Chapter 11

Irrigation Water for Greenhouses

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 recirculated water systems, ponds, and waterways can serve as sources of many waterborne plant pathogens. The most serious of these are the water molds, which look like fungi but require water to complete their life cycles. The most common water molds are Pythium species, which cause damping-off of seedlings and stem cutting rots, and Phytophthora species, which cause root rot, shoot dieback and foliar blight on a wide range of nursery plants. Other troublesome waterborne pathogens transmitted in irrigation water include Colletotrichum, Fusarium, and Ralstonia. Unless recycled or surface water is disinfested before reuse, irrigation water may become an effective delivery system for these plant pathogens. To prevent the spread of pathogens in recycled irrigation water and surface water, it is essential to prevent them from reaching healthy plants using different water disinfestation approaches. Water disinfestation methods can be categorized as filtration, chemical treatment, or physical treatment. The categories and methods within these categories are described below. In general, there is no single method or category that serves most growers as a complete strategy for eliminating pathogens from recycled or natural sources of irrigation water. Using two or more approaches provide the best opportunities for preventing circulation of plant pathogens through irrigation water.

Disinfestation Pretreatment Methods

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 2 reasons. Firstly, larger particulate matter has the potential to clog the irrigation system (e.g. emitters). Secondly, the effectiveness of many pathogen treatments (e.g., all chemical treatments using oxidizers, UV) is decreased significantly in the presence of particulate organic matter.

Disinfestation by Filtration

Physical removal of plant pathogens from irrigation water is accomplished by filtration. Each of the commercially available filtration technologies varies in its abilities to remove pathogens. If properly maintained, several of these can remove fungal spores (2 to 250 µm in diameter) and bacterial cells (0.5 to 2 µm in diameter). None are known to eliminate plant viruses.

Screen Filters

These are the most basic filters. As their name implies, this method relies upon passing water through one or more screens of specific mesh sizes to remove solid material—including larger pathogens—from water passing through. They typically remove only particles greater than 100 µm in size and require frequent cleaning, which make them less effective and efficient to use than other methods.

Filter Cartridges

Woven fibers, wrapped cord, or a solid matrix are contained in a compact cartridge to filter solid particles from irrigation water. Depending on the type, filters in this category may remove particles from 5 to 50 µm in diameter.

Sheet Filters

This class of filters uses flow of irrigation water through a broad layer of cloth or plastic membranes to separate plant debris and pathogens from captured irrigation water.

Disc Filters

These filters are comprised of stacked circular, grooved plastic discs contained in cartridge-like cylinders. Water forced radially through the grooves of these tightly stacked discs provide separation down to 20 µm-sized particles.

Media Filters

These filters trap pathogen cells and spores and plant debris in irrigation water in a deep column of sand, recycled glass, packed mineral or glass fibers, and/or other dense substrates. The most notable of these are conventional sand and recycled glass.

Slow Sand Filtration

Slow sand filtration involves allowing irrigation water to slowly pass through a bed of sand or other porous material such as pumice, rockwool, etc. for treatment. It uses both physical and biological pathogen control mechanisms (See Figure 11.8). As water passes through the fine-grained medium, pathogen propagules are “strained” out as they encounter small pore spaces. However, in addition to this physical filtration mechanism, pathogens are also controlled by a biofilm layer (called the Schmutzdecke) of beneficial microorganisms that, over time, form on the surfaces of sand grains and removes pathogens through antagonistic interactions and competition.

Disinfestation with Chemical Treatments

Several chemical treatment options exist to help growers to destroy or deactivate soil borne pathogens found in irrigation water. These chemical sanitization treatments vary in their persistence in irrigation water.

Chlorine-containing Materials

Chlorine is often added to irrigation water to oxidize and destroy biological microorganisms such as algae, fungi, and bacteria. While these microorganisms may be present in water from any source, they are most likely to exist at high levels in surface irrigation water from rivers, canals, reservoirs, and ponds. Common sources of chlorine are chlorine gas, sodium hypochlorite (a liquid), and calcium hypochlorite (a powder or granules).

Chlorine Gas. 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 and is most active between the pH 6 to 7.5.

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. Most liquid fertilizer injection equipment is capable of injecting liquid chlorine. If the injection point is downstream of the filters, it may be necessary to hand-treat the filters with chlorine (discussed later).

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 of calcium hypochlorite dissolved in 100 gallons of water forms a 1 percent chlorine solution.

Relationship between Chlorine and pH. All three chemicals form hypochlorous acid (HOCl) and hypochlorite (OCI-) in water, both referred to collectively as free available chlorine. These two end products are active oxidizing agents that destroy organic matter. Hypochlorous acid is the more effective of the two and it converts to hypochlorite as pH rises.

Desired Chlorine Concentrations. 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 1 to 2 ppm at the end of the irrigation system (See Table 10.6). 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 1 to 2 ppm at the end of the irrigation system (See Table 11.6).

Affects of Organic Matter on Chlorine. Free chlorine reacts with nearly all organic matter, whether peat moss or living cells. The oxidizing power (disinfectant property) of free chlorine diminishes as it reacts with organic matter. Thus, the concentration of free chlorine that must be injected into the irrigation water system must be increased in accord with increased levels of organic matter, regardless of whether it is nonliving matter, algae, biofilm, iron bacteria, or free microorganisms.

Contact Time and Flushing. Contact time between the chlorinated water and the target biological contamination is important for the treatment to be effective. Chlorine injection times should be at least 2 hours; longer injection periods are preferable.

Injection Considerations. Inject the chlorine upstream from the filter to help keep the filter clean and to remove any precipitates that may be caused by the chlorine injection. Chlorine is a very effective oxidizing agent that causes any iron and manganese present in the water to precipitate and clog the emitters. A filter downstream from the injection point will remove these precipitates from the water and prevent emitter clogging.

Activated Peracids

Peracids, also called activated peroxygen, are a formulation of hydrogen peroxide (H2O2) and acetic acid that produce a highly reactive product called peroxyacetic acid (PAA). While peracids share some of the characteristics of sodium hypochlorite (they are manufactured as liquid concentrates and are classified as oxidizers), they are different from chlorinated products. Peracids combine hydrogen peroxide and organic acids, typically acetic acid, to form a new compound called peroxyacetic acid. This compound is an activated form of hydrogen peroxide and produces a much more stable and powerful oxidizing compound to treat pathogens and algae in water. Formulated peracid products are more stable than hydrogen peroxide and degrade principally through reactions with elemental metals, microorganisms and organic material.

Copper Ionization

Modern copper ionization treatments are more effective, precise and environmentally responsible than their older counterparts (See Figure 10.9). The modern 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.

Ozone

This technology involves generating ozone (O3) and injected into water by means of a venturi tube or diffusion into a contact tank to react with and kill pathogens and algae in irrigation water (See Figure 10.9). In an ozone water treatment system, oxygen from the air is converted into ozone gas, which is then dissolved into water to form aqueous ozone. Ozone controls algae and pathogens in irrigation water by oxidizing constituents of cell walls before it penetrates inside the cell wall and oxidizes enzymes, proteins, DNA, RNA and cell membranes. Ozone is a strong enough oxidizer to remove biofilm from piping, but has a short residual time in irrigation water. Some pesticides are removed by ozone treatment.

Disinfestation with Physical Treatments

There are three major categories of physical irrigation water disinfestation treatment available to growers. Two of these methods have high energy requirements. Two of these methods also require transparent irrigation water to work effectively.

Ultraviolet (UV) Light

Ultraviolet (UV) light is a highly effective option to control pathogens and algae in irrigation systems. The system uses UV lamps (See Figure 11.10), very much in the shape of narrow fluorescent lamps, each contained inside a water-tight quartz sleeve to protect it and allow unfiltered emission of UV light. These tubes are contained inside and parallel to the length of a cylinder. Water flows through the cylinder, over the outside of the UV lamps. In this way, water passes within close proximity to the source of UV radiation. The degree of inactivation is directly related to the applied UV dose, which is a product of UV radiation intensity and exposure time. Most UV systems are designed to provide a dose greater than 40 mJ/cm2. Not all ultraviolet radiation has the same sanitizing effect because different wavelengths have different properties.

Heat Pasteurization

Heat pasteurization in the greenhouse has been used for quite some time. Heat pasteurization of root zone substrates is a common practice, but not so for irrigation water. Yet in The Netherlands, heat pasteurization is the most common water treatment system. Typical recommendations for heat pasteurization of irrigation water requires that the water or nutrient solution pass through a heat exchanger and heated to 203 degrees F (95 ?C) for 30 seconds.

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