Sulfate-Reducing bacteria are a group of anaerobic bacteria that generate hydrogen sulfide (H2S). This product can cause a number of significant problems in water. Problems range from ìrotten eggî odors to the blackening of equipment, slime formations, and the initiation of corrosive processes. SRB microorganisms are difficult to detect because they are anaerobic and tend to grow deep down within biofilms (slimes) as a part of a microbial community. SRB may not be present in the free-flowing water over the site of the fouling.

If SRB activity is present in the BART, sulfate is reduced to H2S, which reacts with the diffusing ferrous iron to form black iron sulfide. This sulfide commonly forms either in the base (as black precipitates) and/or around the ball (as an irregular black ring).