Having been a small municipal water system operator in Upstate New York, I have some experience with basic water treatment. The link provided in ” Chris in West Virginia’s” article is sound in regard to using Sodium dichloro-s-triazinetrione. One would want to use a test kit to measure residual chlorine in the water and maintain the level between 0.3 and 1ppm after initial treatment. To treat water, chlorine is added until the level is at least 0.5ppm after an one hour contact time. It is critical that the chlorine have time to interact with the water and some method of stirring the water during treatment must be employed. Once the water has had time to interact with the chlorine, there will be levels of combined chlorine in the form of chlorides and “free” residual chlorine. Presence of residual chlorine indicates that the water is saturated enough with chlorine that any microbes/contaminants present will continue to be oxidized.
Having clean, filtered water to work with is important as bacteria and other nasties can adhere to microscopic particles in the water making chlorine treatment difficult. There is not a particular specification for the size of particles; however, a bit of research into water treatment will reveal that municipal authorities typically use a flocculant to cause microscopic particles to congeal and sink to the bottom of a clarification basin. They use this method to quickly clarify water. In a survival situation, a good gravity filter for dirty water could be employed prior to chlorination to ensure that most, if not all contaminants are removed from the water. After treatment, the water should be kept still and siphoned from the containing vessel to ensure that any remaining contaminants settle to the bottom of the container. Also, the chlorine level of the water should be maintained at all times to ensure continuous protection.
These methods of water treatment are for surface water as well water typically does not support bacteria growth if the well is in continuous use; however, during a survival situation, treatment of well water is recommended. The need to filter well water prior to treatment is not as important unless debris is drawn up from the well or the well is open to surface contamination.
It is worth mentioning that it is very difficult to remove giardia cysts from contaminated water. To be sure that these parasites are removed, the water must be boiled to kill the cysts or filtered to less than 1micron to remove the cysts. Chlorine does not have any significant effect on giardia cysts. Giardia can be present in the fecal matter of dogs, cats, beavers, cows, and sheep. Infection with giardia causes “beaver fever” in humans.
Also, one should note that ingesting water with chlorine levels above 4ppm can do damage to the digestive system up to the point of death depending on the level of chlorine ingested; therefore it is absolutely critical to be able to test the level of chlorine present in the water before drinking!!
Anyone curious about studying water treatment more in depth can visit www.usabluebook.com  for some very good self-training and reference materials on all levels of water treatment.
Regards, – Drew in Thailand
I have only just rediscovered your blog last night and I am now soaking it up like a sponge.
I’d like to be the “someone with a chemistry degree” to respond to our brother in Christ, Chris’ information regarding use of pool chemicals for drinking water treatment, notably Sodium dichloro-s-triazinetrione. I am a degree qualified chemical engineer who has spent most of the past 15 years in selling industrial chemicals including for water treatment. One of my recent activities was packaging pool and spa chemicals.
It is particularly effective as a water treatment chemical for sterilizing purposes due to its high effective chlorine % (typically >50%) and ease of use due to its powdered or tablet form; when compared with other chlorine sterilizing agents such as sodium or calcium hypochlorite. The other advantage of the powdered isocyanuric acid is that it remains stable over long periods of time provided that the powder is not exposed to moisture or excessive heat. The liquid hypochlorite solutions will lose their activity over any long period of time even when stored in closed drums simply by converting back to chlorine gas and caustic soda. Powdered calcium hypochlorite does not have this same problem provided it is also protected from heat, light and moisture. Liquid sodium hypochlorite (e.g. Clorox) has only 5.7% available chlorine, so more is required per gallon of water treated.
The toxicity of the isocyanuric acid is not 100% known though it is generally thought to be of low toxicity. This link  gives details of toxicity studies completed to date .
Having packaged the product in its finely powdered form, one thing is clear – do not breath the dust as it will cause all kinds of acute (short term, intense) respiratory (breathing) problems. It literally feels like the air is being sucked out of your lungs.
There’s no doubt that the isocyanuric acid is more effective compared to hypochlorite in terms of gallons of water treated per pound of chemical used. I also know that the isocyanuric acid is way more expensive per pound to purchase – at least at the wholesale level. My preference would be to stick with hypochlorite since it’s a little safer to handle, more readily available (not every town has a pool and spa supply but almost every town has a supplier of Clorox brand bleach) and more well researched in terms of toxicity. I’m also concerned whenever I see chemical compounds that have “cyan” and “uric” as part of the chemical structure. Under the right conditions this chemical could break down to form hydrogen cyanide (HCN) as a decomposition product – toxic to humans as well as microbes in water.
It should also be noted that any individual or entity manufacturing or repackaging chemicals for sterilization or disinfection purposes must be registered with the [US] Federal EPA  as a pesticide manufacturer. Retail pool and spa outlets would be exempt from this requirement as they are retailing products that should already comply with this. Consumers should inspect all packaging to look for a federal EPA registration number for any product that has a claim of disinfection. My bottle of Clorox clearly shows the ingredients and also the EPA Reg No 5813-50.
Your readers should also consider hydrogen peroxide as a disinfectant, since its decomposition products are water and oxygen, though it also has problems with long term storage as it will decompose when exposed to moisture, heat and light. Other methods for disinfecting water include chlorine dioxide and ozone (small equipment systems can be purchased to make these and directly inject into the water being treated). Ozone injection is often used in bottled water filling operations for rinsing bottles and also for the water itself, as it decomposes back into oxygen in a few hours and adds no taste to the water once the ozone has depleted. Ozone can’t be stored and you need electricity (probably > 1,500 watts) to run an ozone generator.
As a general rule, I always prefer from a survival perspective to look at disinfection techniques that can be done using physical processes, rather than chemical processes. Filtration (Berkey and activated carbon), boiling and other physical processes can be used to treat water for drinking purposes and use of purification tablets as an additional safety precaution. No chemical disinfection process has shown itself to be 100% effective against all microbes and your readers should consider multiple processes for water treatment prior to use of chemicals.
As another rule of thumb, be careful when buying disinfecting chemicals from pool and spa outlets. The label ingredients are only required to declare the active ingredients that are responsible for the disinfection. You will often see “inert ingredients” listed on the label, sometimes not even these are listed. Typically in powdered and liquid pool chemicals there may be other ingredients included (such as anti-caking agents, stabilizers, surfactants) many of which you don’t want in your drinking water but are included in pool chemicals to enhance the performance in its intended application.
Sincerely, – Graham T.