Experimental determination of the heterotroph anoxic yield in anoxic-aerobic activated sludge systems treating municipal wastewater

Mc Wentzel,Aw Muller,Ga Ekama

doi:10.4314/wsa.v30i5.5111

Abstract

This paper describes experimental research to directly quantify the ordinary heterotrophic organism (OHO) cell yield coefficient under anoxic and aerobic conditions with real wastewater as substrate. Until recently these two values were assumed equal in activated sludge models, despite theoretical predictions that the anoxic yield should be reduced relative to its aerobic value. In this study, parallel aerobic and anoxic batch tests were conducted on the same mixtures of wastewater and mixed liquor drawn from anoxic-aerobic activated sludge systems. By equating the readily biodegradable (RB)COD concentrations in the aerobic and anoxic batch tests, the anoxic yield was determined in terms of the respective measured oxygen and nitrate utilisations and the aerobic yield. Additionally, the aerobic and anoxic yield coefficients were directly determined by using defined quantities of the artificial RBCOD acetate. The wastewater batch tests demonstrated that the OHO anoxic yield is reduced to approximately 81% (0.54) of the aerobic value (0.67 mgCOD/mgCOD). The anoxic:aerobic yield ratio (0.81) was confirmed independently in the acetate batch tests at 0.84, and closely equals that predicted theoretically from thermodynamic and bioenergetic principles (0.83). From these investigations, it is recommended that the OHO anoxic yield be decreased to 0.54 for an aerobic yield value of 0.67 mgCOD/mgCOD. Water SA Vol.30(5)Special 2004: 7-12

Highlights

Integral to the biological nutrient removal (BNR) activated sludge system is the biologically mediated process of denitrification
Biological denitrification has been explicitly incorporated into the steady-state design (e.g. Water Research Commission (WRC), 1984; Wentzel et al, 1990) and kinetic simulation (e.g. Van Haandel et al, 1981; Henze et al, 1987; Dold et al, 1980, 1991; Gujer et al, 1999) models developed as aids to the design and operation of BNR activated sludge systems
In both sets of models, critical as input to quantify the denitrification is the value for the ordinary heterotrophic organism (OHO) cell yield coefficient under anoxic conditions (Y )

Summary

Introduction

Integral to the biological nutrient removal (BNR) activated sludge system is the biologically mediated process of denitrification. If the oxygen and nitrate utilised (OU and NU respectively) for the consumption of the same amount of readily biodegradable (RB)COD under respective aerobic and anoxic conditions can be quantified, YH,NO can be determined as a function of YH,AE; i.e.: OU (1 − YH,AE ) On this basis, anoxic and aerobic batch tests containing the same concentration of RBCOD and acclimatised mixed-liquor from the same source were performed, and the corresponding OU and NU (respectively) measured to determine estimates for Y as a function of YH,AE via Eq [2]. Anoxic and aerobic batch tests containing the same concentration of RBCOD and acclimatised mixed-liquor from the same source were performed, and the corresponding OU and NU (respectively) measured to determine estimates for Y as a function of YH,AE via Eq [2] In this approach, since OU and NU are related to complete utilisation of RBCOD rather than its rate of utilisation (Ekama et al, 1986), exact definition of the OHO biomass in the batch tests is not essential.

Actual Results

Results summary

Conclusions