To compare the effects of low- and high-temperature thermal-alkaline pretreatments (LTTAP, 60 +/- 1 degrees C, pH 12.0 +/- 0.1, 30 min and HTTAP, 160 +/- 1 degrees C, pH 12.0 +/- 0.1, 30 min, respectively) on anaerobic digestion (AD) of waste activated sludge, long-term and semi-continuous experiments were conducted in three laboratory continuous stirred tank reactors. The experimental results showed that the two pretreatments increased the methane yield of sludge from 89.20 +/- 2.41 mL/g added volatile solids (VS) to 117.50 +/- 5.27 mL/g added VS (LTTAP) and 156.40 +/- 2.99 mL/g added VS (HTTAP). After AD, the reduction of sludge (volatile solid) increased from 32.91 +/- 0.27% to 44.17 +/- 1.53% (LTTAP), and 50.86 +/- 1.18% (HTTAP), and the abundance of pathogenic bacteria decreased from 6.53% to 0.38% (LTTAP) and 0.14% (HTTAP). LTTAP enhanced both hydrogentrophic and acetoclastic methanogenis and HTTAP only enhanced acetoclastic methanogenis. Additionally, the energy efficiency of HTTAP and its subsequent AD was lower than that of LTTAP and its subsequent AD.