Environmental Science & Engineering - www.esemag.com - November 2005
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Controlling Legionella with chlorine dioxide

By Thandar Aye

Legio Zon ClO₂ Generator

Legionella bacteria have been known to be the cause of Legionnaires disease and Pontiac fever. Legionella are aerobic bacteria that can take up different forms depending upon the conditions of growth. The infectivity of Legionella is linked predominantly with the L. pneumophila and causes approximately 90% of pneumonia attributed to legionellae. Legionellae are gram-negative rods that stain faintly with the standard Gram stain. They generally appear as small coccobacilli in infected tissues or secretions.

A recent Legionella outbreak in Toronto has caused 21 deaths, and infected many people. Various disinfection methods for Legionella have been discussed. The Legio Zon® chlorine dioxide system from ProMinent® is mainly targeted at the effective destruction of Legionella.

Environmental factors Although traces of Legionella are present in many water services, water can be considered safe if the concentration is at a minimum. Temperature and availability of nutrients are the important factors. Legionella can grow best in water at a temperature between 30 and 37ºC, such as hot tubs, hot water tanks, air conditioners and water-cooling towers. Biofilm present in water pipework and tanks provides optimal growth area for the bacteria.

Within the normal laboratory timescale of days, experiments with cultures of naturally occurring L. pneumophila serogroup 1 showed that over the temperature range 5-45ºC the organism would only multiply in the temperature range 32-42ºC in tap water. In nutrient enriched areas the population of Legionella can double every 3-4 hours. Other Legionella have been shown to be capable of colonizing zones at temperatures up to 63ºC and down to 6ºC (Fliermans et al., 1981).

At lower temperatures the bacteria become dormant but remain viable for months. Once favorable conditions return then these cells restart activity and multiply. Higher temperatures over 45ºC will eventually kill the organisms.

The Legionella infection in humans usually occurs in the respiratory tract, when people breathe in mist or vapour that has been contaminated with the bacteria, and pathologic changes occur primarily in the lung. Despite airborne transmission of the organism, personto- person spread does not occur.

Prevention and treatment Treatment of Legionella involves destruction of the biofilm where bacteria growth is optimal in water distribution systems. Improved design and maintenance of cooling towers and plumbing systems is an important factor to limit the growth and spread of Legionella organisms.

Preventive measures for controlling Legionella include disinfection with various types of disinfectants such as chlorine, chlorine dioxide, ozone, and UV-radiation.

Chlorine dioxide generation technologies Chlorine dioxide can be obtained by either oxidizing chlorite or reducing chlorate. Most commercial generators use sodium chlorite (NaClO₂) as the common precursor to generate ClO₂ from drinking water applications. ProMinent’s Legio Zon uses the direct acid system to produce ClO₂.

Many papers have shown the successful disinfection of Legionella bacteria with chlorine dioxide. Botzenhart et al showed that L.pneumophila was susceptible to chlorine dioxide at higher temperatures and at higher pH. However, the pH effect on inactivation is less pronounced than observed with chlorine. Studies suggested that a complete removal of Legionella in a hot water system at hospitals with a high concentration of chlorine dioxide, 50- 80 mg/L, as a shock treatment for a period of time followed by a residual maintaining at 3-5 mg/L, dramatically reduces the biofilm formation (Walker et al.).

Chlorine dioxide generator Legio Zon, with a capacity up to 5g/hr, is designed for potable water treatment, particularly to combat Legionella. The system works according to the hydrochloric acid/chlorite process. By combining diluted hydrochloric acid, diluted sodium chlorite solution and water, this system first generates a 0.2% chlorine dioxide solution (2g/L). This solution is temporarily stored in the system and supplied to the water to be treated as required.

Functional description Two pumps dose the components Bello Zon® acid and Bello Zon® chlorite into the reactor which is prefilled with a defined amount of water. After a specific reaction time, the controllor empties the content of the reactor into the storage tank. The controllor feeds the required amount of dilution water to the reactor and mixes it with a precise calculated amount of hydrochloric acid and sodium chlorite which is dosed through two dosing pumps. A chlorine dioxide solution with a concentration of 2g/L is generated. This low concentration as compared to other procedures makes the system absolutely safe to operate.

The finished ClO₂ solution is transferred to a storage tank from which it is dosed through the third dosing pump. A preset concentration of chlorine dioxide, controlled via the signal from a water meter, is dosed to the water to be disinfected.

Thandar Aye is an Applications Specialist with ProMinent Fluid Controls

E mail: thandara@prominent.ca

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