Study On Performance And Mechanisms Of Constructed Wetlands With Birnessite-couted Sand Treating Wastewater Containing Typical PPCPs

Pharmaceutical and Personal Care Products(PPCPs),including medicines and personal care products,are widely used worldwide,but at the same time,they are highly toxic and can affect the normal physiological activities of algae,aquatic organisms and even mammals,destroy the ecological balance,and pose a great threat to human health.The effluent of sewage treatment plant is the main pollution source of PPCPs.In order to reduce the pollution discharge of PPCPs,it is necessary to find an economical and effective advanced treatment process to remove such pollutants.Constructed wetlands(CWs)have the advantages of low investment,convenient management and no secondary pollution,the application of CWs in the removal of PPCPs has become a research hotspot.In some places,CWs have been widely used to remove PPCPs.However,the removal efficiencies of PPCPs in CWs is relatively low and unstable.Manganese(Mn)oxides are both adsorptive and oxidative agent,they can be transformed between Mn(II)and Mn(IV)under the action of specific microorganisms,meanwhile,coupling to the removal of organic matter and nitrogen.Mn oxides are mainly used in reactors,filters and constructed wetlands due to their advantages of low cost and easy access.Previous studies have shown that the use of Mn oxides as substrates in CWs can enhance the removal of degradable PPCPs,such as triclosan(TCS),but the removal of refractory PPCPs,such as diclofenac(DCF)and carbamazepine,is not effective,and there is a lack of long-term operational data to prove sustainability and stability.In addition,considering the particularity of PPCPs(exogenous,toxicity),their impacts on CWs and interaction need to be further studied to provide theoretical basis for the practical application of CWs with Mn oxides.In this study,the long-term removal efficiencies and mechanisms of TCS,DCF and conventional pollutants were explored by developing the CWs filled with birnessite-coated sand(B-CWs).The impacts of TCS,DCF and their combination on B-CWs,moreover,the interaction of TCS and DCF were also studied.The main conclusions of this study are as follows:(1)Although it fluctuates with the season,in general,B-CWs have a good removal on conventional pollutants.Compared with the control systems,the removal efficiencies of NH4-N,TN and TP in winter were significawith the maximum average removal rates reaching 80.2%,82.3%and 59.3%,ntly enhanced by biriessite-coated sand,respectively.The average removal efficiencies of NH4-N and TN in the control systems were only 39.1%and 66%,and there was almost no removal of TP.Compared with the control systems,the proportions of the nitrogen(N)and phosphorus(P)removal by media adsorption in the influent load of N and P in B-CWs are higher,the maximum is 12.8%and 48.3%respectively.We also found that the Mn content in birnessite-coated sand decreased after the experiment.At the same time,Mn2+ was detected in the effluent and 10-20cm section,which indicates that birnessite-coated sand participated in the removal process.(2)The removal of TCS and DCF by B-CWs declined when the temperature was low,but on the whole,the removal of TCS was better and the removal efficiency of DCF was lower.The maximum annual average removal efficiency of TCS in B-CWs was 87.8%,while that of the control systems was only 74.7%,which was consistent with the oxidative degradation of TCS by bimessite-coated sand.The highest removal efficiency of DCF in B-CWs was 83.7%in summer,but the maximum annual average removal efficiency was only 59.6%,which was not significantly different from the control systems.The removal efficiencies of TCS and DCF were not affected by different influent PPCPs conditions in both B-CWs and the control systems.In all CWs,the degradation products of TCS and DCF were the same.(3)Under different influent PPCPs conditions,the removal of NH4-N,TN and TP in the control systems were different.The specific differences vary with the season,and are basically consistent with the proportions of N and P removal by microorganisms in influent N and P load.However,the removal efficiencies of these conventional pollutants were almost the same in different systems of B-CWs.The removal efficiencies of NO3-N and TN even increased with the increase of influent PPCPs concentrations.At the same time,the proportions of microbial action in influent N load were positively correlated with influent PPCPs concentrations.We speculate that birnessite-coated sand degraded PPCPs and reduced their toxic effect,providing carbon source for denitrification.(4)Compared with the control systems,the abundances of proteobacteria in B-CWs were higher than that in the control systems at the corresponding height,and the difference of total abundance was up to 37.7%,which was consistent with the better removal performance in B-CWs.In addition,B-CWs has more gamma-proteobacteria at 10-20cm height,which is up to 18.2%higher than the control system.Gamma-proteobacteria has been proved to be related to the Mn cycle.We speculate that gamma-proteobacteria plays a role in Mn transformations in B-CWs.When the influent contains DCF,the abundances of bacteroidetes in B-CWs were less than that of the control system,the gap was up to 7.2%.Bacteroidetes can remove chlorinated organic compounds by reductive dechlorination.We speculate that the addition of bimessite-coated sand can degrade DCF,but inhibits its reductive dechlorination.(5)The microbial abundances were affected by different influent PPCPs conditions in both the control systems and B-CWs,which indicated that the toxicity sequence of PPCPs was compound toxicity>TCS>DCF,but the difference between each system of B-CWs at 40-50cm height was relatively reduced.In the control group,there was no significant difference in the abundance of proteobacteria at the height of 10-20cm,but the abundance of proteobacteria was significantly inhibited at the height of 40-50cm by compound toxicity,which was only 18.57%,while the other two systems were more than 50%.In B-CWs,things were different,there was no significant difference in the abundance of proteobacteria at 40-50cm height,but proteobacteria was inhibited by compound toxicity at 10-20cm height,compared with the other two systems,the abundance was reduced by 14%and 16.7%.