Because bacteria are responsible for the vast majority of treatment, the general success or failure of systems depends on maintaining adequate populations of viable bacteria to treat incoming wastewater as well as sufficient time (hydraulic retention time and sludge retention time) for the microbes to carry out their various functions. While living bacteria are directly responsible for general treatment, there are several crucial ways in which dead bacteria also aid in flocculation and overall treatment. 

Dead bacteria may act as bridging agents between live bacteria and other suspended particles, with the sticky surface of dead bacteria helping to form biological flocs. Dead bacteria release extracellular polymeric substances, polysaccharide and nucleic acids, which act as natural flocculants. Dead bacteria also provide sites for suspended particles to aggregate, and as other bacteria and particles are included, flocs become denser. Lastly, there is some literature that cites factors such as filamentous bacteria, organism decay, and microbial diversity and succession, which may help promote a dynamic microbial community. 

Using Invitrogen LIVE/DEAD BacLight testing, and with experience of viewing thousands of biological wastewater samples, it appears common to see estimated ranges of 40-80% of viable bacteria (per average floc) in most aerobic biological treatment systems. Generally, systems with higher SRT values have a lesser percentage of viable bacteria present, while high-rate systems tend to have higher estimated bacteria viability. These general percentages usually correlate roughly with the bacterial growth curve, with increasing amounts of nonviable bacteria present as endogenous conditions begin to occur. 

Measurement of MLVSS includes viable as well as nonviable bacteria, so while it is a useful tool to distinguish the fraction of inert solids in the MLSS, it is not a reliable tool for bacterial viability. While many systems have successful operations maintaining target MLSS concentrations, there are many factors that make up the MLSS value, including inert solids (generally 15-30% by weight), polysaccharide (generally 5-20% by weight), higher life form organisms (Dr. Jenkins references 5-20% by weight),  and others like archaea and fungi. Based upon conditions present, the amount of viable bacteria present within the mixed liquor may fluctuate, or in some instances, change rapidly (toxic events, slug loads, etc.).

While dead bacteria in adequate amounts have general benefits for flocculation and overall treatment, excess amounts of dead bacteria may pose issues such as undesirable flocs, dispersed dead cellular material in solution, dead bug foams, and if conditions are unfavorable for viable bacteria, loss of overall biological treatment.

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About the author: Ryan Hennessy is the principal scientist at Ryan Hennessy Wastewater Microbiology. He was trained and mentored by Dr. Michael Richard for over 10 years in wastewater microbiology, and serves as a microbiology services consultant. Hennessy is a licensed wastewater treatment and municipal waterworks operator in the state of Wisconsin and fills in as needed for operations at several facilities. He can be reached at ryan@rhwastewatermicrobiology.com. Hennessy's new book "Wastewater Microbiology: Filamentous Bacteria Morphotype Identification Techniques, and Process Control Troubleshooting Strategies" is now available on Amazon.