Abstract
Resource reuse has become an important aspect of wastewater management. At present, use of sludge in agriculture is one of the major reuse routes. Conventional municipal wastewater treatment does not involve any designated process for removal of heavy metals, and these distribute mainly between effluent and sludge. Enhanced removal of heavy metals during primary treatment may decrease the heavy metal concentrations in both effluent and sludge from secondary treatment and promote long-term reuse of secondary sludge. This review considers heavy metal occurrence and removal during primary settling, together with possible treatment technologies for heavy metal removal in primary settlers and their theoretical performance. The variation in total heavy metal concentrations and dissolved fraction in raw municipal wastewater points to a need for site-specific assessments of appropriate technologies for improved heavy metal removal. Studies examining the heavy metal speciation beyond dissolved/particulate are few. Missing or disparate information on process parameters such as hydraulic retention time, pH and composition of return flows makes it hard to generalize the findings from studies concerning heavy metal removal in primary settlers. Coagulation/flocculation and use of low-cost sorbents were identified as the most promising methods for enhancing heavy metal removal during primary settling. Based on the available data on heavy metal speciation and removal during primary settling, sorption technologies may be most effective for enhancing the removal of Cu and Ni, while coagulation may be efficient for Cd, Cr, Cu, Pb, Zn and Hg removal (but not as efficient for Ni removal).
Highlights
Heavy metals, referring to a group of high-density elements including metals and metalloids, are a concern due to elevated concentrations in the natural environment as a result of anthropogenic activities [1]
This review focuses on heavy metal removal during primary treatment of municipal wastewater, with the purpose of reducing heavy metal contents in effluent and in sludge from subsequent biological treatment
The removal of heavy metals during primary settling is determined by the speciation of heavy metals in raw municipal wastewater and the sorption/desorption of heavy metals occurring in the settler
Summary
Heavy metals, referring to a group of high-density elements including metals and metalloids, are a concern due to elevated concentrations in the natural environment as a result of anthropogenic activities [1]. Yoshida et al [5] modelled the ecotoxicity and toxicity impact on humans from municipal wastewater They found that the heavy metal Zn contributed the largest share of toxic impact with regards to both effluent release and sludge use in agriculture. They found that conventional treatment of wastewater significantly reduced the toxicity exposure from inorganic constituents (including heavy metals) on freshwater and seawater. Based on the heavy metal concentrations occurring in sludge from European WWTPs, reuse of sludge is partly limited by current legislation, especially in relation to the stricter regulation in some countries. The review includes: (i) occurrence and speciation of heavy metals in raw municipal wastewater (influent to primary settlers), (ii) mechanisms of heavy metal removal during primary settling, (iii) possible treatment technologies for enhanced heavy metal removal in or directly following primary settling, (iv) calculations to estimate theoretical heavy metal removal if enhanced heavy metal removal was introduced based on current removal of particulate and dissolved heavy metals during primary settlement
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
