Most wastewater systems I have visited target dissolved oxygen (DO) residuals in the aeration basin above 2.0 mg/L - in fact most industrial customers run at 3 - 3.5 mg/L if possible. As energy or electricity costs for aeration are a major part of the operational costs - this has led me to question the wisdom of a "one size fits all" number for dissolved oxygen. In researching this question, I have found some interesting data and I will share some of the information with you.
- Aeration in most biological treatment units functions to both provide dissolved oxygen and to suspend or mix the solids/influent wastewater. Using a diffuser or high oxygen transfer aerator to mix wastewater takes much more energy than a single purpose mixer. So, we can save on power costs by using a true mixer in system where required mixing energy from diffusers results in DO above 3.0 mg/L
- Heterotrophic or COD/BOD removing bacteria can work at DO levels well below 2.0 mg/L. Using a synthetic wastewater with COD = 300 mg/L, biosolids residence time 10 days - the researchers ran the unit with a DO of 2.0, 0.5, and 0.1 mg/L - effluent results were all 10 - 11 mg/L COD. therefore COD removal in non toxic wastewater can be achieved using much lower than 2.0 mg/L DO.
- While heterotrophic can grow well at very low DO, engineers have said the 2.0 mg/L DO is meant to maintain autotrophic nitrifier populations that convert ammonia into nitrite/nitrate. Researchers have found the nitrosomonas (NH4 -> NO2) perform well at 0.5 mg/L DO. Assuming no inhibitory compounds. Nitrobacter (NO2 -> NO3) have lower activity at 0.5 mg/L DO - roughly 12% activity). So a DO <0.5 mg/L with no inhibition to nitrifiers may lead to increased effluent NO2. What the research has found is the amount of inlet COD and inhibitory compounds has more impact on autotrophic nitrifiers than biological unit DO. Running at DO >2.0 mg/L does not give a significantly greater level of activity.
- The greatest impact of lower DO is on biological polymer or floc format
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