Chapter 11

Irrigation Water for Greenhouses

Treating Greenhouse Irrigation Water for Individual Salts

Excess amounts of some nutrients in irrigation water may damage greenhouse crops. Irrigation waters differ widely in concentrations of nutrients. High concentrations of chloride (Cl) or boron (B) can damage crops. Other nutrients supplied by irrigation waters may also satisfy or exceed crop needs.

Iron

Iron chemistry is complex because ionic Fe can exist in two forms. The reduced cationic form, exhibiting two plus charges, is the ferrous form (Fe2+), which is invisible. The ferrous form may be introduced into the irrigation system with the source water because this form of iron is soluble. Chemical conditions may change within the irrigation system itself, resulting in formation of a highly insoluble oxidized form with three positive charges (ferric, Fe3+). It is the ferric or oxidized (rusted) form that becomes apparent through precipitation, and usually appears as brownish red colored particles suspended in the water and causes problems within the irrigation system.

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 below the surface of the water in the holding pond. Intakes too close to the bottom pull settled iron sediment off the bottom of the pond (See Figure 11.11). Those too close to the surface pull more of the oxidized form and other organisms that flourish on iron such as iron fixing bacteria.

Tank Oxidation followed by Filtration

The recommended treatment to remove iron is oxidation, sedimentation, and then filtration (See Figure 11.12). 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, and cascading it over baffles into a settling tank.

Chlorine Injection

Chlorine is often the material of choice as a disinfectant and oxidizing agent. To be effective the chlorine requires a one-minute contact time in the irrigation water to kill iron bacteria or have an oxidizing effect to change ferrous to ferric iron. Chlorination eliminates the food source for the iron bacteria and also eliminates both types of iron deposits on surfaces irrigated. Injection must occur upstream of the filter to remove the precipitate. Usually sand media filters are installed and two media filters generally are recommended so one can be backflushed while the other filter operates during irrigation.

Threshold Inhibitors

Inject an inhibitor that prevents iron precipitation. Compounds such as polymeric acid, polyphosphate, and phosphonic acid have been used as inhibitors. These compounds must be injected prior to any oxidation of ferrous iron to ferric iron, and they do not work if ferric iron has already formed. The injection rate for the inhibitors is usually less than 5 ppm.

Chelation

Another approach to eliminating problems of precipitates is to keep the iron in soluble form. Polyphosphate chelates added to water attach to the soluble iron and keep it from becoming oxidized. Chelation generally works if the soluble iron and manganese concentration in the water is low (less than 1 to 2 ppm).

Manganese

These filters trap pathogen cells and spores and plant debris in irrigation water in a deep column of sand, recycled glass, packed mineral or glass fibers, and/or other dense substrates. The most notable of these are conventional sand and recycled glass.

Calcium and Magnesium

Calcium and magnesium may need to removed from hard water to eliminate salt deposits left on foliage by overhead irrigation. Reducing hardness is called water softening. Ways to soften water include: ion exchange, desalination processes such as reverse osmosis, use of lime, pH adjustment, and controlling water temperatures.

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 degree 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.

Desalination Processes

Hard waters with a salinity level greater than 2.5 dS/m (1500 mg/L) and chloride level greater than 600 mg/L need desalination for domestic and home garden use.

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. The amount of lime required depends on hardness levels.

pH Adjustment

The aim of pH adjustment is to acidify the water to keep scale-forming calcium and magnesium ions in solution and not allow them to precipitate. Even complete blockages can be treated with acid. Acidifying the water can be done by adding acid to the irrigation system at metered rates. Hydrochloric or sulfuric acid is commonly used.

Fluoride

Fluoride can be removed from irrigation water by adsorption using activated alumina or activated carbon. When using activated alumina, the pH of the water is first adjusted to 5.5. The activated alumina unit can be regenerated with a strong base, such as sodium hydroxide, and reused.

Boron

Boron occurs in many irrigation water sources in the anionic borate form. Anion exchange resins similar to those described for deionization systems can be used, but at considerable expense.

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