Greenhouse Water Treatment and Filtration
Water Treatment 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. The principal constituents are usually the cations calcium, magnesium, sodium, and potassium and the anions carbonate, bicarbonate, chloride, sulfate, and particularly in groundwater, nitrate (from agricultural use). Total dissolved solids, the soluble salts measured together as ECw and individually in ppm of the element, can be removed by several water purification systems—reverse osmosis and deionization. Before investing in any costly treatment systems; however, it may be advisable to adopt management practices that can control salinity.
Management Practices for Controlling Soluble Salts
Excessively high soluble salt levels can damage roots, severely restrict plant growth, causing undesirable foliage damage (salt-burn) and possible death of the plants. Managing soluble salts involves an integrated approach to production. The leaching fraction is the most effective method in controlling salt buildup, but other practices can be used to control and/or maintain soluble salt levels too.
Leaching Fraction
The classical textbook solution to salinity management in the greenhouse is through leaching (washing) accumulated salts below the root zone. This is often accomplished by occasional excessive irrigation applications to dissolve, dilute and move the salts. The leaching fraction (LF) is the percentage of applied water (10–20%) that must leave the root zone or container to accomplish the degree of leaching desired.
Selection of Growing Media
Good soil drainage helps to control soluble salt concentrations. Growing media should contain a substantial quantity of large pores to facilitate good drainage.
Use of Substrate Amendments
In some greenhouse soils, some improvement in drainage may be obtained by adding gypsum to the substrate.
Irrigation Method
Different irrigation methods provide different amounts of leaching. Leaching prevents excess soluble salts from building up in the root substrate. The negative effects of high EC are more pronounced for growers who grow on the dry side.
Fertility Regime
Avoid applications of dry fertilizer or highly concentrated nutrient solutions to a dry growing medium and avoid fertilizers that give a high salt stress for a given amount of nutrient (high salt index). Water-soluble fertilizers vary in how much they affect soluble salts levels in the media.
Plant Tolerance
Perhaps the most effective means of managing soluble salts is to avoid producing salt sensitive plants. Each plant species has a distinct response to salt accumulations and growers often can select those with tolerance.
Reverse Osmosis
Reverse osmosis (RO), or commonly referred to as membrane filtration, removes nearly all the total dissolved solids 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 (Figure 19.1). The filter is sufficiently fine to remove ions such as sodium, chloride, bicarbonate, boron, and iron in the water.
Pretreatment Stage
The pretreatment stage is an extremely important part of the system. If algae or pathogens are present, the water is treated by one of several methods such as ultraviolet (UV) designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter radiation, chlorine, or peroxyacetic acid as discussed later in this chapter in the section on Disinfestation of Greenhouse Irrigation Water.
Filtering Efficiency
The amount of purified water delivered in a given time and the degree of salts removed depends on the pressure of the system, membrane type, total dissolved solids of the water being purified and temperature. Efficiency is strongly dependent on the integrity and cleanliness of the membranes.
Deionization
Deionization (DI) is a water filtration process whereby total dissolved solids (TDS) are removed from water through ion exchange (Figure 19.2). Specifically, the purification process removes cations, such as sodium (Na⁺), calcium (Ca2+), magnesium (Mg2+), iron (Fe2+), potassium (K⁺), and anions such as chloride (Cl¯), sulfate (SO42¯), bicarbonate (HCO3¯), and fluoride (F¯). Deionization typically does not remove organics, virus or bacteria except through “accidental” trapping in the resin and specially made strong base anion resins which will remove gram-negative bacteria.
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