title: Evaluating Direct Filtration as an Alternative to Conventional Carbon-Based Advanced Treatment for Indirect Potable Reuse
Creators: name: Flemmer, Savannah M.
Publisher: name: Old Dominion University Libraries
Year: 2023

description: Hampton Roads Sanitation District (HRSD) is recharging purified wastewater into the Potomac Aquifer via the Sustainable Water Initiative for Tomorrow (SWIFT) project. Conventional SWIFT treatment applies coagulation, flocculation, sedimentation, ozonation, biofiltration, granular activated carbon (GAC) adsorption, and ultraviolet disinfection to secondary effluent to produce water that meets drinking water standards for managed aquifer recharge. HRSD is considering implementing direct filtration as an alternative to conventional treatment for two additional SWIFT facilities. Direct filtration presents an opportunity for significant cost savings by eliminating sedimentation, shortening flocculation detention time, and reducing coagulant usage. Without upstream removal of solids and organics, however, biofilters may have difficulty meeting turbidity requirements, and downstream GAC contactors may require more frequent media replacement, potentially increasing operating costs more than estimated savings. Additionally, a lower coagulant dose may not be sufficient to meet permitted phosphorus discharge requirements for when treated water is diverted to a receiving surface water. As a result, a pilot study was developed comparing conventional and direct filtration under variable operating conditions to determine the feasibility of direct filtration for HRSD’s York River Treatment Plant (YRTP) and Nansemond Treatment Plant (NTP).

Conventional and direct filtration pilot operations were evaluated for both YRTP tertiary denitrification filter effluent and NTP secondary clarifier effluent. At a 3.8 gpm/sf loading rate and 10-minute empty bed contact time (EBCT), direct filtration achieved filter effluent turbidity less than 0.15 NTU with a mean filter runtime of 35 hours for YRTP, while a mean runtime of 19 hours was achieved for NTP, a more turbid source water. In comparison, conventional treatment for YRTP and NTP achieved considerably longer mean filter runtimes of 105 and 65 hours, respectively. Mean total organic carbon (TOC) removal efficiency through direct filtration was comparable for both source waters, 35% for YRTP and 34% for NTP. Conventional treatment demonstrated greater TOC removal, 41% for YRTP and 44% for NTP. At a 2.5 gpm/sf loading rate and 15-minute EBCT, NTP direct filtration achieved longer filter runs (43 hours) and enhanced TOC removal (39%). Under similar operating conditions for NTP conventional treatment, mean filter runtime increased to 128 hours and mean TOC removal increased to 48%. Direct filtration with 0.8 mg-Al/L of aluminum chlorohydrate (ACH) addition achieved sufficient phosphorus removal for YRTP but not for NTP. Aluminum sulfate achieved more phosphorus removal per unit aluminum than ACH but resulted in shorter filter runs and less efficient TOC removal for both conventional and direct filtration. Direct filtration managed spikes in total suspended solids and turbidity up to 49 mg/L and 8.7 NTU, respectively. The pilot study results demonstrated direct filtration is a feasible alternative to conventional treatment, while its implementation is dependent on a cost-benefit analysis.