5.7 Nuisance organisms

Nuisance organisms in drinking water are those that are not pathogenic but can cause aesthetic issues and corrosion of the distribution network (AWWA 2014). Nuisance organisms comprise a morphologically and physiologically diverse collection of organisms.

They include:

• bacteria such as non-toxic planktonic and benthic cyanobacteria (commonly known as blue-green algae)

• iron, manganese and sulphur reducing bacteria

• anaerobic (actinomycete) bacteria and fungi

• eukaryotic organisms (organisms with cells containing a nucleus) such as true algae, crustacea and protozoa.

Problems occur when the conditions in source waters, reservoirs or distribution systems support the growth of a particular nuisance organism or group of nuisance organisms. For example, excessive quantities of organic matter will support the growth of bacteria, fungi and maintain populations of protozoa (e.g. Acanthamoebae) and crustacea (e.g. amphipods and copepods). Many invertebrate animals can feed on bacteria, fungi and protozoa.

In addition, a particular nuisance organism may show morphological characteristics or produce some extracellular product that gives the organism a competitive advantage over other aquatic inhabitants. This may include a “holdfast” (a mechanism for anchoring the organism) or sheath (in the case of some iron bacteria) or the ability to produce antibiotic substances (as in some fungal species).

Raw water does not usually contain sufficient numbers of nuisance organisms to create problems. However, manganese-oxidising organisms may be responsible for deposits in groundwater (Section 5.7.3) leading to loss of flow and performance of bores. Water treatment process may also assist their growth. Nuisance organisms concentrate on the surface of filters, inside the filter bed and on mains and water reservoir surfaces. Nuisance organisms lyse and release cellular compounds responsible for colour, turbidity, taste and odour.

Activated carbon filters will contain high amounts of organic matter after some time. This may affect taste, odour and increase turbidity. This organic matter provides an excellent substrate for bacteria. Poorly operated filter systems can be the source of tastes and odours, including activated carbon-based domestic filter systems.

It is not practicable to specify a quantitative limit for nuisance microorganisms.