The main directions of sewage sludge disposal:

• Storage on sludge beds.
• Disposal of SS as fertilizer.
• Direct combustion method.
• Recycling by means of pyrolysis.

   Currently, 90% of the total sludge produced in Russia is disposed on sludge drying beds. Evaporation from  the sludge ponds results in atmospheric air pollution, and seepage into the soil results in pollution of groundwater and nearby bodies of water. Harmful gases released by sewage sludge exceed the maximum permissible concentrations and smell bad. Their smell is equal to 4-5 points on the scale of organoleptic indicators. 

    Numerous studies have confirmed the possibility of using urban sewage sludge as organic fertilizers. The sludge is classified as Hazard Class IV (low-hazardous waste).

The disposal of sewage sludge is generally limited by sanitary-epidemiological and sanitary-chemical factors. Modern technologies make it possible to eliminate the epidemiological danger.

However, the toxic components contained in sludge (Cd, Cu, Ni, Pb, Cr, Zn, Hg, As, Mn, and in some cases Mo, Se, Co, Sr, B, Be, Ba), whose level exceeds the background content in natural objects, prevent the use of sludge in agriculture and make it difficult to use sludge for remediation and recultivation of lands. In order to prevent pollution of the environment with heavy metals, most countries have established national standards for the maximum permissible content of heavy metals in sewage sludge to be disposed of.

However, in Western Europe, many farms that grow organic products refuse this type of fertilizer.

With the fluidized bed technology, the material is burned in a hot sand plume lifted by a jet of air Thus the waste gases carry off slag containing harmful oxides, which must then pass through a multi-stage treatment plant.

Mass development of such plants took place in the United States, Japan and some European countries in the 1980s. However, already in the 90s, the attitude to the technology of sludge combustion has changed dramatically due to its negative impact on the environment.

It should be noted that in recent years, many industrial waste incineration plants, including waste from wastewater treatment, have been closed in Europe and Asia.

European countries are gradually abandoning the technology of sewage sludge combustion and switching to sludge treatment technologies that allow them to produce marketable products while minimizing operating costs.

    This method is the most progressive. Pyrolysis (thermal decomposition of organic matter without oxygen access) at temperatures not exceeding 700°C produces combustible gas (~55%), semicoke (~35%), liquid organic matter (~15%), which at such temperature goes with the gas, and the semicoke undergoes a gasification process and turns into combustible gas as well.

Metal oxides remain in the gasification chamber as pure slag, suitable for use as a mineral filler.

Only the organic constituents of sewage sludge are subjected to gasification and pyrolysis, therefore the emissions do not contain any pollutants, as with direct combustion. Table 1 shows the composition of combustible gas from sewage sludge, showing the absence of harmful constituents..

1. 1. Recycling of sewage sludge by pyrolysis and gasification, using the resulting combustible gas for the production of electricity and heat for the equipment operation.
2. 2. Environmentally friendly disposal without sludge beds and harmful emissions to the atmosphere.
3. 3. Getting commercial products in the form of generated electricity and paving slabs (possible option).

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1. 1. Drying of sludge from a moisture content of 70% to a moisture content of no more than 5%.
2. 2. Pyrolysis without oxygen access.
3. 3. Gasification of semi-coke.
4. 4. Combustion of gas-generating gases in a gas engine gen set to produce electrical energy.

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• Option A: Mixing slag with concrete, producing 100 mm thick paving slabs of different shapes and colours by compression moulding..
• Option B: Removal of slag for use in road construction, filling dumps, etc.