Sulfate Reducing Bacteria
As a general rule of thumb, sulfate-reducing bacteria (SRBs) present in cooling systems can lead to corrosion and other biofilm-related issues, but how are they identified and remediated?
Circulating cooling systems play integral roles in their respective application, whether that be industrial processes or comfort cooling. The formation of biofilm and microbiologically influenced corrosion (M.I.C.) heavily contribute to issues relating to cooling system failures, which unfortunately provide ideal aquatic habitats for bacteria and other microorganisms. The operating cooling tower parameters such as temperature, pH, nutrient load, and dissolved oxygen all contribute to the promotion of an environment favorable for these bacteria to thrive and create biofilm. Of the organisms that are ubiquitous in cooling systems, SRBs are a notable classification of bacteria that contribute to MIC and the reduction of heat-transfer efficiency.
Sulfate-reducing bacteria is a classification of anaerobic bacteria that reduce or oxidize inorganic sulfur compounds such as sulfate, sulfite, and thiosulfate to hydrogen sulfide. In other words, SRBs utilize sulfates to “breathe” alternatively to oxygen. Hydrogen sulfide as a byproduct is acidic and will lead to corrosion of metals utilized in cooling systems, notably stainless and carbon steels. To add insult to injury, biofilms can further increase corrosion rates by isolating SRB growth onto metal surfaces.
There are several methods to identify the presence of SRBs in a cooling system and the most obvious place to start is the evidence of MIC. As mentioned earlier, MIC is defined as corrosion of metals induced by the presence and/or byproducts of microorganisms. Relating to SRBs, this type of corrosion is generally localized pitting, which can become severe when encapsulated within biofilm. Indications of SRB corrosion include reddish or yellowish nodules on metal surfaces that when broken exhibit black corrosion byproducts. If hydrochloric acid is added to the black deposit, hydrogen sulfide (along with a rotten egg odor) will be released2.
While there are a plethora of ways to test for SRBs, it is relatively difficult to rely solely on a single source for verification/validation. There are various methods of testing for and enumerating SRBs, such as lab broth bottles, agar deeps, ATP testing, unique enzyme tests, and more4. Limited personal experience can be attributed to testing for SRBs, but the difficulty of testing comes from collecting samples and cultivating a representative environment in which these bacteria would grow. Samples must avoid having oxygen introduced and need to have sulfate available to either react or monitor. An interesting thing to note is that while several types of bacteria can reduce sulfite, only SRBs have the capability of reducing sulfate which makes the name more understandable. While ATP testing will include SRBs, they are non-exclusive to other living organisms. ATP testing however may help track the overall health of a system. One way to identify SRBs is to measure the amount of sulfate in the cooling system and compare that with the makeup source, provided there are no alternative sources.
Complete eradication of SRBs may be an uphill battle, but biocides are typically deployed to help mitigate biological corrosion along with manipulating water parameters to make conditions less favorable. As SRBs thrive in anaerobic environments with a pH range of 4.5 – 8, increasing system pH may help remove MIC, keeping in mind that hypochlorous acid will dissociate to the less microbiocidal form hypochlorite if being fed in the system. Many bacteria secrete an oocyst-like substance that encapsulates the cell, shielding it from direct contact with water, so that cell is protected. Control of encapsulated bacteria usually requires both oxidation and dispersion of the protective sheath so that the biocide can reach the cell5. Studies have shown that once a biofilm has formed, chlorine is quite ineffective at reaching the protective bacteria, so proactive treatment will be more effective than having to remove a microbiological problem after recognition. Along with routine cooling tower inspections/cleanings, bio dispersants can be slug-fed in conjunction with biocides to allow for a more effective kill and biofilm penetration. Side-stream filtration will help remove the nutrients and reduce contaminant settling required for biological growth. While new methods of biofilm penetration continue to be investigated, glutaraldehyde and isothiazolinone have shown to perform relatively well.
Effective biological monitoring, routine housekeeping, properly dosed biocide additions, and innovative thinking may all be required to keep MIC under control. As SRBs are a living component of cooling systems, adaptation in tandem with complacency can quickly lead to corrosion disasters. Not only have SRBs shown to survive with oxygen present, but biocide resistance can occur over time. Whatever method of monitoring biological activity is chosen, ensure that a strong understanding of its advantages and flaws are understood, and know the corrective actions available.
List of Resources:
 Wikipedia contributors. (2022, May 3). Sulfate-reducing microorganism. Wikipedia. Retrieved June 17, 2022, from https://en.wikipedia.org/wiki/Sulfate-reducing_microorganism
 Sulfate Reducing Bacteria. (n.d.). Chemtex. Retrieved July 2, 2022, from http://www.industrialchemtex.com/docarchive/pub/TT-06%20Sulfate%20Reducing%20Bacteria.pdf
 Paping, Lambèr & Agtmaal, Jacques & Boks, Pieter & Groot, Niels & Veraat, Albert & Brons, Henk & Mourik, Johan & Agtmaal, J.M.C. & Agtmaal, Sjack. (2009). New challenges with the industry, regarding water & energy.
 Vester, F., & Ingvorsen, K. (1998). Improved Most-Probable-Number Method To Detect Sulfate-Reducing Bacteria with Natural Media and a Radiotracer. Applied and Environmental Microbiology, 64(5), 1700–1707. https://doi.org/10.1128/aem.64.5.1700-1707.1998
 Publishing, M., & Company, N. C. (1979). The NALCO Water Handbook. McGraw-Hill Companies.
 Elekhnawy, E., Sonbol, F., Abdelaziz, A., & Elbanna, T. (2020). Potential impact of biocide adaptation on selection of antibiotic resistance in bacterial isolates. Future Journal of Pharmaceutical Sciences, 6(1). https://doi.org/10.1186/s43094-020-00119-w