This research was carried out at both full-scale in aeration tanks and in the microenvironment of activated sludge flocs from two wastewater treatment plants: Mill Creek and Muddy Creek Wastewater Treatment Plants (treatment capacity: 130 and 14 MGD, respectively) in Cincinnati, OH.
The aim of this study was to examine the potential of using ORP to indicate wastewater quality in aeration tanks and the microbial stratification in the activated sludge flocs in medium and low pollutant concentration wastewaters. The fieldwork data show a good relationship between ORP values
and nutrient removal along the length of aeration tanks. Water quality has a provident effect on mixed liquor ORP values. ORP values increased dramatically as organic matter was removed along the aeration tanks, indicating the improvement of the redox status of the bulk liquor. DO higher than
1.0 mg/L was necessary for good biodegradation and improvement of the liquid redox status. Nitrification occurred at higher ORP values (380–420 mV) than was the case for organic substrate oxidation (250–300 mV). The micro-profiles of activated sludge flocs from these two wastewater
treatment plants were studied by using five microelectrodes (ORP, DO, pH, NO3− and NH4+). An upflow chamber was used to keep activated sludge floc suspended during measurements. The activated sludge floc microprofiles showed that, due
to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the top layer (0.1–0.2 mm) of the sludge aggregate at the Mill Creek Plant. When DO in the bulk water was higher 4.0 mg/L, the anoxic zone inside the sludge disappeared, which is helpful
for biodegradation. At the Muddy Creek plant, with lower wastewater pollutant concentrations, the ORP and DO inside the sludge aggregates were higher than those from the Mill Creek plant. Further increasing the DO did not improve treatment efficiency, although the whole activated sludge floc
or aggregate was aerobic and in a high redox status.
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