Focus on membrane technology
Anthony Bennett reviews the threat of pathogens to the public through water supplies and explores integrity testing protection for membrane systems in potable water applications
A number of pathogens exist in water sources and examples of protozoa, bacteria and viruses can be seen in Table 1. Cryptosporidium (Crypto.) protozoa are single celled parasites that live in the intestines of animals and humans. They have a complex life cycle, spreading during the dormant inactive phase, where they exist as oocysts of 4-7µm diameter. Oocysts are excreted in faeces and can survive under a range of conditions. Giardia cysts spread in a similar manner but can be larger at 6-16µm diameter.
The presence of Crypto. oocysts and Giardia cysts in water supplies can cause Giardiasis and Cryptosporidiosis. Some infections can show no symptoms, while others can cause moderate intermittent diarrhoea. Giardiasis can be treated with antibiotics but currently there is no effective treatment for Cryptosporidiosis. Prolonged chronic infections may develop in high risk patients, especially babies and people with lowered immune system responses.
There was a major outbreak of Cryptosporidiosis in Milwaukee, US, in 1993, which affected more than 400,000 people. Outbreaks such as this usually occur in drinking water derived from surface water sources. The difficulty arises in surface water supplies where this contamination is generally associated with a high rainfall event, such as spring run-off, when oocysts and cyst concentrations can be extremely high. The efficiency of pathogen removal is generally measured in terms of the log removal value (LRV). A LRV of four corresponds to 99.99% removal, based on the specified level being present in the feed. The LRV is defined as: