ABSTRACT
The removal of the settleable fraction of raw sewage at the primary settlement stage, and of the settleable solids produced by biological conversion of dissolved nutrients into bacterial cells at the secondary stage, continuously produces a large quantity of concentrated sludge. While the liquid fraction of the wastewater can be fully treated and disposed of safely to surface waters, the accumulated sludge has to be further treated and finally transported from the site for disposal. Sludge separation, treatment and disposal represents a major capital and operational cost in sewage treatment, with dewatering and disposal costs for a medium-sized activated sludge plant representing as much as 50% of the initial capital and 65% of the operating costs.
