Early Adopters Prove Resiliency of Auxiliary Treatment Strategy and Effectiveness of Latest Generation of Multi-Purpose Filtration

BACKGROUND
Many utilities face increasing challenges from (A) high wet-weather flows and (B) stringent effluent limits. Some also aspire to (C) decarbonize and increase resource recovery. Filtration was previously considered feasible only for tertiary polishing (B); however, filter technologies have evolved to address (A) and (C). These can improve performance and resiliency as POTWs mitigate climate change, aging infrastructure, and growing populations. Previous generation pile cloth filters have successfully helped solve (A) and (B) for over 10 years (Fitzpatrick et al, 2018). Adaptations emerging for (C) enhance the capture of influent carbonaceous materials to recover bioenergy, bioplastics feedstock, or other renewable resources and lower downstream energy demands (Hengel-Prom et al, 2022). Several facilities in the US and UK have operated primary filters since circa 2017, more are being installed/designed, and dozens have piloted successfully. The latest generation technologies further increase the viability of multi-purpose applications.

Regulatory policies on (A) vary nationwide and around the world. Within the United States Eighth Circuit Court's jurisdiction, USEPA honors the Court's 2013 decision upholding blending and auxiliary enhanced high-rate treatment (EHRT) technologies, such as pile cloth filters (Iowa League of Cities v EPA). Utilities outside the Eighth Circuit have regulatory precedence for case-by-case permitting, such as USEPA's 1994 CSO Control Policy and NPDES permits for other EHRT facilities outside of the Eighth Circuit. Efforts continue for consistent federal policy that more clearly allows pile cloth filters and other EHRT technologies for auxiliary wet-weather treatment nationwide.

METHODS AND RESULTS
Little Rock Water Reclamation Authority (LRWRA) and Johnson County Wastewater (JCW) successfully overcame these challenges, inside and outside the Eighth Circuit, respectively. Both utilities began operating multi-purpose pile cloth filter facilities in 2021 following comprehensive evaluations, applied research, onsite piloting, design, value engineering and construction. The alternatives evaluations included triple-bottom-line assessments, reference facility visits, and onsite piloting of leading EHRT technologies (Szerwinski et al, 2023).

JCW saved $200M in construction costs with a multi-purpose filtration strategy compared to wet-weather flow storage and equalization. Design and CMAR collaboration saved an additional $40M in construction costs through 60% design value engineering for wet-weather treatment and nutrient removal. The multi-purpose filters are part of improvements that achieve annual average nutrient goals of TP<0.5mg/L and TN<10mg/L at an average design flow rate of 19mgd with peak wet-weather capacity up to 172mgd. A 115mgd AquaStorm MegaDisk filter facility provides direct filtration of coarse-screened influent during peak wet-weather flow events and is also used during normal flow conditions to enhance capture of phosphorus-bearing solids. This system is the largest capacity dual-purpose pile cloth disk filter facility in the US. Monitoring data demonstrate that the multi-purpose filter consistently achieves TSS < 5mg/L during normal conditions and < 20mg/L during peak wet-weather conditions.

LRWRA realized similar advantages as JCW at their Adams Field WRF. Improvements include (1) nitrification and denitrification retrofits and clarifier upgrades to the existing 36mgd activated sludge process and (2) new 58mgd multi-purpose filter, 94mgd UV disinfection expansion, and 94mgd effluent pump station. The filters used for auxiliary parallel wet-weather treatment are also used for effluent polishing during normal flows to improve effluent quality and downstream UV performance and O&M. The stormiest month was March 2023, when four wet-weather events required auxiliary treatment for 16 of 31 days. Effluent TSS and BOD remained at or below 10mg/L.

CONCLUSIONS AND RELEVANCE
Results from JCW and LRWRA agree favorably with reference facilities at over 40 POTWs nationwide and over 100 worldwide operating a variety of EHRT technologies for over 30 years. These data show that pile cloth filters and other EHRT technologies are consistently able to provide 'non-biological secondary treatment' of wet-weather flows, further supporting the US Eighth Circuit Court's 2013 decision and 2021 clarification upholding EHRT applications to wet-weather blending practices for both combined and separate sewer systems. Both facilities co-thicken and anaerobically digest carbon-bearing solids captured by the filters, thus these facilities' operations also add to the knowledgebase for emerging enhanced primary treatment applications aimed at increasing decarbonization and resource recovery.