Pilot Testing of a Membrane Bioreactor for Metals Removal

The City of Davis, California (City) Water Pollution Control Plant (WPCP) is facing stricter effluent permit requirements and compliance with the California Toxics Rule (CTR) effluent limitations. The impact of these requirements will necessitate upgrades to the existing WPCP to include conventional secondary and tertiary processes to control the discharge of biochemical oxygen demand (BOD), total suspended solids (TSS) and metals. The City evaluated the feasibility of a membrane bioreactor (MBR) process to meet the stricter effluent limits through a nine-month pilot study. The MBR system reduced conventional parameters such as BOD and TSS to below potential limits and reduced all metals outlined in the CTR to below regulated limits with the exception of copper and selenium. Varying solids retention time (SRT) had minimal impact on metals removal while coagulant addition generally increased metal removal. Because of the MBR's ability to remove particulates, the particulate metal fraction was removed easily while the dissolved portion passed through to a large extent. Therefore, higher influent metal particulate fractions resulted in higher overall metal removal.