Long-term effects of sulfite pretreatment on the continuous anaerobic sludge digester for improving methane production and volatile solid reduction: Towards sustainable sludge treatment

•Long-term effects of sulfite pretreatment were verified for anaerobic sludge digestion.•Methane production and vS destruction can be enhanced by sulfite pretreatment.•Sulfite pretreatment stimulated more organics extracted from sludge for conversion.•Sulfite pretreatment spurred methanogens and syntrophic bacteria for methanogenesis.•Sulfite-based sludge treatment can save operational cost and reduce carbon emission.