Anaerobic/Oxic/Anoxic Mode Sequencing Batch Reactor: Treatment Performance and Development of Aerobic Granular Sludge

Abstract

Objective: The objective of the present work was to reduce the start-up time of aerobic granular sludge (AGS) formation using medium-strength sewage and to find the most effective anaerobic, aerobic (oxic), and anoxic (A/O/A) cyclic time distribution over the 6-hour cycle time for the removal of nutrients. Methods: A rectangular sequencing batch reactor (SBR) operated systematically in A/O/A cycle was designed for AGS formation. In order to achieve complete nutrient removal, the anoxic phase (5% of the total cycle duration) was introduced in the SBR cycle. Results: Growth in the biomass profile was observed after 20 days of operation, and complete granulation was achieved within 35 days of operational studies. Regular-shaped, compact granules with visible outlines and clear boundaries predominate inside the system, with an average particle size of 220.19 µm. The A/O/A system achieved average removal efficiencies of 70.02 ± 5.98%, 78.21 ± 18.66%, 60.62 ± 9.69%, and 53.49 ± 8.14% for COD, NH4+-N, TN, and PO43--P, respectively. The reasons for poor COD and PO43--P removal performance were the short anaerobic phase (25% of total cycle duration of 6 hours) and higher biomass concentration. However, the long aerobic phase (60% of total cycle duration of 6 hours) with the anoxic phase achieved outstanding (100%) NH4+-N removal efficiency after the system reached a steady state. SEM results inferred that rod-shaped bacteria, cocci, and filamentous fungi were the dominant microorganisms found within the cultivated granules. Conclusions: Sewage with a high COD loading rate accelerates AGS formation and development within an A/O/A cycle SBR reactor. The rapid granulation and simultaneous removal of organics and nutrients by the A/O/A mode SBR without any support materials provide an alternative for wastewater treatment.