Impact of step-feed on COD and BOD₅ removal in rotating biological contactors

A study was conducted to evaluate the effectiveness of step-feed in a rotating biological contactor (RBC) system consisting of two three-stage units (one control and one step-feed) treating synthetic wastewater. The performance of the system was evaluated in terms of organic rates, applied and removed. The impact on sCOD, sBOD₅ removals, and stage-dissolved oxygen (DO) conditions over a range of hydraulic and organic loading rates (HLR = 0.032 to 0.125 m³/m²·day and OLR = 11.03 to 111.6 g sCOD/m²·day) was evaluated. The results indicate a linear relationship with excellent correlation between the organic loading and removal rates. Varying the HLR and the substrate concentration within the tested range had negligible effect on the removal efficiency of the process. Similarly, the overall removal efficiency with varying OLR showed very limited improvement except at the highest OLR tested (111.6 g sCOD/m²·day) where removal increased by 5.2 and 2.4% for COD and BOD₅, respectively. Increasing the HLR and the OLR resulted in a decrease in DO in all the stages of the two units. However, DO values in the step-feed system were higher than those recorded for the control system. In addition, O₂ limiting conditions (DO < 2 mg/L) and heavy bacterial mass growth and possible growth of Beggiatoa were detected in the first stage of the control at high loading rates.