Study On Nitrate Removal From Groundwater With Biofilm Method

Nitrate contamination of groundwater has become an ever-increasing and serious environmental problem. Nitrate concentration in groundwater aquifers has steadily been increasing over the years mainly due to the extensive use of chemical fertilizers in intensive agriculture as well as discharge of domestic and industrial wastes. High levels of nitrate in drinking water will pose a serious threat to human health.Physical, chemical and biological methods have been used to remove nitrate from groundwater. However, physical method only transfer nitrate from groundwater to other environments, and the concentrated brine requires further treatment. Catalytic reduction as chemical method is mainly used to treat nitrate contamination, but the reaction condition is difficult to control and ammonia is easily generated. Biological method is the most economical and effective way, with adapted measures to strengthen natural denitrification process. Nonetheless conventional biological methods have some disadvantages, such as complicated process, hard to manage, low efficiency, high concentration of effluent SS and needing subsequent treatment. So in view of these disadvantages, a fiber-based biofilm reactor has been developed and nitrate removal from groundwater by this reactor has been studied.This study is to investigate the performance of the fiber-based biofilm reactor, and to optimize the running parameters of the reactor to treat nitrate-contaminated groundwater. A series of experiments have been carried out under methanol as carbon source and different HRT, influent nitrate concentration, C/N and pH conditions. The results shows that the optimum HRT is 8 hours, the effluent nitrate and nitrite is up to the quality standard of groundwater with influent NO₃^--N lower than 100 mg/L at HRT=8 h. But when influent NO₃^--N concentration is more than 150 mg/L, HRT should be prolonged to 22 hours to avoid possible nitrite accumulation. The optimum C/N is 1.25, which could assure good quality of effluent and no methanol left. The appropriate pH is between 7.0 and 8.0, and buffer solution is in no demand to keep effluent pH. It can be concluded that, under nature conditions this reactor would achieve effective nitrate removal.Under the optimum reaction conditions we set, this biofilm reactor keeps high performance of nitrate removal at room temperature with no extra N2 to be pumped into the system. Nitrate removal ratio exceeds 99%; the greatest removal loading is 0.45 g NO₃^--N/L·d. The biofilm also acts as a filter to prevent SS from entering the effluent, producing very low SS outflow. Besides, all the materials are easily available like the cotton thread which has a long working life, and the reactor is easy to control and has high and stable nitrate removal efficiency with no nitrite and ammonia accumulation problem. Consequently, this fiber-based biofilm reactor has a good application prospect in nitrate removal from groundwater.