Primary Filtration, Don't Forget the First Flush!

Background: The Water Pollution Control Authority (WPCA), City of Bridgeport, Connecticut is in the process of upgrading their East Side Wastewater Treatment Plant (WWTP). The East Side WWTP currently has an average daily flow of 20.8 ML/d (5.5 mgd) with a 90.8 ML/d (24 mgd) secondary treatment capacity. The WWTP utilizes traditional primary settling tanks, MLE activated sludge process, and sodium hypochlorite disinfection. WPCA's 2020 Facilities Plan recommended increasing the peak flow capacity from 151 ML/d (40 mgd) to 303 ML/d (80 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 conventional primary settling tanks was not feasible. Primary filtration technologies were investigated in the Facilities Plan and recommended for full-scale implementation. Primary filtration offers space savings and due to superior removal efficiency, have an added benefit of reducing the load on the downstream biological treatment system. West Side WWTP Primary Filtration Pilot: A 6-week onsite primary filtration pilot study was completed from March to May 2023 at the WPCA's larger West Side WWTP [1]. Figures 1 and 2 depict the pile cloth media filtration (PMCF) unit used during the pilot study. WWTP primary influent flow served as the influent to the pilot unit. Influent and effluent parameters were assessed to evaluate the performance of the PMCF. The unit was tested under a range of flows and loads, representative of full-scale operating conditions. Through this piloting effort, Aqua Aerobic Systems' (AASI) PCMF was identified as a promising technology for full-scale implementation at both WWTPs. During two wet weather events that occurred during that study period, Total Suspended Solids (TSS) and Biological Oxygen Demand (BOD) concentrations reached a peak of 260 mg/L and 180 mg/L respectively. As design activities progressed from the West Side WWTP to the East Side WWTP, it was initially anticipated that TSS and BOD concentrations at the two WWTPs would be comparable, as both are CSO systems, though with different tributary areas. Operators at the East Side WWTP indicated that the facility could see a sustained first flush of solids during a rain event. They described a significant influx of solids at the start of an event that could last for many hours. CDM Smith and the WPCA decided to complete a wet weather sampling program to define this phenomenon on the East Side WWTP and determine the impact it would have on the design and operation of a primary filter installation. Wet Weather 'First Flush' Profiling: Field sampling activities were completed in October 2025, to better understand when the first flush occurs during an event, and to quantify the spike loading that would occur. It was expected that the first flush would occur prior to peak flow conditions. At peak flow, solids concentrations were expected to be diluted by rainwater in the combined sewer system. An indexing sampler was set up adjacent to the primary influent channel to collect 4 sample increments into an hourly composite sample. Samples were collected from the primary influent channel on October 7, 12, and 29, 2025 during wet weather events. Sampling started prior to the predicted rainfall and ran for 24 hours, and WPCA staff recorded hourly plant flows to correlate to the sample intervals. Table 1 depicts the wet weather events that were captured during this effort. During all 3 events, a sustained 'first flush' condition was observed, where a period of elevated organic and solids loading coincided with elevated plant influent flow. Figures 3 through 8 illustrate the WWTP influent flow during these 3 storms against the influent TSS and BOD concentrations. These events lasted from 3 to 5 hours before solids concentrations diluted from the incoming stormwater. TSS concentrations peaked during all 3 events between 500-600 mg/L. This was a more substantial wet weather response, with higher solids concentrations, than were seen at the West Side WWTP. Of particular note, the results from the October 29th storm showed that the peak WWTP flow during the event (130.2 ML/d, 34.4 mgd) coincided with the peak solids loading (580 mg/L), resulting in a very high solids loading rate of 166,400 lb/d. The solids loading rate during this 'first flush' period during the October 29th storm exceeded the vendor recommendations of 10-12 lbs/ft2/day. This dictated a revision to the design concept. It had been anticipated that providing a filter system with a filter rate of 4.30 gpm/sf to handle the diluted peak flow of the new facility (303 ML/d, 80 mgd) would also have adequate capacity to meet solids loading at any interim flow and load condition. The field sampling proved that this was not true, and the first flush could be so significant that it necessitated the inclusion of additional filter units into the planned primary filter facility. Conclusion: The sampling conducted at the East Side WWTP was an invaluable tool to help the team understand the first flush response of the combined system. The results informed the solids loading rate and drove the decision to add additional filter units to the design. This first flush condition controlled the design sizing, and must be considered in any primary filtration design.