Manipulating dissolved oxygen concentration to control the partitioning between carbon oxidation and redirection in the A-stage high rate activated sludge process

The adsorption/bio-oxidation (A/B) process has emerged as a remedy to the significant energy use from wastewater treatment plants. In such a process, the A-stage's objective is to balance maintaining high COD redirection towards recovery streams with achieving high effluent quality to facilitate energy savings in the B-stage. While SRT was intensively investigated to control A-stage performance, this study investigates the use of dissolved oxygen (DO) concentration under two adverse conditions: short and long residence times (SRT and HRT). The findings indicate that excessive oxidation is impeded at short residence times, meanwhile, high DO concentration (>2 mgO(2)/L) is required for enhanced substrate utilization and high removal efficiencies. Conversely, low DO concentration (< 0.5 mgO(2)/L) is needed at long residence times to control the system's oxidative status and partition more COD towards redirection instead of oxidation. Whereas, high removal efficiency is governed by the long residence times. Notably, this study implies that the low COD removal efficiencies in existing A-stage systems can be referred to the need to deliver oxygen at rates beyond the maximum achievable oxygen transfer rates in full-scale systems. Novelly, this study demonstrates that systems operated at lower rates coupled with low DO concentration can achieve higher COD removal efficiencies while maintaining high COD diversion.