Effect of long-term idle periods on the performance of sequencing batch reactors

Sludge storage can be used as an effective control handle to adjust plant c apacity to large influent variations. The sequencing batch reactor (SBR) te chnology is well suited for temporary sludge storage because reactors can e asily be switched off individually and operated in an idle mode. In this st udy experimental results on the effect of long term (weeks) idle periods on nitrogen removal are presented. The SBRs were operated with idle times ran ging from 6 to 20 days. Batch experiments were performed where sludge was s tored without the addition of any substrate for 7 weeks. In the SBRs, repea ted long-term idle phases had only a minor effect on ammonia oxidation, The nitrite oxidation process was more sensitive to long idle phases resulting in temporary nitrite accumulation in the SBRs. Quantitative gene probe ana lyses demonstrated that the decay of ammonia oxidizers was slower than the decay of nitrite oxidizers which in turn decayed slower than heterotrophic bacteria.