The microbiology of nitrogen removal in activated sludge systems

Abstract

Eutrophication can be defined as a deterioration in the aesthetic and life-supporting qualities of lakes, estuaries and streams, caused by the continued input of nutrients like nitrogen, phosphorus and organic growth factors, which exceed the concentrations limiting the growth of photosynthetic cyanobacteria, eukaryotic algae and/or macrophytes (Parker, 1984; Blanc et al., 1986; Meganck and Faup, 1988; Argaman, 1991; Cole, 1993). In a healthy water body, stable microbial communities exist as components of balanced food chains (Winkler, 1984), and this stability or homeostasis is destroyed when the supply of these nutrients exceeds the metabolic requirements of these communities (Winkler, 1984). Our understanding of what these acceptable levels are is often poor and not always based on extensive scientific data. They will also vary of course with the receiving water body and its intended use and, because of political reasons, differ between countries. However, it is generally believed that phosphorus enrichment is more influential, especially since cyanobacteria can fix atmospheric nitrogen (Kortstee et al., 1994). Much has been written on the environmental effects of eutrophication, now a world-wide problem, on a water body and its ecosystem (Vinconneau et al., 1985; Meganck and Faup, 1988; Argaman, 1991), but the most obvious visual change is the development of a bloom of these photosynthetic organisms Fig. 8.1, which not only decreases the aesthetic appeal but places an increased demand on the utilization of oxygen present in that water. When these blooms eventually die, breakdown of the biomass by aerobic chemoheterotrophic bacteria leads to oxygen consumption and depletion, often followed by death of the fauna (Argaman, 1991). Proliferation of phytoplankton increases water turbidity (Mackenthun, 1973) further threatening the existence of other photosynthetic organisms requiring light energy, and consequently fish species, due to a decrease in 0₂ production.