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.
This paper reviews the current sludge technologies and discusses likely future trends associated with those technologies. It describes the role of thickening, dewatering, pasteurization, stabilization, thermal destruction and product enhancement to satisfy environmental standards. Reference is made to technologies which, for decades, have been associated with sewage sludge but where market conditions have forced R and D initiatives to be introduced and commercialized.
The dewatering of sewage sludges using filter presses has been in common use for the past 80 years. The technology was mainly used in the UK for large waste water treatment plants especially where woollen grease was extracted from the sludge. It was also associated with the predrying of sludge prior to incineration in Multi Hearth Furnaces. To meet the growing demand for drier cakes and better capital productivity the membrane filter press has been developed. With the introduction of improved polyelectrolytes, polymer dosing systems, insitu cloth washing and computerised control the average pressing cycle has been reduced from 4-6 hours to under two hours. At the same time the dry solids content of the cake has become more consistent and for most waste waters in excess of 30%. The use of on-site pilot plant trails to determine the design criteria is now the accepted approach enabling sensible scale up and the setting of performance guarantees.
Sludge arisings in the Hong Kong Territories are predicted to increase from the current level of 44 300 tDS/annum to 314000 tDSlannum. This has led to a need to examine the treatment and disposal facilities required for the future. Disposal facilities were found t o be restricted, with virtually no suitable land for a liquid disposal route and a sea route limited to the present quantities. Only landfill remained as the major disposal outlet, but this is also limited by the moisture absorbence capacity of the emplaced material and limiting metal concentrations.In order to match the sludge production to the disposal outlet capacity, a series of process options was considered. Transportation studies using pumping, road and sea networks were undertaken and the environmental impact assessment of introducing thermal technologies was reviewed. Finally the various options were evaluated using a modified version of the Water Research Centre's WISDOM model.
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