The EPA report nitrification occurs in the network as a consequence of chloramination of the water supply.  They also state that this only occurs up to 2 mg/l of free chlorine and that it may be controlled by break-point chlorination or by purging the network.  A copy of the report is attached.

If you have ammonium in the water in the presence of chlorine,  monochloramine will instantly form,  I therefore consider  that nitrification of ammonium in chlorinated systems is unlikely.  However if there is mineralisation of amino acids or peptides by heterotrophic bacteria, or metabolism of urea by urease to produce ammonium, the nitrification may proceed in the biofilm in the absence of chlorine.  If any ammonium is released at the surface of the biofilm, the micro environmental conditions will still be acidic.  It is therefore postulated that under these conditions reaction with hypochlorous forms trichloramines.

Observations confirm that new sand filters do not form inorganic chloramines,  but once the biofilm develops trichloramine concentration increases by a factor of 10.  Heterotrophic bacterium biofilm will establish very quickly in chlorinated systems,  but autotrophic bacteria including nitrifiers may take several weeks to months to become established.

 

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