Table of Contents - click heading to enter

Principles of oxidation-reduction chemistry
Disinfection in distribution systems

Sulfate reducing bacteria (SRBs)

Iron and manganese oxidizers

EPA Definitions

Microorganisms found in drinking water distribution systems

When water leaves the treatment plant in your distribution system, it has been treated with agents such as chlorine or chloramine to remove pathogenic organisms. Some microbes, especially Cryptosporidium, are resistant to these agents and can survive for long periods in their presence. Others, such as sulfate-reducing and iron-oxidizing bacteria, live within the pipe scale, protected from disinfectants and oxygen, where they mediate important corrosion reactions. Other microbes enter the distribution system from the source water and their remains are incorporated in pipe scales.

Sulfate reducers (SRBs) - in the absence of free oxygen, there is a group of bacteria that use sulfate (SO4=) to oxidize organic matter. They can often be found in abundance within the tubercles that form on unlined cast iron pipes. They leave behind iron sulfide minerals like pyrite (FeS2) or mackinawite (FeS), or native sulfur. The microbial origin of these compounds is revealed by their enrichment in the lighter sulfur isotope 32S.

Iron and manganese oxidizers - Groundwater in many localities contains high levels of iron or manganese in lower oxidation states (Fe2+ or Mn2+). When brought to the surface, interaction of these waters with atmospheric oxygen leads to conversion of the metals to insoluble higher oxidation state forms (Fe3+, Mn3+ or Mn4+) that precipitate on the well casing and can impair well perfomance or break off to clog downstream components. Often this oxidation is mediated by Fe or Mn oxidizing bacteria like Leptothrix. Similarly, cast iron water mains and galvanized iron service branches are subject to corrosion in which metallic iron (Fe0) is converted to ferric iron (Fe3+), which then forms mineral precipitates like goethite or lepidocrocite. These may be abiotic or may be bacterially precipitated.

Diatoms - surface waters are rich in a type of algae called diatoms that secrete a siliceous skeleton. These skeletal remains are resistant to degradation and can pass through water treatment systems to end up embedded in corrosion scales. They are chemically inert and so pose no health problems. However, when found in systems that use groundwater sources, they are evidence that the ground water is 'under the direct influence of surface water' and therefore subject to being regulated as surface water.