Ozone treatment ineffective against water parasite

Ozone treatment is unlikely to kill cryptosporidium cysts in water supplies, research in the USA and UK has shown. The findings will come as a shock to the water industry which has regarded the chemical as the key to removing the chlorine-resistant organism from drinking water.

Cryptosporidiosis has been a problem for the water companies since 1989, when the water supply to the Swindon and Oxford area was suspected of causing hundreds of cases of the diarrhoeal disease cryptosporidiosis (ENDS Report 176, 12-14). There have been several outbreaks since then, the most recent being in Hull (ENDS Report 182, pp 7-8). Thames Water and Yorkshire Water face civil liability actions relating to the Swindon and Hull incidents.

Chlorination is ineffective in destroying the organism, and the industry has been searching for an alternative. Ozone has been regarded as the most likely solution.

But research by Severn Trent Water has confirmed preliminary US findings that ozone doses used in Britain are too small to inactivate the organism at typical water temperatures. The findings were announced in a paper given by Severn Trent's senior microbiologist David Dawson at a Society of the Chemical Industry seminar in May.

Severn Trent's data suggest that to achieve a 99% inactivation of cysts at 10° C would require an ozone contact time of 18-62mg min/l. The typical dosage used in UK water treatment is 6-12mg min/l. As the temperature falls, the dose required to achieve the same effect rises to 47-128mg min/l at 5° C. Typically, water temperatures in the UK are in the range 5-15° C.

Water companies will therefore continue to rely mainly on conventional coagulation and filtration to remove cryptosporidium from supplies. Assuming that these remove cysts as efficiently as other particles of a similar size, Severn Trent believes that conventional treatment can achieve between 99.9% and 99.99% removal.

Monitoring of the river Severn over nine months showed that cyst levels varied from undetectable to two per litre. Expected levels of cysts in drinking water from the river vary from zero to one per 2,500 litres, the data suggest. Severn Trent does not regard this as a significant risk to health. Cryptosporidiosis outbreaks are most likely to occur, it believes, when treatment works are presented with high concentrations of cysts in river water - an event which its own monitoring has failed to detect.

The 1990 report of the official Badenoch committee did not recommend routine monitoring of raw waters for cysts. This may have been because detecting the organism is difficult and expensive, and few laboratories at the time were able to do it.

Severn Trent has now instituted a policy of testing raw surface waters weekly, and treated surface waters monthly. Groundwaters are tested at least once a month.

The company believes that this programme will alert it to high levels of cysts in raw waters and enable it to take additional precautions. These include not recycling filter backwash waters and running filtered water to waste while the filter ripens.

Water from both sources is likely to contain high concentrations of cysts when levels in the raw water are elevated. However, it is not standard practice to discard these waters as they may comprise up to 10% of the works' throughput.

Routine monitoring for cryptosporidium is becoming standard practice in the industry. It enabled Southern Water to issue a boil notice to customers on the Isle of Wight in April after cryptosporidium was found in a service reservoir. About 30,000 people were affected for five days. The company has a policy of testing raw and treated surface waters for cryptosporidium every month, or every week during spring and autumn high-risk periods.

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