Identifying And Dealing With Sulfate Reducing Bacteria
What is SRB?:
SRB stands for Sulfate Reducing Bacteria. SRB is one of many common soil bacteria species that populate our world and perform a valuable ecological role in the environment. SRB lives throughout our ecosystem and can be found in almost any puddle or natural body of water, as well as in the soil around us. While most bacteria are aerobic and require oxygen to survive, SRB is among a group of species that is anaerobic and thrives only in the absence of oxygen. SRB and other anaerobic species have adapted to the oxygen in our atmosphere and in natural waters. These species can survive exposure to oxygen by encasing themselves in a slime coat that protects them from the potentially lethal oxygen. While SRB performs an important function in the wild and does not present a danger to humans or other animals, its effects can cause damage to certain metals in cooling towers, closed loops and even boilers.
But cooling tower water is full of oxygen:
While it is true that cooling tower water is saturated with oxygen, SRB and other anaerobic species such as Clostridium can survive in the oxygen-free deposits that can accumulate in cooling towers. As these deposits accumulate, aerobic bacteria quickly consume all of the oxygen. An anaerobic condition develops since water can not circulate through the deposit to supply more oxygen. When anaerobic bacteria detect the lack of oxygen, they enter their active growth phase and begin to multiply.
How does anaerobic bacteria harm metal?:
SRB colonies can attack all types of ferrous metals including iron, mild steel, galvanized and stainless steel. SRB’s metabolic process uses the conversion of iron to iron oxide to create energy. Clostridium excretes hydrogen ions which react with water to form strong organic acids. Both types of bacteria result in pits in the surface of the metal that are hidden under tubercles of iron oxide. These tubercles, or corrosion nodules, protect the anaerobic bacteria from the oxygen that would quickly kill them. The tubercles also give away the presence of anaerobic bacteria and that microbiologically influenced corrosion, or MIC, that is occurring.
What can I do to prevent microbiological influenced corrosion (MIC) problems?:
Keeping your surfaces clean is the best defense against SRB, Clostridium, or any other anaerobic bacteria, because without deposits to hide under, these bacteria are kept in check by the oxygen-rich water in your cooling tower. In general, when the depth of the deposit in your cooling tower exceeds ½”, MIC problems will tend to develop. The appearance of corrosion nodules “growing” in your sump or elsewhere in the system is also a bad sign.
Can it be stopped once it starts growing?:
Yes. The earlier we can detect the activity of anaerobic bacteria, the better job we can do at protecting your equipment from its harmful effects. A routine cooling tower sump cleaning program will help prevent MIC. An annual chiller inspection and brushing will help keep MIC from occurring in this expensive component. Bromine has also been shown to be very effective at preventing MIC by preventing protective biofilms from developing.
What about my other systems?:
Most closed loop systems use oxygen scavengers to prevent corrosion. The nature of this type of treatment creates the anaerobic conditions that permits SRB to thrive. Fortunately, there is little opportunity for anaerobic bacteria to enter a closed loop since there is no contact with air or other sources of bacteria. Still, there are occasionally cases where anaerobic bacteria have caused significant MIC damage to closed loop system and even boiler feedwater systems.