Pilot-Scale Testing of Anaerobic and Aerobic MBRs at a Molasses-Based Distillery

Wastewater generated during the rum manufacturing process must be treated prior to its discharge to a receiving water. One rum distillery's onsite wastewater treatment plant consists of an equalization tank followed by four fixed-film anaerobic reactors. The distillery wanted to investigate potential long-term treatment options that would either modify or replace the existing anaerobic fixed-film reactors to achieve target effluent concentrations of biochemical oxygen demand (BOD) of less than 250 milligrams per liter (mg/L) and total suspended solids (TSS) of less than 250 mg/L. The average effluent concentrations of BOD, chemical oxygen demand (COD), and TSS from the existing full-scale system during the time period of the pilot-scale testing were 4,340 mg/L, 23,540 mg/L, and 17,125 mg/L, respectively. Based on the high level of treatment needed to achieve the goals stated above, membrane biological reactor (MBR) technologies - anaerobic MBR and aerobic MBR - were selected for testing.In order to test the amenability of the wastewater to MBR treatment, two pilot-scale completely mixed suspended growth MBR test units were set up. The first unit was operated under anaerobic conditions, while the second unit was operated under aerobic conditions. Each unit used three flat sheet 0.40-micrometer (μm) microfiltration membranes, each having a surface area of approximately 0.33 square meters (m2). Flux rate across the filtration membrane was controlled by recycling a portion of the filtrate to the membrane tank. Each MBR was fed using effluent from the full-scale anaerobic fixed-film reactors; toward the end of the test period, the MBRs were operated in series, with effluent from the anaerobic MBR being fed to the aerobic MBR. The feed was dosed with diammonium phosphate so that nutrient limitations would not impede biodegradation within the MBRs. Three MBR configurations were pilot tested as additional treatment of the existing anaerobic system effluent: (1) anaerobic MBR, (2) aerobic MBR, and (3) anaerobic MBR followed by aerobic MBR.1. Anaerobic MBR Configuration: Results showed that the effluent from the existing anaerobic system is amenable to treatment in an anaerobic MBR. The sustainable flux rate achievable in the anaerobic MBR pilot-scale unit was 0.05 m3/(m2*day), with periodic operation up to 0.075 m3/(m2*day). The anaerobic MBR functioned effectively as a solids separation mechanism, with 99 percent removal efficiency; however, only 12 percent additional BOD removal was observed in the 1.3-day hydraulic residence time (HRT) MBR compared with the existing full-scale fixed-film system. Long-term pilot testing at higher solids residence times (SRTs) would be required to determine if additional organic removal can be achieved with the anaerobic MBR. The membranes were not permanently fouled by the existing anaerobic system effluent. Cleanings with citric acid restored the membranes to the original flux rates.2. Aerobic MBR Configuration: Results showed that the effluent from the existing aerobic system is amenable to treatment in an aerobic MBR. The sustainable flux rate achievable in the aerobic MBR pilot unit was 0.1 m3/(m2*day), with periodic operation up to 0.15 m3/(m2*day). The aerobic MBR achieved approximately 60 percent removal of the BOD remaining in the effluent from the full-scale, fixed-film system, and greater than 99 percent TSS was retained by the membranes. Long-term pilot testing would be required to determine if additional organic removal can be achieved. The membranes were not permanently fouled by the existing anaerobic system effluent. Cleanings with sodium hypochlorite and hydrochloric acid restored the membranes to the original flux rates.3. Anaerobic MBR followed by Aerobic MBR Configuration: Results showed a sustainable flux rate of 0.1 m3/(m2*day), with periodic operation up to 0.2 m3/(m2*day) for the aerobic MBR when treating effluent from the anaerobic MBR. The anaerobic MBR followed by aerobic MBR achieved approximately 54 percent additional BOD removal from the full-scale effluent and greater than 99 percent TSS was retained by the membranes. Aerobic MBR treatment of the anaerobic MBR permeate did not result in higher overall treatment efficiency and operating performance of the aerobic MBR; long-term testing is required to determine if higher removal efficiencies are achievable.