Low DO vs. Septicity: Separate Scenarios/ Growth Causes for (often) Undesirable Microbe Types.

There is an interesting and very important correlation with notable differences between the common terms “Low DO” (dissolved oxygen) and septicity. Due to this relatively complex relationship, there is often confusion regarding these terms. The goal of this blog is to briefly explain these differences and how each term should be thought of.

What is Septicity?

In my experience a good way to envision septicity is that “once wastewater has been exposed to septic conditions” it is then forever septic until biological oxidation (treatment). Without going deeply in depth, once available oxygen electron receptors are depleted anaerobic fermentation reactions occur in which carbonaceous “food” is “broken down” into smaller “pieces”, in which we call organic acids or volatile fatty acids. In other terms, once there is no available free dissolved oxygen, or forms of combined oxygen, food (BOD) is converted into simpler and more readily available forms. A good analogy is oatmeal versus Kool-Aid and the differences in how both of these are taken up in your body (Kool-Aid being the organic acids).

Septicity/ Organic Acid Impact on Microbe Competition/ Selection

While the overall loading rate (i.e., lbs./day of BOD) are a factor, the most significant factor appears to be the concentration of organic acids/volatile acids. Literature from Dr. Jenkins and Dr. Richard suggests that bulking from “organic acid filament types” is recognized to occur at volatile acid concentrations >100 mg/L. In addition to filaments, other microbe types that are undesirable at significant abundance (i.e., zoogloea bacteria types, single cell bacteria) appear to have a similar threshold. Based on my personal experience, the 100 mg/L volatile acids concentration appears to be a “ballpark” number due to conditions such as the individual various organic acids that encompass the total volatile acids test. Generally, if organic acid/volatile acid concentrations are diluted below the threshold in which the undesirable microbe gains a competitive advantage, significant changes in the microbes’ present may occur over a long-term period.  Potential control strategies for diluting organic acid concentrations include higher RAS rates, higher internal recycle flows (i.e., BNR plants), implementation of step feed (if possible), and in some instances recirculation of various % of effluent flow to dilute the strength of the incoming wastewater.

Low DO by Definition

The term “Low DO” is relative in that factors such as the oxygen uptake rate and the surplus available oxygen are generally most significant for selection for microbe types we recognize to grow at “Low DO”. A common way to envision this is that when we measure DO, we are measuring “residual DO” that is outside of the floc matrix. The DO setpoint needed to discourage “Low DO” microbe types is based upon the amount of dissolved oxygen needed to maintain aerobic conditions within the interior of the flocs. For example, in lightly loaded municipal systems it is common for DO concentrations as low as 0.5 mg/L to be adequate DO, while in highly loaded systems that are not designed to nitrify the oxygen uptake rate, bacterial growth rates at the front end of the aeration basin are so significant that DO concentrations of 6 mg/L or higher are often needed to discourage Low DO microbes.  For a Low DO microbe type, the applied DO is the most significant factor in controlling the growth of this organism.

Inter-Lap Between Low DO and Septicity

By definition, organic acid microbe types are selected by factors such as the concentration of various organic acids. Applying dissolved oxygen, once organic acids are already formed, is not an effective strategy to oxidize organic acids. Therefore, when troubleshooting a filament or other microbe type that is correlated with organic acids, increasing the DO setpoint is generally not successful if the organic acids are already present in the wastewater being treated prior to the aeration basin.  The only instance in which increasing the available DO is generally successful is if the front end of the aeration basin is septic and organic acids are being formed in the aeration basin itself.

In simple terms, the relationship between low DO and septicity is that once DO is low enough that forms of dissolved oxygen are depleted, fermentation reactions occur, in which organic acids/volatile acids are then produced. In these instances (such as eq basins, collection systems etc.) maintaining aerobic conditions may prevent the formation of fermentation reactions and corresponding formation of organic acids.

Source: https://rhwastewatermicrobiology.com/wastewater-microbiology-book/