Monthly Maximum Discharge Limits (MMDL) have been established for selenium in irrigation drainage by the State of California and the U.S. Environmental Protection Agency following observations of avian teratogenesis at the Kesterson Reservoir in the San Joaquin Valley of California. As a result of these and other adverse effects, farmers and drainage districts on the western side of the San Joaquin Valley must reduce selenium concentrations in irrigation drainage discharged to the San Joaquin River. Drainage treatment will be required in the near future to meet existing MMDL and future Total Maximum Discharge Limits (TMDL) for the San Joaquin River. A 0.4-hectare Algal Bacterial Selenium Removal (ABSR) Facility was designed and constructed at the Panoche Drainage District in 1995 and 1996 using the Advanced Integrated Wastewater Pond Systems or AIWPS Technology. Each of two physically identical systems combined a Reduction Pond (RP) with a shallow, peripheral algal High Rate Pond (HRP). A Dissolved Air Flotation (DAF) unit and a slow sand filter were used to remove particulate selenium from the effluent of each system. The two systems were operated under different modes of operation and the bacterial substrate varied in each system. The rates of nitrate and selenium removal were compared. Microalgae were harvested using DAF and used as a carbon-rich substrate for nitrate- and selenate-reducing bacteria. Mass removals of total soluble selenium of 77% or greater were achieved over a three-year period. Nitrate and selenate were removed by assimiliatory and dissimiliatory bacterial reduction, and nitrate was also removed by algal assimilation. The final removal of particulate selenium is the focus of ongoing investigations. The removal of particulate selenium is expected to increase the overall removal of selenium to greater than 90% and would allow farmers and drainage districts to discharge irrigation drainage in compliance with regulatory discharge limits.
In this study, a pilot-scale experiment was carried out on a pre-settled combined tannery effluent from Modjo tannery, Ethiopia, to evaluate the feasibility of the Advanced Integrated Wastewater Pond Systems or AIWPS Technology, for the treatment of tannery effluent. The pilot-scale AIWPS Facility was comprised of an Advanced Facultative Pond (AFP), Secondary Facultative Pond (SFP) and Maturation Pond (MP) all arranged in series. Three feed phases with low, moderate and overloading organic loading rates were applied to assess the organics and nutrients removal performances of the AIWPS reactors. The overall organics removal performance of the AIWPS Process was high, with removal efficiencies in the range of 90-98% for BOD5 and 86-92% for COD. Among the AIWPS reactors, the AFP attained the highest organics removal efficiency with a BOD5 removal of 70-89%. BOD5 removal efficiencies of the SFP and MP were 34-65% and 30-40%, respectively. The AFP was also able to withstand a much higher volumetric organic loading rate (70% more) than the conventional open anaerobic ponds. The drop in BOD5 removal efficiency of the AFP at the overloading condition was only 7%, while the corresponding drop in the SFP was 29%. AIWPS reactors achieved a cumulative ammonia removal efficiency of 85%. The highest ammonia removal (50-60%) occurred in the SFP, followed by the MP with removal efficiency of 20-26%. At the overloading condition the overall ammonia removal efficiency of the AIWPS Facility decreased by 50%, while the BOD5 organics removal dropped by only 6%, signifying the higher vulnerability of ammonia removal mechanism to high loading conditions than the organics removal. The phosphorus removal in the AIWPS Facility was erratic, with highest removal (up to 75%) occurring in the AFP. The lack of H2S odour nuisance from the AFP was mainly due to the proliferation of sulphide oxidizing anoxygenic photosynthetic pink bacteria of the genera: Thiocystis, Rhodobacter, Rhodospirillum and Rhodopseudomonas in the upper solar illuminated water layers of the AFP.
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