Treatment of tapioca processing wastewater in a sequencing batch reactor: Mechanism of granule formation and performance.

The formation of aerobic granular sludge was carried out in a sequencing batch reactor (SBR) for tapioca processing wastewater treatment. The effect of organic loading rates (OLRs) in the range of 2.5-10.0 kg COD m-3 day-1 on the granulation was investigated. The size and settleability of the aerobic granular sludge increased with increasing OLR from 2.5 to 7.5 kg COD m-3 day-1. The mature granules had an average size of 2.5 mm and good settleability with the sludge volume index (SVI) lower than 50 mL g-1. The granules had a layered structure consisting of anoxic sludge core with nematodes and an outer aerobic layer surrounded by stalked ciliates. Removal efficiencies of chemical oxygen demand (COD) and NH4+-N reached 90.0%-93.0% and 86.6%-92.5%, respectively. Simultaneous nitrification and denitrification at the OLR of 7.5 kg COD m-3 day-1 resulted in the improvement of total nitrogen (TN) removal efficiency to 66.1%.