Earthy-musty-moldy-mildewy-fishy T&O is well known to everybody, even if they don’t associate it with water. It is surprising that molds are not the only source, but algae and certain bacteria also produce some of the same chemical compounds. The differences in smell may simply due to differences in concentration and mixtures. For example, one of the compounds that smells musty in the concentrations found in water smells like – is the aroma of green bell peppers at lower concentrations. They’re exactly the same molecule. These compounds get into drinking water most often from algae blooms in the source water, which are usually seasonal. Filamentous bacteria called actinomycetes are also common causes, but their growths in reservoirs are more continuous and seldom cause unexpected trouble. Actual mold growth is less common and occurs mostly in long-abandoned plumbing or dead-end mains that have not received fresh, chlorinated water for a long time.
Further to the discussion of “sulfur water” (which see, above): One of the bacterial families that is able to reduce sulfate ion (SO4-2) to sulfide ion and hydrogen sulfide (S-2 and H2S) (“‘rotten egg” smell) can also produce spores, which are tough “survival forms” similar to protozoan cysts, only smaller. The bacteria are anaerobic, meaning they cannot tolerate dissolved oxygen, and they grow only in unused, dead-end mains or other nooks and crannies in the distribution system where all the oxygen has dissipated. They produce spores to survive exposure to oxygen if it ever ret urns. When water treatment equipment with good mechanical filtration ability is used on supplies carrying these spores, the spores can become lodged in the media. If the equipment is not used enough to keep the water inside them fresh and oxygenated, the spores can grow into active bacteria and begin producing rotten egg smell. It is usually best to replace or re-bed such media, but it may be possible to “vend” them often enough to kill any of the new bacteria before they can make new spores.
Other anaerobic bacteria can make a different kind of foul T&O when oxygen is absent, but they do not produce spores and are easier to get rid of. The smell they produce is usually characterized as “septic,” referring to odors from sewage that are different from pure hydrogen sulfide. It is not necessarily sewage bacteria that are responsible for this problem in filters; many anaerobic bacteria can make the same odorous compounds.