Effects of mixed alkali-thermal pretreatment on anaerobic digestion performance of waste activated sludge

In the present study, the effect of low-temperature thermal and alkali pretreatments on biodegradability of waste activated sludge was evaluated. Combined with low-temperature thermal pretreatment (80 °C), varied proportions of a mixed alkali (i.e., a mixture of NaOH and Ca [OH]₂) were added. The pretreatment procedures were conducted in different orders; thermal-alkali pretreatment (TAP), simultaneous thermal and alkali pretreatment (STAP), and alkali-thermal pretreatment (ATP). Following pretreatment, the sludge with the alkali-thermal pretreatment was most considerably dissolved. The SCOD concentration was up to 18,000 mg/L in the WAS, and the dissolution rate reached 112.8%. Therefore, the anaerobic digestion (AD) performance of waste activated sludge (WAS) by mixed alkali-thermal pretreatment was investigated. With an increase in sodium hydroxide content in the mixed alkali, the cumulative methane production (CMP) of the sludge increased accordingly. The maximum CMP (i.e., 430 mL/g VS [volatile solids]), increased by 308.7% compared with untreated sludge and was obtained by the pretreatment of the mixed alkali (NaOH/Ca(OH)₂ = 4:1) at 80 °C. The results show that low-temperature thermal treatment combined with the mixed alkali treatment can reduce the inhibitory effect of mixed alkali on AD, thereby shortening the inhibition period (6d) and increasing methane production (308.7%). The preliminary economic feasibility analysis also showed that the ATP was a cost-effective pretreatment method.