for Indoor Growing
By Janeen Wright, Editor, Greenhouse Grower
Water is a precious, finite resource essential for sustaining plant life. Yet not all water is good water. Indoor growers cannot afford to overlook water quality, especially as water resources grow scarcer. Water down the drain, in the form of runoff, is money down the drain. With the rising costs of fertilizer and water, growers benefit economically from recycling and reclaiming water, along with minimizing runoff. On a wider scale, the public has agriculture in its sights as one of the leading sources of water pollution. It is in every grower’s best interest to manage their water efficiently and sustainably with the water treatment methods and tools available to them today, and the promising technologies of the future.
Balancing nutrient loads in recycled water and preventing pathogen development isn’t always a straightforward process, and growers have the challenge of offsetting optimal treatment strategies with economics and practicality. While many operations conduct inline testing for electrical conductivity (EC), pH, and more, testing for individual nutrients is more complex. It takes time to get results back from offsite labs and much can change during that lag time. Onsite testing options would be a grower’s best bet for accurate, up-to-the-minute results.
“Growers and suppliers realize there is a real opportunity here,” says Paul Fisher, Professor of Environmental Horticulture at the University of Florida. “If we can get more onsite options for testing nutrients, it will make balancing nutrients a whole lot easier. There is a lot of development work going into ion-selective probes for onsite nutrient testing, but biofouling [the accumulation of microorganisms on the probe surface] and other ions in the solution makes it a challenge to obtain accurate results. I have also seen growers using both small onsite kits for individual nutrients and Ketos Shield, which is an interesting technology where nutrient solutions pass through a box that measures ions using reagents.”
As more onsite testing technologies come online, barriers such as cost and manually taking and sending samples away to a lab could become less of a problem. And, growers may have access to these options sooner rather than later. Al Zylstra, Division Manager of DRAMM Water, assists with the evaluation and development of future water-testing technology for DRAMM. Some of the water-testing options in development at his company are close to the trialing phase and eventual introduction to the market.
“The ability to do on-site testing of nitrogen, phosphorus, potassium and your major micros is actually here in the present and can be done,” he says. “Within a year, most growers that want to do onsite testing will be able to do a majority of that testing very easily, without having to rely on a person in a white lab coat. They may have to send a water analysis into a lab around four times a year to cross-check themselves and look at other nutrients, but the reality of more regular onsite testing is here.”
Fisher says some of the most interesting water treatment methods on the horizon are ecological, such as bioreactors and constructed wetlands. One type of bioreactor is a pit filled with woodchips and water. Microorganisms from the soil use the woodchips as a food source. As the microorganisms consume the carbon in the woodchips and nitrate from the water, they remove nitrogen, and even some pesticides, from water.
Thomas Fernandez, Professor in the Department of Horticulture at Michigan State University, is studying the potential of bioreactors for removing nitrates, phosphates, and pesticides. He investigates how the pesticides in water that is run through bioreactors influence bioreactor activity.
“We found that bioreactors are very effective at removing nitrates, phosphates, and pesticides, depending on how fast the water moves through and how long it stays in the bioreactor,” Fernandez says. “The longer the water stays in, the more it removes. Pesticides were still removed at fast flow rates while less nitrate and phosphate were removed, making it possible to retain nutrients in recycling systems. The pesticides in the water also didn’t change the activity of the microorganisms or kill them, and they had no effect on nitrate reduction. The bioreactors still had well over 95% effectiveness in nitrate removal. Phosphorus removal was a little lower — around 90% and above.”
Fernandez just finished a project with a large, fast-flowing bioreactor installed in a greenhouse operation. He will publish the full research in the future, but preliminarily, he says the research results are promising, particularly with pesticide removal.
“The greenhouse operation used around 72 pesticides while the bioreactor was in operation,” he says. “Of the 50 that we could test for, we found only nine pesticides in the water that we could identify. The rest we couldn’t find. This tells us the pesticides aren’t moving in the water in this operation. And in the ones that we did find, we found the bioreactor could reduce pesticide levels by as much as 40% at the fast flow rate.”
Sarah White, Professor and Nursery Extension Specialist at Clemson University, is evaluating the potential of floating treatment wetlands as alternative production and remediation systems for nursery and greenhouse operations. This method uses the roots of plants floating on the surface of a pond or reservoir to take up nutrients and clean the water. It is a relatively low cost and effective method if the grower already has a pond or reservoir onsite, but there is some regular upkeep of the floating treatment wetland required.
Fisher is evaluating cold plasma, which is an emerging technology where an advanced oxidation process oxygenates and disinfects water using only air and electricity.
“Cold plasma can both oxidize bacteria, algae, fungi, and oomycetes, and oxygenate the water for root health,” Fisher says. “Cold plasma and ozone are two very compatible technologies within a closed loop irrigation system, because they are very reactive and break down into oxygen without harmful residuals. This increases the dissolved oxygen in the solution.”
Filtration is another area vital to effective water quality treatment. Fisher says he has found activated charcoal filters to be very efficient at removing chemical residues. Another trend he sees is growers turning to disc filters in place of the traditionally used sand filters because they are easier to clean and there is not as much backflushing.
“When you are circulating a valuable nutrient solution through your system, you don’t want to lose a bunch of it due to the difficulty of cleaning out a sand filter,” Fisher says. “I am seeing more use of the disc filters in both the U.S. and Europe.”
Tim Reusch, Dramm Water National Project Sales Manager, is a big proponent of ozone for water disinfection, saying it is a powerful oxidizer that works quickly and efficiently, leaving only a minor residual to eliminate biofilm, with no residual reaching the plant roots. Growers often need to jack up the rates of other chemical oxidizers because they build up resistance and eventually get to the point where they are toxic to plants. Ozone does not build up resistance and has the added benefit of adding dissolved oxygen to the water. Dissolved oxygen is beneficial for the water system and the plants, Reusch says.
Jim Owen, USDA Research Horticulturist, works to improve the quality of water leaving container production sites, particularly at the nursery level, using stratified substances or the layering of fine growing media and coarse growing media to impact how water moves through the soil.
“You can use 20% less water with this approach. If people use controlled-release fertilizers (CRF), it allows them to incorporate the CRF into the top half or strata versus throughout the entire profile, where it may not be utilized as effectively,” Owen says. “If we could look further and figure out what substrates retain more nutrients and dial in that fertility level for nitrogen and phosphorus, we will have a lot less nutrients moving off site and forming algal blooms in greenhouse and nurser reservoirs or in the larger water bodies that surround them.”
Owens, Fernandez, White, Fisher are among the researchers involved with CleanWateR3, a Specialty Crops Research initiative grant focused on helping growers reduce, remediate, and recycle water. The group just completed a round of research examining water conservation and contaminants, as well as the efficacy of water treatment methods. They have now applied to the USDA for a new CleanWateR4 grant, with research projects that will concentrate on scalability and practical application at scale in commercial operations.
“We are looking at combined treatment approaches. No one treatment will work for everything, so we want to know what combinations work best,” Fernandez says. “We are trying to improve the water, not just for growers, but to help with current and future regulatory environments.”
In his work, Fisher is looking at the best way to design treatment systems with controlled-environment agriculture (CEA) operations. Most water treatment methods are not selective, meaning they can destroy many components when they clean water, both the bad and the good. For example, an ozone injector might oxidize pathogens and E. coli (bad contaminants) while also oxidizing chelated micronutrients and beneficial microorganisms (desirable components) that are needed to improve root health.
“I think the next threshold and paradigm shift in water treatment and CEA is around system design, with the goal being to remove the bad guys while maintaining root health,” Fisher says. “Some of the technologies are like a hammer, and you want to whack down the right nails, so practically speaking, I think there are still a lot of the water treatment challenges with CEA that we need to address. For the most part, though, CEA growers are all trying in a challenging situation to close the loop in a sustainable and economically efficient way.”
