- Mineral fertilizers
- Microfertilizers
- Complex fertilizers
- Organic fertilizers
Use of sewage sludge
The growth of urban population, development of industry is accompanied by an increase in the volume of wastewater and its sludge. In industrially developed countries an average of 19-25 kg of dry sewage sludge is produced per inhabitant per year. In the Russian Federation, the estimated average annual volume of dry sludge yield is 2.5 million tonnes.
In towns and large settlements, the volume is estimated to be 0.5-1.0 % of the treated waste water. Fresh sludge from primary sedimentation has a water content of 92-95 %. The sludge is disinfected and dewatered. Depending on the technology, sewage sludge is composted, fermented or thermally dried.
During sludge fermentation decontamination is carried out in digesters at a temperature of 56-58 ° C, where the sludge of primary sedimentation tanks and activated sludge are fed in a ratio of 1:1. Fermented sludge is dried on sludge platforms to a moisture content of 60-80%.
In industrialized countries up to 32% of sewage sludge is used as an organic fertilizer.
Table. Production and use of sewage sludge (V.F. Ladonin, G.E. Merzlaya, R.A. Afanasiev et al., 2002).
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*No data available.
In the Russian Federation only 5-7% of sewage sludge is used for fertilizer, which is connected with insufficient implementation of technologies for the preparation for use in agriculture.
The main technological stage of sludge preparation for use is decontamination. Currently, waste water treatment plants use methods of chemical, thermal and biological decontamination of sewage sludge.
Chemical sludge decontamination consists of treatment with fungicides. The humidity of the product is about 60 %, which makes it difficult to spread over the fertilised area. Thermal decontamination produces dried sludge with a moisture content of less than 60 %. The decontamination takes place at 56-58 °C.
Depending on the treatment technology, sludge for use as fertilizer can be liquid with a moisture content of 92-97%; dried or dewatered with a moisture content of 60-80%; dried with a moisture content of 10-40%.
The optimal timing of sewage sludge-based fertilizers is the fall under the main treatment, as well as the summer for the treatment of fallow and early-release crop rotation fields. During the autumn and spring periods soluble compounds, especially chlorine, contained in sludge are washed out of the root layer. On poorly and medium cultivated sod-podzolic soils, fertilizers are applied at the rate of 10-15 t/ha in dry matter, on well cultivated sod-podzolic sandy loam and black soils – 5-10 t/ha. Application of sludge is combined with the application of mineral fertilizers.
Chemical composition of sewage sludge
The chemical composition of the sludge varies greatly, depending on the production technology and the composition of the treated wastewater.
In thermal drying sludge dewatered on centrifuges or vibrofilters to 80% moisture content is dried at 600-800 ° C to a moisture content of 40%.
The chemical composition of sewage sludge includes nitrogen, phosphorus and calcium, poor in potassium. The amount of organic matter in raw sludge is up to 75% in terms of dry matter and 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. Ash composition is specific and is determined by the composition of industrial water.
Table. Composition of sewage sludge, % on dry matter
| From primary sedimentation tanks | ||||||
| Fermented | ||||||
| Fermented with activated sludge | ||||||
| Thermally dried | ||||||
Sediments contain heavy metals, petroleum products, detergents and other hazardous compounds. Therefore, their use requires continuous monitoring of the composition. It is safer to use sewage sludge on heavy, humusy soils. On light and poorly humus soils they are used in combination with chemical melioration.
According to the content of heavy metals most of the sediments meet the international agro-ecological requirements.
Table. Content of heavy metals in sewage sludge, mg/kg dry matter (Ladonin V.F., Merzlaya G.E., Afanasiev R.A. et al., 2002)
Sludge intended for use as fertilizer must contain not less than 40% of organic matter, 1.6% of nitrogen, 0.6% of phosphorus (P2O5), 0.2% of potassium (K2O), moisture – not more than 82%.
Doses of sewage sludge from the sludge platforms are from 20 to 50 t/ha depending on the chemical composition and content and hazardous substances, as well as the needs of crops and the state of cultivation of soils.
It is advisable to use the sludge for fertilizing parks, tree nurseries, lawns and bast crops, for other crops – with the permission of the sanitary and hygienic control authorities. Sewage sludge is not used for vegetable crops.
Composting of sewage sludge
In composting, fresh sediments are dried to a moisture content of 50-55% and mixed with peat in a ratio of 3:1. Then they are raked in stacks, the temperature in them reaches 60 ° C, which leads to the death of non-spore microorganisms, eggs and larvae of worms and flies.
When using household solid waste and sawdust as fillers, the ratio filler-sludge is taken respectively from 0.5:1 to 1-1.5:1 in the summer and from 1:1 to 2-3:1 in the winter. All types of peat are suitable for composting with sludge. Compost with peat can be made at any time of the year. The proportions of sludge and peat depend on the quality of the peat and the time of composting. In winter, for good warming of the compost, the relative content of peat increases to 2-1.5:1. In spring and summer, the ratio is 1.5-1:1. The quality of the compost increases with the addition of lime at a rate of 15-20 kg/t compost. The maturing 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 samples of compost taken at a depth of 0.5 m. If properly prepared, the compost contains at least 50% organic matter, 1.8-2.0% total nitrogen, 1.0-1.2% total phosphorus and 0.2-0.5% potassium per dry matter at pH 6.7-7.0.
The condition for using sewage sludge and composts as fertilizer is to comply with environmental requirements. They are used on flat areas not prone to water erosion, with a groundwater table not higher than 40 cm from the soil surface. When applying on reclaimed areas along the main canals leave protective strips with a width of at least 30 m.
Sources
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry/Under ed. B.A. Yagodin. – M.: Kolos, 2002. – 584 p.: ill.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.