Post-treatment water stabilization is a critical issue for utilities employing reverse osmosis (RO) based potable reuse of municipal wastewater. Traditional post treatment design and operations target the protection of pipeline infrastructure from corrosion, typically through pH adjustment and/or remineralization. The Orange County Water District (OCWD) has been operating its large-scale RO-based Groundwater Replenishment System (GWRS) indirect potable reuse project since 2008 and its post-treatment process includes partial decarbonation and lime addition. OCWD has experienced challenges in three areas related to GWRS post-treatment: pipeline corrosion, mobilization of native metals during aquifer recharge, and the formation of disinfection byproducts (DBPs) including N-nitrosodimethylamine (NDMA). Initial work has revealed that these issues are interrelated, sometimes in the form of trade-offs that must be managed via optimization of existing post treatment strategies. For example, increasing finished water pH helps avoid pipeline corrosion but has been shown to increase NDMA formation even when the upstream GWRS ultraviolet/advanced oxidation process (UV/AOP) has reduced NDMA to below detection limits. Additionally, temporary increases in arsenic have been observed at monitoring wells immediately downgradient of GWRS recharge locations, followed by consistent attenuation with subsurface travel time and distance. Follow-on controlled laboratory studies have demonstrated that aquifer geochemistry, finished water pH, and the type (e.g., calcium vs. magnesium) and dose of lime added during post-treatment influence the release of native metals from the aquifer matrix. Furthermore, the presence of residual oxidants (e.g., hydrogen peroxide and chloramine) has been demonstrated to affect both the formation of NDMA and aquifer metals release. Deterioration of pipeline fed soft, low alkalinity water similar in quality to waters from advanced potable reuse plants may pose challenges and has been investigated in this work. Field observations and experimental results in all three areas, linked via an ongoing multi-disciplinary optimization project, will be presented along with preliminary findings and recommendations for post-treatment chemistry specific to potable reuse.

Post treatment challenges at advanced potable reuse plants: Corrosion, metals mobilization and the reappearance of disinfection byproducts

253rd National Meeting (2017)

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