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Recently,tetrakis(hydroxymethyl)phosphonium sulfate (THPS) was found to play an important role in the sludge pretreatment process.However,the effects of THPS pretreatment on the characteristics of sewage sludge are still insufficiently understood.The properties of sludge after pretreatment with different concentrations of THPS were investigated in this study.The results showed that pH,dewatering ability,and particle size of sludge decreased with increase in THPS concentration.The volatile suspended solids (VSS) and total suspended solids (TSS) of sludge also decreased slightly with increase in THPS concentration.The specific oxygen uptake rate (SOUR) results suggested that lower THPS concentrations (≤1.87 mg/g VSS) enhanced the activity of sludge,but higher concentrations (≥1.87 mg/g VSS) inhibited it.Gram-negative bacteria with peritrichous flagellation (such as Pseudomonas,Escherichia,and Faecalibacterium) were extremely sensitive to THPS.The decrease in specific most probable numbers (MPNs) of pathogens (total coliforms and Escherichia co/i) with the increase in THPS concentration also proved the sterilization ability of THPS in the sludge pretreatment process.Pretreatment of sludge with concentrations of THPS higher than 37.41 mg/g VSS would meet the pathogen requirements for land application of Class A biosolids.