There is emerging literature about viruses in biological wastewater treatment and how they may impact wastewater bacteria and subsequent processes. Bacteria of vast genetic diversity, the wastewater influent and the characteristics in the aeration basin all play important roles in selection of bacteria, sludge quality and overall treatment performance, but little is known about the impact of viruses on wastewater bacteria.

Bacteriophages (informally known as “phages”) are viruses that replicate within bacteria and archaea. Phages are simple in structure and significantly smaller than bacteria. Phages commonly produce a lytic infection cycle in which the phage adheres to bacteria, injects its genome into the cytoplasm, replicates and is re-released upon cell lysis.

Phages are of emerging interest in wastewater treatment because of their potential to target undesirable bacteria. Phages have been isolated and introduced at a lab scale to successfully infect and reduce targeted undesirable microbes (like foam-forming Nocardioforms), however it is unclear how such application may interact in a highly competitive activated sludge environment, or also, whether or not unintended bacteria may also become infected. 

Although it is still not clear the impact that phages have in biological wastewater treatment processes, it is interesting to consider that phages are naturally occurring at a predicted concentration ranging from 10^8 to 10^9 virus-like particles per milliliter of mixed liquor. It is theorized that phages may have the potential to not only attack undesirable bacteria, but desirable bacteria with environmental conditions (pH, temperature, toxins, various stressors) likely playing a role in infection rates. Bacteria may naturally contain phages without subsequent negative impact, but when various stresses occur, the rate of the lytic cycle of bacteriophages may be increased. 

It is an interesting prospect to consider “biological warfare” for selective reduction of undesirable bacteria in wastewater treatment processes. At the time of this writing, bacteriophages have been shown to be effective in controlling biological foaming in controlled laboratory environments. However, further study is needed to determine how a complex full-scale biological wastewater treatment system could complicate these interactions, and to determine the potential for impacting unintended bacteria.

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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.