Wastewater and Sludge Composition

Pathogenic viruses, bacteria, protozoa and helminths may be present in raw municipal wastewater at the levels indicated in Table 11.3 and will survive in the environment for long periods. Pathogenic bacteria will be present in wastewater at much lower levels than that of the coliform group of bacteria, which are much easier to identify and enumerate (as total coliforms/100 ml). Escherichia coli are the most widely adopted indicator of faecal pollution and they can also be isolated and identified fairly simply, with their numbers usually being given in the form of faecal coliforms (FC)/100 ml of wastewater. 

Municipal wastewater also contains a variety of organic and inorganic substances (Table 11.4) from domestic and industrial sources, including a number of PTEs such as arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), zinc (Zn), etc., which constitute a major issue in terms of abatement and/or treatment (for a review of organic chemicals in sewage sludge, see EC, 2001a Harrison et ah, 2006). 

All the potential contaminants are dealt with during the various phases of the wastewater treatment at WWTPs, with the primary goal of protecting the receiving waters from pollution by harmful effluent. UWW treatment, however, is not sufficient to deal with all the pollution. Although different contaminants end up to varying degrees in effluent from WWTPs, most of the pollutants are concentrated in 

Type of pathogen Possible concentration per litre in municipal wastewater  

The principal health hazards is instead associated with the chemical constituents of the initial wastewaters which can therefore produce contamination of crops or groundwaters. Cu, Cr, Zn and Se are essential trace elements however, they are PTEs, and above certain concentrations may interfere with or inhibit the actions of cellular enzymes. After the treatment, even if toxic materials are not present in the sludge in concentrations likely to affect humans, they might well be at phytotoxic levels, which would limit their agricultural use (Table 11.5). Furthermore, Hillman (1988) has drawn attention to the particular concern attached to the cumulative poisons, principally PTEs, and carcinogens, mainly organic chemicals. World Health 

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