Improving the slurry fuel preparation performance to recycle municipal sewage sludge by combined alkali and ultrasonication pretreatment

Municipal wastewater sludge (hereafter, sludge) is economically recycled by mixing with coal to prepare slurry fuels, called coal sludge slurry (CSS). However, raw sludge needs to be pretreated before being prepared as CSS because of its high viscosity. Alkali and ultrasonication pretreatment can effectively disrupt the sludge flocs and improve slurrying performance. The alkali pretreatment method takes a long time but has a low cost, whereas the ultrasonication pretreatment method needs a high energy input but takes a short time. This study combined these two methods with complementary advantages to reduce the sludge pretreatment time and ultrasonication energy demand to improve the slurrying performance for sludge. Results showed that, compared with only alkali and only ultrasonication, the combined alkali and ultrasonication pretreatment method had stronger effects on reducing slurry viscosity. The characteristic viscosity η c of CSS prepared by raw sludge was 1663.60 mPa s. It was reduced to 1033.44 and 1472.10 mPa s, respectively, after the sludge was only pretreated by NaOH and Ca(OH) 2 at a dosage of 10 % dry sludge (DS) for 2 h, while it was reduced to 1205.43 mPa s after the sludge was only pretreated by ultrasonication of 30 kJ/g DS. After combined pretreatment, η c of CSS was reduced to 774.98 mPa s (NaOH of 10 % DS for 2 h + ultrasonication of 30 kJ/g DS) and 1083.24 mPa s (Ca(OH) 2 of 10 % DS for 2 h + ultrasonication of 30 kJ/g DS), respectively. Apparently, the combined method can effectively decrease the alkali dosage or the ultrasonication energy input.