Aerobic granular sludge treating low-strength municipal wastewater: Efficient carbon, nitrogen and phosphorus removal with hydrolysis-acidification pretreatment

Highlights: • Shortening cycle length to 2 h can trigger aerobic granulation. • Hydrolysis-acidification pretreatment can improve nutrients removal performance. • Underlying mechanism of hydrolysis-acidification pretreatment effect was revealed. Low organic load while high fraction of particulates still challenging the application of aerobic granular sludge process in low-strength municipal wastewater treatment. The feasibility of adopting short cycle length to increase organic load and hydrolysis-acidification pretreatment to enhance anaerobic COD uptake was evaluated. As the cycle length decreased from 4 h to 2 h, the organic loading rate increased from 0.98 to 1.3 g L⁻¹ d⁻¹ and granulation appeared after two weeks. Moreover, with the hydrolysis-acidification pretreatment, the average effluent TN and TP concentrations decreased respectively from 17.8 to 13.7 mg L⁻¹ and 0.76 to 0.41 mg L⁻¹, meeting the Grade IA of the effluent standards in China. Furthermore, cycle tests were conducted to reveal the underlying mechanism of the pretreatment effects. The results showed that the hydrolysis-acidification pretreatment enhanced the COD storage and phosphorus release in anaerobic phase, and improved the simultaneous nitrification-denitrification process, as well as the phosphorus uptake in aeration phase.