The fate and impact of coagulants/flocculants in sludge treatment systems

Coagulants and flocculants have been widely used in various applications, especially in wastewater treatment and sludge dewatering, which result in their high accumulation in waste-activated sludge (WAS). In the past years, the fate and impact of coagulants/flocculants in WAS anaerobic treatment have been extensively studied. Many efforts have been made to explore their effects on WAS anaerobic treatment performance, reveal the underlying mechanism, and establish regulation methods. To date, however, no efforts have systematically summarized or critically considered the fate and impact of coagulants/flocculants in WAS anaerobic treatment systems. In this work, the use and distribution of flocculants/coagulants in wastewater collection and treatment system are summarized, the presence of coagulants/flocculants promoted the formation of large sludge floc, increased the mass transfer resistance of WAS anaerobic treatment, hindered the release of organics from the solid sludge to the liquid phase, and inhibited some related bioprocesses, thus reducing the WAS anaerobic treatment efficiency. In addition, the fates of coagulants/flocculants in anaerobic systems are also clarified, and the regulation strategies (e.g., pH adjustment) to mitigate the harmful effects are analyzed. Herein, the opportunities and challenges of coagulants/flocculants applied in wastewater and sludge treatment systems are rethought, and further research perspectives are put forward.