Effect of heavy metals in mixed domestic-industrial wastewater on performance of recirculating standing hybrid constructed wetlands (RSHCWs) and their removal

Heavy metals (HMs) are frequently detected in wastewaters, especially in industrial discharges or mixed domestic-industrial drainage. Constructed wetlands (CWs) are effective means for the treatment of decentralized wastewater and recirculating standing hybrid constructed wetlands (RSHCWs) provide aerobic conditions for the removal of both nutrients and organic matter. However, their effectiveness regarding HM removal and the effects of HM stress on the performance of RSHCWs remain unclear. The present study examined three types of substrates in RSHCWs with varying HM inflow loads with the aim to evaluate their potential and underlying mechanisms for the removal of HMs from simulated mixed wastewater. The results indicate that HM removal exceeded 85% at 24 h of treatment, and a high inflow load (202-259 mg.m(-2).d(-1)) achieved high removal. HMs mainly accumulated in the substrate (91.0-98.3%), and RSHCWs filled with lava rock and gravel provided higher removal efficiencies of Cd than bio-ceramic at 24 h of treatment. Although plants were minor sinks for HM removal, HMs accumulated in Canna indica L. (grown in RSHCWs) with high bioconcentration factors and translocation factors. RSHCWs quickly co-precipitated HMs in mixed wastewater and decreased their toxicity on plants and microorganisms. Consequently, they achieved consistent and effective removal of total nitrogen, total phosphorus, and organic matter. These findings are useful for the design of CWs (such as RSHCWs) for the treatment of mixed wastewater discharged from both rural and urban areas with fluctuating HM stress.