A Combination Of Biological And Chemical Approach For Nitrate Removal From Simulation Groundwater

There are many reports about biological denitrification and chemical denitrification by Fe⁰ both in and out of China, but the combination of biological and chemical approaches have been rarely reported in the area of remediation of nitrate pollution in groundwater. Preliminarily, batch tests were performed to investigate the differences between denitrification systems. There are systems supported by different solid organic carbon sources, the chemical reduction system supported by Fe⁰, and the collaborative system of biological and chemical. Basing on the former study, the biological-chemical combined denitrification system was further studied through batch tests and column experiments, and conclusions are as follows:(1) The denitrification process in different denitrification system which is supported by sawdust, cotton, corn stalks power respectively has obvious stages. The variation trend of the total nitrogen is almost identical. All the final nitrate removal efficiency is close to 100%. The accumulation and the variation trend of nitrite nitrogen and ammonium nitrogen are all increased at first and then decreased. The nitrate removal rate which uses corn straw as the carbon source is the quickest in all the denitrification systems, then is the system which uses sawdust as carbon source, the last one is the system which uses cotton as carbon source.(2) In the system which combined with carbon source and Fe⁰ the changing trend in various forms of nitrogen has the similar trend with the denitrification system supported by the three carbon sources. Nitrate removal rate in this combined system is significantly higher than that of the single biological or chemical denitrification system. And the results showed that the reaction of autotrophic denitrification using hydrogen produced by Fe⁰ corrosion can be started.(3) In the denitrification system which combined with sawdust and Fe⁰, nitrate removal rate has decreased as the initial concentration increased when other conditions are without change. Correspondingly, the greatest accumulation of the nitrite nitrogen and ammonium nitrogen is also increased. In a certain range, nitrate removal rate and removal efficiency are all increased as the C/N ratio increased, and the greatest accumulation of nitrite nitrogen and ammonium nitrogen are also increased. However, when the accumulation reaches to the maximum value, the removal rate is also speeded as the C/N increases. When Fe⁰/N<160, the nitrate removal rate is increased as the Fe⁰/N increases. When Fe⁰/N>160, the removal rate of the system has not only failed to improve, but declined.(4) The results of single factor experiment showed that C/N and Fe⁰/N and nitrate removal efficiency are highly correlated in the sawdust and Fe⁰ combined denitrification system. The former have a significantly effect on the nitrate removal rate, while the latter highly significantly. In addition, the inoculum size and removal rate is significantly correlated but not significantly. By three factors and three levels of the optimal orthogonal experiments, the optimal denitrification system is when C/N is 60, Fe⁰/N is 150, inoculum size is 6g in an ambient temperature of 25℃, 50mg/l nitrate system.(5) In the column reactor different packing modes had different influences on denitrification, the column reactor filled with completely mixed sawdust and Fe⁰ had higher nitrate removal rate than the two filled in layer modes. Moreover, adding activate carbon can not only promote the removal of nitrate, reduce the system of accumulation of ammonium nitrogen, but also can improve effluent quality.(6) On certain nitrate concentrations, the increase in reaction time can increase the nitrate removal efficiency and reaction time had some effect on the effluent DOC, the more reaction time, the higher effluent DOC concentration. In a certain range,increase the amount of inoculum can increase nitrate removal rate, in this experiment,when the system inoculated surpass more than 150ml, it can not promote the removal of nitrate but lead to reduction in effluent flow rates. On certain reaction time, nitrate removal efficiency, effluent DOC concentration and the effluent colonies decreased with the initial concentration increased, while the accumulation of ammonium increased with nitrate concentration increased.(7)Throughout the process, the nitrite could be nearly detected in the effluent, the bacrteria colonies changed from 10³ CFU.ml^-1 to 10⁶ CFU.ml^-1, further treatment is needed to apply practices. The nitrate removal efficiency of the reaction column all exceed 95%, indicating that the combination of biological and chemical approach is a promising method for nitrate removal from groundwater.