Combined pretreatment with simultaneous ultrasound (40 KHz, 50 W) and lime (560 mg/L) was carried out to improve the performance of subsequent mesophilic (35 ± 1°C) two-phase anaerobic digestion (TPAD) processes for waste activated sludge with low-volatile solid (VS) content. The mesophilic TPAD process was operated in a semi-continuous mode, fed with thickened sludge every 24 h at hydraulic retention times (HRT) of 10, 15 and 20 days, and the organic loading rate (OLR) was in the range 1.1–2.2 kg VS/m3/d. The ammonia concentrations in the two phases increased with OLR. However, there was no significant phosphorus release in the two phases. The VS removal decreased with increase in OLR, and was in the range 22.3–29.7% during the operation. The average specific methane yield decreased from 0.38 to 0.35 l CH4/g VSrem with the increase in OLR. The bioactivities characterised by the specific 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride–electron transfer system (INT-ETS) were enhanced in the...