Effect And Mechanism Of Rhamnolipids-citric Acid Compound On Bio-clogging Mitigation In Constructed Wetland

As a comprehensive wastewater treatment system,the constructed wetlands(CWs)plays important role in the reclamation and landscape of wastewater treatment.However,during the long-term operation,a large number of extracellular polymeric substances(EPS)secreted by microorganisms and the organic matter in the water are excessively aggregated in the substrate layer,which would lead to bio-clogging in the CWs.The occurrence of bio-clogging would significantly affect the sustainable and stable operation of the CWs.In order to alleviate the bio-clogging of CWs systems,this study proposed a new environmentally friendly solvent that combined rhamnolipids(RLs)with citric acid(CA)for restoration of clogged CWs.In this paper,through a combination of batch experiments and continuous experiments,the solubilization effect of CA-RLs solvent on EPS were investigated,and the mechanism was explored.In the batch experiments,the suitable dose of CA-RLs was selected and the optimal action time was determined.Four laboratory-scale vertical flow CWs were constructed to monitor the changes of CWs performance and hydraulic conductivity(porosity and permeability coefficient)during the four processes of “initial operation-wastewater purification-system clogging-clogging mitigation”.The aims of this study are to analyze the effects of CA-RLs on the composition and characteristics of DOM in effluent as well as the structure and diversity of microbial communities,to elucidate the mechanism of CA-RLs in alleviating bio-clogging,and to assess the feasibility of CA-RLs application in alleviating bio-clogging in the CWs.The main findings of the study are as follows:(1)Effect and mechanism of CA-RLs on EPS dissolution.Compared with the RLs alone,CA could promote the solubilization effect of RLs on EPS,in which CA-RLs(0.03 g/g CA and 0.10 g/g RLs)increased the solubilization of S-EPS by about 16.33 mg/g.The solubilization amount of EPS by CA-RLs increased gradually during 0-8 h,and reached the best at 8 h.After solubilization treatment,Zeta potential increased,indicating that CA-RLs destroyed the stable structure of EPS,changed its electronegativity,and generated electrostatic repulsion inhibited the aggregation of EPS.In addition,CA-RLs increased the contact angle of microbial cell surface and weakened the cell surface adhesion,thus alleviating the clogging.(2)Alleviation effect of CA-RLs on bio-clogging in the CWs.When the reactor was clogged,the effective porosity of the system decreased about20%.After solubilization treatment with CA-RLs in-suit,the effective porosity recovered to 83.0% of the initial value.SEM results showed that CA-RLs reduced the lamellar structure and increased the pore size of EPS,which improved the permeability of biofilm.In addition,the content of tryptophan species was less in the effluent by EEMPARAFAC analysis,indicating that CA-RLs were relatively mild in solubilizing the plugging material.(3)Effects of CA-RLs on microbial community structure and diversity in the CWs.The results of high-throughput sequencing showed that the dominant genera Pleomorphomonas,Rhodopseudomonas,Saccharimonadales and Rubrivivax varied significantly in the CWs.After solubilization treatment with CA-RLs,the increasing porosity of the surface layer promoted the relative abundance of Proteobacteria,and created suitable conditions for the growth of aerobic denitrifying bacteria(Rhodopseudomonas and Rubrivivax),which accelerated the recovery of the treatment performance of CWs.In addition,the use of CA-RLs also reduced the relative abundance of Pleomorphomonas easily to form biofilm,which was conducive to reduce the risk of biological clogging in CWs.