From Water Technology: An Introduction for Environmental Scientists and Engineers, Second Edition


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.


Products & Services
Dewatering Equipment
Dewatering equipment uses force, including vacuum and centrifugal motion, to separate water from solids.
Clarifiers and screens are used to remove solids from liquids through gravity settling or a mechanical screening process.
Oxidation Equipment
Oxidation equipment sterilizes wastewater via oxidative chemical processes.
Sequencing Batch Reactors
Sequencing batch reactors (SBR) are fill-and-draw activated sludge systems for wastewater treatment. Wastewater is added to a single "batch" reactor, treated to remove undesirable components, and then discharged.
Fixed Growth Treatment Systems
Fixed growth treatment systems attach microorganisms to a surface that is exposed to water.

Topics of Interest

INTRODUCTION Wastewater treatment systems for individual houses are required to be cheap, robust, compact, hygienic at site, odourless, require little maintenance and be installed, and operated, by...

3.1 Thickening Thickening is normally the first processing operation after raw sludge has been generated at the wastewater treatment plant. Thickening is defined as the removal of water from sludge...

OVERVIEW All biological treatment systems generate excess sludge or biosolids that must be removed (eventually) from the system to maintain optimum treatment efficiency. Some high rate activated...

3.4 Dewatering Dewatering is the removal of water from sludge to achieve an overall volume reduction. Sufficient water is removed during the dewatering process to produce a sludge that is no longer...

3.3 Conditioning In many cases, stabilized sludge may be applied directly to land. If it is desired to land apply stabilized sludge in liquid form, sludge from anaerobic digestion, aerobic digestion,...