In-Tank Carbon Generation as a Primary Benefit of RAS and MLSS Fermentation for Stabilizing Biological Phosphorus Removal Performance

As effluent nutrient limits get more stringent, water resource recovery facilities (WRRFs) will continue to need to implement biological nitrogen (N) and phosphorus (P) removal. BPR systems in particular are susceptible to upset and many plants struggle to maintain BPR year-round because of a multitude of factors including, insufficient bioavailable carbon for PAOs, suboptimal aeration control, solids recycle impacts, excessive solids retention times, suboptimal pH operation, and secondary phosphorus release. Recently, it has been suggested that current BPR designs are inadequate for stable performance and that sidestream RAS fermentation is needed for stable BPR (Barnard et al., 2017). The authors propose that RAS fermentation selects for unique BPR organisms (e.g., Tetrasphaera) that can perform anoxic P uptake and also ferment in the presence of deeply anaerobic conditions redox conditions. Barnard et al., 2017 have also suggested that the influent wastewater characteristics can become irrelevant if RAS fermentation is implemented. Interestingly, recent work has shown that the microbial community in conventional BPR systems has less diversity than sidestream RAS or MLSS fermentation systems; however, the functional community (primary organisms responsible for BPR) were similar regardless of configuration (Onniss-Hayden et al., 2019). These authors were also careful to suggest that the "superior" performance of RAS/MLSS fermentation systems may be due to positive bias in their comparison of facilities. Given these findings, it is still not clear to practitioners what may be driving RAS/MLSS fermentation systems to have stable BPR. This work investigates the underlying reasons for scenarios where implementing RAS or MLSS fermentation as part of a flexible BNR design can help to stabilize BPR. Specifically, this work documents how RAS/MLSS fermentation can be used to ensure that sufficient carbon is provided to a BPR system that is subjected to shifts in wastewater characteristics in response to peak flow conditions and cold temperatures.