Chapter 11

Irrigation Water for Greenhouses

Treating Greenhouse Irrigation Water for Total Dissolved Solids

Total dissolved solids (TDS) are a measurement of a variety of compounds like minerals, salts and organic compounds that are dissolved into water through contact with rock and other surfaces. TDS comprise inorganic salts and small amounts of organic matter that are dissolved in water. The principal constituents are usually the cations calcium, magnesium, sodium and potassium and the anions carbonate, bicarbonate, chloride, sulphate and, particularly in groundwater, nitrate (from agricultural use). Two commonly used systems to remove of total dissolved solids are reverse osmosis and deionization.

Reverse Osmosis

Reverse osmosis (RO) removes nearly all alkalinity and impurities from the water and can render almost any water source usable for greenhouse production; however these systems are expensive to install and operate so identifying an alternative water source is always advised if such extreme water treatment is deemed necessary (See Figure 11.6). The filter is sufficiently fine to remove ions such as sodium, chloride, bicarbonate, boron, and iron in the water. The system works by osmosis, which is the passage of a solvent (water) through a semi-permeable membrane separating two solutions of different salts concentrations. A semi-permeable membrane is one through which the solvent can pass but the solutes (salts) cannot. If pressure is applied on the solution with a high salt content (the irrigation source water), the solvent (water) is forced to move through the membrane leaving behind the salts. Relatively pure water accumulates on the other side of the membrane. Because pores in the filter are this small, they are easily plugged by particles or ions in the water that could precipitate on the filter. The filter and its maintenance can be costly if the system is not engineered properly. For this reason, particulate matter is removed prior to the RO filter. Irrigation waters differ vastly in the types of particles and precipitating ions, thus the physical and chemical treatments within the RO system are equally variable.

Deionization

Deionization entails removal of electrically charged (ionized) dissolved substances by binding them to positively or negatively charged sites on a resin as the water passes through a column packed with this resin (See Figure 11.7). This process is called ion exchange and can be used in different ways to produce deionized water of various qualities. Examples of cations are: sodium (Na+), calcium (Ca2+), magnesium (Mg2+), iron (Fe2+) and potassium (K+). Examples of anions are: chloride (Cl-), sulfate (SO42-), bicarbonate (HCO3-), and fluoride (F-).

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