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Partial nitritation/anammox (PN/A) is a novel pathway for nitrogen removal in wastewater treatment that offers advantages of low oxygen and organic carbon demands as well as high potential for energy recovery. However, the partial nitritation process remains a key hurdle for the widespread implementation of the PN-A process in mainstream treatment due to the difficulty in washing out nitrite-oxidizing bacteria (NOB) from active sludge. Exposing biomass to high concentrations of free ammonia (FA) has been reported as an effective strategy to achieve partial nitritation. This study examined the effectiveness of treating 20% of return activated sludge with synthetic centrate containing FA at 200 mg N/L for 24 hours to promote partial nitritation in mainstream wastewater treatment. Experimental and control bioreactors were operated under two different conditions, with or without FA treatment, respectively, after reaching similar nitrification performance. Biokinetic parameters of ammonium-oxidizing bacteria (AOB) and NOB were estimated by performing respirometric batch tests with activated sludge biomass from the two bioreactors under different operational conditions, and calibrating a process model based on oxygen mass balance. The bioreactor performance showed that the FA treatment strategy promoted the PN process, with a maximum nitrite accumulation ratio (NAR) of 41.9 ± 2.1% after treating return sludge with high FA solution for 37 days. However, this nitrite accumulation was not stable, and the NAR decreased to 10.9 ± 6.0% after 33 days, indicating that NOB were able to acclimate to the temporary exposure to a high FA concentration. The biomass yield coefficient (Y) of AOB increased during FA treatment, while the maximum specific growth rate (μmax) of AOB and NOB decreased under this condition. Microbial community analysis on activated sludge under FA treatment, and further investigations on the optimization of the FA treatment strategy combined with other NOB out-selection strategies are required to better facilitate the application of PN-A to full-scale mainstream wastewater treatment.
Biofilm photobioreactors rely on cooperation between algae and bacteria within a single biofilm to treat wastewater. Algae growth produces oxygen, which can subsequently be utilized by aerobic bacteria to degrade organic matter and produce carbon dioxide, which is then utilized as a carbon source by algae. Due to their relatively low maintenance and energy inputs, biofilm photobioreactors could be amenable for decentralized wastewater treatment. However, the impact of biofilm photobioreactor design on treatment efficiency has received little attention. Here, it was hypothesized that open (i.e. unsealed) versus closed (i.e. sealed) photobioreactors could promote different nitrogen removal pathways by altering redox conditions throughout a diel cycle. This study explored the effect of open versus closed photobioreactor configurations on nitrogen removal and the microbial community structure in two parallel photobioreactors treating microscreened decentralized wastewater. The reactors were intermittently lit in a 16hr-8hr light-dark cycle, and operated at an HRT of 2 days and SRT of 9 days. The influent feed regime and alkalinity addition were varied over three successive 30-day experimental phases. Microscreening was an effective primary treatment step, removing 70 ± 6% (95% c.i.) of suspended solids and 39 ± 9% of COD. The photobioreactors removed 90 ± 6 % and 83 ± 3% of the remaining suspended solids and COD respectively, independent of operating conditions. Alternating oxic and anoxic conditions were observed in both reactors during the lit and unlit periods, respectively, resulting nitrification and denitrification. Optimal nitrogen removal conditions were observed under a sequencing batch feed regime with alkalinity addition. Under these conditions, TKN removal was significantly higher in the open reactor at 93 ± 5% compared to 78 ± 6% in the closed reactor due to higher rates of nitrification and N assimilation. TN removal was similar at 77 ± 9% and 76 ± 8% in the open and closed systems, respectively. The dominant bacterial genus in the reactors was Tychonema, a cyanobacteria which comprised up to 87% of 16S rRNA gene amplicon reads. Overall, this study demonstrated that nitrogen removal pathways differ significantly in open and closed photobioreactors when operated at the same COD loading rate.