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 g L and 1400 g 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 g 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 of selenium 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 algalbacterial 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 of 60 to 90 per ton Quinn et al 2000 Once in the reduction pond local bacteria strains first remove the nitrate After the nitrate is removed the bacteria reduce selenate to selenite then to elemental selenium in anoxic
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