background and general features

Chronologically, we have a number of different periods:

• end of the 19^th century and 1^st half of the 20^th century: widespread application of the extensive biological treatment system or slow filtration to surface water (see slow filtration);

• mid 20^th century (40’s to the 70’s): slow filtration was progressively replaced by processes based on a coagulation-flocculation treatment and which mainly comprised a sedimentation stage and a fast filtration stage; pre-chlorination was then applied almost systematically and set to above the critical point (see the section the oxidants and disinfectants) so that residual available chlorine could be measured in clarified water and in filtered water; under these conditions, no biological phenomenon could take place in this type of treatment plant. At the same time, some slow filtration water treatment plants did continue to operate but with gradually reducing options (deteriorating raw water quality, increasingly stringent standards);

• after the 70’s: biology featured again in drinking water treatment, in three ways:
  • gradual elimination of pre-chlorination from water treatment plants processing surface water that was polluted and/or too rich in OM, due to the problem caused by THM and other oxidation by-products (see pollution generated by water treatment); this approach has favoured the development of useful bacteria on supports formed by sludge masses in fast clarifiers (see sludge blanket settling tanks and sludge recirculation settling tanks) and by granular matter (sand and other filtering matter, and especially the GAC that has been increasingly installed as the second filtration stage after that period), thus contributing to the removal of OM (mineralisation) and nitrogen ammonia (nitrification). Therefore, these types of water treatment plants have a combined operating mode that is both physical-chemical and biological, especially when they also include polishing based on the [O₃ + GAC] sequence (see the BAC (biological activated carbon concept)):
  • refurbishment of some slow filtration treatment plants that still exist, for instance by installing a clarification pre-treatment and an O₃ and GAC polishing post-treatment (as in the case of the Ivry and Joinville plants upstream from Paris);
  • development of specific biological processes: iron-manganese removal (see biological iron and manganese removal ), nitrification and denitrification (see nitrogen conversion); these processes mainly apply to groundwater.

At present, all biological processes applied to drinking water treatment can be compared to attached growth process on turbulence-free granular beds and are, therefore, used in biofilters (see attached growth processes). With the exception of denitrification, these processes are aerobic and work because of the oxygen initially dissolved in the water or because air is injected simultaneously. With the exception of rarely used, so-called "dry" filters (where water trickles through the filtration mass), all these filters operate, by gravity or under pressure, with a totally submerged filtration medium.