Integrating Wet-Weather Resiliency and Advanced Treatment With Dual-Purpose Filtration

BACKGROUND
Johnson County Wastewater (JCW) and Little Rock Water Reclamation Authority (LRWRA) completed major treatment improvements in 2021 to mitigate wet-weather flows, increase treatment performance, and improve effluent quality. Both utilities added dual-purpose auxiliary treatment systems featuring largest-scale units of latest generation pile cloth filters, an innovative enhanced high-rate treatment (EHRT) technology that does not require chemicals. This allows the filters to cost-effectively treat at high flow peaking factors and polish normal dry-weather effluent, providing year-round water quality benefits instead of just during peak wet-weather events. JCW upgraded its Tomahawk Creek Wastewater Treatment Facility to achieve annual average effluent limits of 79.2 lb/day total phosphorus and 8.0 mg/L nitrate + nitrite nitrogen, and annual average effluent goals of 0.5 mg/L total phosphorus and 10 mg/L total nitrogen. The improvements also expand average design capacity to 72 ML/d (19 mgd) and peak design flow capacity to 651 ML/d (172 mgd). New facilities include pile cloth disk filters for tertiary treatment of flows up to 216 ML/d (57 mgd) and auxiliary treatment of wet-weather flows up to 435 ML/d (115 mgd), making it the world's largest capacity dual-purpose pile cloth disk filter installation to date. The improvements are a major milestone in JCW's Integrated Plan to identify, assess, and prioritize infrastructure initiatives and investments. The Kansas Department of Health and Environment authorized the improvements through major modifications to its National Pollutant Discharge Elimination System (NPDES) permit with no objections from the United States Environmental Protection Agency (USEPA). LRWRA added a similar 220-ML/d (58-mgd) filter system to its Adams Field Water Reclamation Facility to provide auxiliary treatment in parallel with 136 ML/d (36 mgd) of nitrifying activated sludge treatment for a total peak flow capacity up to 356 ML/d (94 mgd). When not being used for auxiliary parallel treatment, the new filter system can be used to polish secondary effluent to increase the performance and reliability of the downstream UV disinfection system. The Arkansas Department of Energy and Environment's Division of Environmental Quality authorized the improvements through NPDES permit renewal with major modifications and no objections from USEPA. A key consideration for regulatory approval was the 2013 decision by the Eighth Circuit of the U.S. Court of Appeals in Iowa League of Cities v. EPA that upheld the practice of parallel treatment and blending.
OBJECTIVES
To maximize stakeholder benefits, both utilities sought auxiliary treatment systems that increase wet-weather treatment capacity most cost-effectively and that can also provide tertiary treatment during normal flow conditions to decrease effluent total phosphorus (JCW objective) or improve operation and maintenance of UV disinfection and possibly lead to effluent reuse opportunities (LRWRA objective).
METHODS AND RESULTS T
he JCW team conducted applied research and assessments of technologies prior to design and delivery under a construction-manager-at-risk framework. Activities and milestones include: - 2005/7/8 - Jar testing of chemically enhanced settling and ballasted flocculation at the Douglas L. Smith WWTF. Full-scale trials of chemically enhanced settling at the 75th and Nall Peak Excess Flow Treatment Facility. Side-by-side pilot testing of three different high-rate filter technology alternatives at the Nelson Complex (see Figure 1). Study results previously reported by Fitzpatrick et al (2010). - 2016 – Conceptual design studies to refine previously recommended alternatives, project costs and schedule; develop a conceptual level site plan to support public outreach and site permitting efforts; negotiate and finalize a discharge permit for the new facilities. Results summarized in a Preliminary Design Report. - 2018 – Detailed facility design completed. Pile cloth disk filters selected after evaluation of 60% designs with both pile cloth and compressible media alternatives. - 2021 – Startup and testing of dual-purpose filters demonstrates achievement of specified performance criteria and effluent limitations (see Figure 2, Table 1, and Figure 3). The LRWRA team conducted similar applied research and assessments to support design-bid-build project delivery, which included: - 2001 – Onsite pilot studies demonstrated performance of chemically enhanced high-rate settling (CEHRS) technologies. - 2016 – Onsite side-by-side piloting of pile cloth and compressible media technologies demonstrated similar removals as CEHRS alternatives but did not require chemicals, were found to be simpler to operate and control, and improved downstream UV disinfection performance (see Figure 4). Conceptual facility designs for five different technology alternatives were evaluated based on economic and non-economic factors. Follow-up CEHRS jar testing was conducted to estimate dosages for an alternate coagulant to mitigate alkalinity risks (aluminum chlorohydrate) and defoamer feed conceptual designs to mitigate effluent foaming risks for CEHRS alternatives. The team conducted further investigations of the top two candidates (ballasted flocculation and pile cloth filtration) including a number of site visits to reference facilities and recommended pile cloth filter technology. The new filter system eliminated 132 million liters (35 million gallons) of collection system storage and equalization expansion and was found to be more feasible because it: (1) provides better resiliency by optimizing both storage and treatment capacities, (2) does not limit peak flow management to a finite volume, (3) requires a much smaller site, (4) does not require additional odor control, and (5) has lower life-cycle costs. Study previously reported by Fitzpatrick et al (2018). - 2018 - Detailed designs issued for construction including nitrification and clarification upgrades to existing activated sludge treatment process, new dual-purpose filters, expanded UV disinfection and effluent pump station. - 2021 – Substantial completion and startup of the improvements (see Figure 5).
CONCLUSIONS AND RELEVANCE
Results from JCW and LRWRA agree favorably with those found from other auxiliary EHRT applications. Wet weather capacity with EHRT technologies is in many cases more affordable than other storage or treatment alternatives, and their small footprint makes EHRT facilities more feasible for constrained sites in urban and suburban settings, whether located at existing treatment sites or remotely at key points in the collection system. The simple physical process of filter-based EHRT alternatives offers operational advantages over chemical and biological alternatives, particularly for remote facilities or intermittent operations. EHRT effluent quality when treating peak wet-weather flows is equivalent, for all practical purposes, to that from conventional secondary treatment technologies. EHRT alternatives should not be pigeon-holed into the same regulatory category as traditional 'bypass' or 'blending' technologies that achieve primary treatment equivalency. Existing NPDES regulations appear to have the flexibility to further legitimize EHRT technologies as 'auxiliary treatment' facilities within the codified definition of 'secondary treatment' (40 CFR 122 and 133). This regulatory flexibility is critical as integrated wastewater and stormwater utility plans gain momentum and are implemented. Some utilities challenged with water scarcity are considering these same EHRT technologies for stormwater reuse. Over 100 POTWs worldwide operate auxiliary EHRT facilities with the earliest adopters having over 25 years of experience.