Improvement of nutrient removal by optimizing the volume ratio of anoxic to aerobic zone in AAO-BAF system

•High nutrient removal was achieved in AAO-BAF at low COD/N.•Aerobic phosphorus uptake in AAO reactor was essential for long term operation.•The favorable volume ratio of Vano/Vaer for AAO reactor was between 2.5:1 and 6:1.

A two-sludge system, combining anaerobic/anoxic/aerobic process with biological aerated filter (AAO-BAF), is used to treat domestic wastewater with low COD/N ratio (around 3.6). The volume ratio of anoxic to aerobic zone (Vano/Vaer) of the AAO reactor gradually increased from 2:5 to 6:1, during which the nutrient removal was improved. However, phosphorous removal began to deteriorate noticeably while Vano/Vaer increased to 7:0. The favorable Vano/Vaer was between 2.5:1 and 6:1. When Vano/Vaer was 6:1, the average removal efficiencies of COD, TN and PO43- were 89 ± 4%, 83 ± 3% and 99 ± 1%, respectively. This study suggested the AAO-BAF system could achieve efficient nitrogen and phosphorous removal with limited carbon source.