What are the Basics that Wastewater Operators Should be Aware of Regarding Organic Acids? 

A brief look at a table demonstrating the various bulking filamentous bacteria types commonly observed in wastewater treatment process will show that a high % of filamentous bacteria (and other bacteria that may potentially be undesirable at certain amounts) are recognized to “proliferate at elevated concentrations of low molecular weight organic acids”. This produces a common question “What are organic acids?”

Organic Acids in Wastewater 

A serviceable analogy for how bacterial cells “eat” is to think of their cell wall as a “filter”. (Dr. Richard liked to describe the cell wall or cell membrane as the “mouth” of the cell). 

In order for “food” to be accessible for the bacteria, it must be in “small enough pieces” that it may pass through the filter. For example, larger “pieces of food” (such as particulate BOD) are not readily available for absorption into the bacterial cell and are retained on the outer edge of the cell (adsorption) while enzymes “break them down” into small enough pieces in which they eventually become available. In wastewater “food” (BOD) is not all the same, and a major component of bacterial selection is how readily available the food is to be utilized by the bacteria.  

Using something like the glycemic index in nutrition as an analogy, Kool-Aid is taken up quickly by cells producing quick and often short-lived energy (ATP), while food like oatmeal is more complex and takes longer to be digested as there are many steps needed to reduce these “larger pieces” into “small enough pieces” in which they may pass through the cell membrane and become available.  

Which Bacteria Compete Best for Organic Acids (Volatile Acids)

Due to the practical analogy of organic acids being “readily available food”, bacteria that have the highest kinetic growth rates as well as the best availability to store food for later (Poly-β-hydroxybutyrate or PHB) gain a competitive advantage when there is a surplus of organic acid/volatile acid substrate (food). Generally, as volatile acid concentrations exceed 80 mg/L, this is where “organic acid” filaments gain a competitive advantage over many floc forming bacteria. Note that previous literature mentions 100 mg/L as a threshold, however we suspect that there are many factors (type or organic acids, temperature, F/M ratio etc.) In addition to certain filamentous bacteria, other bacteria types which may be undesirable at various amounts (i.e., single cell bacteria, zoogloea bacteria types) also favor elevated organic acid concentrations. 

Often, respirometry methods such as oxygen uptake rate may correlate well with a general idea of “how fast the bugs are taking up the food”. A useful and admittedly simple/basic way to think of oxygen uptake rate as it often correlates with organic acids is as “the speed limit”, where at >60 mg/L/hr. undesirable growth may often occur. 

Sources of Organic Acids 

In many industrial wastewater treatment systems and septage, high concentrations of organic acids may be naturally present in the influent. Once organic acids are present, they are “there” and must then be oxidized/treated by bacteria. In areas in which the wastewater become septic and there are no longer available oxygen acceptors (i.e., collection systems, lift stations, holding tanks, primary clarifiers), “fermentation” reactions occur in which “bigger pieces” of food are “broken down”, into “smaller pieces” (organic acids). These fermentation reactions resemble the first stages of anaerobic treatment, however in full/complete anaerobic treatment methane and carbon dioxide are produced as final end products, where formation of organic acids may be generally thought of as “incomplete or partial anaerobic treatment”.  

Organic Acids “Double Edged Sword” 

Having sufficient organic acids is necessary for processes such as denitrification and enhanced biological phosphorous removal, and in some instances treatment plants may even have specific zones (i.e., fermenters) to help convert BOD into organic acids to help produce sufficient readily available food in selector cells. Since septicity and wastewater go hand in hand, there is always a percentage of BOD that is available as organic acids. Lastly, it may be useful to note that in the Krebs cycle, all “food” is eventually converted into acetyl coenzyme A or other intermediates (which requires acetic acid, an organic acid) in which glucose is fully oxidized and energy is stored in ATP and other high energy compounds. (So basically, everything is eventually broken down into acetic acid during “treatment”). Often, where organic acids become problematic, is when they become too highly concentrated in the initial contact zone (first 15-30 minutes) of the aeration basin. There are various short- and long-term operational approaches for troubleshooting problems related to elevated organic acid concentrations and their selection is based on individual circumstances and their logistics. 

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