Greenhouse Water Treatment and Filtration
Water Treatment for Specific Ions
Specific ions can be toxic to plants and/or detrimental to the substrate’s physical structure. Certain ions (e.g., sodium, chloride, and boron) can cause direct root injury, accumulate in shoot tissues and cause shoot toxicity problems, or cause direct foliar toxicity on plant leaves. These problems are almost always present when total salinity is high. Tolerances to specific ions vary among plants and their cultivars.
Boron
Boron (B) is an essential element for plant life, but it can be toxic even at very low concentrations. Levels of 0.2 to 0.5 mg/L are considered normal in irrigation water. However, levels of above 0.3 can be harmful to sensitive crops. Plants have different levels of tolerance ranging between the two extreme values. The toxic effects of boron are initially apparent in old leaves in the form of yellowing, chlorotic spots or dried tissue at the tip and edges of the leaf.
Calcium and Magnesium
Water hardness is a measure of the amount of calcium and magnesium dissolved in the water expressed as if it were calcium carbonate (CaCO3), commonly referred to as limescale or lime. The precipitation and deposition of calcium carbonate (lime scale) in greenhouse micro-irrigation systems is one of the most common causes of system plugging and the associated loss of irrigation efficiency. Precipitation of calcium carbonate can occur in one of two ways: evaporation of water leaving the salts behind, or a change in solubility due to changes in solution characteristics (mainly temperature or pH).
Ion Exchange
The most effective way to treat hard water is to install an ion exchange resin softener. This softening equipment works best when pH is between 7.0 and 8.0 and water temperatures are less than 0 degrees F (32°C). In this process, the resins are used to remove calcium and magnesium from the water by exchanging their ions, in a sense, with the soft ions of sodium or potassium.
Lime Softening
In lime softening, hydrated lime, that is, Ca(OH)2 calcium hydroxide, is added to water to precipitate calcium carbonate before the water is used.
pH Adjustment with Acid
Acid can be used to lower the pH of irrigation water to reduce the potential for chemical precipitation and to enhance the effectiveness of the chlorine injection. Since precipitation occurs more readily in water with a high pH (above 7), precipitation of these compounds can be prevented by continuous injection (whenever the system is operating) of a small amount of acid to maintain water pH just below 7.
Chloride
Though not usually considered an essential micronutrient, chloride is needed in small quantities by plants. However, chloride levels greater than 70 ppm (2 meq/L) can become a production problem. The principal effect of too much chloride (Cl¯) is to increase the osmotic potential of the substrate solution, which reduces the availability of water to plants and can lead to wilting.
Fluoride
Fluoride is often added to municipal water at a concentration of 1 ppm to prevent tooth decay. This level is safe for most crops but not for members of the lily family such as the genera Chamaedorea, Chlorophytum, Ctenanthe, Dracaena, Marantha, Spathiphyllum and a few other plants. Toxic levels of fluoride can scorch of the tips of older leaves.
Iron and Manganese
Iron can be present in a water supply in many different forms (soluble, chelated, organic, and precipitated) and may or may not be apparent to the eye. These forms include ferrous (Fe2⁺) or dissolved iron, which is invisible, while the ferric (Fe3⁺) or oxidized (rusted) iron becomes apparent through precipitation, and usually appears as brownish red colored particles suspended in the water. Irrigation water with iron levels above 0.1 ppm may cause clogging of drip irrigation emitters and above 0.3 ppm may lead to iron rust stains, and discoloration on foliage plants in overhead irrigation applications.
Depth of Irrigation Intake from Holding Pond
Greenhouse growers can reduce the problem of iron deposits by making sure that their irrigation intakes are located 18 to 30 inches (46–76cm) below the surface of the water in the holding pond.
Basin Aeration Pump
If pond water has a high, iron content, then consider using a basin aeration pump. Aeration may be the only practical method for dealing with high iron concentrations. It causes oxygen to dissolve in the water, which changes the iron or manganese to the insoluble forms; it releases dissolved carbon dioxide gas, increasing the pH of the water; and it releases or partially releases other dissolved gases such as hydrogen sulfide.
Tank Oxidation
Another option in removing iron is tank oxidation, followed by sedimentation, and then filtration. If enough space is available, pump the source water into a tank where the insoluble iron compounds can precipitate and settle out. The water is often pumped in as a spray for rapid oxidation of the iron to an insoluble form. Other ways of aerating the water include bleeding air into the intake side of a pump, agitating the water with propellers or paddles, or cascading it over baffles into a settling tank.
Oxidizing Agents
Other methods of oxidation include the use of oxidants such as chlorine, chlorine dioxide, ozone, and potassium permanganate. For a complete precipitation of iron, it is recommended to add a base to raise the pH. Iron precipitates more readily as the pH is raised above neutral. Chlorination is widely used for oxidation of divalent iron and manganese. To reduce the amount of chlorine injected, removal of organic residue is recommended, which requires filtration.
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