Carcinogenic substances are the greatest concern with tap water quality in Japan. However, it is important to note that the water system in Japan has improved greatly in the area of disease prevention. In order for the water system to supply safe drinking water, disinfection is the most important requirement to be considered, as imperfect disinfection can cause an epidemic outbreak of an infectious disease.
2. Legal response to disinfection in Japan
In Japan chlorination is required by law. The maintenance of 0.1mg/l of free chlorine concentration or 0.4mg/l of combined chlorine is required. This is the minimum level required to prevent dysentery bacillus and cholera, which are known as bacterial pathogens. Bacterial epidemics often breakout when this standard is not adhered to. The most significant problem with chlorine treatment is that it can produce harmful chlorinated organic compounds such as trihalomethane as a by-product. In addition, pathogens which are not inactivated by chlorination have become a subject of discussion. As regards the former, the use of chlorine dioxide as a supplementary measure has been permitted since 2000. The concentrations of chlorine dioxide and chlorite must be set at 0.6mg/L and below. Eventually chlorine feeding is required.
3. Recent challenge on disinfection
Pathogens which resist disinfection by chlorination must be considered. The following backgrounds are presented: 1. Outbreak caused by pathogens that have strong chlorine-resistance, such as cryptosporidium, 2. Outbreak caused by virus, 3. Anti-biological terrorism measures.
3 -1 Measures against cryptosporidium
In 1996, 8812 cases of cryptosporidium infection were identified in Ogose, Saitama prefecture, whose population was 13800 people at the time. Measures against cryptosporidium were taken. Cryptosporidium cannot be inactivated by conventional chlorination because 99% of CT values (disinfection concentration x contact time) responding to chlorine of this protozoa are 7200mg･min/L. Therefore, the treatment standard was changed. Turbidity of filtrated water of the possible contaminated water was established at less than 0.1 degree. Because of this, the use of separation membranes has increased at small-scale drinking water treatment plants. High costs surrounding equipment and maintenance/operation have lead to the use of ultraviolet exposure methods at these small-scale plants. However, the maintenance of the residual chlorine is conducted as usual. Extremely small amounts of １mJ/cm2 ultraviolet radiation makes inactivation at 2log possible. The effect is not influenced by water temperature. The maintenance is easy because it can be manipulated just by ON-OFF of electric power supply. Although it is feasible for simple disinfection, the addition of some additional treatment such as chlorine is needed after irradiance. Japan’s renewed ultraviolet application conditions for cryptosporidium are indicated as follows: The irradiance level to raw water such as groundwater where indicator fungi have been detected is over 10mJ/cm2 and turbidity is less than 2 degrees. An ultraviolet strength meter and a turbidity meter with constant measurements must be installed.
In Japan, waterborne viruses have not been a consideration of much recent concern, but, in 2006, 29 cases of diarrhea were caused by norovirus in the small water supply system of Nitsuichou, Akita prefecture. Most viruses have a weaker chlorine-resistance than cryptosporidium and generally show approximately one digit higher resistance in CT values than bacteria. Therefore, with virus contamination, a slightly higher amount of chlorine feeding or prolonged contact time must be considered. In Japan ozone is used for advanced processes like deodorizing. However, ozone creates high side effects. Therefore, at the sites where advanced processes are conducted, viruses must be neutralized but ozone infusion techniques cannot be used.
3-3 Anti-microbiological terrorism measure
In the case of microbiological terrorism, anti-disinfection pathogens are highly likely to be used. Japan’s current manipulation for disinfection alone may be unable to respond to a bio-terrorist attack. Additionally some pathogens are present in which safety is not ensured just by raising the chlorine concentration. Therefore, the following integrated countermeasures are needed to be considered: high concentrated disinfection infusion, ozone infusion, and the introduction of membrane filtration as well as the prevention of inputting pathogens into the water supply system, the inspection system, and the surveillance system using fish. In Japan various measures are currently taken based on the present conditions of drinking water treatment plants.