The effects of sulfite pretreatment on the biodegradability and solubilization of primary sludge: Biochemical methane potential, kinetics, and potential implications

•Sulfite pretreatment deteriorated methane production from primary sludge.•The release of soluble substances in PS was stimulated by sulfite pretreatment.•Sulfite pretreatment facilitated the dewaterability of primary sludge.•Sulfite pretreatment is not suitable for PS solo treatment before the AD process.