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Thread: What is the process that chlorine kills algae/pathogens?

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    jrs_diesel's Avatar
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    Question What is the process that chlorine kills algae/pathogens?

    I am curious as to how chlorine kills algae/bacteria/viruses. What does it actually do to the cells?

    And a second question to go along with the first, if my water utility is using chloramine as a sanitizer, how does that process work?

    Something I've always been curious about.
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    JasonLion's Avatar
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    Re: What is the process that chlorine kills algae/pathogens?

    Roughly speaking, chlorine steals electrons, which tends to destabilize bonds in various organic molecules, essentially it tears the molecule apart. However, last I looked no one really knows exactly how chlorine kill bacteria in detail. The part they are clear on is that cell membranes become much more permeable in the presence of chlorine, leading to cell death. Regardless the basic "tears it apart" principal kind of covers it.

    Roughly speaking again, chloramine is essentially the same, except it carries around a nitrogen that is (mostly) along for the ride. The extra nitrogen changes some of the other chemical properties in ways that are sometimes desirable to water systems, without interfering with the sanitizing/oxidizing aspects too much.
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    Re: What is the process that chlorine kills algae/pathogens?

    Adding to the Chloramine part of your question, they create Chloramines by injecting a precise amount of Ammonia to combine with the Chlorine to create the Chloramine. It's an aside, but some forms of life are particularly susceptible to it such as fish. However it functions, Chloramines are very potent in terms of sanitizing drinking water systems.
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    Re: What is the process that chlorine kills algae/pathogens?

    Quote Originally Posted by Patrick_B View Post
    Adding to the Chloramine part of your question, they create Chloramines by injecting a precise amount of Ammonia to combine with the Chlorine to create the Chloramine. It's an aside, but some forms of life are particularly susceptible to it such as fish. However it functions, Chloramines are very potent in terms of sanitizing drinking water systems.
    yea, our local utility just switched to chlormine. At least the notified us so I knew to treat the water different,y for my fish tank.
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    Patrick_B's Avatar
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    Re: What is the process that chlorine kills algae/pathogens?

    Yep, I remember when you had to start using a different scavenger to take care of that when I had fish. Lethal to fishes.
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    Re: What is the process that chlorine kills algae/pathogens?

    For more details about how bleach (hypochlorous acid) kills bacteria, see this post. It refers to an article and scientific paper where the mechanisms are described.

    What Jason described with respect to disruption of the cellular wall to create leakage is how common algaecides work (e.g. quaternary ammonium compounds including Polyquat) and more details for that are in this post. Hypochlorous acid, however, looks very much like a water molecule so tends to get passed through cell membranes and does most of its damage inside the cell, mostly reacting with proteins that have amine (nitrogen) or sulfur groups and this causes the proteins to unfold and then aggregate. Cells need proteins to function properly to survive so disruption of proteins prevents cell reproduction and the cell essentially dies.

    There is also a much slower attack on cell walls by hypochlorous acid but such attack shows up mostly for pathogens with thicker cell walls including protozoan oocysts that take far longer to inactivate. The chlorine reaction is much slower because such cell walls are composed mostly of sugars that do not contain nitrogen or sulfur groups subject to faster attack by chlorine (the peptidoglycan, lipopolysaccharaides, lipoproteins, and chitin have their most common nitrogen component in an amide group that reacts very slowly with chlorine). Also, for protozoan oocysts, they do not pass much water through their shells and therefore not much chlorine. Such cysts are activated after ingestion by the low pH in the stomach followed by pancreatic proteases in the small intestine (see this paper for more info).

    Many water utility districts have switched the chemical used as the residual in the mains from chlorine to monochloramine. The reason is two-fold. First is that they wanted to reduce the amount of disinfection by-products. Second is that they wanted the disinfectant residual to last longer. Monochloramine reacts more slowly (so lasts longer) than chlorine and does not produce as many disinfection by-products. It will kill many bacteria faster than they can grow, but it will not generally kill them fast enough to prevent person-to-person transmission of disease nor will it inactivate viruses or protozoan oocysts that may be introduced by bathers. What's good for drinking water is not so good for swimming pools.
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    Re: What is the process that chlorine kills algae/pathogens?

    Great Post chemgeek, thank you!
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