Introduction to the microorganisms found in activated sludge processes

Abstract

Sewage can be defined as a suspension of faeces in the domestic and industrial wastewater of the community, and it has been estimated that an average human being excretes about 100 g fresh weight of faeces daily. Gray (1989) suggests that more than 10⁶m³ of domestic sewage and seven times that amount of industrial wastes are produced daily in the UK. Extrapolating these figures to global production illustrates why wastewater treatment is the largest biotechnology industry in the world. This domestic sewage is made up of undigested food remnants, still rich in an array of organic compounds, and bacterial cells. A typical composition of domestic sewage is given in Table 1.1 (Painter, 1983). In a healthy person, the bacteria present in faeces would usually be mostly harmless, but are still present in very large numbers (about 10⁹ cells per gram of faeces). However, people with gastrointestinal upsets or who carry certain pathogenic organisms will also continually excrete disease causing viruses (Tyler, 1985), bacteria (especially Salmonella spp. and Vibrio cholerae, which are both serious health hazards), protozoa like Giardia lamblia (Gillin et al., 1996), and other intestinal parasites in large numbers as shown in Table 1.2 (Jones and Watkins, 1985). As expected, the concentration of faeces in sewage — and hence its bacterial content — is not a constant, but changes continuously with time, weather conditions and patterns of human activity.