Piloting Primary Filtration: Impacts on Removals, Secondary Loading, and Disinfection

The Water Pollution Control Authority (WPCA), City of Bridgeport, Connecticut is in the process of upgrading their West Side Wastewater Treatment Plant (WWTP). The 2020 Wastewater Facilities Plan completed by the WPCA and CDM Smith recommended increasing the WWTP's wet weather peak flow capacity from 340 ML/d (90 mgd) to 757 ML/d (200 mgd). This increase was recommended to enable treatment of wet weather flows that reach the plant and cost-effectively reduce overall untreated CSO volumes upstream in the collection system. Due to the tight footprint of the site, construction of traditional primary settling tanks was not feasible. Advanced dual-use primary filtration technologies were investigated in the Facilities Plan and recommended for full-scale implementation. Primary filtration offers a substantial space savings and due to superior removal efficiency, have an added benefit of reducing the load on the biological treatment system. To evaluate and select a primary technology for the West Side WWTP upgrade, an onsite side-by-side pilot study of two technologies was completed from March to May 2023. CDM Smith and WPCA identified Aqua Aerobic Systems' (AASI) AquaPrime cloth media filtration and Tomorrow Water's (TW) Proteus upflow media filtration as two promising technologies for full-scale implementation. The AquaPrime system consists of pile cloth media filter disks with an inside out flow path, and the Proteus system consists of a layer of expanded cross-shaped polypropylene media. Both vendors provided trailer mounted units for the pilot study, shown in Figure 1 and Figure 2. During the pilot study, screened primary influent flow from the West Side WWTP served as the influent to both pilot units. Various influent and effluent parameters including TSS, BOD, COD, NH3-N, TKN, TP, Turbidity, and UVT were assessed to evaluate the treatment performance of both units. Collimated beam testing and bacteria counts were also completed to determine the suitability of the effluent for ultraviolet (UV) disinfection. Operational parameters, such as backwash frequencies, backwash volumes, backwash settleability, along with fats, oils and grease (FOG) and solids generation were also monitored throughout the study. Both pilot units were tested under a range of hydraulic loads, representative of anticipated full scale operating conditions. The hydraulic loading rates for each pilot unit are presented in Table 1. Additionally, the pilot study captured two significant wet weather events, and at times, additional solids were used to artificially increase solids loading, simulating a 'first flush' condition. TSS and BOD Removal The two piloted primary filtration technologies met expectations for both TSS and BOD removal. During ambient conditions across all flow conditions, AquaPrime averaged 81% TSS removal and Proteus averaged 71% removal. Ambient was defined as incoming screened primary flow without the addition of spiked solids. For BOD removal during ambient conditions, AquaPrime averaged 64% removal and Proteus averaged 53% removal. The influent and effluent data are reported in Table 1 and Table 2 in the following format: Median value (min. value — max. value) [Number of values] Figure 3 and Figure 4 depict the primary influent and pilot effluent conditions in a graphical format. Generally, AquaPrime's TSS removal effectiveness remained constant between the lowest and highest hydraulic loading rates, while Proteus saw a decline in removal from the highest to lowest rates of about 15%, as expected. Hydraulic loading rate had little to no impact on BOD removal effectiveness for both pilot systems. Backwash and Solids Waste Production For both systems, as hydraulic and solids loading rate increased, both the backwash frequency and volumes increased. The study defined an efficiency parameter to measure the impact of backwash volumes on process efficiency. AquaPrime demonstrated consistent efficiency, ranging from 70 to 100%, with most trials between 90 and 100%. Proteus efficiency generally decreased with increased hydraulic loading rates, ranging from 68 to 85%. The backwash from both AquaPrime and Proteus was observed to be much thinner than primary sludge from traditional settling tanks, averaging 1400 mg/L and 700 mg/L during ambient conditions respectively. Disinfection Effectiveness The facilities plan recommended dual-use advanced primary filtration, with primary effluent flows beyond the secondary capacity of the plant (wet weather bypass) receiving UV disinfection prior to discharge. Collimated beam testing for UV disinfection effectiveness revealed that UV could not reliably inactivate enterococci in the primary filter effluent for both AquaPrime and Proteus to the WWTPs permit limits. This was true for both primary effluent, as well as primary effluent that was 'blended' with secondary effluent from the WWTP to match a real-world condition. These enterococci results can be compared to the West Side WWTP's instantaneous and monthly geomean permit limits in Figure 5 and Figure 6. AquaPrime effluent is shown, but Proteus produced comparable results. Subsequent dose testing with hypochlorite disinfection proved successful, meeting all WWTP permit limits within 30 minutes. These results can be seen for AquaPrime in Figure 7. Overall, this hypochlorite disinfection was successful for both AquaPrime and Proteus.