Solid-liquid separation: an emerging issue in heavy metal wastewater treatment.

Solid-liquid separation (SLS) plays a dominant role in various chemical industries. Nowadays, low efficiency of SLS also become a significant problem in heavy metal (HM) wastewater treatment, affecting the effluent quality (HM concentration and turbidity) and overall process economy. In this context, we summarize here the occurrence of solids in HM wastewater, as well as typical SLS operations used in HM wastewater treatment, including sedimentation, flotation, and centrifugation. More important, this article reviews the improvement of the SLS operations by some technologies, including coagulation, flocculation, ballasted method, seeding method, granular sludge strategy, and external field enhancement. It is noted that abiological granular sludge strategy and magnetic field enhancement often possess higher SLS efficiency (faster settling velocity or shorter separation time) than other methods. Hence, the two strategies stand out as promising tools for improving SLS in HM wastewater treatment, but further research is required regarding scalability, economy, and reliability.