Sanitation and Treatment for Pathogens in Recycled Water
by Donald J. Merhaut
Water recycling conserves water and reduces irrigation and fertilizer costs. However, the presence of plant pathogens in recycled water such as Phytophthora and Pythium, as well as bacteria and viruses, can lead to the spread of diseases throughout the nursery with potentially disastrous consequences. Cultural practices to prevent diseases in the nursery such as good sanitation practices are the most effective ways to reduce pathogens in recycled water. In addition, recycled water should be treated to kill pathogens as a standard sanitation and preventative control practice. There are several physical and chemical methods to treat recycled water. One method, discussed by Loren Oki in his feature article, is slow sand filtration. I will discuss the other methods for treating recycled water: chlorination, ultraviolet light, ozonation, copper ionization and heat treatment.
Chlorine kills organisms through the oxidation of cell membranes. The three methods of incorporating chlorine into irrigation water include sodium hypochlorite, calcium hypochlorite, and chlorine gas. Always check with local agencies regarding the regulations associated with the use and storage of chlorine products.
Effectiveness of treatment depends on several factors, including water cleanliness, concentration of chlorine, pH and temperature. The dirtier the water, the more chlorine it requires to treat it, because chlorine binds to other mineral and organic particles. As the concentration of chlorine increases, the effectiveness increases along with the rate of disinfection. Most crops will tolerate 100 ppm chlorine. Certain pathogens require higher chlorine concentrations or longer exposure times for effective treatment.
When using chlorine, you should strive for a residual of 0.5 ppm available chlorine for water not containing ammonia. Water containing ammonium-N will form chloramines, and your total residual combined chlorine should range from 0.5 to 1.0 ppm. Typically the dosage ranges from 4.5 to 8.0 ppm. The residual will always be less than the dosage since much of the chlorine reacts with contaminants in the water. Too much residual chlorine can damage sensitive crops.
Chlorine molecules are most stable, and therefore most effective, at a neutral pH (7.0). Low water temperature (less than 50°F) or high temperature (greater than 68°F) may reduce chlorine effectiveness. The disadvantages of chlorine are that residual chlorine can damage or kill plants, and chlorine does not break down or remove most pesticides or herbicides. Also, brown coloration, due to dissolved organic matter and acids, will not be removed with chlorination. Moreover, the fumes are highly dangerous, leading to a worker/operator exposure concern.
Ultraviolet light (UV)
Ultraviolet light, which has a wavelength from 100 to 400 nanometers (nm), may be used to kill pathogens. The mode of action is the production of free radicals disrupt cell membranes and kill organisms. In order to be effective, the water being treated must be relatively clear and colorless. Nurseries that use hydroponic or soilless systems are candidates for UV treatments. Nurseries using traditional methods of producing plants in soil would need to subject the water to other filtration processes to remove any suspended fractions or discolorations in the water.
Four types of lamps can be used to emit UV: low-pressure mercury lamps, high-pressure mercury lamps, excimer lasers and xenon flash lamps. Low-pressure mercury lamps emit a wavelength of about 254 nanometers. High-pressure mercury lamps emit a wavelength of 190 nanometers, which also causes the formation of sanitizing ozone in the water. Excimer lasers emit pulses of light at 248 nanometers. Xenon lamps emit light over a larger spectrum, some of which is not UV; therefore, this light source is not as energy efficient.
There are several advantages of using UV for water disinfection. The cost of operation is low if the water source is relatively clean. No chemicals are used in this process, and very few components must be maintained. Finally, even though it kills all organisms in the water, there is no resulting residue that is toxic to plants.
A major disadvantage of using UV is that the water must be fairly clear and clean of debris for effective treatment. If the water source is not clean, a UV-exposure time of longer than 30 seconds will be required for complete pathogen kill. Light sources may chemically denature some chelates that are used to keep micronutrients in the soluble form. UV does not remove other chemicals from the water, such as herbicides and pesticides. UV treatment also does not remove discoloration caused by organic acids.
Ozone is an oxidant that, like chlorine, kills organisms by disrupting cell membranes. The ability of any specific chemical to cause oxidation is measured as oxidation reduction potential (ORP). ORP values of 700 millivolts should provide complete disinfection. ORP values less than 300 millivolts are usually considered safe for most aquatic life.
The advantages of ozonation are that no residual chemicals remain after treatment, no chemical storage is required since ozone is manufactured on-site, the system efficiency is inexpensively monitored by measuring ORP values, and ozonation effectively oxidizes most pesticides. A major disadvantage of ozonation is that it requires a fairly clean water source to work properly. Therefore, water with large amounts of organic matter, clay, or other debris requires increasing the ozone exposure time (20 minutes to 1 hour). Other disadvantages of ozonation include an increased cost due to the use of electricity to produce ozone, the risk of some chelated nutrients precipitating out, and an increase in water pH, which may require acidification. Furthermore, ozonation is not completely effective in killing chlamydospores and microsclerotia of some pathogens.
Copper ionization is the process of adding copper ions to water. Copper electrodes are inserted into the water, and an electric current is passed through the electrode, releasing copper ions into the water. Effective pathogen treatment requires an ion concentration of approximately 50 to 300 parts per billion (ppb). While copper ionization has been used to treat algae in greenhouse coolant pads, there has been limited use for treatment of irrigation water. In hydroponic systems, a copper concentration of approximately 50 ppb is recommended, which also contributes to plant nutrients since copper is a necessary plant nutrient. However, a related disadvantage is that certain crops are sensitive to the concentration of copper recommended for effective treatment, and toxic levels may accumulate in closed recirculating systems.
An advantage of copper ionization is the relatively low cost for installation and maintenance of the system. The copper ionization system is also portable, alleviating the need for large holding tanks; there is also no chemical storage necessary with this process. However, like many other disinfectants, the effectiveness of copper ionization is reduced if the water contains excessive amounts of organic matter, clay, or other debris.
Heat treatment has also been successfully used by the nursery industry to sterilize soils and growing media. European countries use heat to sterilize water, and this method has also been used in California greenhouses (fig. 1). Viruses are killed at temperatures of 130°F, with an exposure time of 1.5 hours. As the temperature is increased, exposure time decreases. Heat exchangers (for heating water) are located at one point of the water system. Prior to heating, the water pH is reduced to approximately 4.5 to prevent calcium accumulation on the heat columns.
The primary advantage of heat treatment is that the water does not have to be as clean as is required in other chemical treatment processes. The other major benefit is that no chemicals are used, alleviating chemical storage and residue issues.
Some disadvantages of heat treatment include the following: water must be cooled before being applied to plants, heating water adds to the cost, heating duration may be as long as 1 hour, calcium removal from heat exchangers needs periodic maintenance, additional space is required for tanks to hold treated water, and high heat denatures chelates that are used for micronutrients.
Don Merhaut is UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.