UV treatment with lamps
UV irradiation (using lamps in particular) provides a quick and effective way of disinfecting water supply from harmful bacteria without leaving a chemical residue, like the one chlorine leaves. Usually it is used as one step in the process of making water safe to drink. Other steps include filtration and/or reverse osmosis. But in general, UV light (whether by sun of lamps) has demonstrated efficacy against pathogenic organisms, including those responsible for cholera, polio, typhoid, hepatitis and other bacterial, viral and parasitic diseases.1 Special low-pressure mercury vapor lamps produce UV radiation at 253.7nm, the optimal wavelength for disinfection.2
The set up for UV disinfection of small or household water supplies consists of submerged lamps or lamps in the air and mounted above a thin layer of the water to be irradiated. In the units with submerged lamps, the lamps are covered with a protective, UV-penetrable as protection from the electrical hazards associated with water. Water can be treated on a batch basis by placing the lamp in a container of water for several minutes or longer, or on a flow-through basis in a housing or channel, with the water flowing parallel or perpendicular to the lamp(s).
In units having the lamps mounted in the air, the UV lamps are in a metal housing with reflective surfaces that direct the UV radiation downward onto a thin layer of water flowing through a channel or tray below the lamps. The advantage of the submerged systems is intimate lamp contact with the water, water-mediated cooling of the lamps, and the use of housing designs that maximize UV exposure of the water.
Low maintenance
However, the protective sleeves over the lamps must be mechanically or chemically cleaned on a regular basis to overcome fouling by a physical, chemical or biological film that can forms on the sleeve surface, reducing UV passage into the water. The non-submerged, in-air lamp units have the advantage of no need for lamp cleaning due to lamp fouling, but there is some loss of UV radiation due atmospheric and surface absorption. However, both types of UV disinfection system designs are available for disinfection of household water at point- of-use, point-of-entry or at the community level.
Suitable conditions
| Advantages | Disadvantages |
|---|---|
| - effective for inactivating waterborne pathogens - simple to apply at the household and community levels |
- the need for a source of lamps, which have to be replaced periodically (typically every year or two - the need for a reliable source of electricity to power the lamps |
Construction, operations and maintenance
There are not many known homemade UV lamp systems, most are small units that are commercially available. However, an alternative to a lamp UV is to use direct sunlight, and there are several methods on how to build your own UV treatment / Solar disinfection (SODIS) system.
Otherwise commercial units can be bought, for instance, the Naiade water purification unit or the UV WaterBox. Technical aspects of the UV WaterBox by Aqua Aero.
Costs
Because the energy requirements are relatively low (several tens of watts per unit or about the same as an incandescent lamp), UV disinfection units for water treatment can be powered at relatively low cost using solar panels, wind power generators as well as conventional energy sources. The energy costs of UV disinfection are considerably less than the costs of disinfecting water by boiling it with fuels such as wood or charcoal.
UV units to treat small batches (1 to several liters) or low flows (1 to several liters per minute) of water at the community level are estimated to have costs of 0.02 US$ per 1000 liters of water, including the cost of electricity and consumables and the annualized capital cost of the unit. On this basis, the annual costs of community UV treatment would be less than US$1.00 per household. However, if UV lamp disinfection units were used at the household level, and therefore by far fewer people per unit, annual costs would be considerably higher, probably in the range of $US10-100 per year. Despite the higher costs, UV irradiation with lamps is considered a feasible technology for household water treatment.
Field experiences
The Power of UV Light
During the summer of 1993, Gadgil and a graduate student investigated the effectiveness of UV light and whether it was economically feasible. "We were completely amazed," he says. "Using the simplest engineering, we could disinfect water for half a cent per ton. That's shockingly cheap. You could disinfect one person's drinking supply for a full year for a couple of cents."
From his experiences in India, Gadgil knew that any system would have to require little maintenance and not take for granted basic infrastructure like electricity and water pressure. The system he and his student built, later named UV Waterworks, is remarkably simple. In a compact, enclosed box, a UV lamp is suspended above a shallow pan. Water runs into the pan under the force of gravity, where it is exposed to the UV light, then into a holding tank. The only power that is needed is about 40 watts to power the light; this can come from a car battery. The system can disinfect four gallons of water a minute, killing 99.999 percent of bacteria and viruses. This produces enough clean water to serve more than 1,000 people.
Water Health International, the company founded to bring the technology to market, now makes several different versions of Gadgil's disinfection system, for small and large applications, for emergency use, and for locations that also need to filter out silt and other large contaminants. Prices start at about $1,500.
UV Waterworks systems have been used in India, South Africa, the Philippines, Honduras, and other countries. Since 1998, the Mexican government has installed about 100 in Guererro, a state on the Pacific in southwestern Mexico. The results have been very positive. In the summer of 2000, Gadgil reports, people from Water Health International returned with stories and data showing a dramatic decline in the incidence of diarrhea among children and adults. And preventing deaths and illness are just the most visible effects of purifying water—it also protects children from stunted physical and mental growth. "This is the kind of story that really makes my day, really makes me happy," Gadgil continues. "It makes me feel good when I get up in the morning."
Manuals, videos and links
- Sharon Davis Design and UV Water Filtration in Rwanda.
- Everything You Need to Know about Ultraviolet Water Purification.
- Commercial UV units: Ultraviolet Water Purification Systems Available from Purest Filters.com. Prices start at USD$ 365.
- Commercial UV unit: UV Waterworks: Reliable, Inexpensive Water Disinfection for the World.
- Low Cost UV Disinfection System for Developing Countries: Field Tests in South Africa. Ashok Gadgil, Peter Miller, Ndirangu Kibata.
- What microorganisms are deactivated by germicidal ultraviolet light? Ultraviolet.com.
References
- ↑ Trojan UV: Introduction to UV Disinfection
- ↑ HiTech Ultraviolet Pvt. Ltd.
- ↑ Tech Brief: Ultraviolet Disinfection National Drinking Water Clearinghouse (NDWC) newsletter. Sept. 15, 2000.
Acknowledgements
- Managing Water in the Home: Accelerated Health Gains from Improved Water Supply. Heat and UV Radiation or (alternative link). World Health Organization (WHO) 2002.
- Courage: Ashok Gadgil. From the book: Inventing Modern America: From the Microwave to the Mouse. David E. Brown. MIT Press.