Biological Nutrient Removal

Abstract

Biological nutrient (BNR) is mediated mainly by three groups of organisms functioning in the three different zones – aerobic, anoxic, and anaerobic – of the BNR wastewater-treatment plant (WWTP) – the ordinary heterotrophic organisms (OHOs), which break the influent wastewater biodegradable organics (COD), the autotrophic nitrifier organisms (ANOs), which nitrify ammonia to nitrate, and the heterotrophic phosphorus accumulating organisms (PAOs), which take up P in excess and store it intracellularly as polyphosphate chains. The OHOs subdivide into three subgroups: one aerobic which utilizes oxygen in the aerobic zone; one facultative which mediates denitrification of nitrate and nitrite to nitrogen gas in the anoxic zone; and one fermentative which produces the volatile fatty acid substrate for the PAOs from the influent readily biodegradable organics the anaerobic zone. In this chapter, by applying mass balance, continuity, and bioprocess modeling principles, steady-state models that describe quantitatively the behavior of these three organism groups are developed and the experimental data on which these models are based are described. Because these steady-state models are based on the same principles and bioprocess kinetics as the dynamic kinetic models for WWTP simulation, only their simplified versions, they not only give great insight into the inner workings of the dynamic kinetic models but also give closely similar predictions as the kinetic models. Therefore, steady-state models serve a very useful complementary role to the dynamic kinetic models. This chapter distills much of the developments in BNR with activated sludge over the past three decades and so is essential reading for anyone involved in WWTP modeling. Studying this chapter will provide advanced knowledge and deeper insight into the modern approach of BNR WWTP design and operation via modeling and simulation so that these techniques can be used with greater confidence.