Greenhouse Water Treatment and Filtration
Disinfestation of Greenhouse Irrigation Water
Some sources of irrigation water need to undergo disinfestation prior to use in the greenhouse. Municipal water is clean and generally does not require treatment. Well water usually requires only filtration to remove suspended solids or specific salts such as iron. However, irrigation water from closed irrigation systems, ponds, and waterways can serve as sources of many waterborne plant pathogens. To prevent the spread of pathogens, it is essential to prevent them from reaching healthy plants using different water disinfestation approaches. Water disinfestation methods employed in greenhouses include pre-filtration, chlorination, chlorine dioxide, hydrogen peroxide-based materials, copper ionization, ozonation, ultraviolet (UV) irradiation, or heat treatment by pasteurization.
Pre-Filtration
Pre-treatment or pre-filtration of irrigation water involves the use of various filter types to remove organic and inorganic particulate matter (debris, sediment, soil particles, algae, etc.) from the water prior to treatment for pathogens. Pre-filtration is important for two reasons. Firstly, larger particulate matter has the potential to clog the irrigation system (e.g., emitters).
Chlorination
Chlorination is a widely used technology for disinfecting water in greenhouses. Chlorine is very effective for removing almost all microbial pathogens and is appropriate as both a primary and secondary disinfectant. Chlorine oxidizes ferrous iron (Fe2+) to ferric (Fe3+). As a disinfectant, chlorine kills algae, bacteria and other organisms in the water supply. Chlorination also eliminates the energy source for iron bacteria. Chlorine is added to water as either chlorine gas, sodium hypochlorite (a liquid), or calcium hypochlorite (a powder or granules). Chlorine added by these methods reacts with the water by hydrolysis to form hypochlorous acid—the main active ingredient of chlorination.
Chlorine Gas
Chlorine gas is injected from gas cylinders. Dissolving chlorine gas in water produces hypochlorous acid, hydrogen, and chloride. Chlorine gas contains 100 percent available chlorine because it lowers the pH of the water to a level that results in mostly chlorine and hypochlorous acid. It typically is injected at 25 to 200 ppm. Adequate mixing and contact time must be provided after injection to ensure complete disinfestation of pathogens.
Sodium Hypochlorite
Sodium hypochlorite (liquid bleach) is usually available with up to 15 percent available chlorine. Household bleach is sodium hypochlorite with 5.25 percent available chlorine. Using liquid bleach is often preferred as this requires no special training or equipment and is easier to handle than gaseous chlorine or calcium hypochlorite. Sodium hypochlorite is transported by tanks.
Calcium Hypochlorite
Calcium hypochlorite is a solid form of chlorine available as granules or tablets contains about 65 to 70 percent available chlorine. It is an effective disinfectant that eliminates bacteria, algae, slime, fungi, and other microorganisms. Note that 12.8 pounds (5.8kg) of calcium hypochlorite dissolved in 100 gallons (378L) of water forms a 1 percent chlorine solution. A 2 percent chlorine solution therefore requires adding 25.6 pounds (11.6kg) of calcium to 100 gallons of water. Any chlorine stock solution can be mixed following the same pattern.
Factors to Consider with Water Chlorination
Water chemistry also affects the effectiveness of several disinfestation treatments. For example, the sanitizing activity of chlorine is strongly dependent on pH. For water amended with either calcium hypochlorite or sodium hypochlorite, the optimal pH range is 6.0 to 7.5. At a higher pH, more chlorine is needed for the same result or the water must be acidified for chlorine injection to be effective.
Continuous injection of chlorine should be used if the irrigation water has high levels of algae and bacteria. The recommended level of free chlorine is 2 to 3 ppm at the end of the irrigation system (Table 19.4). Contact time between the chlorinated water and the target biological contamination is important for the treatment to be effective.
While chlorine dioxide has chlorine in its name, its chemistry is radically different from that of chlorine. Chlorine dioxide (ClO2) has been successfully used for water treatment in greenhouses. Chlorine dioxide is a very effective material for removing biofilm in pipe as a shock treatment, is less sensitive to pH than chlorine, and can also be applied on a continual basis. Although a chlorine compound, chlorine dioxide does NOT act by chlorination. It dissolves readily in water and does not react to form hypochlorous acid and, unlike hypochlorous acid, chlorine dioxide’s mode of action is solely by oxidation.
Chlorine Dioxide
While chlorine dioxide has chlorine in its name, its chemistry is radically different from that of chlorine. Chlorine dioxide (ClO2) has been successfully used for water treatment in greenhouses. Chlorine dioxide is a very effective material for removing biofilm in pipe as a shock treatment, is less sensitive to pH than chlorine, and can also be applied on a continual basis. Although a chlorine compound, chlorine dioxide does NOT act by chlorination. It dissolves readily in water and does not react to form hypochlorous acid and, unlike hypochlorous acid, chlorine dioxide’s mode of action is solely by oxidation.
Hydrogen Peroxide-Based Materials
A strong oxidizing agent widely known for its sterilant and antiseptic properties, pure hydrogen peroxide can be used as a water treatment but is quite unstable and has a short shelf life. It is, therefore, marketed in a number of stabilized forms or peroxygens. These forms of hydrogen peroxide degrade slower, allowing it to have a residual effect. This means it remains in the water in low concentrations all the way down the irrigation line for a certain period of time, allowing for extended disinfection throughout the system.
Copper Ionization
The process of copper ionization uses electricity to harness the natural molecular properties of copper. Because soluble copper ions lack two electrons, they are eager to bond with other suitable atoms that can supply the missing electrons. When copper ions encounter organic matter, including plant pathogens, they firmly attach themselves and disrupt the pathogens’ cell walls, killing the organisms. A concentration of 0.5 to 1 ppm is effective against pathogens, while 1 to 2 ppm may be required for algae. Copper ions have a residual effect. Copper ionization is widely used by plug and liner greenhouse growers. Copper ionization can be combined with other technologies, such as filtration and ultraviolet light. Copper ionization seems to be less affected by organic matter in the water than some other treatment options.
Ultraviolet Light Treatment
Ultraviolet (UV) light is a highly effective option to control pathogens and algae in greenhouse irrigation systems (Figure 19.10). The treatment works because UV light penetrates an organism’s cell walls and disrupts the cell’s genetic material, making reproduction impossible. It is effective against algae, fungi, bacteria, and viruses. Water is treated in UV systems by passing it through a steel treatment chamber containing a UV lamp enclosed in a quartz sleeve.
Ozonation
AOzone is the most efficient disinfection agent available and replaces traditional chemicals such as hypochlorite, peracetic acid (PAA) and hydrogen peroxide. Ozone is a powerful oxidizer which destroys fungi, pathogenic bacteria and viruses which cause waterborne diseases. Once dissolved in water, ozone destroys pathogens and then safely reverts into oxygen. Ozonation deploys the unstable gas ozone (O3), which is produced on site by an ozone generator (Figure 19.11) and is bubbled through the water where it rapidly reacts with microorganisms and organic matter.
Heat Pasteurization
Pre-heating (or pasteurization) of water prior to reuse is a very effective and reliable method of treating irrigation water for pathogens in greenhouses (Figure 19.12). This method involves heating irrigation water to a specific temperature that inactivates microorganisms. Pasteurization does not require the water to be pre-filtered and does not have any residual impact on plants. Pasteurization, however, uses a high amount of energy to heat the water. This becomes expensive for production facilities that use large volumes of water and as such this treatment is better used for smaller volumes of water.
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