The Decision To Use UV For Municipal Water Disinfection In Busselton, Western Australia – A Case Study
Busselton Water is an independent water authority supplying water to domestic, commercial and light industrial consumers in the town of Busselton in Western Australia. The town’s population is approximately 25,000, but during weekends and holidays this can increase to 65,000 people. Annual water consumption is about 950 million gallons (3.6 Gigalitres) with an annual growth rate of 8%.
All the water is produced from artesian bores between 980-2600 feet (300-800 meters) deep drawing on two aquifers. The water is slightly alkaline and is produced at temperatures ranging from 79-100oF (26-38oC). The peak flow rate from each of the four plants operated by Busselton Water is currently about 60 gallons/second (225 liters/second).
Busselton’s water supply differs from most municipal sources in Australia because the deep aquifers offer high microbiological integrity and very low organic carbon loadings. Most other municipal water providers draw water from shallow production bores, open water bodies, or a combination of sources that contain particulates such as dispersive clays, plant material and algal debris.
The requirement for water disinfection
In the past, Busselton Water has not disinfected its water supplies and, historically, the water has been free from microorganisms. However, under the Australian Drinking Water Guidelines, set up by the National Health & Medical Research Council and the Agricultural & Resource Management Council of Australia & New Zealand, it was recommended that all water supplies should be disinfected.
The search for suitable disinfection technologies
To assist them in identifying the most suitable disinfection technology, Busselton Water commissioned local groundwater consultants Rockwater Pty Ltd to investigate the available methods and make recommendations for a suitable system. The objectives for the study were to determine a disinfection system that effectively met the Australian Drinking Water Guidelines. Rockwater found that none of the available disinfection methods it investigated met all of the requirements, so a compromise was necessary to obtain the optimum process for Busselton’s reticulation system.
The five most commonly used methods of disinfection – chlorination, chloramination, chlorine dioxide, ozonation and ultra-violet (UV) irradiation – were all investigated as they all offer good bactericidal activity.
The decision to use UV
UV was finally selected because it met all of the design criteria except for one – maintaining a persistent residual barrier. However, the water is maintained in a closed system following treatment, and the chance of contamination is minuscule given that in the past the water supply maintained compliance without any disinfection.
Chlorination achieves only one of the conditions in the 1996 guidelines: it maintains a residual barrier, but it breaks down rapidly. To maintain sufficient residual disinfection at the outer extremities of the system, therefore, it may have been necessary to exceed the maximum dose rate at the point of disinfection. Chlorination is also a less effective disinfectant than UV at the recommended dose rates. UV installation costs at Busselton were estimated to be about 2.2 times those for chlorination and chloramination disinfection systems, but UV was estimated to be about half the cost of chlorine dioxide systems. UV operational costs, however, were estimated to be only about 25% of those for chlorine.
The Guidelines state that the implementation of a disinfection system should be a consultative process involving the community. In a previous survey, residents of Busselton had strongly objected to a proposal to chlorinate the town’s water supply, so it was decided to adopt UV as the sole method of disinfection. To achieve a confident level of bacteria destruction, a minimum dose rate of 30 mWsec/cm2 was recommended.
UV system installation and testing
Following a successful 12 month trial of two single-lamp, medium pressure UV units, it was decided to install eight systems. Each of the UV systems contain a single, medium pressure UV lamp housed in a protective quartz sleeve. The lamps are polychromatic, producing a UV output at wavelengths between 200nm to 300nm, with a peak output between 240nm and 280nm, which has been shown to be wavelength range most effective against most bacteria. UV monitors detect the UV output of each lamp and variable power controls allow output to vary as flow rates increase or decrease. Automatic wipers keep the quarts sleeves surrounding the UV lamps free from organic fouling.
Water transmissivity is measured using a dedicated transmittance monitor – a patented device that continuously measures water transmittance and feeds the data to a control unit. For a high quality, stable groundwater source like Busselton’s, however, transmittance was been assumed to be constant, so a figure of 90% (when measured against double distilled water) was used. The UV dose is displayed in mWscm-2, or mJcm-2. The UV dose is displayed on a control panel which is capable of data-logging variables such as water flow, UV dose, UV intensity and wiper frequency; this record allows any faults to be date- and time-stamped.
Each UV system operates in duty/standby mode. The power to each lamp is continuously varied, ensuring that the specified UV dose is always delivered. Both systems may be operated at 50%, and in the event of failure a ‘hot’ standby is available immediately. The operating protocols are strictly fail-safe and will not under any circumstances allow untreated water forward. The control panel provides a broad range of signals to actuate valves, initiate pumping, and generally provide meaningful monitoring and control of the system.
Following the introduction of the Australian Drinking Water Guidelines, the Busselton Water Board in Western Australia was required to disinfect its municipal water supply. The water had not previously been disinfected because it originated from deep artesian aquifers and was microbiologically very pure. Following an extensive investigation of available disinfection technologies, UV was chosen as most appropriate method as it met all the design criteria except for one – providing a residual disinfection. This was not seen as a drawback, however, as Busselton’s water supply is maintained in a closed system after treatment, (the chance of contamination was also seen as minuscule seeing as previously the water supply had been compliant without the use of any disinfection). Following a 12 month trial, eight medium pressure units were installed to treat the entire Busselton network.