Ultraviolet technology has potential to treat ditch water for agricultural irrigation.
By Lori Lovely
The availability of irrigation water for food crops is crucial to berry and vegetable farmers; but before it can be used, irrigation water—and the UV system to treat it—must meet regulatory agency standards. In fact, the relationship between water demand and water safety always requires a delicate balancing that must, as Lynn Lashuk of the British Columbia Agriculture Council points out, include affordable treatment solutions.
 |
Photo: UV System |
| UV Systems used to reduce E. coli and other bacteria in bodies of water. |
 |
Photo: UV System |
| Water samples are taken pre-UV
treatment and post-UV treatment. |
British Columbia’s agricultural credentials are quite impressive: According to a 2006 StatsCan census, the region logged $2.7 billion in farm receipts, and a farm area growth rate of 9.6% (well above the 0.1% national average). Within this context, it makes sense that the Agriculture Environment Partnership Initiative awarded the British Columbia Vegetable Marketing Commission a one-year $18,000 research grant in July 2007 to enable the commission to evaluate the effectiveness of ultraviolet (UV) technology in treating ditch water for agricultural irrigation. More specifically, the commission hopes to demonstrate UV’s effectiveness in reducing microbial pathogen levels in irrigation water, thus allowing it to meet the Provincial (British Columbia) water quality guidelines.
In 2002, the BC Ministry of Water, Air, and Land Protection tested the water in and around Surrey. The results indicated that E. coli levels exceeded Provincial irrigation water quality guidelines in the Nicomekl River, which serves as the irrigation water supply for vegetables and berries in the Cloverdale area. According to Stephanie Tam, from the Ministry of Agriculture and Lands in Abbotsford, those E. coli levels “could potentially create a human health hazard. Preliminary water samples in several areas in the Lower Mainland, including Matsqui Prairie, where surface water is the primary source of irrigation water supply, showed similar results.”
According to the Ministry of Agriculture, Food, and Fisheries, many of British Columbia’s surface water supplies contain pathogens that constitute a risk to human health. Agricultural drainage ditches often supply irrigation and crop wash-water, but these supplies are particularly prone to poor quality. Of course, all surface water sources can be contaminated with pathogens from septic fields, animal manure, milk house wastes, and wildlife.
Because—as Lashuk points out—irrigation (or surface) water is subjected to contamination from many sources, there is an extensive and diverse list of microorganisms in the aquatic environment that makes identification and monitoring complicated and expensive. To simplify matters, fecal coliforms are used as an easily detectable indicator organism for overall fecal contamination, because these organisms are present only when other pathogens are present. This method helps distinguish false tests and aids in similar responses to the pathogens of interest.
The drawbacks of using fecal coliforms include the fact that these tests detect both potentially harmful bacteria like E. coli and less harmful bacteria. If effluent sources derive from dairy farms or industrial food processing, there’s a chance the effluent may indicate a high positive fecal coliform test without the presence of fecal matter. In addition, fecal coliforms will not identify pathogens of nonfecal origin, and are not useful as indicators for pathogens responsible for eye, ear, nose, throat, or skin infections or the presence of viruses, protozoa, or worms.
E. coli is also frequently used as an indicator, because it is more specific to human fecal contamination than any other group of organisms. The correct combination of UV light intensity, duration of exposure, and extent of penetration through the water can destroy E. coli.
Applying Ultraviolet Light
Although part of the natural light spectrum, ultraviolet light is invisible to the human eye. Nevertheless, it can destroy microorganisms such as bacteria, fungi, and algae. Sunlight does not contain ultraviolet light in sufficient quantities to kill pathogens quickly, so high intensity UV lamps are used to kill pathogens in a shorter amount of time.
The measurement of intensity is microwatts per square centimeter and the factors that affect intensity are lamp output and water quality. Regularly cleaning the quartz sleeve and annually replacing lamps helps maintain peak lamp output. Monitoring equipment can also be added to ensure that proper outputs are maintained.
The flow rate determines the necessary exposure time required. Increasing the flow rate decreases the exposure time and dosage, which requires higher intensity lamps to ensure effective treatment.
Proper dosage can be determined by multiplying the light intensity by the exposure time. Increasing either requires a corresponding increase in dosage. The measurement for dosage is microwatt-seconds per square centimeter. According to the Ministry, the standard to disinfect water contaminated by bacteria and viruses is 38,000 microwatt-seconds per square centimeter, although the dosage required will vary according to the species.
One challenge of treating water is that light penetration is limited by the quality of the water. If the water contains high levels of suspended solids or dissolved organics, or does not have good clarity, light penetration will be limited. In addition, dissolved organics have a high ultraviolet absorption capacity and iron can reduce light intensity by coating quartz sleeves. To improve clarity and ensure adequate penetration of UV light through the entire flow profile, filtration of surface water is necessary.
Putting UV to the Test
When the Ministry of Agriculture approached the British Columbia Vegetable Marketing Commission in 2003, the BC Vegetable Marketing Commission agreed to fund (in part) and administer funding for research, with the expectation of discovering that UV technologies to reduce microbial pathogen levels are 99.9% efficient.
Other funding came from the British Columbia Ministry of Agriculture and Lands, the British Columbia Blueberry Council, the Lower Mainland Horticultural Improvement Association, and the British Columbia Agriculture Council—Agriculture Environment Partnership Initiative. In addition, the participating farm owners (one vegetable farmer in Cloverdale and blueberry farmers in Matsqui Prairie, who share the same surface water and UV system) paid approximately one-quarter of the cost in exchange for being allowed to keep the equipment.
“It started with a problem encountered by a spinach grower in Cloverdale,” explains Jack Wessel of the BC Vegetable Marketing Commission. “That caused us to look at water quality.” Initial water testing was conducted over that summer as the BC Vegetable Marketing Commission sought solutions. “When a lot of green leafy vegetable growers experienced water problems, it made us ask what they could do,” Wessel says. “One answer we got was ‘don’t irrigate for two weeks,’ but that wasn’t practical. We wanted practical solutions.”
When the Ministry of Agriculture identified the UV light treatment as a potential method of reducing microbial pathogen levels, the BC Vegetable Marketing Commission agreed to explore the possibility. They promptly selected growers to participate in the study, as well as the time period for the research, but left evaluation of the program to the Ministry of Agriculture.
Project Profile
H2O Irrigations installed UV treatment systems at both locations. Owner Doug Jarvie explains the design criteria.
Ditch water passes through a set of two sand filters to separate out particulates. Sand filters are required prior to UV treatment to ensure maximum transmission of ultraviolet light through the water. The filters are designed not to exceed a flow rate of 15 gallons per minute per square foot of bed area. After being filtered, the water is estimated to have a transmission level of 50%.
Next, the water proceeds through a screen filter, although Jarvie says it’s not strictly necessary, and is included mainly as a precaution in case of a failure with the sand filters. The filtered water then passes through a set of 12 UV lamps. Turbulent flow is required through the UV units; if the flow is smooth, microorganisms may be able to pass through the outlet without having sufficient contact time.
For crop-washing facilities, an ultraviolet dosage of 40,000 microwatt-seconds per square centimeter is recommended. Wash water still requires chlorine or chlorine dioxide, Jarvie notes, because the UV process cleans water but doesn’t kill bacteria, which will regrow. Wash water must be 100% bacteria-free. However, before being discharged into the environment, all the residual chlorine must be removed.
Irrigation water doesn’t need to be completely free of bacteria. Jarvie indicates that bacteria levels must be below a count of 70%, and that it’s “easy to get below 70.” For irrigation system treatment, an ultraviolet dosage of 16,000 microwatt-seconds per square centimeter is advised. Irrigation water treatment should be started 30 days prior to harvest.
Working with the manufacturer, Jarvie designed and tested the systems installed on the farms in the project. He says they’re not difficult to install, and that, if designed and sized properly, they’re very effective. To determine the proper size, he checks water samples for clarity. The necessary size of the unit depends on the flow rate, light intensity, and water quality.
Maintenance is equally simple. “They need to be cleaned every couple weeks,” Jarvie recommends. Smaller units can be easily disassembled for cleaning, but bigger systems feature an automatic cleaning system to clean the dirt that sticks to the glass tubes, using nontoxic citric acid, which can safely be released into the environment. “It’s too difficult to disassemble the larger systems.”
Spatial requirements are also minimal. Although it depends on the water flow, most need only an 8-foot-by-8-foot pump house. “They don’t take a lot of space,” Jarvie reiterates. “The sand filter takes less space than the UV unit, which is 12 inches in diameter and about 6 feet long.”
Rough Waters Precede Smooth Sailing Toward Results
Water samples were taken pre-UV treatment and post-UV treatment weekly from the end of May to mid-September. Trial results indicated that the UV system was effective in reducing the levels of E. coli and fecal coliforms in the ditch water, meeting Provincial irrigation water guidelines.
One concern during the study was that other factors might mask the true effectiveness of the UV technology; e.g., leaks along the distribution lines in certain areas of the field causing some levels of contamination of the treated water or water turbidity masking the UV lamps, reducing treatment efficiency. According to Tam, “It is very difficult to try to fix or even to locate where the leaks occur when you are dealing with permanent pipes installed underground.”
Other issues arose because the operation and maintenance of the UV facility was the sole responsibility of farm owner. If the UV lamps weren’t cleaned regularly, the treatment efficiency was reduced. If the sand filters weren’t flushed regularly, microbial growth might occur within them, thereby increasing the microbial pathogen levels in the water after being filtered, but prior to going through the UV lamps. The UV dosage needed depends on the quality of the untreated water. If microbial growth is allowed to occur in the sand filters, the UV dosage in the system might not be enough to treat the water to its full effectiveness.
In fact, maintenance did somewhat affect the results. Water samples were taken at both farms during three consecutive irrigation seasons from 2003 to 2005, inclusive. The results met Provincial guidelines, except when UV lamps were clouded or out of order. Tam says they learned that check-ups of treated water should be performed periodically—as often as once every two weeks during the irrigation season and especially after rainfalls or non-irrigation season. The project results indicate, “when special maintenance of materials in sand filters or UV lamps should be done,” she concludes.
Rewards
“Two farms in the Lower Mainland now have good-quality water for crop washing and for irrigating their crops that are eaten raw, as long as the UV systems are maintained regularly,” Tam reports. She cautions growers, however, that “purchasing and maintaining a UV system is costly,” but emphasizes that they need to be aware of the fact that using untreated surface water that might be contaminated with microbial pathogens can potentially put public health in jeopardy. She urges growers to make every effort to ensure that their crops are safe for public consumption.
Jarvie confirms that the capital investment cost of the system is high, but long-term, the solution is cost-effective, especially considering the alternatives. “Actually, there aren’t a lot of options: you either add something to kill the bacteria or you take out the bacteria at the source,” he says. “We recommend against adding large amounts of chlorine because chlorine byproducts can end up in the soil. Why add something that has to be taken out?” In addition, he notes that a UV system requires a one-time investment and minimal maintenance costs, whereas chlorine applications will add annual costs.
One farm with a need for a large volume of wash water had two options prior to installation of a UV system: buy city water or truck in water. Both were expensive options, Jarvie says. In that case, the UV system paid for itself in one month.
He doesn’t do that kind of work anymore, claiming his small operation couldn’t keep up with the heavy demand, but in a two-year period, Jarvie installed at least eight UV systems. “The first farm installation I did required a lot of research,” he says. “It had been shut down due to an outbreak in its spinach crop caused by contaminated ditch water. You can’t have bacteria in irrigation water.”
Ditch water for irrigation purposes isn’t the only successful application of UV light treatment systems. Using the UV systems to treat wastewater is “very common,” Jarvie adds. “It’s used for sewage treatment, because it doesn’t have to be a pressurized unit in a closed chamber. You can have an open channel to oxygenate the water. That allows it to grow bacteria to break the sewage down prior to UV treatment.”
Wessel indicates that because there have been no recent incidents regarding water quality, there haven’t been any additional tests of UV treatment in British Columbia, although he does mention related experiments conducted in Ontario to evaluate the number of hours of sunlight required to kill bacteria. “It’s terrible to say, but since we haven’t had any incidents, there hasn’t been any focus on the issue—although I’m sure it will be a matter of increasing concern in years to come.”
Based in Indianapolis, IN, Lori Lovely writes on technical subjects.
OW - March/April 2008 |
