Developments in the Thermal Drying of Sewage Sludge

The application of heat to dried sewage sludge has been practised by the water industry for many years. Early technologies were effective but their utilization of energy was not so efficient. Other industries have faced up to this problem and, as a result, have developed thermal-drying techniques which are more efficient and therefore more cost effective. In recent years, the transfer of this technology to sewage-sludge drying has been taking place, and some companies have designed dryers specifically for this purpose. Commercial and legal pressures have also focused attention on the final product and its use in agriculture, horticulture, land reclamation, landfill, incineration, and as a substitute for fuel. This paper reviews the work which has been undertaken to determine the changes which occur as sewage sludge is dried, and discusses the technology which is available at present. Factors influencing capital and operating costs and product choice are also included.