Elucidating the role of ferric chloride on carbon capture from low-strength municipal wastewater in high-rate activated sludge process: Parameter optimization and crucial mechanism

Abstract

To enhance organics capture efficiency from low-strength municipal wastewater of China, FeCl3 enhanced high-rate activated sludge (FC-HRAS) process was established in this work. Effects of sludge concentration and FeCl3 dosage on carbon capture in FC-HRAS were investigated. The optimum sludge concentration of 2.0 g/L and FeCl3 dosage of 30 mg Fe/L were obtained, with total COD removal efficiency of 76.8%. Carbon balance analysis showed that FC-HRAS increased organics capture proportion by 91.8% while reducing loss from oxidative metabolism compared to HRAS. The potential mechanism of enhanced carbon capture for FC-HRAS was further explored. Adding Fe(III) caused coagulation of organics by increasing the Zeta potential in wastewater. Impacted by charge neutralization “compression” and coagulation floc “encirclement”, sludge particle sizes decreased, leading to increases of internal surface area and adsorption site of sludge for binding with bacteria; it facilitated the biological flocculation of FC-HRAS. Meanwhile, Fe(III) binding with proteins in EPS induced formation of “protein shell – β polysaccharides kernel” sludge structure and reduction of sludge negative charge, which intensified sludge stability and agglomeration. These factors explained the increase in organics capture proportion. The decrease in metabolism loss of organics could be attributed to the increase in organics transfer resistance in sludge EPS and selective elimination of heteromorphic bacteria (e.g., denitrifiers) in sludge. This work provided guidance for energy-neutral and cost-saving wastewater treatment in China.