Pilot Study of Advanced Water Treatment Processes to Purify Secondary Effluent

摘要

The Metropolitan Water District of Southern California (Metropolitan) entered a collaborative effort with the Sanitation Districts of Los Angeles County (Districts) to evaluate the feasibility of a regional indirect potable reuse program. This potential indirect potable reuse program would apply advanced water treatment (AWT) processes to purify secondary effluent produced at the Districts' Joint Water Pollution Control Plant (JWPCP). This pilot study tested two separate advanced water treatment process trains. One process train consisted of ultrafiltration (UF), followed by reverse osmosis (RO) and advanced oxidation process (AOP) consisting of UV oxidation in combination with hydrogen peroxide (H_2O_2). This train was tested to produce-treated waterfor groundwater replenishment. The second process train consisted of a membrane bioreactor (MBR) followed by RO and AOP and was tested to produce water that can be used for surface water reservoir augmentation. The water quality results show that both treatment trains produced high quality recycled water for parameters relevant to groundwater replenishment and surface water reservoir augmentation; such as nitrogen, TOC, trace organic compounds including volatile organic compounds, pesticides, herbicides, pathogens indicators, and contaminants of emerging concern. UF-RO#1 train treated these compounds to levels either below the laboratory reporting limits or relevant regulatory limits; the only exceptions were N-nitrosodimethylamine (NDMA), 1,4-dioxane, and boron. NDMA and 1,4-dioxane can be removed by AOP to meet the Title 22 Draft Groundwater Recharge Reuse Regulations. Boron levels in the RO permeate was slightly above the Basin Plan limit of 0.5 mg/L. Source control may be required to reduce the boron levels to meet this limit. Similar to UF-RO #1 treatment, the MBR-RO #2 system achieved a high degree of rejection for all constituents tested except NDMA, 1,4-dioxane, and boron. While NDMA and 1,4-dioxane can be removed by AOP, boron and low levels of nitrate nitrogen in the product water need to be removed if the water was to be considered for surface water reservoir augmentation. This study also demonstrated differences between MBR and UF for treatment of RO feed water. The MBR nitrified the secondary effluent, so nitrate was the dominant form of nitrogen; ammonia was the dominant nitrogen form in the UF filtrate. Nitrification also reduced alkalinity and pH, so less sulfuric acid was required to reduce the pH of the RO influent to the target level. In addition, the MBR was more effective in removing total organic carbon (TOC), NDMA, NDMA formation potential (NDMAFP), and biodegradable pharmaceuticals, personal care products (PPCPs).