HEAVY-METALS AND HEAT VALUE IN SEWAGE-SLUDGE

Of the 50 x 10(6) Mg of sewage sludge (5% dry solids) produced per year in the FRG, 60% is dumped, 30% used in agriculture or composted, and 10% is combusted. For all the different types of sludge utilization, thermal sludge treatment and disposal or dumping, major problems are created by pollutants in the sludge. There may be serious levels not only of salts and organic pollutants, but also especially of heavy metals. The content of these pollutants causes environmental dangers in soil, plants, groundwater and in the air. For the thermal treatment or disposal of sludge, knowledge of the salt and heavy metal content is important for the choice of the right flue gas cleaning system. Only by this means can hazardous emissions into the air be avoided. The advantage of thermal sludge treatment as opposed to the dumping and utilization systems must be seen in the thermal destruction of the organic pollutants. At the same time, as a large part of the heavy metals are evaporated by the high burning temperature, they are taken up into the flue gas and can be captured by condensation on the filter-ash or in the scrubber water. In the combustion of sludge its heat value is very important, and is dependent on the organic matter and the water content of the sludge, which can be influenced by dewatering systems. Comparative results on heavy metals and heat values for different sorts of sludges have been collected in a great number of investigations. The results of these investigations are presented and should serve as a basis for the prevention of negative environmental influences by the utilization, dumping or burning of sludge.