Comparative analysis of floc characteristics and microbial communities in anoxic and aerobic suspended growth processes

Strict anoxic conditions significantly impact the metabolism of particulate and colloidal organic matter and bio-P compared to aerobic conditions

Fully anoxic suspended growth treatment of domestic wastewater is rarely performed in practice at large scale. However, recent advances in membrane aerated biofilm reactor (MABR) technology can enable the "hybrid" concept that couples nitrification in the MABR with anoxic suspended growth for biological nitrogen removal. Small scale sequencing batch reactors were constructed to compare high-rate anoxic metabolization of influent carbon and biological phosphorus removal side-by-side with a conventional aerated system in a low-strength domestic wastewater (COD/TN ratio of approximately 6). Little differences existed in the oxidation of soluble readily biodegradable organic material between the two systems, but hydrolysis of particulate and colloidal organic matter in the anoxic reactor over a range of solid retention times was 60 % of the aerobic reactor. Reduced hydrolysis limited the amount of carbon available to ferment to volatile fatty acid (VFA), adversely impacting anoxic biological phosphorus removal (bio-P) process rates, and ortho-P removal performance was diminished by more than half at equivalent SRTs. At optimal growth conditions, i.e., an SRT of approximately 8 days and with supplementary VFA, ortho-P removal from the influent averaged roughly 75 %. Experimentation with supplemented acetic acid showed reduced anoxic metabolic efficiency, quantified via a P/O ratio of 0.90 versus 1.7 for the aerobic system, although overall anoxic bio-P removal demonstrably increased with external carbon.

The hybrid MABR process achieves intensified nitrogen removal while N2O emissions remain low

The hybrid membrane aerated biofilm reactor (MABR) process represents a full-scale solution for sustainable municipal wastewater treatment. However, most of the existing hybrid MABR processes retain large aerobic bioreactor volumes for nitrification, which is undesirable for energy and carbon savings. In this study, we used the plant-wide modeling approach with dynamic simulations to examine a novel hybrid MABR configuration with aeration controls that change the anoxic and aerobic fractions of the bioreactor volume. Result showed that the novel hybrid MABR showed "swinging" nitrification and denitrification capacities in response to diurnal loadings, achieving intensified nitrogen removal performance under both warm and cold temperature scenarios. N2O emissions from the hybrid MABR were only 1/5 of the emissions from the conventional activated sludge. The model predicted higher CH4 emissions from the hybrid MABR than the activated sludge process due to the methanogen growth in the oxygen-depleted MABR biofilm layer. Future measurements for CH4 emission are needed to obtain a holistic picture of the carbon footprint of the hybrid MABR process.