August 2011

Sustainable Solutions to Greenhouse Irrigation Management By Thad Humphrey

Water is vital to plants. It's the lifeblood of a greenhouse and how it's managed has a direct impact on the success of a grower. As the demand for water increases, finding sustainable solutions to capturing, containing and using it become critical to both plant quality and the financial success of the grower.

Water is all around us and recycling this water is no new concept. Our environment has been constantly recycling the same water since its existence. Collected sources of water evaporate into the atmosphere; the water condenses and precipitates and is then recollected for use once again. But just because water may be near us doesn't mean it's readily available for use. As a result, growers commonly use traditional sources of water, such as local wells or municipal supplies, which may be treated beyond your needs (meaning you could be paying too much for water). In addition, these water supplies could have withdrawal restrictions or limitations. So how can we find alternative ways to sustainably manage water with minimal chemical use to match each individuals needs? The answer may be right under your feet or just over your head.

Step 1: Recognize your available water source alternatives.

Where you get your water depends on where your greenhouse is. There are many alternative sources for obtaining water but for purposes of this discussion, we'll stick to some common and practical solutions. Ponds can be a great source of stored water and can potentially accommodate a high capacity delivery system in addition to be a redundant supply of water.

Another alternative is a rain water collection system. Capturing rain from roof gutters (which is common place in Europe) is a natural consideration as the collection system tends to be built into common greenhouse structures. Creating a gravity distribution system to divert this captured water to nearby storage tanks can be accomplished at reasonable costs and low operations and maintenance costs.

The hurdle that prevents most from using these sources of water is the corresponding water quality. The key to making this water usable is being able to cost effectively treat it.

Step 2: Use sustainable methods to treat your water.

There are a variety of ways to properly treat water for greenhouse use. Choosing a treatment method that minimizes consumable resources would help to increase its sustainability. Filtering is the appropriate solution to clean up the bulk of the undesirable suspended solids and particulate in the water. But before we can filter, we need to consider our water source.

For sources of stagnant water (such as ponds, large cisterns, etc.), aerating the water is a low cost means to produce a balanced dissolved oxygen (DO) level throughout the water column from surface to bottom. Managing the waters DO content means you're also managing the water's biochemical oxygen demand (BOD). Preventing the water from being oxygen depleted also prevents: algae blooms, anaerobic pathogens, anoxic release of nutrients, metals and phosphates, release of hydrogen sulfide, etc. Essentially, properly aerating the water will remove the bulk of large suspended solids and provide an oxygen-rich water supply.

A traditional method of aeration is installation of fountains in ponds. This method brings water from a central collection point and redistributes it to a point in the oxygen rich atmosphere above the pond surface. It also induces mixing of this oxygen-rich water with the surrounding water within a fixed distance. The drawback to this method is that you're essentially bringing the entire body of water up to the atmosphere to be oxygenated with a pump. This requires many fountains in addition to corresponding electrical costs to operate the pumping system.

A sustainable and highly effective alternative to fountains is bringing the water to the pond surface by air. Air (which can be augmented with trace amounts of ozone) is introduced at the bottom of the pond (by means of an air compressor system) through a series of weighted air diffusion tubing. The diffusion tubing has integral micro cuts along the top that act as check valves preventing water from entering the tubing. Once pressurized air fills this tubing, the microcuts open allowing micro bubbles to escape. As the micro bubbles rise, they move water from the bottom of the pond to the top where oxygen is introduced. Essentially, air bubbles gently mix the pond water by taking advantage of gravity and the surface tension of the bubbles. The result is oxygen rich water made from a simple system that has low operating and maintenance costs.

Now that the aeration has removed the BODs from the water, it's ready for filtering. There are many filter alternatives (such as pressure filtration, roll media filtration, vibratory filtration, etc). These are very effective methods and are commonplace with most growers. However, all these tend to be energy intensive processes or may have filter media requiring constant replacement. Drum filters are a sustainable solution to filtration as they can filter below 30 micron, they use media that requires infrequent replacement and they filter by gravity (to minimize energy costs).

Step 3: Recycle your water.

One method to cut down on the use of potentially expensive pretreated water is to recycle it. This requires your growing surface to be designed to contain, collect and distribute the water. Typically, this is achieved through flood floor systems, flow floor systems, flood bench systems, troughs, etc. Water is collected and held in storage tanks. This water is then fertigated and distributed to the floors or benches through a pumped distribution system. Water is typically introduced to the plant from the bottom up, allowing the water to be wicked up by capillary action. Water is then drained and recaptured in drainage pipes, passed through a filtration system and reintroduced into the storage tanks to be reused again.

Of course, a primary concern in reusing water is managing potential pathogens that could enter the system, which brings us to our next step.

Step 4: Use sustainable methods for pathogen removal.

Pathogen management is critical to the health of your plants. This step is most effectively done after filtration to minimize the introduction of chemicals into your system. However, there are sustainable alternatives that can achieve the same result (or better) and virtually eliminate the need for storing chemicals. Here are a few options for consideration.

Slow sand filtration. This process involves passing water (by gravity) through a bed of sand at low velocities resulting in substantial particulate removal by means of physical and biological mechanisms. The bed of sand provides the support medium for the biological treatment layer (biofilm). Because flow through the sand must be controlled to let the biofilm perform, the process is slow. The advantage is no chemicals are required and the biofilm adapts to the pathogens that are introduced to it.

Ozone. Ozone (O3) is an inherently unstable molecule that is ready to give up one atom of oxygen making it the most powerful commercially available oxidizing agent. It can be made through passing oxygen through a tube which discharges electricity. The produced ozone is then distributed as a gas and injected into water by means of a venturi tube or diffusion into a contact tank. With proper contact time, the water is disinfected. Determination of how much ozone your system needs varies depending on your regional water quality and the water quality of your recycled water (which varies throughout the year). Ozone systems can be installed to disinfect as a batch process or on demand. Because ozone is a disinfectant, it will drop out anything in the solution it oxidizes including some components of water soluble fertilizer. Ozone systems typically are installed with filters to remove the particulate that drops out of the solution. Fertigation should always occur downstream of (if on demand) or after (if batch treated) the ozone process. Because ozone is created as it's needed, no chemical storage is required on site. Also, the ozone process requires very little electricity making its operating costs low.

Ultraviolet irradiation (UV). Using electromagnetic radiation, water is passed across an ultraviolet light emitting a wavelength that's considered germicidal. This breaks the molecular bonds within the pathogens and can destroy them. For the light to properly irradiate, the water passing across it must be highly filtered to minimize suspended particles from blocking the light (called "shadowing").

Augmenting or replacing water with sustainable sources of treated water can make a more robust and cost effective irrigation system. Since there is no simple solution to managing irrigation water, applying sustainable solutions is a consultative process and should be well planned to ensure the methods selected best fit your greenhouse needs. So next time you're looking for water management alternatives, look up to the sky or look down on the ground with a new perspective. That extra water you've been searching for may have been there all along.