TABLE OF CONTENTS 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.
At moisture contents >90% sludges behave as liquids, while below 90% they behave as non-Newtonian fluids exhibiting plastic rather than viscous flow (Fig. 21.1). In sludges with a moisture content of >95% most of the water is in a free or readily drained form (70%) while the remainder is bound to the sludge and more difficult to remove (Fig. 21.2). While most of the free water can be removed by gravity settlement, some of the bound water must also be removed if the sludge is to be mechanically handled rather than pumped. This is normally done by coagulation using chemicals and dewatering equipment which removes moisture by altering the particle formation of flocs and the cohesive forces that bind the particles together, thus releasing floc and capillary water.
