Bio-clogging mitigation in vertical subsurface flow constructed wetlands using rhamnolipids-citric acid compound

Abstract

Sustainable operation of constructed wetlands (CWs) is challenging due to bio-clogging caused by the excessive aggregation of extracellular polymeric substances (EPS) in substrate interspaces. In this study, rhamnolipids (RLs) and citric acid (CA) were used as solubilizers in situ with the influent to mitigate bio-clogging in CWs for the first time. Four lab-scale CWs with vertical subsurface flow were set to validate the efficiency of CA-RLs. After solubilization treatment with CA-RLs, the effective porosity of CWs restored to about 83.0% of the initial. RLs could solubilize and disperse blockage, and CA further promote the effects by destroying the stable structure of EPS. The surface contact angle became larger after CA-RLs treatment, as did the absolute zeta potential of EPS, attributed to the solubilization of hydrophobic EPS. CA-RLs inhibited the surface adhesion of cells and dissociated EPS into small particles, reduced the aggregation ability of EPS. The EEM-PARAFAC analysis showed solubilization treatment increased the tryptophan-like or protein-like components in effluent. Furthermore, the rehabilitative porosity after CA-RLs solubilization treatment provided suitable conditions for the growth of denitrifiers (especially Rhodopseudomonas and Rubrivivax), conducive to recover the performance of CWs. After bio-clogging mitigation, the removal rates of NH4+-N (25–30%) and COD (50–55%) in CWs gradually recovered to 55–70% and 70–75%, respectively. These findings provided a better understanding of the bio-clogging characteristics, which were helpful to mitigate bio-clogging in CWs.