The sources of solids in a wastewater treatment plant vary according to the type of plant and its method of operation. Usually there are two sources of Sewage sludge within the treatment process:

• solids in the affluent to the treatment plant which consist of settable organic matter and mineral substances which are not trapped in the grit chamber.

• biomass that has grown on the organic load (BOD)

Sewage sludge is separated from the treatment process by sedimentation or flotation. In many cases there is a primary sedimentation (primary sludge) and secondary sedimentation (secondary sludge). Smaller treatment plants often have only one sedimentation tank in which the entire sludge is separated from the treated water.

Sewage sludge consists of water and solids that can be divided into mineral and organic (volatile) solids. The quantity and characteristics depend very much on the treatment processes. The following table contains some of the most important information about Sewage sludge for different treatment processes: the quantity per population equivalent (PE) and day, the water content and the percentage of volatile solids.

Table 1: Quantity and water content of Sewage sludge from activated sludge plants

As you can see, the biggest amount of sludge is produced by primary treatment which, of course highly depends on the hydraulic retention time in the primary sedimentation tank.

A big advantage of anaerobic wastewater treatment is the production of much less sludge than in aerobic systems because of low growing rates of anaerobic bacteria. If the primary and secondary treatment is replaced by an anaerobic step like a UASB reactor the sludge production is less than 10% of the aerobic system (see also lesson B3).

Among the unspecified “mineral� and “organic� solids are hazardous materials. Most of the pollutants that enter the wastewater get adsorbed to the Sewage sludge. Therefore, Sewage sludge contains heavy metals, many organic pollutants (pesticides, hydrocarbons etc.) and pathogens. The concentration depends on the industry, workshops or hospitals connected to the sewer system and their efforts to reduce the emission.

A very important class of substances are nutrients. Sewage sludge contains nitrogen (av. 2.6 % of TS), phosphorus (av. 2 % of TS), potash (av. 0.2 % TS). These elements, various trace elements and organic substances make Sewage sludge a valuable fertilizer.

The purpose of sludge treatment is, besides hygienisation, to change the figures in the table above. Easily biodegradable volatile solids cause odour, that is why they should be reduced by stabilisation. A high water content (ï‚® low percentage of total solids) makes the handling difficult, causes high cost for transportation and storage and should therefore be reduced.