Activated Sludge Dynamic Filter Achieves Quality Effluent in Small Footprint

Similar to a membrane bioreactor, an activated sludge dynamic filter can replace secondary clarification and tertiary filtration in a smaller footprint. This filter can also be operated in parallel with secondary clarifiers to produce higher quality effluent and increase treatment capacity. Two pilot tests have validated this technology for achieving high quality effluent in a small footprint. The pilot tests demonstrated consistent effluent total suspended solids (TSS) concentrations less than 5 mg/L, over a wide range of flows, loads, and temperatures - a key requirement for meeting stringent nitrogen and phosphorus effluent limits. The low effluent TSS is similar to that produced by tertiary filtration and provides an opportunity for non-potable water reuse and lower energy consumption in UV disinfection. Installed directly in aeration basins, the activated sludge dynamic filter develops a dynamic biomass cake layer on micro mesh panels to produce filtrate from the mixed liquor solids. The filtration rate is controlled by the rotational speed of the discs and a continuous filtrate backwash system that removes the cake layer after each rotation. The activated sludge dynamic filter can increase the capacity of existing bioreactor tanks by operating at higher MLSS concentration than a conventional activated sludge process. To evaluate the activated sludge dynamic filter technology two pilot tests were conducted. The first pilot test was conducted at the Hampton Roads Sanitation District Nansemond Treatment Plant in Suffolk, VA which has a 5-stage Bardenpho process to meet stringent nitrogen and phosphorus limits. The pilot plant had an average effluent flow of 4m3/h. At Nansemond, the activated sludge dynamic filter was operated in parallel with the existing secondary clarifiers. Mixed liquor settling characteristics at Nansemond were very good, resulting in an average secondary clarifier effluent TSS of only 4.1 mg/L. The average effluent TSS from the activated sludge dynamic filter was 3.2 mg/L (Figure 1). It was also encouraging to see that the activated sludge dynamic filter effluent TSS did not increase when the RAS flow rate was decreased and the MLSS concentration in the filter tank increased to more than 10,000 mg/L. The Nansemond pilot test also showed that flux rates could be increased while maintaining low TSS effluent by applying a low dose of polymer during limited duration simulated peak flow events. The initial pilot test at Nansemond indicated that the activated sludge dynamic filter could be a viable alternative to other high biomass concentration, low footprint nutrient removal technologies such as polymeric or ceramic-membrane MBR and granular activated sludge. The Nansemond pilot test showed that the activated sludge dynamic filter could offer potential benefits such as a low replacement cost of the filtration mesh panels, the ability to operate without a fine headworks screen, and low operating power in terms of kWh/m3 water treated. While the pilot test at Nansemond yielded positive results, there was uncertainty of how the system would operate in a closed system (without secondary clarifiers operated in parallel) and with a biomass grown exclusively with a dynamic MBR. Additional testing was also needed to optimize polymer conditioning for handling peak flow events, and to determine how to produce more consistent effluent TSS results. A new pilot unit with dedicated bioreactor tanks was constructed in 2021 and installed at the Western Butler County Authority (WBCA) wastewater treatment plant in Pennsylvania. At the WBCA, the average flow treated by the pilot unit was 4.9 m3/h with a return activated sludge ratio of between 2 and 4X the effluent flow. Raw wastewater was fed directly to a pilot bioreactor with an anoxic zone, swing zone, and an aerobic zone before flowing downstream to the activated sludge dynamic filter for liquid/solids separation and biomass return to the process. Adjustments in design and operation of the filter, including utilizing a different mesh fabric, were made to produce more consistent effluent quality. Throughout the pilot test period at WBCA, from August 2022 to August 2023, the activated sludge dynamic filter produced significantly lower TSS effluent than the plant's secondary clarifiers (Figure 2). At WBCA, effluent TSS concentrations less than 5 mg/L (on par with cloth-media tertiary filter effluent) were consistently produced. This low TSS was achieved with MLSS concentrations in the bioreactor more than 8,000 mg/L and of up to 14,000 mg/L in the filtration tank (Figure 2). At WBCA, the ability to increase flux during peak flows using polymer was also validated, including several tests when peak flow rates were sustained for as long as 24 hours (Figure 3). Throughout the WBCA pilot, the activated sludge dynamic filter successfully demonstrated its ability to reduce TSS. For utilities that have limited land availability, as well as construction challenges for the existing land, the use of an activated sludge dynamic filter as part of the activated sludge process can provide a flexible alternative to increase treatment capacity and meet stringent nitrogen and phosphorus limits.