Todd Stong email on using chlorine for water purification
This is a one page guide for using chlorine in water purification. Here in Mexico I find the use of a $5 testing kit that may be bought at any pool supply to be the final word.
In this paper I suggest a 2 PPM dosage of raw water, assuming it is water similar to that that comes in the city pipelines. If the water is not that clean and is cloudy, it could take more. The key is that you would like to see a residual level of about 0.5 PPM (parts per million) when the water comes to your faucet. No one really knows what that residual may be until you make a test. That is the crud in the water or the pipe lines could consume a lot or a little from the trial addition of 2 PPM. Thus, with one of these low cost chlorine testers you can tell in a minute what you have.
Where I live the owner places one concentrated chlorine tablet (3 inch diameter) into the water storage tank each 2 weeks. I not with my tester that seems to keep some very low residual in the water, perhaps 0.1 - 0.2 PPM. Thus, I have no qualms over drinking the city water here, but we still actually use the bottled water since it is low in cost and has a good taste.
If there is a taste of chlorine that is not desired one may let the water sit in the refrigerator over night and all the chlorine should be gone into the air. Alternatively, one can pour water back and forth between 2 pitchers about 10 times and the chlorine will also outgas.
Chlorine will kill 100% of the bacteria and viruses.
Cysts, which are microbes encased in a hard shell take long contact time with chlorine, often over 30 minutes to get the Giardia and seldom successful on the cryptosporidium.
Accordingly surface water should be filtered. The slow sand filter will do the job with 24 inches of sand, but who is going to carry that on the trail! In the case of the typical under the sink filters, the best such filters are good to 3 microns, the giardia will be filtered out since it is typically 5-20 microns in size. But the under the sink filters, regardless of cost will not filter out the cryptosporidium since the are in the 1-5 micron range.
Back packers will most often carry a pump filter with may use ceramic elements that may filter down to 0.5 microns and thus take out the crypto. These filters are good only for a fill 10s of liters, thus not practical for home use.
The everyday USA city water plant takes care of the bacteria and viruses with chlorine, and takes care of the giardia with its sand filters. But every plant in the nation lasses on some level of cyptosporidium.
When the level are high one is advised to boil the water.
For the normal person in the USA the presence of cryptosporidium in the water is handles just fine by the body. The problem is when crypto is taken in by immune deficient persons (AIDS, chemo therapy and organ transplant patients). There bodies are most often unable to fight the crytpto. In the case of AIDs patients the mortality is often near 70%.
What the author of the article does not know is that you can kill crypto with a special concentration of iodine for a certain contact time. I was an advisor last year to a firm in FL developing in home treatment systems with this technology. It is yet to reach the market.
In sum most people have no problem handling city water with crypto in it.
Hikers in the mountains should be just fine with a 3 micro filter to get out the giardia. They should nevertheless use chlorine or iodine to kill the bacteria and virus.
In general no filters take out all viruses, that is you need the chlorine or iodine, or the natural biological action of a slow sand filter, where by the surface layer on the sand eats these viruses to do the job.
Recall a gallon of Chlorox will treat 100,000 liter of water for bacteria and virus!
If you are an immune deficient patient it may be best to drink distilled water or water from a reverse osmosis device and then add back minerals with pills, or take your water from a mature operating slow sand filter, or look for this new device I was working on in FL.