A study of nitrate versus oxygen respiration in the activated sludge process

Abstract

Utilization of the activated sludge process is widespread although many of the mechanisms involved are still relatively misunderstood. Incorporation of nitrate respiration (denitrification) into the activated sludge process can have many advantages, but little is known about microbial growth and substrate removal when nitrate respiration is employed.

The primary objective of this study was to evaluate and compare microbial growth and biokinetic coefficients in an aerobic and an anoxic (anaerobic with nitrate as the terminal electron acceptor) activated sludge process. Two bench-scale continuous flow reactors were operated over a range of mean cell residence times with organic carbon as the limiting nutrient. Alkalinity changes were monitored and compared with theory. Engineering applications of the results were discussed.

The maximum microbial yield and endogenous decay coefficient were lower, and the maximum substrate utilization rate was higher for nitrate versus oxygen respiration. Alkalinity production during denitrification was very near the theoretical stoichiometric value of 3.57 mg as CaCO₃ per mg NO₃⁻-N denitrified.

It was concluded that single-sludge systems incorporating organics removal, nitrification, and denitrification can potentially achieve a high degree of nitrogen and organic carbon removal at lower cost than a similar size system incorporating organics removal and nitrification only. Aeration energy savings and reduced sludge production obtained by the utilization of nitrate respiration in single-sludge systems should result in significant cost savings.