Water treatment series, Part 1: Treatment options

Over the last two years many parts of the continent have experienced problems with water quantity or quality. Water is a major issue for horticulture. Drought conditions and water restrictions in the Southeast are forcing companies to adapt or go out of business. With increasing urbanization, water-management authorities are regulating supply and runoff as growers and landscapers compete with both rising consumer and commercial water demands.

Growers recognize the importance of water conservation and recycling. However, the potential spread of waterborne pathogens in irrigation water has also highlighted the need to treat and manage water quality, especially in recycled water. Choosing the best water treatment method can be confusing.

Water treatment is an area of emerging technologies and innovation, with gaps in our industry’s current knowledge. Water treatment for horticulture involves perspectives of water chemistry more common to the swimming pool and municipal treatment industries than horticulture, in addition to plant pathology, engineering and financial analysis.

(PDF chart: Treatment options for waterborne pathogens in greenhouse irrigation systems)

Define goals and priorities

The goal for water treatment is not to sterilize the system. This is unrealistic and undesirable because many microorganisms are beneficial or benign. The real goal is to minimize the pathogen risk from water as part of an overall sanitation program -- without blowing the grower’s budget.

Systems approach

A big-picture perspective is required to evaluate the overall flow of water in an operation. A water-treatment specialist can help to identify the potential points of contamination that can lead to the correct combination of technologies needed.

There is no one silver bullet when it comes to water treatment. Many technologies work. The questions that need to be answered are which are the most cost-effective, how do they interact and how can they be used effectively?

Filtration

Filtration and water pretreatment underlies all other treatment technologies. Treatments such as ultraviolet light require clear water for wavelengths to penetrate pathogen cell walls. Oxidizing materials such as chlorine will react to any organic material, whether it is peat or a pathogen cell wall, and are therefore less effective in the presence of growing media and plant debris. This series will explain treatment methods ranging from reverse osmosis that removes “almost everything,” including all pathogens, to options such as screen, sand and media filters.

Water pH and electrical conductivity

Chemical characteristics of the water source affect most treatment technologies. For example, if the water electrical conductivity is low, then a copper ionization would need to be engineered correctly to increase the copper electrode surface area in order to provide the desired parts per million of copper. If the water pH is above 7.0, using chlorine may require acid injection for pH control.

Technologies available

Many water-treatment options are available (see Table 1). The challenge is to decide which one is best for your setup. Specific details on each of these technologies will be provided in upcoming articles.

Some equipment, such as copper ionization, ultraviolet light and ozone, have high initial investment costs, but operating costs are low and they can treat large water volumes cost-effectively. Other injectable materials such as chlorine dioxide have low initial investment costs, but have a higher operating cost and may be better suited for plants that require the highest-quality water.

Treatment considerations

Cost is only one consideration in choosing a water-treatment method. For example, technologies also vary in their mode of action, for example, oxidizers versus toxins. Some treatments, including copper and chlorine, have a residual, downstream effect, whereas other treatments are a single-point treatment. Some materials are designed only for continual low-level treatment, whereas others are also an effective shock treatment to reduce biofilm or chlorine dioxide or are suitable for surface sanitation.

Systems may be combined. An example is the addition of activated peroxygen in conjunction with ozone and/or ultraviolet light increases the sanitizing power of all these technologies.

Flexibility is needed. Increasing levels of bacteria, fungi and algae have been measured as greenhouses operate through winter into spring in response to increasing temperatures, high light levels, increased plant debris and more employee movement. Because of these seasonal variables and periodic disease events, growers may need to increase or decrease water treatment.

Water treatment choices

Water treatment is part of an overall sanitation program. A systems approach is needed to correctly engineer the flow, filtration and treatment. Every application is different. For a specific example of how one greenhouse operation decided to combine treatment technologies, visit the Water Education Alliance for Horticulture Web site, www.watereducationalliance.org.

- Paul Fisher, William Argo, Ratus Fischer, Peter Konjoian, Rob Larose, Alan Miller, Gary Miller, Robert Wick and Rick Yates

Paul Fisher, University of Florida, Environmental Horticulture Department, (352) 392-1831, Ext. 375; pfisher@ufl.edu; William Argo, Blackmore Co.; Ratus Fischer, Fischer EcoWorks; Peter Konjoian, Konjoian’s Floriculture Education Services; Rob Larose, BioSafe Systems; Alan Miller, Whitmire Micro-Gen; Gary Miller, PPG Industries; Robert Wick, University of Massachusetts; and Rick Yates, Griffin Greenhouse and Nursery Supplies.

Notes on water treatment options

* Desired concentration depends on the application. See product label and manufacturer’s instructions for specifics.

* All treatment methods mentioned are nonspecific and react with any type of organic matter, whether it is a pathogen, algae or a particle of peat. In all cases, the cleaner the water is before the treatment, the more effective the disinfection method is at removing pathogens.

* Bromine, chlorine products, ozone, peroxyacetic acid and hydrogen peroxide are strong oxidizing agents. Metal micronutrients (copper, iron, manganese and zinc) are easily oxidized (particularly iron). It is likely that long-term exposure (over 20 minutes) of metal micronutrients to these oxidizing agents will decrease their solubility. Chelated micronutrients should only be slightly less affected than sulfates.

* Ultraviolet radiation is a photo-oxidizing agent. Studies by Cornell University researchers on photo-oxidation of iron in fertilizer solutions indicates that the greater the light exposure, the less iron that will remain in solution.

* Quaternary ammonium compounds such as Green-Shield, Physan 20 or Triathlon are listed for disinfection of walkways, benches, tools and flats, but are not for use with irrigation water.

* Liquid hydrogen peroxide/hydrogen dioxide (H2O2) solutions (35-50 percent H2O2) are not U.S. EPA-registered for water treatment in greenhouses. H2O2 solutions are less effective and stable compared with registered activated peroxygen products.

Partnerships lead to water alliance

To help growers make effective water-treatment decisions, University of Florida has partnered with many leading industry and university experts, companies and organizations to form the Water Education Alliance for Horticulture.

Partners in the alliance include TrueLeaf/Aerotech, BioSafe Systems, Griffin Greenhouse and Nursery Supplies, Hanna Instruments, PPG Industries, Whitmire Micro-Gen and the Young Plant Research Center partners.

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Workshops on water treatment will be conducted at the OFA Short Course, Plug and Cutting Conference and other events supported by the alliance. Information on these events and additional water treatment resources are available online at the alliance Web site, www.watereducationalliance.org.