How do I decide on a water treatment system?

Originally published in Issue 13

Know your goals before investing in a water treatment system.

A water treatment system is not going to add value to your product. It’s all about reducing the risk of crop losses.

One of the advantages that ornamental plant growers have over growers of hydroponic edible crops is that ornamental crops are usually produced with some kind of root substrate.

“Most ornamental plant growers are not purely hydroponic,” said Paul Fisher, who is University of Florida professor and floriculture extension specialist. “That means ornamental growers have more options they can use for water treatment compared with a hydroponic system where the roots are bathed in the recirculating solution. For instance, with hydroponics, a grower needs to be especially sensitive to the accumulation of chloride from chlorination or copper from copper ionization in the recirculating nutrient solution.”

Know your water concerns first

Fisher said one of the challenges that growers face with water treatment is the tendency to choose a solution without first finding out what the problem is.

“There are many different potential water quality problems that growers can have,” he said. “These can be broken down into microbial problems (plant pathogens or biofilm), chemical problems (salts, alkalinity and occasionally pesticide residues) and particle (filtration) problems. Growers should think in terms of these three different types of potential problems.

“They should test their water and only then decide on the appropriate solution. No single technology is a silver bullet. In some instances, water treatment companies are aggressively pushing one particular technology that they sell, which may be a good solution for one problem, but not others.”

Fisher said before growers make any decision about water treatment, they need to define what issues of water quality they want to address.

“When a grower sends a water sample to an analytical testing lab, the most common water test is to measure the concentration of dissolved ions,” he said. “These tests could include alkalinity, sodium and chloride, electrical conductivity (EC), hardness (calcium and magnesium) and other ions such as iron or boron in the water.

“A complete lab analysis will help growers select the best fertilizer recipe, because the nutrient solution is a combination of the water source and added fertilizers. For example, if growers have enough calcium and magnesium in their irrigation water, then they may not need to add these nutrients in the fertilizer. Chemical water analysis also helps decide if additional treatment is necessary, such as acidification if water alkalinity is high or reverse osmosis if the EC is high.”

Photo courtesy of Paul Fisher, Univ. of Fla.

Common, uncommon water issues

Fisher said if growers are using well water or a municipal water source, the most likely problems to treat for are alkalinity or high salts, depending on where a grower is located.

“High alkalinity is a very common water treatment issue in our industry,” he said. “Irrigating with highly alkaline water is like adding lime to a crop with each watering. The pH climbs over time leading to iron deficiency. Injecting an acid such as sulfuric, nitric, or phosphoric acid may be needed.”

Fisher said another common issue with water is high EC. Typically the most common cause for this is sodium chloride. He said reverse osmosis is one of the treatment options for high EC where ions are removed when water is passed at high pressure through a membrane.

“One of the biggest differences from one hydroponic location to another is the incoming water quality,” he said. “For example, in the Midwest if there is a limestone aquafer and growers are using well water, there may be enough calcium and magnesium that these nutrients don’t need to be in the fertilizer solution. In contrast, in parts of the Northeast and North Carolina where the water has a low EC, growers must choose a fertilizer that is going to contribute most of the nutrients.

Fisher said another challenge with EC management that is important for hydroponic growers is to know what is making up the EC in their recirculating solution.

“For example, nutrient levels drop over time because of uptake by plant roots, but the water source contains a significant amount of dissolved ions,” he said. “Then much of the EC may be coming from sodium and chloride rather than nutrients such as nitrogen, phosphorus and potassium. These growers will have to do a certain amount of replacement of their nutrient solution. For example, they may have to dump a certain amount of their nutrient solution every two weeks to prevent the sodium chloride from accumulating. This can be an environmental hazard (encouraging eutrophication of water supplies) and also increases fertilizer costs.”

Fisher said once growers deal with common water quality issues they may face issues that are unique to different parts of the country.

“I am working with a grower in Indiana and another grower in Florida who have high iron in their water,” he said. “The iron is clogging filters either directly because of rust particles or because of bacteria growing on the iron. There can be a mix of iron that is already a solid particle, which is rust, and there is also dissolved iron.

“The process of removing iron is to oxidize it and turn it into rust. This can be done using chlorine or potassium permanganate or some other oxidant. Ozone could also be used. Once the iron is turned into rust the water can be run through a sand filter. The filter will trap the iron particles. The filter will have to be washed out periodically to remove the particles. These are examples of why it is important to test the irrigation water first, identify the issues, and choose appropriate solutions.”

Biological issues

Fisher said if growers are using well water or municipal water it is very unlikely that the water is going to be the source of a plant pathogen. These water sources may be helping to distribute a pathogen if growers are recirculating the water, but the incoming water is likely to be very clean. He said when the water source is surface water, from a pond, or from a recirculation tank, it’s more likely that the water could be a significant source of pathogen inoculum.

Fisher said one of things that can happen with any of these water sources is that there are three types of biological problems:

1. Plant pathogens

2. Biofilm

3. Human safety bacteria (i.e. E. coli)

“The most common pathogens that would be favored in irrigation water are the oomycetes of Phytophthora and Pythium,” Fisher said. “If growers have root disease problems and suspect that their irrigation water may be a part of the disease distribution, they can send a water sample to a university extension lab for testing. However, it can be hit-or-miss as to whether or not a pathogen is going to be present in a particular water sample. Routine sampling of irrigation water for disease detection is not something that most growers normally do because of the time and cost.”

Fisher said many of the state extension plant diagnostic testing labs are able to run samples for plant pathogens.

“The labs typically plate organisms out to the genus level of the organism, identifying whether it is Pythium or Phytophthora,” he said. “It really matters a lot what the species is, which many labs are able to analyze, although this may take longer and cost more. Pythium can be quite ubiquitous. Phytophthora tends to be more aggressive than Pythium.

“The University of Guelph diagnostic lab will check the DNA fingerprint of what’s in the water. The lab can compare a sample with a data base of other plant pathogens.”

Photo courtesy of Mike Evans, Univ. of Ark.

Dealing with biofilm

Fisher said when growers contact him with a biofilm problem, he asks them to send samples to a water testing lab to measure the aerobic bacteria count from different sampling points in their irrigation system. Usually, but not always, he said, well water has a low bacteria count.

“If growers are using pond water, it is very likely that there is going to be a high bacteria count,” he said. “These high bacteria counts occur because of the presence of microbes including cyanobacteria and other algae. When there are very high bacteria counts, growers usually have to treat for microbes if they use mist nozzles or drippers. The microbes may not be plant pathogens that cause disease, but they may clog irrigation emitters and filters.”

Fisher said if growers have a biofilm problem, they need to determine where the bacteria are coming from.

“Growers would collect water samples from the water source, after the water is chlorinated, after the fertilizer is added to the water, and out in the greenhouse,” he said. “By testing samples from these different locations will identify where the bacteria are growing in the irrigation water and where the water treatment needs to occur. It will also tell growers, whether the treatment systems they are using, for example, chlorine, chlorine dioxide or ozone, are effectively controlling the microbes.”

Particle issues

Fisher said particles in the water could include algae from pond water or sediment (clay, silt or sand). These particles can clog up filters and water emitters.

“Water testing labs should be able to provide a measurement of turbidity, which is the clarity of the water, and also the amount of total suspended solids (TSS),” he said. “A lab will take a specific water sample volume, filter it through a very fine filter and then dry it down and weigh it. This will determine the TSS in terms of milligrams (weight) of particles in a liter of water.

“From experiences with growers, if there is more than 5 milligrams of suspended solids per liter of water, it is quite likely that there are enough particles in the water to cause some issues in the irrigation lines.”

Fisher said growers who are using municipal water typically use screen filters.

“It is unlikely that a high concentration of suspended particles will come from a municipal water source,” he said. “For risk management purposes, however, growers usually install one or more screen filters with enough filtration to remove any suspended particles that are large enough to clog up the finest irrigation emitters in the system.”

In the case of well water, Fisher said growers occasionally may pull up some suspended particles like silt that may require they install some additional filtration.

He said there are two kinds of recirculated water. Pond water usually comes from the water that is drained off outdoor areas or as runoff from a greenhouse. The other source of recirculated water drains off from ebb-and-flow concrete floors or troughs/benches in a greenhouse and is stored in concrete tanks. Water from these sources has similar needs in regards to filtration.

“Pond water will contain algae and other bacteria,” Fisher said. “With ebb-and-flood systems there can be root substrate and plant debris. With pond water there are usually pumps that are pumping water through a filter and then the water, which is under pressure, goes all the way to the greenhouse. There is usually a series of filters for organic materials, including disc filters, sand filters and sometimes screen filters.

“The greenhouse that is being filled with water and then drained back is filtered and stored in another supply tank. This is typically where paper filters, vibrating screen filters and rotating drum filters are used. This is usually a gravity-fed system.”

Agrichemical residues

Fisher said if growers suspect they are having a problem with their crops that is not related to nutrition or disease, it may be an agrichemical issue.

“Growers may suspect there is something toxic in their water that might be herbicide runoff from a neighboring farm or it may be growth regulator residues from past applications,” he said. “There are special labs that are able to test for these chemicals. But growers need to know what chemicals to specifically ask a lab to test for.

“In my research program we are doing a lot of work on removing paclobutrazol residues from irrigation water using carbon filtration. Paclobutrazol has a half-life of about six months in irrigation water. It is normally applied in the parts per million range. But the chemical has activity in the parts per billion range, even as low as 5 parts per billion, on sensitive crops like begonia. There can be some leachate from the spraying or drenching of paclobutrazol that gets into recirculated irrigation water that can then impact untreated plants.”

Keep the system clean

Fisher said growers should try to keep their irrigation systems clean, but they don’t have to sterilize them.

“Cleaning out the recirculating tanks, greenhouse surfaces and irrigation lines several times a year is good idea,” he said. “Although most of the microbes in a recirculation system are likely to be beneficial or benign, the equipment can start to clog. There is going to be algae growth and there is the possibility of pathogen spores getting embedded in biofilm. The goal is to keep the system clean, but there is no need to continually kill all of the organisms in the system.

“Growers who are not using fine drippers or mist nozzles are less likely to have a problem with clogging from biofilm.”

Fisher said after power washing the water storage tanks growers can apply an agricultural cleaning product, such as Strip-It, which is widely used. This helps to remove biofilm.

“This treatment may keep the system clean enough that it is not necessary to continually inject some type of sanitizing agent,” he said.

Maintaining dissolved oxygen levels

Fisher said dissolved oxygen is mainly an issue for hydroponic growers because roots are bathed in the nutrient solution. In contrast, when growing in a root substrate with a high level of air porosity (from large particles in the substrate), the roots will receive adequate oxygen so long as the plants are not overwatered.

“If growers are using a nutrient film technique (NFT) system, then the movement of the water helps oxygenate the nutrient solution,” he said. “Aeration of the nutrient supply tank may still be required.

“With floating pond systems, low oxygen conditions are likely to occur. If the water temperature is warm, there is going to be a lot of biological activity occurring and respiration by the microbes. Warm water also holds less oxygen than cool water. It is a good idea to install some type of bubbler. A bubbler creates small bubbles that add oxygen to the water and raise the dissolved oxygen level. If the oxygen level becomes low in a hydroponic solution then it can favor pathogenic organisms such as Pythium. The water should contain at least 5 parts per million of dissolved oxygen.”

Investing in a treatment system

Fisher said growers should place their emphasis first on ensuring their plants are healthy and growing well. Their incoming water should be from a high quality well or municipal source and there should be a high level of overall sanitation. With this foundation in place, he said, an expensive water treatment system may not be needed.

Fisher said growers need to think in terms of profitability of their business when considering water treatments.

“Margins are tight for most growers so they need to think about how they are going to generate a positive return on their investment,” he said. “If there is an existing chemical, biological or physical water quality problem that has been clearly identified (including lab testing), then investing in a targeted water treatment solution to that problem will rapidly be paid back.

“However, if growers are spending money on a water treatment system that they don’t need, then they don’t have that capital available to spend on an alternative investment such as supplemental lighting that could increase their yields.”

Fisher said a water treatment system is not going to add value to growers’ products.

“No one is going to pay growers more for their product just because they have installed a water treatment system,” he said. “It’s really about crop losses and reducing the risk. When growers have a root rot problem caused by a water-borne pathogen, then they can very quickly pay back the benefits of a water treatment system.”

 


For more: Paul Fisher, University of Florida, Institute of Food and Agricultural Sciences Extension.

David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.

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