Effect of Salt on Bacterial Denitrification of Agricultural Drainage Water

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.