Sidestream EBPR is About the Fundamentals: Simple Strategies to Enhance Biological Phosphorus Removal

Over the past decade, increased attention has been placed on enhanced biological phosphorus removal (EBPR) configurations that include fermentation of a portion of the activated sludge biomass. Often this is accomplished in a sidestream reactor, and thus is referred to as sidestream enhanced biological phosphorus removal (S2EBPR). Recent research suggests that S2EBPR configurations promote more stable and lower effluent orthophosphate concentrations due to a more diverse microbial ecology compared to 'conventional EBPR' configurations. Such research suggests that S2EBPR encourages the selection for a larger prevalence of fermentative phosphorus-accumulating organisms (PAOs) which can metabolize a wide range of carbon sources in addition to volatile fatty acids. This scenario may improve process performance by reducing the reliance of the EBPR process on influent carbon in the form of VFAs. Four S2EBPR configurations that promote biomass hydrolysis and fermentation include: (1) sidestream RAS fermentation, (2) sidestream RAS fermentation with supplemental carbon addition, (3) sidestream mixed liquor suspended solids (MLSS) fermentation; and (4) mainstream unmixed in-line MLSS fermentation (UMIF). A Water Research Foundation (WRF) project (Project 4975) studied S2EBPR configurations to provide practical design guidance, operating tools, and modeling best practices for EBPR processes using biomass fermentation. Results of a UMIF full-scale pilot conducted at the Geneva WWTP in 2020 as part of this Project will be presented. The 5.0 mgd Geneva (Illinois) Wastewater Treatment Facility (WWTF) completed a major process improvements project in 2019, including modifications to the conventional activated sludge process to provide EBPR to meet tightening discharge permit requirements, including a total phosphorus limit of 1.0 mg/L. Bioreactor upgrades were recently completed to promote EBPR to meet this limit. The bioreactor design provides the flexibility to operate in either a 'conventional EBPR' design using an A2O configuration, or to operate with A2O in a UMIF configuration by turning off mechanical mixers in the second of two anaerobic zones and only operating them for 15 minutes per day. The retained sludge ferments at the bottom of the tank and generates VFAs. This sludge is continuously pumped at a rate of about 10 percent of the forward flow from the UMIF zone to the first anaerobic zone. This internal return flow distributes VFAs into the mixed liquor, where they become available to PAOs. During the commissioning of the EBPR process, the secondary treatment process was operated in three configurations: A/O, A2/O with nitrate recycle to Anoxic Zone A, and A2/O with nitrate recycle to Anaerobic/Aerobic Swing Zone B. Ortho-phosphorus profiles through the bioreactors are shown in Figure 1 for each of these configurations before operating in a UMIF configuration. Note that in this figure, each color reflects a different operating configuration, and the two lines for each color reflect the two individual bioreactors used in the study. The phosphorus profiles in each bioreactor were similar for each BNR configuration sampled, with effluent averaging about 0.6 mg/L in each. Following the commissioning period, the operators began piloting the UMIF mode to increase the anaerobic SRT, provide seed biomass for downstream anoxic/aerobic treatment, and encourage hydrolysis and fermentation of the settled MLSS which can be recycled to the head of the anaerobic zone with a recycle pump. The plant operated a UMIF pilot train in parallel with a conventional A2O train to compare their performance. Ortho-phosphorus profiles through the bioreactors under UMIF conditions are shown in Figure 2, with effluent averaging about 0.06 mg/L. The following observable trends were noted:

*Significant decrease in ortho-phosphorus concentration in the anoxic zones. This is primarily attributable to dilution by the nitrate recycle stream, though it may also be possible that some phosphorus uptake may occur by denitrifying PAOs due to the introduction of nitrate.

*A sharp decline in the ortho-phosphorus concentration in the first oxic zone. This is attributable to the sheer size of the Oxic Zone A, which is the single largest zone in the bioreactor, allowing for the longest hydraulic retention time in which phosphorus uptake can occur in an aerated zone.

*Operation with UMIF has resulted in excellent EBPR performance, with UMIF resulting in significantly lower orthophosphate concentrations in the final aerated zones in the bioreactors. This UMIF pilot demonstrates how a simple operational modification, like turning mixers on and off, can improve phosphorus removal with no capital improvements or time-consuming operational modifications. The UMIF configuration has positioned the WWTF well for upcoming further tightening of the TP limits to 0.5 mg/L. This presentation will provide not only a description of the anaerobic zone mixer operating strategy and how it improved EBPR performance, but will also include lessons learned and guidelines for operators to consider to fine tune the mixing strategy for their plant to avoid unintended negative consequences, such as increased ammonia release and resulting nitrite accumulation in the aerobic zones.