Dissolved oxygen and idle time effects on sewage bioremediation

This paper explores the effects of dissolved oxygen (DO) concentrations and idle time on the performance of aerated-sludge portable sewage systems for rapid deployment during natural calamities or warfare. Using synthetic sewage, three different reactors were operated at DO levels of 2-6 mg/l to determine the optimum DO for reactor performance, chemical oxygen demand (COD) and total organic content (TOC) removal, pH, conductivity and biomass concentrations. Low DO values result in very low COD removal efficiency, while DO levels over 6 mg/l are not significantly more efficient. The oxygen utilisation rate and specific oxygen utilisation rate were in the ranges 0.87-1.08 mg O₂/h and 0.44-0.895 mg O₂/g MLSS (mixed liquor suspended solids) per hour respectively for the three reactors. The highest achieved specific removal rate was 14 mg/g MLSS per hour and 6 mg/g MLSS per hour for COD and TOC respectively. Investigation of idle time (no-aeration) revealed that an idle time of less than 24 h results in low removal efficiency (COD and/or TOC) but the system recovers in the next 24 h; however, an idle period of 48 h or greater results in very poor efficiency and may cause permanent failure of the bioremediation process. The results of the present study can be applied to avoid system failure in a sewage treatment system as a result of, say, intermittent or sudden power shutdown.