Due to the limited number of disposal sites and limits required by the management of only mechanically drained sewage sludge will become both more difficult and more expensive. Therefore, it is to be expected that a reduction of mass/volume of sewage sludge before depositing will be demanded in the future. Extensive discussions about organic and inorganic toxic agents in the sewage sludge have led to a declining acceptance of dry sludge in the agriculture. Some European countries have introduced stringent national legislations which prohibit usage of sludge in agriculture. In order to decrease area used for landfilling, sludge drying can be used, which require considerable amounts of energy. Another alternative to landfill disposal is incineration, i.e. high temperature combustion of waste.
Waste of land, odour problem
Due to the huge amounts of sludge, the amount of the landfilled sludge should be as low as possible and cannot be considered as the main option.
May contain hazardous stuff
Usage of sludge as fertilizer or soil conditioner related to several issues
- Handling problems due to odor and high water content
- Soil contamination by bringing in pathogens, parasites, seeds of weeds;
- Heavy metal contamination of soil.
Composting may solve first two issues, however this option requires considerable space and takes times. Moreover, several European countries banned agricultural use of sludge.
Energy consumption and further disposal of dryed sludge
Main goal of drying is to evaporate the excess of water, thus decreasing the volume of sludge cake to be disposed at the landfill site. Since drying does not solve the problem related to the soil contamination, restrictions imposed on agricultural use are remained. Large amount of energy required for sludge drying is a very high price just for facilitating of landfilling and cannot be considered as option as well.
Fuel required, environmental impact of
combustion products and ash
Performs sludge drying and mass combustion, decreasing volumes of sludge to be disposed. Incineration consumes fuel to maintain stable mass combustion of sludge. Waste heat energy can be used to generate electricity. Air/oxygen supply is a source of dioxins formation; incineration ash should be disposed at special landfill site. Additional problems are associated with large amount of gas emissions, including greenhouse gases (water vapors, carbon dioxide, nitrous oxide), and dust filtering.
Waste-to-energy and useful solid products generation
The advantage of the process is the effective reduction of volume and mass of the dewatered sludge and vaporization of organic toxic agents from the sludge (e.g. PCB, PAK ) into harmless substances in the combustion chamber. Mercury and its compounds are thermo decomposed and vaporized as well. Pyrolysis also performs energy recycling; moreover, no supplement fuel supply is required for the normal operation – significant reduction of running costs.