Between 1974 and 1996, over 160 waterborne disease outbreaks involving some 8000 people were reported in Canada. It is estimated that only 10 per cent of all waterborne outbreaks occurring in Canada are ever reported. Disinfection of Water Depending on the source of the water, conditions of use, and magnitude and extent of microbiological contamination, disinfection may be needed occasionally over short periods of time or on a continuous basis. Unscented household bleach, which contains four to five per cent sodium hypochlorite, will disinfect water when at least two drops are added to each litre of water and the water is left to stand for 30 minutes. Water purification tablets that release iodine or chlorine are especially useful for travellers, when used according to manufacturers' directions.
Protozoan cysts are often present in surface waters.
Because cysts are more resistant than bacteria and viruses, iodine and chlorine should not be relied upon to inactivate them. When water must be continuously disinfected because of the unacceptable quality of the supply, the possibility of sporadic contamination or the presence of cysts, a water treatment device incorporating filtration and disinfection should be used rather than short-term disinfection methods. Water Treatment Devices Water treatment devices can be divided into two groups, according to function. There are several types of devices within these two groups, each suited to a specific water quality problem. Water treatment devices that disinfect water will be discussed here.