Distribution and removal pathways of heavy metals during the operation of sludge treatment wetlands

The distribution and removal pathways of heavy metals within different sludge treatment wetlands (STWs) during different running periods in Northeast China have not been well studied. In this study, we examined three STWs, i.e. an STW with aeration tubes only (unit 1; U1), an STW with reeds and aeration tubes (unit 2; U2), and an STW with reeds only (unit 3; U3). The results showed that the levels of Cu as well as Zn accumulated faster within STW residual sludge, whereas the levels of Cd, Cr, Ni, and Pb accumulated more slowly and decreased slightly over time. The removal rates of heavy metals from the influent sludge by STWs ranged from 64.5% (Cr) to 92.2% (Zn). Reeds removed heavy metals from the STWs by direct absorption, and Zn was highly enriched in the reeds. The presence of reeds also promoted the spreading of heavy metals to the substrate layer and improved the removal of heavy metals in STWs. The mass of each heavy metal accumulated within the residual sludge of U2 and U3 was lower than that of U1, indicating that reeds could facilitate the removal of heavy metals. The STWs removed heavy metal mainly by substrate adsorption, and the mass percentage of heavy metals accumulated in the substrate ranged from 35.8 to 63.6%.

Automated summary

The distribution and removal pathways of heavy metals within different sludge treatment wetlands (STWs) during different running periods in Northeast China were investigated. The results showed that Cu and Zn accumulated faster within STW residual sludge, while Cd, Cr, Ni, and Pb accumulated more slowly and decreased slightly over time. The removal rates of heavy metals from the influent sludge by STWs ranged from 64.5% (Cr) to 92.2% (Zn). Reeds removed heavy metals from the STWs by direct absorption, and Zn was highly enriched in the reeds. The presence of reeds also promoted the spreading of heavy metals to the substrate layer and improved the removal of heavy metals in STWs. The mass of each heavy metal accumulated within the residual sludge of U2 and U3 was lower than that of U1, indicating that reeds could facilitate the removal of heavy metals. The STWs removed heavy metal mainly by substrate adsorption, and the mass percentage of heavy metals accumulated in the substrate ranged from 35.8 to 63.6%.