Rethinking Water Recycling - Alternative Technologies for Indirect Potable Reuse
- Jayson Page, Tara Fishbain, Benjamin D. Stanford, Erik Rosenfeldt - Hazen and Sawyer
Rising wastewater effluent discharge standards have led to more advanced wastewater treatment while growing demand has taxed existing drinking water supplies, making planned indirect potable reuse a feasible scenario for more utilities. A majority of planned indirect potable reuse schemes currently in operation or planned to be employed in the U.S. make use of advanced oxidation processes (AOPs) and reverse osmosis (RO), the fully advanced treatment (FAT) technology, to ensure significant removal of trace and emerging contaminants.
The City of Hollywood owns and operates a reclamation facility that uses an open ocean outfall to discharge secondary effluent. Recently enacted legislation requires the City to ultimately eliminate its ocean outfall and increase water reuse by over 20 MGD. Floridan Aquifer recharge has been identified as an option to achieve this level of recharge. Due to the relatively high total dissolved solids (TDS) levels of the Floridan Aquifer (around 3000ppm), any drinking water applications require low pressure RO membrane treatment to achieve potable quality. This aspect of the aquifer may allow the utilities that surround the Floridan Aquifer to consider indirect potable reuse schemes where RO or NF membranes are not incorporated at the reuse facility, thereby deviating from the FAT approach.
Therefore, the City performed an indirect potable reuse pilot study to investigate the advanced treatment of wastewater for groundwater recharge to a brackish aquifer using ultrafiltration, ion exchange, ozone and ultraviolet light (UV) based AOPs, as well as biofiltration. Since this study is focusing on recharge options to a brackish receiving water body (TDS > 500 mg/L), these treatment processes were examined as an alternative to the FAT approach of RO and UV AOP in multiple treatment schemes to test performance in meeting known regulatory limits, including primary/secondary drinking water standards as well as stringent nutrient limits (total nitrogen < 10mg/L; orthophosphate < 0.01 mg/L). Additionally, the regulatory community remains concerned about the potential for trace organic contaminants (TOrCs) entering the environment and water supply. Since Florida currently does not have TOrCs regulations, California’s Title 22 Groundwater Replenishment Reuse Draft Regulation which focuses on log removal for indicator compounds from nine functional groups, was used to determine the efficacy of the different treatment schemes.
The pilot operated for over ten months and results demonstrated adequate TOrCs oxidation without the use of RO membranes. Some nitrosamine formation was found in the biological activated carbon (BAC) filters which can be mitigated by moving the UV process post BAC filtration. In addition to TOrCs oxidation, the pilot evaluated state of the art ion exchange technologies to meet the low orthophosphate effluent limits. Results demonstrated significant orthophosphate removal (below 0.01 mg/L) without the use of membrane treatment.
The avoidance of RO membranes in the reclamation scheme could save the City upwards of $100 million (US) in capital expenses, in addition to the savings from operation and maintenance of a high pressure membrane system.
This approach will also reduce the carbon footprint of the overall system. This paper and presentation will provide an overview of the project, data on selected TOrC removal, nitrosamine behavior, and orthophosphate removal through each process, as well as the presence/removal of regulated and unregulated contaminants. The material presented will be of interest to regulators, operators and managers considering high quality recharge or supply augmentation of high TDS receiving waters.
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