Seite - 77 - in Clean Water Using Solar and Wind - Outside the Power Grid

Water treatment 77 5.4.2 UV light disinfection One major advantage of using UV light in remote areas is that it does not require any consumable chemicals. Maintenance is straightforward and there is no risk of overdosing. UV radiation does not leave any residuals in the water. UV light has been used quite extensively for water supply disinfection in small communities. It is one of the few affordable technologies for small-scale water supply that effectively kills most bacteria, viruses and other harmful microorganisms. A UV lamp will imitate sunlight. In nature sunlight will destroy some bacteria, purifying water naturally. The efficiency of UV disinfection depends on the intensity and the wavelength of the radiation. If the water contains colour or turbidity, then the exposure of the microorganisms will decrease, and the disinfection becomes less efficient. This is of course a disadvantage. Therefore, some pre-filtering before UV radiation may be needed. Another problem is that there is no simple test of the disinfection result. Disinfection with UV is usually done so that the water is passing though transparent pipes. It is sufficient to have a contact time of  a  few  seconds. A UV dose (energy) is normally expressed in mJ/cm2 = mWs/cm2, the product of the UV intensity in mW/cm2 and the contact time. A common dose is 20–40 mWs/cm2 (EPA, 2011) to inactivate most waterborne pathogenic bacteria. The Department of Health and Human Services (U.S.) has established a minimum exposure of 16 mWs/cm2 for UV disinfection systems. Most manufacturers provide  a lamp intensity of 30–50 mWs/cm2. In general, coliform bacteria, for example, are destroyed at 7 mWs/cm2 (Oram, 2014). The usual wavelength is 254 nm. A typical low-pressure UV lamp has a power rating of 40–85 W and will last for about 12,000 hours (15 months). It has an operating temperature of about 40°C (ibid.). Typical power requirement to disinfect water with UV light is 10–20  W/m3 ⋅ h. For example, assume that we need to disinfect 1 m3 of water, which is produced during six hours of sunlight. With a flow rate of 1/6 m3/hour this will require a UV light power source of 1.7– 3.4 W, a very small amount of power compared to the requirement for desalination. The cost of a UV disinfection system is much lower than ozonation and membrane filtering. For ozonation the capital cost is roughly five Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user