Solid-liquid separation and its type greatly affected the stability and performance of the anaerobic sequencing batch reactor (ASBR) for municipal sludge digestion. Flotation thickening occurred in mesophilic ASBR, while solid-liquid separation in thermophilic ASBR followed a gravity thickening. The hydraulic retention time (HRT) and cycle period as well as the type of thickening were key parameters governing sludge thickenability and critical solids accumulation. Thickened sludge volume was a critical operating variable in the ASBR with the gravity thickening, which had a poor performance because of the loss of thickened solids, and sludge interface disruption or instability of sludge bed due to internal gas evolution. A cyclic mutual effect between thickened volume and gas production was a serious in the gravity thickening, whereas it was insignificant in the flotation thickening.