Research On Electricity Generation And Nitrogen Removal Performance Of Floating Bed-Microbial Fuel Cell With Plant-Cathode

The problem of eutrophication caused by excessive emission of nitrogen and phosphorus has been widely concerned.The nitrogen removal is of great significance for the treatment of eutrophicated river.Therefore,it has become a key point in water pollution control.As an in-situ remediation technology for rivers,ecological floating beds have some advantages for nitrogen removal,but there are still some problems such as poor plant growth in winter,low activity of root microorganism and reduced effect of nitrogen removal.Micro-current generated by MFC can promote the growth of plants at low temperatures and improve the growth status of the floating bed plants to strengthen the nitrogen removal.In this study,the ecological floating bed and the microbial fuel cell are coupled?hereinafter referred to as the coupling system?,and the windmill grass,the goldfish algae,the water hyacinth and the water spinach are selected as the cathodic floating bed plants to explore their effect on the electricity production efficiency and nitrogen removal.Combined with the plant root secretion,the absorption kinetics of NH₄⁺-N and the enzyme activity of microorganisms attached on the surface of electrode,the performance of electricity production and the mechanism of enhanced nitrogen removal in each system are clarified.Below are the key research findings:?1?Under the same operating conditions(influent COD concentration is 100 mg·L^-1,NH₄⁺-N concentration is 30 mg·L^-1,HRT=2 days),planting plants can reduce the internal resistance of the system by 21.23%-67.66%?compared to the blank group?,and the electron transfer efficiency of the anode and cathode is improved,so as to increase average voltage by26.26%-62.63%.Water spinach system has the best performance of electricity production,followed by the water hyacinth,goldfish algae,windmill grass and blank system.The performance of electricity production of each system is consistent with the total amount of organic secretion and radial oxygen loss from the plant roots in each system.?2?The effect of COD removal in water spinach system is better than that of goldfish algae system and water hyacinth system,but the overall improvement effect is not significant?2.49%-8.61%?.The reason may be that MFC relies mainly on microorganisms to degrade COD and has good removal rate.Therefore,the planting of plants has less effect on the removal of COD.Planting windmill grass result in an increase in the effluent concentration of COD,which is probably due to the decay of the windmill grass roots to release organic matter.There is no significant correlation between electricity production and COD removal efficiency.The reason may be that dissolved oxygen concentration in the anode chamber is high,so some electrons are captured by oxygen to affect the electricity production and some COD is degraded by aerobic bacteria.?3?The nitrogen removal effect of coupling system could be enhanced by the cultivation of water spinach and water hyacinth.The water spinach can increase the removal rate of NH₄⁺-N and TN by 16.40%and 16.86%respectively,and water hyacinth can increase the removal rate of NH₄⁺-N and TN by 12.40%and 12.47%respectively.Windmill grass has adverse effect on nitrogen removal,which may be caused by the decay of roots.With the prolongation of time,the specific activities of ammonia monooxygenase,hydroxylamine oxidase,nitrate reductase and nitrite reductase in each system are improved.Enzyme activity analysis finds that HAO has a low specific activity,resulting in a weak nitrification ability of the systems,which affects the nitrogen removal.Based on the electricity production performance and the removal of COD,NH₄⁺-N and TN of each system,it is found that water spinach is more suitable as the floating bed plant.?4?Nitrogen removal in the blank group without plants mainly relies on the microorganisms.The removal rate of TN is 38.74%.The goldfish algae,water hyacinth and water spinach increase the removal rate of TN by 2.91%,12.47%,16.86%respectively.The microorganism plays a major role in the process of nitrogen removal.Planting plants has positive effect on the removal of nitrogen,because root exudates enhance the activity of rate-limiting enzymes affecting nitrogen removal and plants can absorb nitrogen.The system with better electricity production has higher activity of enzymes related to nitrogen removal.On one hand,electricity production may promote the enzyme activity related to nitrogen removal.On the other hand,plant root exudation can promote electricity production and enhance the activity of nitrifying bacteria and denitrifying bacteria,thus improving the activity of enzymes related to nitrogen removal.