Enhanced combined sewer overflow treatment by rapid magnetic flocculation–magnetic sedimentation: efficiency and mechanism†

Abstract

Magnetic flocculation has been gaining interest as a potential method for treating combined sewer overflows (CSOs) because of its short settling time, small footprint, and dense sludge. This study developed a rapid magnetic flocculation method involving 30 s rapid stirring and 90 s slow stirring combined with magnetic sedimentation to treat CSOs. Compared to traditional magnetic coagulation, the entire process had a reaction time of only 2 minutes. Despite the low floc density that was insufficient for settling by gravity alone, the flocs rapidly settled in a magnetic field due to the flocs enveloping magnetic particles, significantly reducing the required reaction and settling time for the treatment. Meanwhile, the optimal parameters of the process were determined. Under optimal conditions, the removal efficiencies of chemical oxygen demand, total nitrogen, and total phosphorus can reach 90%, 75%, and 80%, respectively. Besides, the treatment efficiency of rapid magnetic flocculation–magnetic sedimentation on CSOs under different weather conditions was also investigated to demonstrate the feasibility of the process in practical applications. The results suggest that the rapid magnetic flocculation–magnetic sedimentation technique is a promising strategy for the treatment of CSOs.