Co-Combustion of Sewage Sludge in Grate-Based Combustion Plants
Various feeding systems are available for co-combusting municipal sewage sludge in thermal waste-to-energy plants using grate-based systems. Dried sewage sludge can be fed directly into the refuse pit. Mechanically dewatered sewage sludge is intermediately stored in silos and then conveyed by means of solids pumps to the specific feeding system in the waste chute of the grate-based combustion system or furnace.
Disposal of sewage sludge is the subject of much controversial and emotional discussion.
On the one hand, there is a clear preference for an agriculture-based approach to sewage sludge treatment, involving the return of nutrients and humus to the land or use of the sludge in landscaping. On the other hand, however, regulations with more stringent limit values are being promoted to protect the general public, and these regulations mean that an agriculture-based approach is no longer possible.
Sewage sludge consists of 50 - 70 % carbon, approx. 7 % hydrogen, 21 - 24 % oxygen, 15 - 18 % nitrogen, 1 - 1.5 % phosphorus and up to 2 % sulphur, i.e. by all means valuable substances for soils, such as nitrates, phosphates and potassium. As waste water treatment has become increasingly complex and fewer pollutants are permitted in cleaned waste water, these materials can be found in concentrated form in sewage sludge after the cleaning process. In particular, heavy metals such as mercury, cadmium, and lead, as well as halogen compounds, organic substances, medication residues and hormones are critical.
Nevertheless, agriculture-oriented sewage sludge treatment continues to be common practice because the relevant directives and ordinances essentially permit the application of sewage sludge on agriculturally or horticulturally used soils as long as specified concentration limits for heavy metals are adhered to (Sewage Sludge Ordinance - Council Directive 86/278/EEC). However, the trend in many countries is to restrict the use of sewage sludges as fertilizers because the effects of introducing the specified pollutants into the soil and groundwater is seen as an extremely critical issue.
Valuable materials such as phosphate and nitrate can already be separated in the waste water treatment plant and returned to the cycle as raw materials. The remaining sewage sludge can be treated thermally.
This paper outlines the options for treating sewage sludge from municipal sewage treatment plants in the grate-based combustion systems of waste-to-energy plants.
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