|【Cryptosporidium control in drinking water】|
Anan National College of Technology
An outbreak of cryptosporidiosis occurred in Ogose Town, Saitama Prefecture, Japan, in June 1996. The number of patients with Cryptosporidiosis exceeded 8,700 individuals and most people of the town were diseased. Subsequently, it was revealed that cryptosporidiosis was caused by public water contaminated with Cryptosporidium parvum. Group infections from public water contaminated with this Cryptosporidium parvum broke out in England and the US many times in the early 1980-90s. In Japan the first incident of infection caused by contamination of the water receiving tank in a small water supply system occurred in Hirazuka City in 1994. This was the second recorded incident. The group infection that occurred in Ogose Town was a significant incident that damaged public confidence of water supply. It led to a fundamental reconsideration of two defensive measures， “physical removal” and “chlorine disinfection”, which have greatly contributed to the decrease in waterborne diseases resulting from pathogenic microbes in the water supply. Cryptosporidium parvum is an enteric protozoon which belongs to Sporozoa. Cryptosporidium is parasitic in human and various animals’ digestive tracts, and causes diarrhea. There are reports that C. hominis, C. parvum, C. meleagridis, which belong to the genus Cryptosporidium, can cause infection in humans. Cryptosporidium is host specific from the range of infection in only humans, infection in only animals such as cows and pigs and infections in both humans and animals. Therefore, livestock and human wastewater can also be a source of water pollution. When Cryptosporidium is excreted from the intestinal tract of an infected animal into the environment, an infection type of oocyst with approximately 4-6μm diameter is formed. The oocyst has an impregnable oocyst wall and resists various stresses and chlorine disinfection. The number of oocysts excreted in acutely-ill patient’s stool is more than 107/g.
What is Cryptosporidium?
Removal by chlorine-resistance and rapid filtration
In the group infections that occurred in Ogose Town, Europe, and the United States, drinking water treated with the conventional chlorine disinfection is the source of infection. This indicates that the normal level chlorination is not adequate as an effective measure against cryptosporidium. In the assessment of the inactivation using the mouse bioassay after exposing oocysts to high chloride concentration (80mg/L), it was reported that 7200mg･min/L at CT values for 2log inactivation was an impractical value for running water supplying drinking water (Korich et al., 1990). On the other hand, there was a report that CT values for 2log inactivation resulting from a long exposure experiment with low concentration of practical level (1.0mg/L) were 1600mg･min/L (Shimura et al., 2001). Considering the detention time in clean water reservoirs and water distribution reservoirs, this value shows that the inactivation of as much as 1log is possible in the current system. However, this level is not enough to prevent group infections. The examination in a large scale of adequately controlled water treatment plants indicated that the removal ratio by using rapid filtration, which is the conventional water purification method, is approximately 3log (Hashimoto et al., 2000). In case of raw water contaminated with acutely-ill patient’s stool that contains oocysts, there is yet no proven remedy.
Cryptosporidium control in drinking water
After the outbreak of cryptosporidiosis occurred in Ogose Town in 1995, the old Ministry of Health and Welfare drafted a “Guideline for Cryptosporidium control in drinking water”, which was a tentative guideline, and each waterworks undertaker implemented various measures. The tentative guideline states that “Contaminated levels of water source” is evaluated by Cryptosporidium or detecting anaerobic sporeforme, an indicator for high residual fecal contamination. Additionally, when contamination is likely to occur, the guideline requires the introduction of physical water purification processes such as rapid/slow filtration and membrane filtration, and reduces the control of turbidity for purified water to 1.9 degrees, 0.1 degree down from the 2 degree water quality standard for drinking water. Because detecting contamination of Cryptosporidium, which is likely to be present with extremely low-concentration, during water purification is not practical, securing safety of water based on control of water sources and raw water and control of water purification process is required. In March 2007, the tentative guideline was amended as the “Guideline for Cryptosporidium control in drinking water” and further measures were included. The control of turbidity followed this tentative guideline: “Opportunity” for water source to be contaminated by Cryptosporidium was classified into four levels, from “Level 1”, low possibility of contamination, to “Level 4”, highest possibility of contamination, by conditions for inspection of indicator fungi (anaerobic sporeformer and coli bacterium) and conditions for water resources (surface water, confined groundwater and etc.). The guideline requests implementations such as frequency of detecting Cryptosporidium and an indicator fungus, and physical treatment such as filtration for each level. This guideline allows the use of ultraviolet treatment that is effective in the inactivation of Cryptosporidium as an alternative to the filtration process, in case raw water is not surface water. This guideline led to the first ever use of ultraviolet treatment as disinfection that complements chlorine disinfection in waterworks. Many waterworks undertakers that supply water by only using chlorine disinfection without physical treatment like filtration, when raw water retains high water quality, are minor waterworks or small water supply undertakers. It is not easy for these facilities to introduce and maintain physical treatments such as rapid filtration and membrane filtration because they need a large amount of capital, even if measures against Cryptosporidium are needed. From a financial perspective, the option of ultraviolet treatment is worthy of special mention as well.