Effect of organic loading rate on aerobic granulation. I: Reactor performance

The effect of organic loading rate (OLR) on the aerobic granulation process was investigated using laboratory-scale sequential aerobic sludge blanket reactors (SASBRs). Reactors R1, R2, R3, and R4 were operated at OLRs of 1, 2, 4 and 8 kg chemical oxygen demand (COD)/m(3) day, respectively. Aerobic granules could not be formed at the relatively low OLRs in RI and R2. Stable aerobic granules were successfully cultivated at the mid-range OLR of 4 kg COD/m(3) day tested in Reactor R3. These granules first appeared 14 days after startup and eventually grew to become the dominant form of biomass in R3. The granular biomass stabilized at a mixed liquor volatile suspended solids (MLVSS) concentration of 12,000 mg/L, with a food-to-microorganism (F/M) ratio of 0.33 kg COD/kg MLVSS day and a mean cell residence time of 31.1 days. Aerobic granules were first observed on Day 18 in Reactor R4, which operated at the highest OLR tested of 8 kg COD/m(3) day. However, these granules were unstable and eventually washed out of R4. The best reactor performance was achieved in R3 with a COD removal rate of 99%, an observed yield coefficient Won) of 0.10 mg MLVSS/mg COD, and a sludge volume index 24 mL/g MLVSS. The volumetric specific oxygen utilization rate was highest in R3, at 356 mg O-2/L h. An optimal choice of OLR was found to favor the cultivation and retention of well-settling granules and enhanced the overall ability of the reactor to remove COD. This study contributes to a better understanding of the role of OLR in aerobic granulation.