Simultaneous nitrification, denitrification, and phosphorus removal at low temperatures

Simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) has been demonstrated to be a promising technology for carbon, nitrogen, and phosphorus removal. However, SNDPR has not been fully studied at low temperatures. This study was the first to investigate the performance of SNDPR at 10℃ to treat a complex synthetic wastewater and real municipal wastewater. A comprehensive floc model was developed, calibrated, and validated to quantitatively understand the transformation of carbon, nitrogen, and phosphorus in the SNDPR system at 10 ℃. Nitrogen removal pathways of SNDPR at low dissolved oxygen (0.3 mg/L) and temperature (10℃) were explored to understand nitrogen removal mechanisms. Biological nitrogen and phosphorus removal were sustained with total inorganic nitrogen removal, phosphorus removal, and simultaneous nitrification and denitrification (SND) efficiencies of 62.6%, 97%, and 31%, respectively. The SND was observed in the first 2 h of the aerobic phase and was attributed to denitrifying ordinary heterotrophic organisms (OHOs) using readily biodegradable chemical oxygen demand and denitrifying phosphorus accumulating organisms (DPAOs), which removed 15% and 12% of influent nitrogen, respectively. A phosphorus accumulating organism (PAO)-rich community was indicated by stoichiometric ratios and supported by 16S rRNA gene analysis, with Dechloromonas, Zoogloea, and Paracoccus as DPAOs, and Ca. Accumulibacter and Tetrasphaera as PAOs. Even though Ca. Competibacter (10.4%) was detected, limited denitrifying glycogen accumulating organism (DGAO) denitrification was observed, which might be due to low temperatures. This research was the few researches that investigated the SNDPR process at 10℃ by using a complex synthetic wastewater, investigated the nitrogen removal pathways in the aerobic phase using an experimental method, and integrated microbial community analysis with experimental findings. The feasibility of SNDPR at a low temperature (10℃) when treating real municipal wastewater was explored by implementing two process configurations (anaerobic/aerobic (AO) and anaerobic/aerobic/anoxic (AOA)). It was found that SNDPR in the AO configuration failed, however, SNDPR in the AOA configuration was achieved with total nitrogen removal, phosphorus removal, and SND efficiencies of 91.1%, 92.4%, and 28.5%, respectively. The main nitrogen removal pathways were denitrification by DPAOs in the aerobic phase and denitrifying OHOs using hydrolyzed carbon in the anoxic phase, which accounted for 16% and 56% of influent nitrogen, respectively. A PAO-rich system was indicated by stoichiometric ratios and supported by 16S rRNA gene analysis, with Dechloromonas and Ca. Accumulibacter as dominant DPAOs and PAOs. Ca. Competibacter was detected, whereas limited denitrifying GAO denitrification was observed, which might be due to low temperatures. This research was the first to 1) investigate the performance of SNDPR when real municipal wastewater was treated under low temperature conditions (10℃); 2) investigate whether operational conditions that have been successfully employed to treat synthetic wastewaters can also be applied to real municipal wastewaters; 3) compare the performance of SNDPR when operated in different process configurations (AO and AOA). A comprehensive floc model was designed to investigate SNDPR at 10℃. Results show that only boundary layer thickness in the floc-related parameters established a minor impact on nitrite, and seven new incorporated parameters (fP,VFA, fPP,PHA,ox, and intrinsic half-saturation coefficients of oxygen of ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), OHOs, PAOs, and GAOs) were deemed as sensitive parameters. The model calibration and validation were demonstrated successful based on R2, mean square relative error, and residual analysis. After model validation, intrinsic KO values of AOB, NOB, OHOs, PAOs, and GAOs were estimated to be 0.08, 0.18, 0.03, 0.1, and 0.07 mg/L, respectively. Based on model analysis, 87% of volatile fatty acids were stored by PAOs and GAOs, leading to successful PO4-P uptake through PAO aerobic growth (85%) and PAO denitrification via nitrite (12%). In the aerobic phase, 93% and 5% of consumed readily biodegradable chemical oxygen demand were used for OHO aerobic growth and OHO denitrification via nitrite, respectively. Regarding to SND, nitrite was the dominant electron acceptor for denitrification by PAOs (75%) and OHOs (25%), indicating NO2-N was easier to be used by PAOs and OHOs for denitrification than by NOB for nitrification. This study was the first to design a comprehensive floc model that incorporated PAOs and GAOs, intrinsic half-saturation coefficients of each microorganism, external mass transfer terms, internal diffusion, and intra-floc movement, to simulate SNDPR. A set of intrinsic half-saturation coefficients of oxygen of each microorganism was estimated for the first time