Effects of thermal and thermal-alkaline pretreatments on continuous anaerobic sludge digestion: Performance, energy balance and, enhancement mechanism

The effects of thermal pretreatment (TP) and thermal-alkaline pretreatment (TAP) on a continuous mesophilic anaerobic digestion (AD) of sewage sludge were investigated. Both TP (134 +/- 1 degrees C and 30 min) and TAP (pH 12, 134 +/- degrees C and 30 min) released, hydrolyzed, and mineralized the organic matter of sludge, but TAP provided superior results. The methane yields of thermal pretreated sludge (TPS) and thermal-alkaline pretreated sludge (TAPS) were 1.88 times and 2.20 times that of raw sludge (RS), respectively. Moreover, the average methane content of biogas produced from TAPS (73.4 +/- 3.4%) was higher than that from TPS (69.1 +/- 3.1%) and from RS (67.2 +/- 2.9%). The energy balance of the total process with TAP was the highest (4.326 kJ/g VS) and that of the RS AD was the lowest (2.045 kJ/g VS). Both pretreatments enhanced the sludge hydrolysis with TAP presenting higher efficiency, and shifted the rate-limiting step of the anaerobic sludge digestion from hydrolysis to methanogenesis. (C) 2019 Elsevier Ltd. All rights reserved.