High organic loading influences the physical characteristics of aerobic sludge granules

Aims: The effect of high organic loading rate (OLR) on the physical characteristics of aerobic granules was studied. Methods and Results: Two column‐type sequential aerobic sludge blanket reactors were fed with either glucose or acetate as the main carbon source, and the OLR was gradually raised from 6 to 9, 12 and 15 kg chemical oxygen demand (COD) m−3 d−1. Glucose‐fed granules could sustain the maximum OLR tested. At a low OLR, these granules exhibited a loose fluffy morphology dominated by filamentous bacteria. At higher OLRs, these granules became irregularly shaped, with folds, crevices and depressions. In contrast, acetate‐fed granules had a compact spherical morphology at OLRs of 6 and 9 kg COD m−3 d−1, with better settling and strength characteristics than glucose‐fed granules at similar OLRs. However, acetate‐fed granules could not sustain high OLRs and disintegrated when the OLR reached 9 kg COD m−3 d−1. Conclusions: The compact regular microstructure of the acetate‐fed granules appeared to limit mass transfer of nutrients at an OLR of 9 kg COD m−3 d−1. The looser filamentous microstructure of the glucose‐fed granules and the subsequent irregular morphology delayed the onset of diffusion limitation and allowed significantly higher OLRs to be attained. Significance and Impact of the Study: High organic loading rates are possible with aerobic granules. This research would be helpful in the development of aerobic granule‐based systems for high‐strength wastewaters.