Combined use of polymeric ferric sulfate and chitosan as a conditioning aid for enhanced digested sludge dewatering

In this study, the effects of polymeric ferric sulfate (PFS) and chitosan (CTS) on the dewatering characteristics of the digested activated sludge (e.g. filtrate turbidity, specific resistance of filtration (SRF), and cake moisture content) were investigated. The combined conditioning of PFS and CTS improved the settleability of the digested sludge more effectively than when using of PFS alone. The SRF value of the digested sludge decreased to 2.08 × 10¹² m/kg after conditioning with 50 mg/g PFS and 10 mg/g CTS. Furthermore, a minimum moisture content of 68% was obtained when 40 mg/g PFS and 10 mg/g CTS were used to condition the digested sludge. At a fixed dose of PFS, the concentrations of extracellular polymeric substances, including polysaccharides (C/C₀ = 2.1), proteins (C/C₀ = 2.7), and deoxyribonucleic acids (C/C₀ = 7.8) in the supernatant were increased considerably with an increase in CTS dose (0–10 mg/g). Phosphorus could be recovered efficiently as a result of charge neutralization and the adsorption-bridging effects of CTS, which promote the release of phosphorus from the digested activated sludge. The concentrations of heavy metals in the digested sludge conditioned with 40 mg/g PFS and 10 mg/g CTS satisfied the agricultural safety requirements. These results indicate that conditioning the digested sludge with combinations of PFS and CTS improves its dewatering performance and enables its direct use as a fertilizer.