Disinfection

Disinfection is defined as an integrated system of treatment processes that reliably reduces the population of viable pathogenic microorganisms to levels deemed to be safe by public health standards.

The use of chlorine and its compounds is the most common disinfection method in water supply systems in Germany. It is inexpensive, readily available in several forms, and effective against bacteria. Its effectiveness is easy to test by measuring the chlorine residue in a system. However, in a small system the time between adding chlorine and using water is so short that relatively high concentrations are required. Larger retention tanks can increase contact time before use and reduce required concentration. Research findings indicate that carcinogenic and mutagenic halogenated organic compounds (halomethanes) can actually be formed during chlorine disinfection when organic substances are present. With this discovery, activated carbon filtration or reverse osmosis units should become a part of all up-to-date home chlorination systems.

Small amounts of water can be disinfected by boiling for 15 minutes. However, the process is energy intensive and may even increase the concentration of other contaminants due to evaporation.

There are other methods of water disinfection. Most of them are still too complex or too expensive for home water supply. They are discussed here for a few reasons. These methods are effective and they are being constantly improved. With the development of new technology they may quickly become a good, feasible solution for water disinfection in individual water supplies. They include ultraviolet radiation, ozonation, iodination, and distillation.

Ultraviolet radiation, in order to be effective, must pass through the water in order to control the bacteria. The water therefore cannot have any turbidity or suspended particles. Ultraviolet radiation adds nothing to the water and does not produce any taste or odor. It is very effective on pathogens but not on protozoan cysts such as those responsible for giardiasis. Because of the possible presence of protozoan cysts, a 5-m filter must be added to the system. Ultraviolet radiation disinfection also requires a safety system, where a photoelectric cell activates an alarm system and/or stops the water pump if the ultraviolet radiation intensity is not sufficient for safe disinfection. The major problems with such a system are cost, fouling of the chamber, collection of sediment, and growth of algae. In the latest ultraviolet radiation systems, Teflon tubes are used instead of quartz tubes and seem to decrease these problems.

Ozone is a very strong oxidizing gas and is very effective in killing bacteria even with short exposure times. In water, ozone (0 ₃) breaks down to O ₂ and O ^- and combines with organisms and chemicals. It also does not leave any taste or residue, and is therefore very difficult to detect to determine its effectiveness because a residual amount of O ₃ is needed to assure disinfection. With new developments in electronic technology, detection of the short-lived residual ozone in the water may become economical in home water purification systems, but for the present it is not a practical solution.

Addition of iodine into drinking water is a relatively new approach to home water disinfection, though the technique has been around for years. It is very effective on a wide variety of bacteria and does not affect the water taste any more than chlorine. However, iodine is not readily available and the cost is relatively high. It is less reactive than chlorine and has less tendency to form halogenated organics. Physiological effects of prolonged use of iodine, especially on children, are unknown. However, in a newly developed system (the resin-sequestered iodine system) the iodine remains attached to the resin particles. It contacts the organisms in the water and kills them. It does not move beyond the filter or alter the taste of the disinfected water.