Effect of Mixing Speed on Sludge Characteristics, Treatment Efficiency, and Microbial Profiling in an Anaerobic Sequencing Batch Reactor Treating Domestic Wastewater

Anaerobic biological processes for wastewater treatment are gaining attention due to relatively lesser energy demand than aerobic processes. Since aeration is absent in anaerobic bioreactors, sludge mixing is usually done through mechanical mixing. Mechanical mixing speed is significant in terms of providing proper mixing and calculating the specific energy requirements of the plant. Moreover, the mixing speed affects the presence of different microbial species in the bioreactor. Therefore, this study investigates the effect of mechanical mixing speed in anaerobic sequencing batch reactors (AnSBRs) while treating synthetic domestic wastewater. The aim of the study is to find the optimum mixing speed, which gives the highest contaminant removal efficiency, stable sludge characteristics, higher gas production, and better microbial diversity in AnSBRs. Four AnSBRs were operated in parallel at different mixing speeds of 100 rpm, 200 rpm, 300 rpm, and 400 rpm at laboratory temperature (32 ± 2 °C) continuously for 30 days. Regular analysis of treated water quality and sludge characteristics and microbial profiling were performed. Colony morphology, biochemical tests, and 16 S rRNA based analysis were performed to isolate and identify the bacterial cultures. Mixing speed demonstrated a significant role and showed the highest contaminant removal at 400 rpm with 83% chemical oxygen demand, 74% biological oxygen demand, and 78% total suspended solids removals. The gases collected from the top of sealed reactors were identified as CH₄, CO₂ and H₂. Overall, from mixed liquor volatile suspended solids growth and gas production point of view, 400 rpm was the most suitable speed. Different bacterial species were isolated from the sludge in the form of circular, paired irregular, and filamentous colonies. The dominant species identified were Acinetobacter, Aeromonas sp, Citrobacter, Clostridium, Salmonella sp, Bacillus and Clostridium sp bacteria, confirming the maintenance of anaerobic conditions inside reactors. Overall, the 400 rpm speed provides more stable conditions to operate the AnSBRs.