Berkeley ETHSTD 196 - Effect of Salt on Bacterial Denitrification of Agricultural Drainage Water

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Effect of Salt on Bacterial Denitrification of Agricultural Drainage Water Rick Huang Abstract The Algal-Bacterial Selenium Removal (ABSR) system has demonstrated an ability to efficiently remove nitrogen from agricultural drainage water in the San Joaquin Valley. However, it is not known whether the ABSR system can effectively reduce nitrate and nitrite concentrations in drainage water with high levels of salinity. In order to determine whether the ABSR system can be used to treat salty drainage water in the future, the effectiveness of nitrogen removal from drainage water with high salt concentrations must be examined. Two laboratory experiments were conducted to study the effect of salt on bacterial denitrification of drainage water. The first experiment involved evaporating and diluting typical drainage water into bottles with different salt concentrations and equal amounts of molasses and bacteria. Dissolved oxygen, pH, and turbidity were monitored and the bottles were periodically tested for nitrate and nitrite concentrations. Similar procedures were used for the second experiment, but four different salt concentrations were achieved through the addition of salt. Denitrification was slower at higher salt concentrations for the first experiment, however, nitrogen removal may have been affected by higher nitrate concentrations at higher salinity. The results from the second experiment showed that bacterial denitrification occurred at the same rate for all four different salinities, and that salt had no significant effect on nitrogen removal. The results from this study suggest that salt does not hinder bacterial denitrification of drainage water, and that the ABSR system can be used to remove nitrogen from agricultural drainage water with salt concentrations of up to 22 g/L. Future research can help determine how other factors may affect bacterial denitrification of agricultural drainage water.Introduction The San Joaquin Valley in California is one of the most productive agricultural regions in the world. However, agriculture in the San Joaquin Valley relies on heavy crop irrigation because of low annual precipitation in the dry region. Subsurface drainage systems are constructed beneath agricultural fields in the valley to transport excess drainage water and runoff to a main canal (Botkin and Keller 2000). The drainage of agricultural wastewater has been a problem in the San Joaquin Valley since the 1950’s, when agricultural activities increased dramatically. A tiny percentage of salt minerals are present in freshwater resources, however, salt is a problem in the Central Valley because of heavy irrigation and over pumping of groundwater for farming. Salt minerals accumulate in soils and groundwater over time because water used in agricultural irrigation evaporates quickly and salt is left behind. The build up of salt from evapotranspiration is further compounded because more and more salty water is pumped from underground and used for irrigation. As a result, large amounts of salt are accumulating in the San Joaquin Valley. According to the California Department of Water Resources, salt is accumulating at a rate of 2.45 million tons per year in the Central Valley (Follette 2002). The buildup of salt in soil and groundwater is problematic because many agricultural crops cannot tolerate salty water and some aquatic organisms cannot survive in high saline water (Meng and Moyle 1995). In response to the drainage problem in the San Joaquin Valley, the San Luis Drain project was initiated in 1968 to construct a long irrigation drainage canal from the San Joaquin Valley to the Pacific Ocean (Botkin and Keller 2000). The drain would have carried salty drainage water from the San Joaquin Valley and disposed it into the San Francisco Bay. However, the San Luis Drain project was never completed due to budget cutbacks and environmental concerns from the Bay Area community. Instead, the 132-kilometer long partially completed drain stopped at Kesterson Reservoir and agricultural wastewater was dumped there. In 1983, it was discovered that the occurrence of deformed and dead migratory birds at Kesterson Reservoir was due to high levels of selenium in the water (Ohlendorf et al. 1986). The San Luis drain and the Kesterson Reservoir were closed in 1985 due to concerns over the threat of toxic selenium on wildlife and public health (Botkin and Keller 2000). Ever since the closure of Kesterson Reservoir in 1985, there have been many attempts by the government to manage the drainage problem in the Central Valley and to deal with the selenium in agricultural wastewater. Agricultural waste water from the Central Valley cannot be exportedor disposed of elsewhere without first removing the toxic selenium in the water. One of the ways that selenium can be efficiently removed from drainage water at low cost is by the Algal-Bacterial Selenium Removal (ABSR) system. The ABSR system, designed by UC Berkeley professor William Oswald, consists of two artificial ponds that utilize bacteria and algae to remove nitrate and selenium from agricultural drainage water (LBNL 2000). Bacterial denitrification is an important process in the ABSR system. The presence of nitrogen hinders selenium removal from drainage water because bacteria prefer reducing nitrate and nitrite before converting selenate to the more easily removable selenite and elemental selenium (Lee 2002). Therefore, nitrogen must be removed before the removal of selenium can successfully occur. Past research studies have shown that the ABSR system can effectively remove nitrogen and selenium from drainage water in the Central Valley. A pilot ABSR facility constructed near Los Banos in the Central Valley has demonstrated an ability to remove 95% of the influent nitrogen and 80% of the influent selenium (LBNL 2000). Although the ABSR system can effectively remove selenium and nitrogen in typical drainage water with salt concentration ranging from 6 to 8 g/L, the ability of the ABSR system to remove selenium and nitrogen from high saline water is unknown. One of the ways to remove salt from drainage water is through reverse osmosis, also known as hyperfilitration. Many reverse osmosis plants have been built in the Central Valley to help remove salt minerals from agricultural drainage water. However, reverse osmosis leaves behind concentrated impurities that are high in salt and contain high levels of selenium.


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Berkeley ETHSTD 196 - Effect of Salt on Bacterial Denitrification of Agricultural Drainage Water

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