Technically speaking

Peter Minting looks at new research from the US on Cryptosporidium disinfection

If raw water supplies are close to freezing, Cryptosporidium oocysts have a very good chance of making it through the treatment process

If raw water supplies are close to freezing, Cryptosporidium oocysts have a very good chance of making it through the treatment process

Scientists in the US have discovered a way of improving Cryptosporidium disinfection. At many WTWs in the UK the final treatment stage is slow sand filtration followed by disinfection with chlorine, but unfortunately chlorine or ozone alone will not kill Crypto at low temperatures. Customers can claim compensation for undrinkable water and a big outbreak of cryptosporidiosis can cost water companies a lot of money.

Some water companies have installed ultrafiltration equipment to remove oocysts at high-risk sites, but this is an expensive option. At lower-risk sites where there is no legal requirement for 24hr monitoring or ultrafiltration, supp-liers can still be fined if the water contains Crypto and causes an outbreak.

There are many ways to improve a treatment process but scientists at Illinois University have shown that by fine-tuning the disinfection process, WTW operators could reduce the risk of outbreaks without installing extra filtration stages. The team, led by Jason Rennecker, has been studying sequential disinfection schemes for some time. To date the team has tried using ozone as the primary disinfectant, followed by monochloramine or free chlorine (derived from hypochlorous acid) as the secondary disinfectant. Synergistic effects have been found; firstly ozone pre-treatment cuts out the lag phase normally seen with monochloramine and free chlorine, making the process more effective. The secondary inactivation rate also increases as a result of pre-treatment.

It is almost impossible to kill off Crypto at low temperatures with just one dose of disinfectant. According to Rennecker: "The high CT requirements make free and combined chlorine ineffective" (CT = average disinfectant concentration x contact time). Ozone and chlorine dioxide disinfection is said to be more effective but if relied upon Rennecker claims: "Many existing ozone and chlorine dioxide disinfection systems will need upgrading." If ozone and chlorine dioxide are used most water companies also add free chlorine to act as a residual in the supply network, as the first two substances break down rapidly.

The effectiveness of ozone, chlorine dioxide and free chlorine as a sequential disinfection method is still uncertain, partly because the high cost of oocyst viability tests has hindered research. Rennecker's team has been using both established assessment methods, in-vitro excystation and the more expensive animal infectivity tests. Comparisons were made between each method to see if the in-vitro excystation technique could be used to reduce the cost of research. The in-vitro excystation method was found to overestimate infectivity when compared with animal infectivity data, but the difference (log-0.7) remained fairly constant with variations in the dose of disinfectant. So with careful experimental design and data analysis, research costs could probably be reduced.

In the first study, Rennecker's team found the synergistic effects of pre-treatment followed by secondary disinfection increased with decreasing temperature. The effects were found to be three or four times greater at 10°C than 30°C.

In a second study carried out between 1°C-20°C similar results were found. The inactivation rate with monochloramine after ozone pre-treatment was five times faster at 20°C than without pre-treatment and 22 times faster at 1°C. Although inactivation still decreases with decreasing temperature, the effect is much less dramatic with ozone pre-treatment and a suitable secondary disinfectant. Manageable amounts of disinfectant can then be applied which will ensure inactivation.

Using just monochloramine at 1°C the scientists found a CT of 64,600mg/min/l was necessary to inactivate Crypto by one hundred times (log-2). But with ozone pre-treatment, only 1,350mg/min/l was needed to achieve this level of disinfection. At 20°C the synergistic effect reduced the required dose from 11,400mg/min/l to 720mg/min/l.

Rennecker's team has put forward a suggestion as to why pre-treatment reduces the temperature dependence of the inactivation: "The low apparent activation energy observed for secondary monochloramine disinfection after ozone pre-treatment suggests the secondary inactivation step may be the result of permeation within oocyst wall layers with relatively lower reactivity. In contrast, in single-step disinfection and in the first step of a sequential process, inactivation may be a permeation process within more reactive oocyst wall layers." In other words, pre-treatment somehow increases the damage that is done to the organism by the secondary disinfectant.

The physiology of the process remains a mystery, but water companies should take note that: "The synergy levels observed present a potential solution for inactivation of Cryptosporidium oocysts during wintertime, when the water temperature approaches freezing point".

See: Jason Rennecker et al (2000): Water Research Vol. 34 (17) p4121-4130, and Amy Driedger et al (2001) Water Research Vol. 35 (1) p41-48.



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