ET
Jun 7, 2003, 2:13 PM
Post #25 of 29
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The idea of randomly dumping some quantity of bleach into a drinking water supply, let alone chucking in a toilet mint ("....one of those chlorine tablets intended for use in toilet tanks....") makes me twitch. I'm repeating the mantra of a coworker ("people do all kinds of weird things to their bodies") over and over again and looking for something healthy like a half-dozen Krispy Kremes or some nice black tar heroin to chill with.
Chlorine is a two-edged sword. As a disinfectant, it's extremely effective against a wide range of pathogenic organisms (organisms which are capable of causing diseases) and is very cheap. On the downside, it's quite toxic, rapidly inactivates when it comes into contact with organic (carbon-based) materials, can react with these same organic materials to form potentially carcinogenic organochlorine compounds such as chloroform (CHCl3), can impart an unpleasant taste and odor to drinking water, and can damage certain materials such as stainless steel and some elastomers. What this translates to is that you want to use doses that will create the appropriate concentration of chlorine in your water supply, but neither overshoot nor underrun significantly.
With this in mind there's two distinct and different uses of chlorine in a potable water system, which require very different concentrations of chlorine in water. These applications are shock chlorination, used to disinfect the components of a potable water system, and routine disinfection used as a public health safeguard or barrier to reduce the possibility that a pathogenic organism can survive in the water stream itself.
When you properly shock chlorinate a potable water system, the water used during the process in rendered unusable for drinking and other "domestic" applications. The chlorine concentrations that remain at the end of a successful shock chlorination sequence are far too high to be acceptable or pleasant for routine consumption and the reaction products if present can be anywhere from distasteful and malodorous to carcinogenic.
When you shock chlorinate a system, you normally introduce enough chlorine concentrate (depending on the size of the system anything from consumer-concentration (5.25%) sodium hypochlorite bleach, to chlorinated lime (CaOCl2), to high-test calcium hypochlorite (Ca(OCl)2, aka swimming pool chlorine) to create a chlorine concentration of somewhere between 100 and 200 parts per million (PPM). At these concentrations there's typically enough chlorine present to survive the inactivation by organic materials and destroy algae colonies and biofilms (slime) which harbor all kinds of microorganisms. To verify that sufficient chlorine was present to effectively disinfect the system in hardcore commercial applications you'd test residual chlorine levels (typically using a field colormetric technique) at the end of the disinfection sequence to verify that all of the chlorine wasn't gobbled up by the organics present.
The typical shock chlorination sequence runs something like:
- you remove vulnerable components from the path of flow (carbon filters, elastomer lined pressure tanks, etc.).
- you verify that they system is isolated from the municipal water system (it should be through vacuum breakers anyways, but in reality....).
- you introduce the chlorine concentrate at the wellhead, tank, or other point of delivery.
- you run the water at various points in the system so that the highly chlorinated water is distributed.
- you let the highly chlorinated solution sit for several hours (in contrast to the 15 minutes somebody recommended earlier in the thread, the shortest hold time I've seen is 2 hours, 6 is more common, and for domestic applications I've seen "overnight" as a common recommendation).
- (optional) you test the system to verify that elevated chlorine concentrations are still present.
- you thoroughly flush the system, dumping the water into the municipal sewer system or somewhere where the remaining chlorine won't kill anything. Generally you'd use flow time, combined with a sniff test to verify that the system has been properly purged, in sensitive applications you might use a field test to verify chlorine concentrations are down to levels typical of the supply stream.
In contrast to shock chlorination, municipal water system operators normally add very low concentrations of chlorine to their drinking water at the end of their processing sequence before feeding it into the distribution system, with the target of having somewhere between 0.5 and 1.5 PPM chlorine remaining in the water when it gets to the consumer's tap. These concentrations will not sterilize pipes or plumbing, but rather will kill any stray microorganisms that either remain in the water itself following processing at the treatment facility, or are introduced into the flow downstream because of minor cases cross-contamination.
Maintaining the low concentration of chlorine in the drinking water supply is a fine art, particularly considering the large volumes of water being processed and that it's a continuous flow rather than batch feed system. Rather than a straight dilution calculation, parameters such as temperature, pH, mineral content, and condition of downstream pipes need to be taken account, and the variable length of plumbing between the treatment plant and various end users drives operators nuts, as a half-PPM extra chlorine will have people complaining about the terrible taste and odor of their water, and no chlorine at the point of use raises the risk of either actual or perceived public health problems. In the past several years many municipal water treatment facilities in the US have converted to a chlorine compound called chloramine because it's less reactive and consequently has more predictable behavior in water supply systems (it also reduces the already trace formation of the potentially carcinogen halomethane compounds).
There's no way that either periodically adding a slug of bleach or floating a cake of pool chlorine in a water storage tank will simulate the process of routine chlorination used in a water treatment plant. I wouldn't be at all surprised if it's actually a case of the water supply coming into your home system is perfectly fine and free of biological contaminants and that the addition of the chloro is simply a case of appeasing the gods. I also mildly wonder if this random dumping in of a bottle of bleach actually is causing some of the intestinal maladies attributed to the water supply or food contamination.
Time to go skin pop....
PS - Whoever was tossing the toilet cake into their tank should check and verify that that really is just a chlorine source. At least the thing I think of as tank mints are actually paradichlorobenzene (moth flake) cakes impregnated with various color and odor agents. PDCB really isn't something I want in my personal water supply.
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