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Key Takeaways:

  • Biofilms are resistant to chlorine and can significantly increase chlorine demand.
  • Biofilms may increase the rate of creation of some disinfection by-products.
  • There is controversy over whether bacteria can be killed by chlorine before forming biofilms though in residential pools and spas this appears to be the case based on a lack of symptoms.
  • There are questions about the frequency at which biofilms form in sand filters in high bather-load pools such as commercial/public pools.


Biofilm is an aggregate of microorganisms in which cells adhere to each other and/or to a surface.  For our purposes in pools and spas, biofilms are on surfaces and excrete chemical compounds (such as alginate slime) that make the microorganisms harder to kill from sanitizers such as chlorine.


Large colonies of bacteria in biofilm can significantly increase chlorine demand and may result in higher disinfection by-products such as nitrogen trichloride (trichloramine).


There are questions (well, at least I have questions) about whether planktonic (free-floating) bacteria are able to be killed quickly enough in properly chlorinated pools and spas before biofilms can form and the effect of different filter types, especially sand filters, on biofilm formation.  The posts in this discussion thread relate to such issues as well as others regarding biofilms.

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The 6:1 Cl:N ratio is a stoichiometric one.  It just says how much chlorine will get used up when fully oxidizing nitrite.  One can have a lower Cl level if one keeps adding chlorine to maintain such a level.  Nitrite will only show up if it is getting produced/introduced faster than it is getting oxidized by chlorine.  So even 0.2 ppm FC should get rid of nitrite quickly unless for some reason it is getting introduced quickly (as with the UV example you give).  There are many papers in the photolysis of nitrate to nitrite (see this one, this one, this one, this one and several of Blatchley's papers including this one as well as presentations at WAHC).



The pool in question was a public pool, not operating in compliance to DIN. Sand filters and medium pressure UVc,  FC around 1 mg/l, pH 7.3.


The pool has a heavy load,  total volume approx 600 cubic metres, bather load between 500 and 1000/day.

The water was analyzed by a certified laboratory,  subsequent tests have shown the nitrate levels dropping and nitrite concentrations very low. There seems to be a pattern that follows the  UV  unit and bather load.


We need more information so any feed-back from public pools would be useful.


Also is there any chance  of nitrosomine  being produced, has anyone tested for the product ?


There are more questions than answers at the moment,  but I don’t think this area has ever been investigated for pools. 


I'm still wondering if the sample was taken before or right after the UVC unit, or was it taken from the actual pool basin itself? Was the 1 mg/L of FC in the actual sample that the nitrite was tested from? Still not sure how nitrates would affect the stability of pH and alkalinity, or what the maximum levels should be. I've heard of 25 ppm for algae control, but that's about it? Have you come across any recommendations?

You mentioned:

The water was analyzed by a certified laboratory,  subsequent tests have shown the nitrate levels dropping and nitrite concentrations very low. There seems to be a pattern that follows the  UV  unit and bather load.

What pattern are you seeing? I would think that the photolysis of bather related inorganic chloramines would raise the nitrate levels in the pool, and the perhaps the nitrite levels right after the UVc unit due to the conversion of nitrate to nitrite under 240nm wavelengths? Is that the pattern you are seeing? How would you explain the drop of the nitrate levels besides from water loss? The rapid conversion of nitrite to nitrate after the UVc shouldn't really raise or lower the overall levels, as it is just a never-ending cycle of chlorine demand. It's not changing it to something it?  


You can google "nitrosamines in swimming pools" and get a few hits, so someone is discussing it. This one is rather interesting.

There is this paper on nitrosamines in chlorinated pools.  Outdoor pools showed 1/6th lower concentration of N-nitrosodimethylamine (NDMA) compared to indoor pools and those indoor pools showed 1/10th lower concentration of NDMA compared to indoor hot tubs.  Both outdoor and indoor pools showed lower concentrations of N-nitrodimethylamine (DMNA) compared to indoor hot tubs.  This data as well as analysis of outdoor spas and indoor pools with retractable roofs is consistent with photolysis (i.e. the UV in sunlight) helping to control nitrosamines.


DMNA is highly correlated (r2 = 0.91) with nitrite concentrations while NDMA is less correlated with nitrite (r2 = 0.61).  So the question will be whether the effect of UV destroying nitrosamines outweighs the UV creation of nitrites that increase nitrosamines.


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