biological impurities

All water is likely to be polluted by autochtonous or allochtonous micro-organisms. These are described in greater depth in chapter aquatic organisms.

bacteria and viruses

Faecal contamination test bacteria are brought in with urban wastewater that is discharged into the environment, with or without treatment. These bacteria are indicative of potential contamination by bacteria or viruses that are pathogenic to Man (see pathogenic micro-organisms).

The development of common germs can create serious problems in mains networks: consuming chlorine and dissolved oxygen, corrosion, developing unpleasant tastes.

miscellaneous micro-organisms (phytoplankton and zooplankton)

Surface water contains many organisms, both vegetable and animal, making up phytoplankton and zooplankton respectively (see micro-organisms for which freshwater is their natural habitat). They form a biomass that has to be eliminated.

Some of these organisms (especially Actinomycetes and Cyanobacteria) secrete compounds (such as geosmine) that can impart an unpleasant taste and/or smell to water.

Additionally, during their development or when they die, some Cyanobacteria emit metabolites that are toxic to higher orders of animals (neurotoxin, dermatotoxins and hepatotoxins including microcystins).

Furthermore, when phytoplankton, zooplankton and invertebrates are present in the network (Asellus, Copepodes, Nematodes, Oligochaeta, insect larvae), they are visually displeasing to the consumer and their development can result in major problems (proliferation, deposits, emergence of anaerobic conditions) that elicit complaints from users.

Finally, almost all surface water carries protozoa and parasites (some of which sometimes take the form of cysts which are highly resistant to chemical oxidants) that are particularly responsible for gastro-enteritis epidemics (amoeba, Giardia, Cryptosporidium, Cyclospora).