33. Sludge Treatment Technologies and Systems, an Introduction

Sewage sludge is the largest by-product of used water treatment. Its production is dependent upon influent characteristics and the processes applied. It is composed of a complex, heterogeneous mixture of microorganisms and readily biodegradable organic and inorganic materials. Sludge treatment is a highly complex, expensive, and environmentally sensitive problem. At present, there are a wide variety of sludge treatment processes in operation, which can be employed in various combinations in line with the goal and end usage. Volume reduction and sludge stabilization represent two main sewage sludge treatment objectives. Volume reduction begins with the thickening process during which the sludge solids are concentrated in order to increase the efficiency of further treatment. Sludge dewatering and drying are other methods for volume reduction that go far beyond thickening and reduce water content considerably. Sludge stabilization can be achieved by biological, chemical, and thermal processes and except in the case of alkaline stabilization leads to mass reduction. Other positive effects of sludge stabilization are pathogen removal, a cut in odors, and improved sludge dewaterability.