Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
The Stories

More ways to save water with hydroponics                                           

By Brandon Merrill

Hydroponic greenhouses use about 10 times less water than a field crop, said Pat Rorabaugh, who works at the University of Arizona’s Controlled Environment Agriculture Center. However, even hydroponics wastes water. Using data from a greenhouse at the center shows just how much water can be saved.

Capture the water vapor in the air
All plants transpire, meaning they release water vapor into the air to keep cool. Of all the water that the plants take up, more than 95 percent will be transpired – releasing water vapor from the plant as part of its water cycle. That water vapor is sucked out of the greenhouse by the exhaust fans. A water vapor condenser in front of the fans could capture much of this water before it leaves the greenhouse.

Chieri Kubota, a Plant Science professor who works at the center, tested this on its one-tenth-acre greenhouse during the summer of 2007. The captured water vapor could cover half of the water needed by the plants during that summer, she reported in a 2007 American Society of Agricultural and Biological Engineers meeting proceedings.

Reuse the excess water that plants didn’t use
Not all the irrigated water is used by the plants. What is not used will be lost as runoff, which will be released into the ground. That water could be captured fairly easily and cleaned by filtering it through a fine-sand tank, and then recycling it back into the system. This could save another 20 percent of the water supply.

Harvest local rainwater
Greenhouses don’t usually collect the rain that falls on the roof. This is a free resource that could be harvested and added to the water supply. On a one-tenth-acre greenhouse in Tucson, the 12 inches of annual rainfall in an average year could provide about 18 percent of the water needed for the hydroponics system. The water could be cleaned as a precaution.

Use wastewater rather than fresh water
Rather than trying to conserve the water so that less of the aquifer or water source is used, changing to another source might be better for conservation. Treated sewage effluent could replace the use of freshwater altogether. As an added bonus, the treated sewage contains extra nutrients for the plant, meaning fewer nutrients would need to be added later. The water must be cleaned, possibly through the same sand filter mentioned earlier, to remove certain residuals from the water, such as heavy metals. The plant can tolerate the residuals, but they can be dangerous for people.

The first three techniques can save about half the water used in a greenhouse, resulting in nearly a 95 percent savings over field crops. Including the treated sewage, the greenhouse becomes a poster child for water conservation.


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