Enhancement Of Constructed Wetlands Performance By The Synergistic Effect Of Submerged Plant And Benthic Fauna

In these years,the traditional wastewater treatment technique can not meet the requirement of water treatment today with the increase of sewage discharge and strict environmental standard.The constructed wetlands(CWs),an ecological technology for wastewater treatment,are widely used because their less investment,convenient management and supply of landscaping.However,there are some problems faced by the technique.For example,given that the influent of CWs have low C/N ratio,the nitrate(NO3--N)removal efficiency is low and then total nitrogen(TN)removal efficiency is insufficient.Besides,nitrogen removal is also limited by the limited nitrification of CWs which is always anaerobic.Furthermore,the total phosphorus(TP)performance of CWs is limited by the situation of plants and microbial as well as the adsorption capacity of substrate.In winter,the performance of CWs are usually disappointing because that most wetland plants wither and microbial activity is limited.Benthic fauna are an important part of natural wetland ecosystem,but they are neglected in the CWs.They can strengthen NO3--N and TP removal of CWs by promoting denitrification,stimulating microbial activity and influencing phosphorus flux between sediments and the overlying water.But adding benthic fauna into the CWs is faced with the negative effect of ammonia(NH4+-N)release.Based on the important role of benthic fauna in the aquatic system and the physiological characteristics of submerged plants,this paper proposes to scientifically improve the artificial wetland ecosystem by adding benthos and matching them with submerged plants in CWs,which is a strengthening measure of water purification in CWs.The advantages of submerged plants in constructing CWs of benthic fauna were investigated,and the benthic fauna of different life types were applied to CWs and compared.By making full use of the seasonal growth characteristics of plants and the seasonal activities of benthic fauna,the seasonal submerged plants and benthic fauna were matched scientifically and rationally.Then,the coupling CWs of submerged plant and benthic fauna were constructed and operated,and the effect of nitrogen and phosphorus removal was studied.The feasibility of stable operation of submerged plant-benthic fauna constructed wetland was discussed.Finally,a typical submerged plant-benthic fauna constructed wetland was used as an example to study the mechanism of nitrogen and phosphorus removal.The main conclusions of this paper are as follows:(1)Compared with emergent plants,submerged plants can release oxygen directly into water,and dissolved oxygen(DO)in the sediment-water interface of CWs matched with benthic fauna was 38.29%higher and DO in the superincumbent substrate layer(SS layer)profile is also higher,which can eliminate the negative effect brought by the NH4+-N resealse of benthic fauna.The average removal efficiency of NH4+-N and TN in the submerged plant-benthic fauna CWs systems were 72.93%and 47.13%,respectively,which were significantly higher than that of the benthic fauna system without plants and that of the benthic fauna wetland system with emergent plants.There was no significant difference in the removal efficiency of NO3--N and TP among the three systems.This provides direct evidence for the construction of wetlands with the combination of submerged plants and benthic fauna.(2)The chironomid larvae of meat-eaters,mussels of filter feeders and viviparus of skimmer showed different rules in the construction of submerged plant-benthic fauna CWs.In winter,the removal of NH4+-N in mussel system was slightly higher than that in chironomid larvae system,viviparus system and blank system.The chironomid larvae systems showed significantly better effects in the removal of NO3--N,TN and TP,with removal rates of 51.98%,59.78%and 97.94%,respectively.In summer,the mussel system showed better pollutant removal efficiency,and the removal rates of NH4+-N,NO3--N,TN and TP were 94.64%,74.91%,82.83%and 98.54%,respectively,among which the removal rates of NO3--N and TN were significantly higher than those of other systems.(3)In winter,the selected chironomid larvae were used to construct submerged plant-benthic fauna CWs and the results showed no NH4+-N accumulation compared with the blank system,and the removal rates of NO3--N,TN,and TP in constructed wetland increased by 29.51%,15.16%,and 37.62%,respectively.In summer,submerged plant-benthic fauna CWs system constructed by mussel showed significantly better treatment effects in the removal of NO3--N and TN,with an average removal rate of 37.3%and 23.79%higher than that of the blank system,respectively,and no NH4+-N accumulation was occured,too.According to the performance of Potamogeton crispus-chironomid larvae CWs in winter and Ceratophyllum demersum-mussel CWs in summer,it was analyzed that after the addition of two kinds of benthic fauna,the submerged plant-benthic fauna CWs systems could provide guarantee for the stable pollutants removal of CWs through the replacement of natural ecological niche,and could realize the purification of sewage all year round.(4)Take Potamogeton crispus-chironomid larvae CWs as an example to explore the mechanism of submerged plant-benthic fauna CWs systems.The results showed that the burrows created by the bioturbation of chironomid larvae increased the nitrogen,phosphorus flux of overlying water and deep substrate and increased the substrate effect on the absorption of nitrogen and phosphorus.The TN adsorption capacity increased 42.51 mg N/m2/day,and the TP adsorption capacity increased 5.64 mg P/m2/day.The proportion of TN input stored in the substrate was 6.92%higher.Plants absorbed nitrogen and phosphorus from the substrate as nutrient elements,so the TN absorption of plants increased by 11.03 mg N/m2/day,and the TP absorption increased by 1.67 mg N/m2/day.Plant growth was promoted,and the proportion of nitrogen removal caused by plant absorption in TN input increased by 1.80%,the proportion of plant absorption in TP input increased by 4.54%.At the same time,the nitrogen loss caused by the gas generated in the nitrification and denitrification process of microorganisms in the Potamogeton crispus-chironomid larvae CWs was 32.86mg N/m2/day higher than that in the non-animal CWs,which increased the proportion of nitrogen removal caused by other loss in the TN input by 5.35%.The results showed that the addition of chironomid larvae enhanced the nitrogen and phosphorus absorption capacity of wetland substrate and plants,and the nitrification and denitrification of microorganisms were also enhanced.(5)The results of microbial analysis showed that chironomid larvae could increase the quantity of de nitrifies and nitrifies in the SS layer and underlying substrate layer(US layer),but had little effect on the quantity of whole bacteria.Certain amount of nitrifying and denitrifying microorganisms were found in the body of chironomid larvae,indicating that the benthic fauna themselves could be the place where nitrification and denitrification occurred in the submerged plant-benthic fauna CWs systems.High throughput sequencing revealed that chironomid larvae changed the structure of microbial community of the CWs,and the relative proportion of microbial community related to nitrogen-transformation increased,in which the denitrifying microbial significantly increased in the US layer,and nitrifying microbial slightly increased in the US layer and SS layer.