Optimized alkaline pretreatment of sludge before anaerobic digestion

► NaOH pretreatment was systematically analyzed for anaerobic digestion. ► The optimized condition of NaOH dose and pH control was concluded. ► The effects of high pH and high salinity on anaerobic digestion were investigated. ► The influence of alkaline pretreatment on sludge settleability was compared.