Effect of rotifers on the stability of aerobic granules

The current study evaluated the effect of rotifers on the stability of aerobic granules. Two sequence batch reactors (SBRs) with airflow rates of 4 (R1) and 6 (R2) 1 min(-1), respectively, were used to develop aerobic granules. Granules were well developed with excellent settleability in terms of SVI30 (sludge volume index,) of about 50 ml g(-1) in both reactors at the beginning. With the outgrowth of rotifers, granules completely disintegrated in R1 around cycle 500 (a cycle was 3 hours). However, after the rotifers disappeared, i.e. cycle 550, granules re-appeared with a slow settling rate in R1 (SVI30: 200-300 ml g(-1)). The rotifers mechanically damaged the structure of granules, resulting in disintegration. However, granules developed under high shear force seem to have strong resistance to rotifers. During re-granulation, a long time lag between the improvements of morphology and settleability suggested that re-granulation resulted from entanglement more than bio-attachment or bio-growth. Additionally, it was confirmed that the ratio of carbohydrate to protein extracellular polymeric substances (EPS) could well indicate the strength of granules. Protein EPS well correlated with the difference between SVI5 and SVI30 in R1, therefore, decreasing protein EPS would increase the compactness of granules.