The mixed solution was manually dra

The mixed solution was manually drawn off and then immediately fed with the same volume of gelatinized PPW slurry once a day according to the SRT and SC (for example, SRT of 2 d and SC of 30 g L1 reactor, 400 mL mixture was removed and same volume of gelatinized PPW with SC of 30 g L1 was added daily), no sedimentation phase was conducted. Sampling and analysis was conducted after at least 3 SRT cycles to ensure steady state conditions of the bioreactors were obtained. A total of thirteen conditions composed of 4 cubic points and 4 axial points and 5 replicates at the center point were conducted. The experimental data were processed using Design Expert 8.0 software (Stat-Ease, Inc., MN, USA).