R. E. Loewenthal

Heterotroph anoxic yield in anoxic aerobic activated sludge systems treating municipal wastewater.

As input to the steady state design and kinetic simulation models for the activated sludge system, the correct value for the heterotroph anoxic yield is essential to provide reliable estimates for the system denitrification potential. This paper examines activated sludge anoxic yield values in the literature, and presents experimental data quantifying the value. In the literature, in terms of the structure of ASM1 and similar models, theoretically it has been shown that the anoxic yield should be reduced to approximately 0.79 the value of the aerobic yield. This theoretical value is validated with data from corresponding aerobic OUR and anoxic nitrate time profiles in a batch fed laboratory scale long sludge age activated sludge system treating municipal wastewater. The value also is in close agreement with values in the literature measured with both artificial substrates and municipal wastewater. Thus, it is concluded that, in ASM1 and similar models, for an aerobic yield of 0.67mg COD/mg COD, the anoxic yield should be about 0.53 mg COD/mg COD. Including such a lower anoxic yield in ASM1 and similar models will result in a significant increase in denitrification potential, due to increased denitrification with wastewater RBCOD as substrate. In terms of the structure of ASM3, for the proposed substrate storage yields and the aerobic yield of 0.63 mg COD/mg COD, experimental data indicate that the corresponding anoxic yield should be about 0.42 mg COD/mg COD. This is significantly lower than the proposed value of 0.54 mg COD/mg COD, and requires further investigation.

A hypothesis for the cause of low F/M filament bulking in nutrient removal activated sludge systems

Abstract Laboratory research has indicated that a possible cause of low F/M filament bulking in ND (nitrification-denitrification) and NDBEPR (nitrification-denitrification biological excess phosphorus removal) systems occurs as a result of competition for substrate between filamentous and floc-forming organisms which have different denitrification pathways. In ND and NDBEPR systems alternating anoxic-aerobic conditions prevail and continuous utilization of particulate slowly biodegradable COD (SBCOD) by floc-forming organisms in these systems leads to accumulation of the denitrification intermediates nitrite (NO2−) and nitric oxide (NO) under anoxic conditions. It is proposed that a cause for low F/M filament bulking is that the intermediate NO inhibits the utilization of SBCOD by floc-formers under subsequent aerobic conditions, with high concentrations of NO2− exacerbating this effect, thereby allowing filamentous organisms, which do not accumulate NO, to dominate. Some experimental evidence to support this proposal is presented.