Part three of a four-part series

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Do you treat your irrigation water with chemicals to keep your crops free of infection by plant diseases? Do you want to clean your irrigation water to limit your potential for infection by plant pathogens? Do you want to sanitize your irrigation lines to prevent clogging by biofilm? Do you want to ensure your water is free from disease agents that can infect your plants? Do you need an organic water treatment method to remove plant pathogens from irrigation water? Keep reading — this article is for you. We will focus on three newly designed tools: (1) a chlorine dilution calculator, (2) a chlorine contact time calculator and (3) a slow sand filter sizing tool. These three tools complement the tools discussed in the previous two articles in this series that focused on how long your irrigation water would last and irrigation system design. You can access these tools and many others for free through the www.cleanwater3.org website under the “Tools” section.

Chlorine dilution calculator

Injecting the right concentration of chlorine into irrigation water or into irrigation lines is critical for plant pathogen and biofilm control. If the concentration is too low, you may not get complete control of plant pathogens or biofilms. If the concentration is too high and excess free chlorine is in the line, phytotoxicity can occur. Before injecting chlorine into irrigation water, it is best to filter the water using rapid sand or mechanical filters that remove organic matter from the water. If organic matter (like peat, bark, algae, etc.) is not sufficiently removed before chlorine injection, the chlorine will react with these particles and may be used up before the plant pathogens and bacteria have been destroyed. If an inadequate concentration of chlorine is injected, plant pathogens or bacteria may remain in the water, potentially infecting plants or clogging irrigation lines.

The proper chlorine dose to inject depends upon your initial water quality, crop and the temperature outside. Generally, warmer temperatures mean higher biological activity (i.e., more algal growth) and thus higher chlorine demand. Maintaining 2 ppm of free chlorine at the end of the irrigation line and ensuring 2 minutes of contact time in the pipeline before irrigation water is applied through the closest emitters or sprinklers, is adequate to control most plant pathogens. Chlorine injection should be coupled with measurement of oxidation-reduction potential (ORP, read in millivolts — mV). An ORP reading of 650 to 750 mV generally indicates adequate free chlorine presence to kill plant pathogens. Typically, chlorination chemicals are stored in a stock tank before being injected into irrigation water, but they can also be directly mixed with irrigation water in a mixing tank.

Figure 1. The chlorine dilution calculator helps you determine how much of a concentrate to add to a stock tank to maintain the desired concentration of sanitizing agent.

The chlorine dilution calculator can help you determine both how much chlorine concentrate to add to a mixing tank and how much it will cost you to chlorinate your water (Figure 1). Figure 1 shows how to mix a concentrated solution of germicidal ultra-bleach (6% of active ingredient sodium hypochlorite) into a 500-gallon mixing tank with a target concentration of 2 ppm chlorine in the tank. For most concentrated solutions, the label will tell you the % active ingredient (similar to a pesticide label); this information is generally provided in a chart that looks like the blue inset text in Fig. 1. Also entered in the tool is the concentrate container size (1 gal) and the cost per container ($3.10). After entering this information in the Concentrate Source section, enter the information about your mixing tank. How big is it? 500 gallons (you can also enter this information as liters). What is the desired concentration? 2 ppm. (1 ppm = 1 mg/L). You can then click on the blue button that says “Recompute” — and it will calculate the required dosage, or the volume of concentrated solution that needs to be added to the stock tank to attain the desired chlorine concentration.

For this example, we need to add 2.13 oz. (you can also calculate gallons and liters) of concentrate solution to make 500 gallons of solution at 2 ppm chlorine, costing $0.05 to mix the tank and $0.10 per 1,000 gallons of water. The cost per tank is how much it costs you to fill the stock tank, given the information you entered. The cost per 1,000 gallons is a way to compare the costs of other products or sanitizing agents you might consider. This calculator could also be used to determine how to mix solutions other than chlorine, as long as % active ingredient is provided on the label and you know the stock tank size and desired application rate. The calculations for adding chlorine to a mixing tank are the same as for injecting chlorine into an irrigation water system, therefore it is possible to calculate how much chemical (concentrate source) is required to treat a certain volume of water (mixing tank).

Once you know the desired chlorine concentration, how long does the chlorine need to be present in-line to provide adequate control? The next calculator helps you determine how long your irrigation lines need to be to provide that contact time prior to your first application point.

Chlorine contact time calculator

Figure 2A/B. Chlorine contact time calculator helps you determine how to achieve the desired contact time in your irrigation infrastructure to completely sanitize your water. Scenario A represents irrigation infrastructure where a mixing tank is needed provide adequate contact time. Scenario B represents irrigation pipe length alone provides adequate contact time.

A minimum of 2 minutes of contact time with all types of chlorine is recommended to control plant pathogens. Your chlorination system should be designed with either an adequate length of “mixing” pipes to provide contact time or have a combination of inline mixing with a mixing tank that is appropriately sized to provide adequate contact time.

The chlorine contact time calculator asks you to input your pumping rate (gallons per minute) and the diameter and length of pipes through which water injected with chlorine flows and uses this information to calculate contact time. In Figure 2A, an 8” diameter pipe with a 300 GPM flow rate that is 120 feet long was not long enough to provide 2 minutes of contact time; it provided only 1 minute of contact time. A 287-gallon mixing tank was needed after the chlorine injected water flowed through the 8-inch pipe to meet the 2-minute contact time prior to irrigation. In Figure 2B, the same flow rate and pipe diameter were used, but the length of pipe was 230 feet, providing 2 minutes of contact time in-line, prior to water distribution into irrigation lines. If a particular plant pathogen or biofilm management strategy requires a chlorine contact time greater than 2 minutes — you can adjust pipe length to get to desired contact times. It is worth mentioning that chlorine reacts with fertilizers just like it is organic matter or pathogens; residual or added fertilizer will be removed by chlorine injection.

If you are concerned about the presence of plant pathogens in irrigation water, but are under organic production constraints or want to preserve the microbial diversity of the water you apply to crops and cannot or do not want to use chlorine to sanitize water, slow sand filters may be the right treatment technology for you.

Slow sand filter sizing tool

Figure 3A/B. Diagram of the components of a slow sand filtration system (A) and the slow sand filter sizing tool (B) that calculates the surface area of sand bed. The output includes the diameter of the tank needed to meet the recommended surface area to treat a specified volume of water on a daily basis.

Slow sand filters are used to remove human pathogens from drinking water and can be used to remove most plant pathogens (e.g., Phytophthora spp.) from irrigation water. Just as with chlorination, slow sand filters will work better if organic matter is removed from water before water enters the filter. The raw (untreated) water flows into the sand, passing through the “schmutzdecke”, the German word for the filter-skin or bio-filtration layer. The bio-filtration layer forms between the water column and the sand, and microbial communities entrap, slow and remove contaminants from the water (Figure 3A). Water treated by the slow sand filter should be stored in a cistern or reservoir dedicated for storage of irrigation ready, clean water. To find more information about slow sand filters and their operation download this article (ucnfa.ucanr.edu/files/251648.pdf). But how do you size a sand filter that will function for your operation?

To use the tool, first enter how much water you need to apply on a daily basis or the daily water volume to treat. For the example in Figure 3, a grower needs to treat 15,000 gallons per day; this is the total daily volume of irrigation water pumped out of a storage pond in a single day that requires treatment before use. The average recommended flow rate (1.7 gal/hr – ft2 of sand) is a default value, but you can enter a different flow rate if desired. If the selected flow rate is too high, a prompt lets you know that this rate exceeds the recommended flow rate and will result in inadequate disinfection. The tool calculates the surface area or tank diameter of the slow sand filter needed to effectively treat the indicated daily volume of water. It is wise to install two slow sand filters onsite to ensure disinfection capacity. Installing two tanks permits continuous water treatment, even if one filter is shut down for maintenance. If installing two units, each unit can be slightly smaller (the 15.3-foot diameter recommendation with 2 tanks) than the single tank, or similarly sized to maintain redundant water treatment capacity and to expand capacity to treat additional volumes of water for future production needs.

These three tools help simplify how you calculate (1) how much chlorine (or concentrate) to add to a mixing tank to get to a desired sanitizer concentration, (2) how long the irrigation pipes need to be or how large a mixing tank needs to be to provide adequate contact time to completely sanitize water prior to irrigation, and (3) how to size a slow sand filter so you can remove plant pathogens from water without the use of chemicals. Please let us know how you use the information from these tools at your operation.

About the authors: John Majsztrik is a research assistant professor at Clemson University, jmajszt@clem son.edu; Bruno Pitton is a staff research associate in the Department of Plant Sciences at the University of California, Davis, bjpitton@ucdavis.edu; Lorence Oki is a cooperative extension specialist at UC Davis, lroki@ucdavis.edu ; and Sarah White is a professor and nursery extension specialist at Clemson University, swhite4@clemson.edu.