Irrigation water disinfestation

 

Water disinfestation methods can be categorized as filtration, chemical treatment, or physical treatment. The categories and methods within these categories are described below.

 

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 (2–250 µm in diameter) and bacterial cells (0.5–2 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. These filters are frequently installed in-line in series as arrays or cascades of larger to smaller particle size separation to prevent finer filters from blinding or clogging quickly. Bacterial cells, small fungal spores or cells, and viruses may pass through these filters, requiring physical or chemical destructive treatments, thereafter.   Because these filters may clog quickly and typically cannot be cleaned, frequent replacements may be required. As expected, frequent replacements translate into higher filtration costs than other methods over time.

·         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. These filters are capable of simply and efficiently filtering particles as small as 10–25 µm in diameter from water with longer periods of use before replacement than filter cartridges. Bacterial pathogens and small spores of fungal pathogens can pass through these filters. 

·         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. Clogged disc filter units are back-flushed to remove collected debris. As with the other filtration methods described above, these filters typically do not remove bacterial cells and small fungal spores from recycled irrigation water. These filters work best when arrayed in series from separating larger to smaller particle sizes to extend the periods of time between back-flushing and other cleaning steps.

·         Depth 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, recycled glass, and slow-sand filters. Conventional sand and recycled glass filters pump irrigation water through a filter matrix column to remove particles, may retain down to 10–20 µm-sized particles, and require periodic back-flushing to remove collected debris from each filter matrix. Slow-sand filtration systems require wider filtration columns in large-volume tanks or reservoirs with greater horizontal surface area than conventional sand and recycled glass filters, which rely upon gravity to move water gradually through a living biofilm layer (called the Schmutzdecke) and the filter matrix to trap plant debris and pathogenic microorganisms down to 1–5 µm in size. Slow-sand filtration systems are the most effective at removing pathogenic bacteria and fungi from irrigation water and typically require the least maintenance and fuss over time than other filtration methods.

·         Water channel and wetland filters: Pathogens and plant debris are removed from irrigation water via settling and/or passage through submerged living aquatic or wetland plant material in systems where the water winds through water channels or passes through marsh-like cascading terraces. These are the most passive and unpredictable, and least understood of all pathogen removal systems currently in use by growers. Though many growers report fewer disease issues when using these systems, other growers have experienced little to no improvement or, sometimes, greater soilborne disease incidence and severity. More research is underway in North America and several European countries to identify requirements for construction and maintenance of these systems to obtain effective, predictable results.

 

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

 

Hydrogen peroxide-based materials: These products are based on H₂O₂, known by several names including hydrogen peroxide (the most common), hydrogen dioxide and dihydrogen dioxide. The basis for deactivation or destruction of pathogens with hydrogen peroxide-based products is the oxidizing effects and reactive effects of this active. EPA-registered products containing this active ingredient typically include a stabilizing agent for longer term storage and may include other ingredients such as peracetic acid and peroxyacetic acid with the aim of improving efficacy. The addition of either of these two extra ingredients to hydrogen peroxide can increase risks of phytotoxicity on several crops. Hydrogen-peroxide products kill most bacteria, fungi and algae in irrigation water on contact, though have little to no effect on bacterial endospores, encapsulated bacterial cells, thick-walled fungal spores, and fungal sclerotia. Intact viruses are not affected by hydrogen peroxide. Effects of hydrogen peroxide-based products are short lived in irrigation water due to the active ingredients reactivity with metals, organic matter, and other substances in irrigation systems.

 

Chlorine-containing materials: This class of materials includes chlorine dioxide, sodium hypochlorate, and sodium hypochlorite. The efficacy of all of these materials is dependent on the reactivity of the Chloride ion on bacterial and fungal cells. Levels of chlorine in these products can remain reactive and toxic to many microorganisms several hours after treatment. 

 

Metal ionization methods: This strategy relies upon releasing reactive silver or copper ions into irrigation water from charged silver or copper plates contained in flow cells installed in irrigation systems. The positive silver or copper ions attach to and disrupt vegetative bacterial and fungal cell walls and membranes. These ions can persist and possibly accumulate in irrigation ponds as well as growing media and soils.

 

Quaternary ammonium compounds: These materials are the most persistent and effective of all chemical disinfestation treatments. They destroy most vegetative bacterial and fungal cells on contact and can kill certain persistent fungal spores and viruses.   They can be toxic to several crops if used at higher recommended application rates.

 

Ozonation: This technology involves generating ozone (O₃) and mixing with irrigation water to react with and kill pathogens in irrigation water. Similar to H₂O₂, ozone kills vegetative bacterial and fungal cells as well as thin-walled fungal spores on contact and is very short lived in irrigation water due to the reactivity of the ozone molecule with organic matter and the substances in and different components of irrigation systems.

 

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.

 

Heat pasteurization: This method involves heating irrigation water for 30 seconds at 203 °F (95 °C) to kill soilborne bacterial and fungal pathogens in irrigation water. While this can be an effective treatment for non-persistent forms of bacterial and fungal pathogens, it is less effective on persistent bacterial endospores, fungal spores and mycelia, and viruses than the other methods presented below. In addition, this approach is quite expensive to use because of its high energy requirement.

 

High-pressure ultra-violet flow cells: One of two methods for destroying pathogen based on ultra-violet (UV) light involves passing irrigation water through a high-energy, high intensity emission flow cell. This method has been shown to destroy bacterial and fungal pathogens and diminish or denature plant viruses in recycled irrigation water. These effects are due to both the exposure of bacterial and fungal cells to UV light and to the high heat generated by the UV flow cell (as much as 600 °C). For the UV light to be effective, it is important for growers to remove as much color and solid matter from the recycled irrigation water. Like heat pasteurization, high-pressure UV treatments are fairly expensive due to the high energy demand of these systems.

 

Low-Pressure UV Arrays: By increasing the exposure time of irrigation water to UV light, low-pressure UV arrays are considered as effective or more effective than high-pressure UV flow cells though using only a fraction of the energy. Each flow cell in the array emits UV light at the same wavelengths as the high-pressure systems with a much lower intensity, energy demand, and operating cost. Low-pressure UV flow cells are linked in series to provide a sufficient exposure of water passing through to UV light for deactivation or kill of microorganisms and virus particles. As with high-pressure UV systems, it is critical for irrigation water to be transparent (not cloudy) and as colorless as possible for the UV light to be effective.

 

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. Many greenhouse and outdoor nursery growers in Europe and, more recently, in different parts of North America are able to successfully clean their irrigation water using recycled sand filtration followed by ozonation or slow-sand filtration followed low-pressure UV arrays that are properly scaled for their water needs of their operations. Biofungicides such as RootShield and RootShield Plus applied or injected into irrigation water after treatment with either of these examples of combined disinfection approaches provide greater preventive control of root diseases in recycled irrigation water systems than using the biofungicides alone. Filtration methods combined with ozonation or with hydrogen peroxide can also enhance the efficacy of RootShield and RootShield Plus, though these and other biofungicides should not be applied through the irrigation water disinfestation system.