Most wastewater treatment plants treat the settled sewage liquor aerobically, that is, in the presence of dissolved oxygen. The activated sludge system relies on a mixed culture of bacteria to carry out the basic oxidation of organic materials. The main biological groups are bac teria, fungi, protozoans, and rotifers. Major biological groups, with some examples of genus and species, and their functions in various wastewater treatment processes are listed in Table 5.

Heterotrophic microbes predominate in the sludge and cause the removal of both organics and nitrogen, using soluble organic sources of carbon from proteins, fats, and carbon hydrates for energy and reproduction. Consequently, the removal of carbon BOD is accom plished through the growth of heterotrophic bacteria such as Anabaena sphaerica, a species of Cyanobacteria. As an example, the structure of Anabaena sphaerica is shown in Figure 1 . Heterotrophic nitrification is carried out by a

Table 1 Composition of fresh municipal sludge

Organic component (Painter, 1983)

Concentration in solution (mgl 1)

Concentration in suspension (mgl 1)

Organic carbon Carbohydrates Fats

Free and bound amino acids Acids Detergent Uric acid Creatine Amino sugars Amides

90 70

211 34 140 42

Inorganic component

Cd Co Cr Ni

Solid distribution of sewage (Rickert and Hunter, 1971) SettIeabIe soIids Super-colloidal solids CoIIoidaI soIids Soluble solids Total solids

Whole sewage USA (Painter, 1971) (mgl 1) 21.10 3.90 0.80 0.13 9.80 10.30 5.90 23.00 0.47 1.56 0.36 0.48 10.30 6.60

Settled sewage UK (Painter, 1971) (mgl 1) 68.00


109.00 6.50 20.00 100.00 0.05 0.20 0.65 0.08 22.00 22.00

Total solids (mg l 1J 74 57 31 351 513

Mean value (Gould, 1976; cited in Gray, 2004) (mgl 1)


Organic content (mg l 1) 59 43 23 116 241

Value range (Gray, 1980; cited in Gray, 2004) (mg l 1)

COD (mgl 1 120 87 43 168 418

Data from Gray NF (2004) Biology of Wastewater Treatment. London: Imperial College Press; Painter HA (1971) Chemical, physical and biological characteristics of wastes and waste effluents. In: Ciaccio LL (ed.) Waterand Water Pollution Handbook, vol. I, pp. 329 363. New York: Marcel Dekker; Painter HA (1983) Metabolism and physiology of aerobic bacteria and fungi. In: Curds CR and Hawkes HA (eds.) Ecological Aspects of Used Water Treatment, vol. 2, pp. 11 75. London: Academic; Rickert DA and Hunter JD (1971) General nature of soluble and particulate organics in sewage and secondary effluent. Water Research 5: 421 436.

wide range of heterotrophic microbes including bacteria, fungi, and algae. Cyanobacteria are able to reduce nitro gen and carbon simultaneously in aerobic conditions. Autotrophic microbes are responsible for nitrification and denitrification. They can be further divided into autotrophic ammonia oxidizers and autotrophic nitrite oxidizers. The heterotrophic bacterium Thiosphaerapanto tropha is able to nitrify and denitrify simultaneously. The phosphorus removal mechanisms is covered in the next section.

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