikov G. P., et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: D. N. Pryanishnikov VNIIA Publishing House.

2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.