A large-scale oxidation ditch (OD) plant had been upgraded by modifying the aeration and stirring technique of the ditches. Through upgrade, surface turbine aerators have been installed together with the existing aeration rotors. A turbulent stirring was created by the addition of the new turbine aerators, while the recirculation motion achieved by the original rotors. The two aeration techniques (turbulent stirring and recirculation motion) were operated at different ratios ranging from 0.0–1.0. As a result, by operating the turbulent stirring and gradual dispensing with recirculation motion, the system version gradually converted from an OD to a conventional activated sludge. Firstly, the gradual conversion from recirculation motion to turbulent stirring was performed under constant oxygenation level (DO) approximately 1.0 mg/L. Secondly; the turbulent stirring was increased with constant running of recirculation motion associated with gradual increase of DO level up to 2.5 mg/L. The effect of aeration and stirring on removal efficiency, sludge production, and sludge settleability has been revealed. It was found that turbine aerators could be added successfully inside ODs to increase the whole system efficiency. Firstly, at constant DO level, under same aeration power, turbulent stirring increased the removal efficiency of COD from 65 to 74%. The observed sludge yield (Yobs) came down from 0.31 to 0.25 gm volatile suspended solids (VSS)/gm BOD. Furthermore, the sludge settleability was enhanced from SVI = 137 to 115. Secondly, increased DO level with the new turbulent stirring, increased removal efficiency of COD from 65 to 95%. Moreover, the system achieved a nitrification efficiency of 70%. Finally, the sludge settleability was improved from SVI = 164 to 92 with minimum sludge yield.

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