[Eriophorum (OP)]

The Drinking Water Inspectorate say that residual chlorine in tap water is there to preserve the microbiological quality of tap water, but as far as I can tell 0.5mg/l of chlorine (0.5ppm) is not enough to kill anything. My bleach-based disinfectants recommend (if my maths is correct) about 200ppm. Also, I have found a couple of studies on the survival times of Leptospira in tap water which seem to show they can survive for over 100 days (Chang, S. L., Buckingham, M. & Taylor, M. P. (1948) J. infect. Dis., 82, 256) and (Sawyer and Bauer, Am J Trop Med Hyg January 1928 vol. s1-8 no. 1 17-28). As far as I know, Leptospira are not particularly resistant to chlorine (quite the opposite according to Leptospirosis.org).

So if 0.5mg/l chlorine can't even kill Leptospira after 100days, then how does the DWI justify its claim? Is there something I'm missing about the above experiments that explains the results. Both were conducted at room temperature whereas tap water is much colder, but I thought this makes chlorine less effective.

Any help understanding this would be gratefully received.

[hamdani yusuf]

You can start from here https://en.wikipedia.org/wiki/Water_chlorination
Some important points from the source :

As a strong oxidizing agent, chlorine kills via the oxidation of organic molecules.[13] Chlorine and hydrolysis product hypochlorous acid are neutrally charged and therefore easily penetrate the negatively charged surface of pathogens. It is able to disintegrate the lipids that compose the cell wall and react with intracellular enzymes and proteins, making them nonfunctional. Microorganisms then either die or are no longer able to multiply

Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount of hypochlorite into the water. The hypochlorite can be in the form of a powder or a liquid such as chlorine bleach (solution of sodium hypochlorite in water). Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three parts per million (PPM) or less.

In summary, dillute chlorine is to prevent, while concentrated chlorine is to kill microorganisms that have already grown.

[Eriophorum (OP)]

It is able to disintegrate the lipids that compose the cell wall and react with intracellular enzymes and proteins, making them nonfunctional. Microorganisms then either die or are no longer able to multiply

That sounds like a perfect explanation thanks, although a little counter-intuitive. What I don't understand is why the bacteria's reproductive ability is so sensitive. Basically it seems to be that damage to their reproductive ability is sustained at 0.5ppm chlorine, whereas damage to any other part of their structure (like digestion, energy production, waste disposal; all of which would kill them) is only sustained at levels 400 times greater.

Basically, why is their reproductive system 400 times more vulnerable than any other system?

[chiralSPO]

I suspect that the reason that high concentrations are needed to kill existing populations but only low concentrations are required to prevent microorganisms from gaining ground has to do with partitioning (auto-concentration) of the chlorine.

Chlorine is only sparingly soluble in water, and it has a higher affinity for organic environments. Therefore it can reach concentrations significantly higher in cells (before reacting) than in the surrounding water. If there are only a few cells in a very large amount of slightly chlorinated water, there is still plenty of chlorine around to kill them, but if there are many, many cells in the same amount of water, it will not be as effective.