Preliminary Study On The Processes Of Nitrate Removal In Surface Water And Its Correlation With Inorganic Sulfur

In recent years,with the rapid development of industry and the rapid population growth,the amount of nitrogen compounds in water has increased dramatically.Most of the nitrogen compounds are converted into relatively stable nitrate nitrogen under suitable environmental conditions.In general,due to the direct input of high-load nitrogen-containing wastewater,the carbon and nitrogen ratios in the surface water are often relatively low.At the same time,nitrate nitrogen accumulates in surface water due to the relatively weak self-purification of natural water bodies.Nitrate has become the most prominent pollution factor for surface water in some areas,which seriously affects the value of surface water resources and endangers people's health.Therefore,the effective removal of nitrate nitrogen has become one of the main technical bottlenecks in the field of water pollution prevention and control.The traditional methods of removing nitrate nitrogen in polluted water mainly include physical,chemical,biological and composite technology methods.However,high running cost and secondary pollution exist in the traditional methods.Although the hydrogen autotrophic denitrification technology is considered to be a cost-effective nitrate removal technology,the traditional hydrogen autotrophic denitrification reactor has the disadvantages of small contact surface between the electrode and the substrate,the asymmetric water current and the low utilization of cathode.At present,many researches have focused on the nitrate removal of urban sewage,but have not yet found a special process for the removal of nitrate nitrogen in surface water.Therefore,it is very important to identify the nitrate nitrogen source and its formation process,and develop the technology of nitrate removal in surface water accordingly.The thesis that use the high concentration of NO₃^--N,low ratio of C/N and the difficulty in denitrification using conventional methods in the surface water of Jiaodong area as the research orientation,optimally designed the hydrogen autotrophic denitrification reactor with up-flow hree dimension electrodes and realized highly efficient removal of NO₃^--N in surface watrer.The main research results are as follows:Based on calculation of nitrogen flux and total nitrogen amount of different sources,such as input of reservoir basin,land runoff and atmospheric deposition,combined with 15 N stable isotope tracer technique,the source of nitrate in Menlou Reservoir?MR?was identified and the causes for excess were studied.The main research results are as follows:?1?The NO₃^--N?> 90%?pollution is the main form of nitrogen pollution in the Menlou Reservoir.the higher Dissolved Oxygen?DO?value?> 3%?and the lower ratio of C/N have a negative impact on denitrification in water.?2?Upstream river input?coastal livestock,poultry breeding and other pollution discharge?and the surrounding soil surface runoff?fertilizer loss?are the main sources of nitrogen in the MR area.?3?Although the reserves have some self-purification ability,the rate of denitrification is only 20% indicating that artificial treatment measures should be used to remove the excessive amounts of nitrate nitrogen in study area.According to the surface water characteristics of Jiaodong Peninsula such as the Menlou Reservoir?MR?,the simulated water samples were made and the main construction and parameters of hydrogen autotrophic denitrification reactor were improved and optimized.The main research results are as follows:?1?The traditional electrochemical reactor is changed into up-flow bioelectrochemical reactor?UBER?in order to increase the utilization of its matrix.?2?Adding carbon particles in the reactor,build the three-dimensional electrode to increase the contact area between the electrode and the substrate,and finally improve the efficiency.?3?Using two UBER reactors in series provide the sufficient hydraulic retention time and increase their water stability.The results show that the removal rate of nitrate is the high?⁹5%?when the current intensity is 10 m A and the HRT is 6 h.The high denitrification rate?⁹0%?can be achieved at the lower temperature?15??which is of great significance to the high nitrogen removal during the winter.However,the removal efficiency of nitrate in the reactor is significantly decreased under the condition of high ammonia concentration.In order to achieve the simultaneous removal of nitrate and ammonium in surface water,the reactor is further improved in this study.The nitrification zone was set at the bottom of the original UBER reactor to simultaneous removal of nitrate and ammonium.The simulated wastewater firstly conducts nitrification,and then hydrogen autotrophic denitrification is carried out.The results show that:?1?the improved UBER system can achieve simultaneous nitrification and denitrification,and the denitrification rate is high?90%?.The oxygen consumption area is dominated by heterotrophic Nitrification,and the autotrophic nitrification zone is mainly dominated by hydrogen autotrophic denitrification?2?the vertical distribution of dissolved oxygen?DO?in the heterotrophic nitrification zone and the hydrogen autotrophic denitrification zone shows that the aerobic and anoxic layers coexist in the biofilm to promot efficient denitrification.The surface water in the coastal area usually contains a certain amount of sulfate,and the sulfate reduction process could inhibit the denitrification.For that reason,the studies on nitrogen and sulfur coupling are conducted.In order to investigate the impact of sulfur on hydrogen autotrophic denitrification and improve the denitrification efficiency,this study concentrates on the effect of S/N on the hydrogen autotrophic denitrification.The results show that:?1?sulfate at low S/N?1:2?can not inhibit hydrogen autotrophic denitrification reactor,and simultaneous removal of NO₃^--N and SO₄^2--S can be achieved;?2?with the increase of S/N?1:1?,the hydrogen autotrophic denitrification can be inhibited by the accumulation of sulfide and the removal rates of nitrogen and sulfate are about 80%;?3?at higher S/N?2:1?,high sulfide can inhibit both of hydrogen autotrophic denitrification and sulfate reduction and critical inhibitary concentration of the hydrogen autotrophic denitrification and sulfate reduction were 10 mg/L and 15 mg/L,respectively.Besides,low pH can also inhibit the hydrogen autotrophic denitrification.Our results show that S/N has a significant effect on the denitrification efficiency in warer containing sulfate and the low ratio of S/N can promot simultaneous removal of NO₃^--N and SO₄^2--S.