J. Gen. Appl. Microbiol., 51, 361–368 (2005)

The ecology of microbial populations in water and wastewater environments has been extensively studied (Barcina et al., 1997; Scott et al., 2002; Sinton et al., 1994). Commonly, the reduction of fecal indicator bacteria is attributed to biotic and non-biotic factors such as predation, nutrient scarcity, temperature, osmotic stress and visible light. But these factors could affect these bacterial populations to different degrees. For instance, the survival of Clostridium perfringens is higher than that of fecal coliforms or enterococci in river waters and in swine wastewater (Hill and Sobsey, 1998; Medema et al., 1997). Gram negative bacteria are selectively preyed upon by ciliates (González et al., 1990). It has also been reported that in sewage treatment plants, bacteria are removed by inactivation, grazing by ciliated protozoa and adsorption to sludge solids and/or encapsulation within sludge flocks followed by sedimentation (Bitton, 1994). Attending to these complex processes of depuration, it should be expected that the predominant species or strains of different microbial indicators and their proportions in treated sewage would change in respect to raw Distribution and persistence of fecal bacterial populations in liquid and dewatered sludge from a biological treatment plant