ABSTRACT

Dissolved oxygen concentrations affect the rates of nitrification, denitrification, and phosphorus removal in a variety of ways. Enough DO must be present in the aerobic zone to allow growth of nitrifiers and PAOs at adequate rates. In general, the DO requirements for nitrifiers are controlling due to their high half-saturation coefficient for oxygen. Although a DO concentration of 2 mg/L is often specified to obtain efficient nitrification, effluent quality goals can be obtained at lower DO concentrations if aerobic SRTs are sufficiently long. This effect is illustrated in Figure 3.3 where it can be seen that many combinations of DO and ammonia-N concentrations can exist for a specified nitrifier specific growth rate, i.e., for a given system aerobic SRT. At a fixed aerobic SRT, a reduction in the DO concentration will result in an increase in the ammonia-N concentration, but the increase may be small if the aerobic SRT is sufficiently long. Consequently, DO concentrations must be evaluated on a relative basis and adjusted in accordance with system performance requirements. Operation at aerobic zone DO concentrations below 2 mg/L may result in adequate nitrification and phosphorus uptake, while also encouraging additional denitrification. Low DO operation, on the other hand, may encourage the growth of Group IV filamentous bacteria, as discussed previously. The addition of DO to anoxic and anaerobic zones should be minimized because it is used preferentially as a terminal electron acceptor, thereby reducing the amount of readily biodegradable substrate available for denitrification or for uptake by the PAOs. If the quantity of readily biodegradable organic matter in the influent wastewater is high, however, larger oxygen inputs can be tolerated while still producing acceptable process performance because sufficient quantities will still be available for the other needs. Care should

be taken not to introduce too much oxygen, however, because DO concentrations will be low in those situations and significant oxygen inputs can lead to excessive growths of low DO filamentous bacteria. In spite of the effects of oxygen entry into anoxic and anaerobic zones, both are often uncovered and simply rely on low surface turbulence to minimize oxygen transfer rates. As a consequence, care must be exercised in the selection and placement of mixers to minimize surface turbulence. In situations where interfacial oxygen transfer must be kept as low as possible, covers of various construction can be used. Since they need not exclude all air, but merely need to reduce surface transfer, the opportunity exists for the design engineer to be innovative in solving the problem.