The Effect of Molasses Concentration on Bacterial Treatment of Selenium in Agriculture Waste Water in the San Joaquin Valley Lela Fischer Abstract In the San Joaquin Valley, selenium concentrations in agricultural waste water have become a serious concern as an environmental pollutant causing birth defects and death in birds, small mammals, and fish. One method of lowering the concentration of selenium in the agriculture discharge is to use biological treatment. A pilot treatment system was developed at Panoche Drainage District using local bacterial strains in reduction ponds to minimize the amount of selenium being discharged into the San Joaquin River. In order to keep operation costs down and determine optimal growth for the bacteria, this project examines how molasses substrate concentration and trace nutrient additions for bacterial growth change the total selenium reduction. Three different concentrations of molasses (0.1g/L, 0.2g/L and 0.4g/L) were prepared in triplicate using influent collected at Panoche and processed at Lawrence Berkeley Laboratory. These samples were analyzed periodically (4 or 7 days) in a controlled environment, using atomic absorption spectrometry to determine selenium concentrations. Final results for varying concentration of molasses between 0.1g/L, 0.2g/L and 0.4g/L did not differ for total selenium concentration. Nor did the total selenium concentration of 0.4g/L with trace nutrient buffer differ from the other concentrations of molasses. Total selenium reduction averaged ten percent from the initial concentration. These results did not meet expectations. Previous experiments had a reduction of eighty percent under similar conditions. This implies that at low concentrations of molasses inadequate carbon is available for maximum bacterial growth and therefore selenium reduction.Introduction Sources of selenium pollution are various and include industrial effluents from thermal power plants, oil refineries, smelting plants, and in the production of semiconductors, pigments, and solar batteries (Kashiwa et al, 2000). Finding a reliable treatment may be applicable to a broad range of industries. The focus of this project is the analysis of the current selenium laden agricultural waste water problem in California. In California’s San Joaquin Valley, an area of extensive agriculture, high levels of selenium have been found naturally occurring in soils (Oswald et al, 2000). For California to maintain its high level of food production, adequate water supplies must be available. Extensive canal systems were built to bring water to the fertile valley, but no canals were built to remove the waste water. Unfortunately, while irrigating fields with selenium rich soils it has been found that selenium leaches into the water. Concentrations between 75 µg/L and 1400 µg/L are measured in the subsurface drainage water (Fan et al, 2001). This excess drainage is then pumped up to the surface and sent to lakes or discharged into the San Joaquin River (Quinn et al, 2000). Some of the effects on organisms when selenium is present in aquatic environments are reproductive dysfunction, deformities, anemia, and death in many species of birds, fish and mammals (Amweg et al, 2003). Since the discovery of selenium accumulation in vertebrates, law makers have tried to establish safe levels of selenium in discharge waters (Amweg et al, 2003). Because much of the selenium cycle is not clearly understood, the establishment of safe levels of selenium in water has been difficult to determine (Fan et al, 2001). The result of this situation is that the Environmental Protection Agency (EPA) has tried to reduce the amount of total selenium entering the watershed as a means to reduce the risk to the environment (Quinn et al, 2000). In 1987, the EPA set a chronic exposure level for freshwater aquatic life at 5 µg/L of total selenium (Fan et al, 2001). Selenium can be found in four different oxidation states (-II, 0, IV, VI). The chemical form of selenium will determine its solubility and availability to organisms (Zhang, 1999). Selenate (selenium VI), selenite (selenium IV), and selinde (selenium –II) are all water soluble and therefore considered to be the most important sources ofselenium in water (Amweg et al, 2003). Though its solubility is agreed upon, there are differing opinions about which forms are most toxic. Zhang, Moore, and Frankenberger cite Mikkelsen, Bingham, and Page (1999) to assert that selenate is generally considered to be the most toxic. Whereas Amweg, Stuart, and Weston (2003) assert that organic forms of selenium are thousands of times more bioavailable than selenate and therefore pose the most important risk to the environment. Since most of the selenium from agriculture runoff in the San Joaquin Valley is primarily in the form of selenate, a problem arises as to how to appropriately manage selenium discharge without impacting agriculture production. Methods such as chemical precipitation, catalytic reduction, and ion exchange are effective for the removal of selenite but are not effective in removing selenate (Kashiwa et al, 2000). These methods are also costly (Kashiwa et al, 2000). Due to a lack of affordable treatment of selenium to meet concentration objectives there has been a regulatory shift to reducing the selenium load (Quinn et al, 2000). It may prove to be that bioremediation of selenium by bacteria into less toxic and more a stable form (elemental selenium) is the most cost effective method of reducing the selenium load (Quinn et al, 2000). In Panoche Drainage District near Firebaugh in the San Joaquin Valley an algal-bacterial selenium removal system was created to treat drainage water (Oswald et al, 2000). The waste water is not only high in selenium but also in nitrate. Algae were originally used to remove the nitrate from the influent before the reduction pond where bacteria reduce the selenium. The old algae could then act as a carbon source for the bacteria and minimize external inputs into the system. However the algae component has since been discontinued due to experiments that showed better selenium reduction with out the drainage first passing though the algae system (T. Lindqust, 2003). The carbon source for the bacteria could be replaced by many sources found from byproducts of food production, and in the San Joaquin Valley molasses is readily available at the price
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