More and more facilities are looking at how they can cut costs and become more environmentally responsible, whether it’s through recirculating their leachate or treating available local water sources.
Unfortunately, added provisions must be taken in terms for disinfection. This is because bacteria, fungi, and viruses and build up in these recirculated systems to the point where they can damage the crop. In commercial growing systems the two most common ways to prevent this build up is through ultraviolet (UV) and ozone water treatment.
UV works by the emission of UV-C light (light with a wavelength of 254 nm) into a water sample. The UV light damages the DNA and RNA in organisms preventing them from reproducing. UV is a popular option for water disinfection because it is reliable and only affects the water’s microbes.
The most important step for UV unit selection is determining the needed UV dose. UV dose is determined by the equation:
Dose = Intensity x Transmittance x Exposure Time
Where intensity is the amount of power produced by the UV bulbs (as 254 nm light) measured in mW/cm^2, transmittance is the percentage of light reaching the microorganisms in the water (certain amount of light absorbed by particulates in the water), and the exposure time is the amount of seconds the water sample has been in contact with the UV light. All of these factors together impact the UV dose delivered to the water. In most systems the desired UV dose is 250 mj/cm^2 as this is the amount of UV needed to destroy several common viruses including tobacco mosaic.
Ozone is a molecule with the chemical formula O3. It disinfects by oxidizing organisms, which often entails reacting with the cell’s wall. Ozone doesn’t just attack organisms though, but rather any material that can be oxidized, for this reason it is incredibly important to design systems around ozone, and not simply add ozone to an already running irrigation system. To determine the amount of ozone needed to disinfect an irrigation system it is important to conduct a variety of water test. One of the tests used to determine the amount of organics in the water is the COD (chemical oxygen demand) test where the amount of oxygen needed to convert all of the organic carbon to carbon dioxide is measured. Approximately 2.5mg of ozone is needed per 1mg of COD.
While each application is different, the cost of UV and ozone disinfection typically come to approximately the same up front and maintenance cost. Therefore choosing between the two is best done by looking at how UV disinfect, but doesn’t alter its composition, and ozone does alter. While UV only kills pathogens, ozone will aerate, clear biofilm, and precipitate out impurities. Unfortunately szone will also corrode/react with steal, rubber, fiberglass, and plant nutrients. For these reasons ozone is best used to treat incoming local water sources and UV is best for recirculation systems.
While the initial cost may look scary, UV and ozone disinfection systems will pay for themselves in no time by saving water and nutrients. Facilities looking to take advantage of local streams can apply ozone treatment and those exploring recirculation should look towards UV.
Jacob Carson is a Water Systems Engineer within the Commercial Greenhouse Division at RBI. He believes that every growing operation is unique and should be treated as such. His aim is to engineer based on efficiency and sustainability. Jacob is currently in his third year at the University of Cincinnati studying Environmental Engineering.