5.7.1 Organisms causing taste and odour problems

Objectionable tastes and odours can result from compounds produced by certain types of algae, cyanobacteria (blue-green algae), bacteria and sometimes protozoa. For instance, Actinomycetes and cyanobacteria produce geosmin and methylisoborneol (MIB), which have an earthy taint and a taste and odour threshold of approximately 0.00001 mg/L (10 ng/L). Development of anoxic conditions in tank sludges and stagnant water (e.g. dead-ends) can lead to bacterial sulphate reduction, often producing hydrogen sulphide.

Several groups of protozoa produce odorous compounds in culture. Certain species of the amoeba genera Vannella, Saccamoeba and Ripidomyxa that carry dense bacterial symbionts also produce either geosmin or MIB. Symbionts are different organisms that share a close long-term relationship with other organisms. They may be mutualistic, communalistic or parasitic through the process of symbiosis. Most previously described sources of these compounds have been cyanobacteria or actinomycetes, so it seems likely that the symbionts are the immediate source. The mechanism of symbiont contribution to odours in waters is unknown. However, they should be considered as the likely source of a problem if no other biological source of these strongly smelling compounds can be identified.

Free-swimming protozoa with hair-like cilia (ciliates) can contribute to odours in water if they reach high densities. Examples include Climacostomum and certain Stentor species that bear the algal symbionts zoochlorelle, although such incidents are not often reported.

Consumers often detect taste and odour problems before analytical methods have detected the compounds responsible. It is therefore advisable to use trained panels to detect taste and odour and undertake remedial measures before a problem becomes significant. Section 3.5.2 and the fact sheet on Taste and Odour in Part V discuss such panels.

Another method to pre-empt taste and odour problems is to use microscopy to regularly examine the type and number of organisms present in the water. When a group of organisms known to cause taste or odour problems is dominant, measures should be taken to overcome the problem.