No abstract

In the external nitrification (EN) biological nutrient removal (BNR) activated sludge (AS) system, the nitrification process is removed from the main BNRAS system to a fixed media system external to the AS system (Hu et al., 2003). The ENBNRAS system provides considerable advantages over the conventional BNRAS system, e.g. reduced bioreactor volumes, secondary settling tank surface area and oxygen demand. Further, the ENBNRAS system provides opportunity for substantial system intensification. The performance and characterization of the ENBNRAS system has been successfully demonstrated at lab-scale (Hu et al., 2000, Sötemann et al., 2002, but has not yet been tested in full-scale implementation. In collaboration between the City of Tshwane Metropolitan Municipality (CTMM) and the University of Cape Town, ENBNR activated sludge is being implemented at fullscale at the Daspoort Waste Water Treatment Works (DWWTW) in Central Pretoria, South Africa. This paper describes the preliminary design of this full-scale plant and initial implementation.

The full-scale single-stage tertiary nitrifying trickling filter (NTF) at the Citrusdal Wastewater Treatment Plant provides for external nitrification of unclarified effluent from the facultative aerobic lagoon in order to meet standard effluent ammonia concentration requirements. The apparent ammonia nitrification rate (ApANR, gN/m 2 media surface•d) of the NTF was sensitive to particulate organic loading rates which were predominantly in the form of algae, and the soluble COD removal rates increased under cold climates. Installation of forced-air ventilation fans improved the nitrification efficiency from 15% to 43%. An increase in hydraulic loading rate (HLR) by effluent recirculation significantly improved the ApANR, eradicated filter flies and decreased the prevalence of worms. Maximum ApANR of ~1.0 gN/m 2 •d was achieved yielding an ammoniaremoval efficiency of approximately 71%. Profile samples collected along the NTF media depth indicated poor media wetting at low HLR resulting in low ApANR (<0.5 gN/m 2 •d). Also during the cold and rainy winter period, poor biofilm activity and prevalence of motile algae were observed, and under low hydraulic loading rates and warmer temperatures, a dominance of filter flies and fly larvae were observed. In contrast, in controlled laboratory studies, ApANRs up to 1.72 gN/m 2 •d (22.1 mgN/ℓ removal) were attained, which, in conformity with full-scale, was also found to be sensitive to hydraulic loading conditions.

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.

In the external nitrification (EN) biological nutrient removal (BNR) activated sludge (AS) system, the nitrification process is removed from the main BNRAS system to a fixed media system external to the AS system. The ENBNRAS system provides considerable advantages over the conventional BNRAS system, e.g. reduced bioreactor volumes, secondary settling tank surface area and oxygen demand. Further, the ENBNRAS system provides opportunity for substantial system intensification. The performance and characterization of the ENBNRAS system has been successfully demonstrated at lab-scale, but has not yet been tested in full-scale implementation. In collaboration between the City of Tshwane Metropolitan Municipality (CTMM) and the University of Cape Town, ENBNR activated sludge is being implemented at full-scale at the Daspoort Wastewater Treatment Works (DWWTW) in Central Pretoria, South Africa. This paper describes the implementation and initial performance of the system.