Part 4 of 8: Dissolved Oxygen
Dissolved oxygen is a critical growth pressure. The presence or absence of dissolved oxygen determines the type of metabolism. Much has been written and discussed the proper dissolved oxygen residual for Aerated Stabilization Basin (ASB) operation. The textbook guidance for proper ASB residual is 1-2 mg/L dissolved oxygen. While this may be proper for some facilities, it is not proper for all systems. As the organic loading increases, the target dissolved oxygen residual may need to be increased accordingly.
Highly loaded plants have found that low-DO stress occurs even though 1-2 mg/L DO has been achieved. On the other hand, plants operating at low organic load may perform very nicely at 0.5-1.0 mg/L DO. The best rule is to be open-minded about dissolved oxygen and adjust basin residual in accordance with the organic load.
One of the biggest expenses in the operation of an ASB is the energy required for aeration. Conscientious control of dissolved oxygen can pay tremendous economic dividends.
Because of ASBs, particularly those with surface aerators, are non-homogenous, dissolved oxygen monitoring can be tricky at best and misleading at worst. Environmental Business Specialists has extensive experience in assisting our clients in optimizing and upgrading aeration systems. By combining basic profiling with horsepower input and performance data, we can assist clients in determining whether their performance issues are related to insufficient dissolved oxygen input or some other growth pressure.
There are several indicators that oxygen is inadequate. The first, or course, is the pass through of soluble Biochemical Oxygen Demand (BOD). This refers to the failure of the system to convert soluble BOD into cell mass (TSS). The second is the appearance of low DO filaments in the biomass. Its microbiology is always an excellent indication of the conditions in the ASB. Also, most protozoa and metazoa are strict aerobes. A loss or lack of these higher life forms may indicate low dissolved oxygen levels. If in doubt about conditions in the basin, ask your bugs. A final indication of inadequate dissolved oxygen distribution is the appearance of gas bubbles in low flow areas of the basin. These bubbles are usually hydrogen sulfide and are indicative of undesirable anaerobic activity.
This article is adapted from “Aerated Stabilization Basins in the Pulp and Paper Industry” by Paul Klopping and Mike Foster.  2003. Callan & Brooks. For information on purchasing a copy of this manual, contact EBS.
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