Tuesday, October 28, 2008

Disinfecting Irrigation Water for Disease Management

High quality water for agricultural use is becoming limited. Runoff and irrigation return flow from containerized nursery and greenhouse facilities may contain nitrogen, phosphorus, pathogens, certain pesticides, various salts, and trace metals. Traditional greenhouse irrigation practices recommend watering to 10% excess, with application scheduled just prior to incipient wilting. The use of trickle tube irrigation can reduce the volume of irrigation water consumed in a greenhouse.

Closed and subirrigation systems are widely used by European greenhouses to comply with government imposed regulations to limit environmental contamination from pesticides and fertilizers from greenhouses. Ebb and flood irrigation is one subirrigation system that is being adapted by U.S. growers to reduce fertilizer and water waste. Closed recirculating irrigation systems can reduce the consumption of water and fertilizer.

Plant disease can potentially be introduced through irrigation water. Fungicides are often considered to be the first line of defense and often applied to the substrate, but not the irrigation water. Therefore alternative disease control strategies or integrated pest management (IPM) should be employed when using recirculated irrigation water.

Many growers using recirculated irrigation water employ several water disinfection methods to prevent the spread of soil borne disease organisms, including UV-C radiation, heat treatment, chlorination, ozone, and activated hydrogen peroxide.

Ultraviolet Radiation of Irrigation Water

Ultraviolet (UV) light is electromagnetic radiation with wavelengths between 200 and 700 nm. Visible light is from about 400 to 700 nm. UV disinfection water systems use UV-C radiation at 254 nm. Microorganisms absorb most of the energy at this wavelength resulting in a germicidal effect. The photochemical reaction alters essential molecular components (DNA and RNA). This process essentially eliminates most fungi, bacteria and viruses.

Heat Pasteurization of Irrigation Water

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 require that the water or nutrient solution pass through a heat exchanger and heated to 203°F (95°C) for 30 seconds, yet viruses can be inactivated at 131°F (55°C) and 158°F (70°C) for 90 and five minutes, respectively.

Oxidation Reduction Disinfection of Irrigation Water: Chlorine, Ozone and Hydrogen Peroxide

Oxidation reduction reactions originally referred only to reactions that involved the reaction of oxygen with another element or compound and reduction was used to indicate the removal of oxygen from a compound. Common oxidizing compounds include chorine, bromine, ozone, sodium and calcium hypochlorite, hydrogen peroxide. Common industrial uses of oxidation-reduction include water disinfection, odor control, cyanide destruction, chrome reduction, and metal etching.

Chlorine is the most typical water sanitizing agent and its oxidation characteristics can best describe oxidation reduction chemistry. Chlorine activity is typically reported as free residual chlorine or total chlorine. Chlorine exists in water as HOCl (hypochlorous acid) or OCl- (hypochlorite).

Ozone is a form of oxygen O3) typically generated by a corona discharge system by passing dried, oxygen-containing gas through an electrical field. The electrical current splits the oxygen molecules (O2). The resulting oxygen atoms (O-), seek stability and will bond to other oxygen molecules (O2), forming ozone (O3). The ozone is then injected into irrigation water where it inactivates microorganisms by disrupting cell membranes through oxidation.

Hydrogen peroxide (H2O2) a strong oxidizer, but is not very stable. Hydrogen peroxide is water (H2O) with an extra oxygen molecule attached and is a natural compound found in trace amounts in rain and snow. One commercially available hydrogen peroxide formulation is currently labeled as a greenhouse pesticide/disinfectant. Hydrogen dioxide (ZeroTol) kills bacteria, fungus, algae and their spores immediately on contact.

For the complete article, click here.