Energy savings and higher volumetric loading rate achieved in the conventional anoxic-oxic process for sewage treatment by enhancing biomass retention in the secondary clarifier

•Longer settling time (≥4 h) enhanced biomass retention in A/O process for sewage treatment.•Final volumetric loading rate was ∼4 times higher than that of conventional A/O process.•The advantages also included 22% energy savings, smaller footprint and reactor requirements.

This is the first study to achieve a short HRT (∼2 h for the A/O reactor), high MLSS (∼10 g L−1), and high volumetric loading (∼3.7 kg COD m−3 d−1 and ∼0.6 kg NH4-N m−3 d−1) in the A/O process for sewage treatment by enhancing biomass retention in the secondary clarifier (∼4 h settling time), which we refer to as the high-performance A/O process (HP-A/O) in this paper. Over 258 days of continuous operation, with a decrease in HRT from 12 to 2 h, remarkable COD (95 ± 3%), NH4+-N (98 ± 2%), TN (79 ± 5%), and TP (74 ± 10%) removals were stably achieved, while the air requirement significantly decreased by 22%. The HP-A/O process offers advantages over the conventional A/O process (6-8 h for A/O reactor, 3-5 g L−1 MLSS, and ∼1.0 kg COD m−3 d−1) for sewage treatment in terms of its lower energy consumption, smaller footprint and reactor requirements.