Exploring the Carbon Balance in a Sidestream Enhanced Biological Phosphorus Removal (S2EBPR) Demonstration Facility

Introduction
Conventional design of enhanced biological phosphorus removal (EBPR) focuses on providing volatile fatty acids (VFAs) to phosphate accumulating organisms (PAOs) in an influent anaerobic zone. As an alternative, sidestream EBPR (S2EBPR) diverts a portion of RAS flow to a longer hydraulic retention time (HRT) sidestream reactor. In this sidestream reactor, biomass and particulate COD is fermented to generate the VFA required for PAO metabolism. In a strict RAS fermentation reactor, this results in selection of PAO metabolism without direct use of influent readily biodegradable COD (rbCOD) or VFA. In a carbon focused wastewater paradigm, this provides increased flexibility for COD diversion and capture; use of influent rbCOD for nitrogen removal; and potentially more stable phosphorus removal due to a lack of sensitivity to influent COD characteristics. In addition, moving the PAO selection to the sidestream provides increased infrastructure flexibility for incorporation of EBPR into existing facilities. The Wisconsin Rapids Wastewater Treatment Plant (WRWWTP) began pilot testing of RAS fermentation in April 2021. The facility moved through a series of operational setpoints (Figure 1) to optimize phosphorus removal. One of the keys was the addition of a cranberry waste to provide additional carbon for RAS fermentation. By the end of the testing period, effluent total phosphorus was averaging less than 0.4 mg/L with no chemical addition (Figure 2).
Carbon Balance Exploration
To better understand why the S2EBRP configuration was working at the WRWWTP, a series of bench-scale tests were completed to explore the carbon balance for the facility. Batch RAS fermentation rate tests were completed as previously described (WRF Project UR13). The fermentation rate tests provided an indication of the amount of soluble biodegradable COD (sbCOD) produced per mass of volatile solids in the fermenter. On average, a fermentation rate of 1.5 mg sbCOD/g vss-hr was observed over the testing period (Figure 3). This is a critical piece of information, as it provides insight into how much carbon can be released from RAS fermentation. Phosphorus release and uptake rate testing (Figure 4) provided insight into the use of carbon by PAOs in the system. As EBPR was established, the sbCOD:P ratio was reduced to 13:1 (Figure 5).
This information provides insight into the ratio of carbon required by PAOs in the S2EBPR reactor. The last set of batch experiments explored the phosphorus release rate of the ecology with various external carbon sources. The full-scale system was fed a cranberry juice wastewater, with was dominated by sugars. During bench scale testing, the cranberry juice exhibited the highest release rate of the three carbon sources tested (Figure 6). This difference is likely driven by an adaption of the ecology to the carbon source, and not specifically tied to the cranberry juice. Although, cranberry juice is well noted for its overall health benefits, and the antioxidants from Wisconsin grown cranberries may provide additional benefits to PAOs. But this detail was not explored during this research. The bench scale testing provided the information to complete the carbon mass balance for an S2EBPR system (Figure 7). Using the measured rates, the sbCOD deficiency can be calculated. If a deficiency is occurring, a reduced level of effluent phosphorus removal performance would be anticipated. If the deficiency is zero, or if extra carbon is available in the RAS fermenter, effluent phosphorus concentrations should be reduced. As shown in Figure 8, the mass balance around carbon correlated to observed effluent phosphorus performance.
Benefits and Significance
This presentation will provide a detailed review of an extensive testing of S2EBPR. The following benefits will be provided to the audience: - High-level review of bench testing protocols - Value proposition for adding bench testing to operational data for EBPR and S2EBPR facilities - Outline of how to complete a carbon balance for an S2EBPR facility - Target values for fermentation and sbCOD:TP in S2EBPR facilities These results will be significant for anyone planning, designing, or operating an EBPR facility.