Unit 4 water quality Flashcards
Importance of water quality
solvent/carrier for nutrient,
Provides moisture to the bacteria beneficial to the plants,
Provides moisture which are essential for the chemical action within the plant leading to growth,
Some salts present in the soil reacts to produce essential nutrients only in the presence of water,
Water cools the soil and atmosphere making favourable environment for healthy plant growth,
Controlled irrigation water supply helps to washout or dilute salts in the soil, and
Water softens the tillage pans.
Factors contributing to water quality in organic agriculture
Main impurities in irrigation water includes:-
Concentration of sediments,
Total Concentration of soluble Salts (TDS),
Proportion of sodium ions to other cations,
Concentration of toxic elements- boron,
Concentration of bicarbonate in relation to the concentration of calcium and magnesium,
Bacterial concentrations,
Effects of water quality on crops
(a) Effects of sediment in irrigation water:-
The effect of sediments in irrigation water depends on:-
Type of sediments, and
Type of soil in the irrigated field
In a irrigated field with Sandy soils:-
Generally fertility is improved by deposition of sediments in general,
Both fertility and permeability of soil will reduce if the sediments are derived from eroded areas.
(b) High concentrations of salt in water may become harmful to plants in the long run
(c) High concentration of sodium ions in irrigation water leads to:-
When wet- Soils become plastic and sticky,
When dry- Soils crust and from clods.
(d) High percentage of toxic elements in irrigation water the soil become harmful to certain- nut, citrus and deciduous plants.
(e) High concentrations of bicarbonate leads to precipitation of calcium and magnesium bicarbonates from soil increasing relative proportions of sodium ions sodium hazards.
(f) Bacterial contamination may not directly harm plants but may affect persons or animals eating these plants.
Methods to improve water quality for irrigation
Three major water quality problems can be improved by chemical or physical treatment system:-
Alkalinity of water can be neutralized by addition of acids,
High TDS water can be reduced by several purification systems, and
Individual elements can be remove by purification process.
Before investing in treatment— worth investigating on
alternative water source or mixing water sources.
Water purification to remove Total Dissolved Salt
Reverse Osmosis (RO)
Deionization
Removing Individual Salts Iron and Manganese Calcium and Magnesium Fluoride Boron Blending with Rainwater and Other Non-Problem Water
Reverse Osmosis (RO) for TDS removal
- Passage of a solvent (water) through a semi-permeable membrane separating two solutions of different salts concentrations,
- A semi-permeable membrane allow solvent but not salt under pressure- a relatively pure water is accumulated on other side of membrane,
- Maintenance/ replacement of membranes are a significant part of the cost. Less efficient and less costly membranes are available that require less energy because of their lower operating pressures,
- Efficiency depend on pressure of the system, membrane type & cleanliness, and temperature,
- Chlorine can cause rapid degradation of the membranes and sediments cause clogging,
- TDS removal can be 95 to 99 percent, individual salts are removed with varying efficiency.
- Generally, calcium, magnesium and sulfate are removed more efficiently than potassium, sodium, lithium, nitrate, chloride and borate.
- A disadvantage of reverse osmosis systems is that salty wastewater is produced. Disposal of this waste may fall under government regulation.
Deionization for TDS removal
Soluble salts in water carry either positive [cations: sodium (Na+), calcium (Ca++), magnesium (Mg++), iron (Fe++) and potassium (K+)] or negative [anions: chloride (Cl-), sulfate (SO4=), bicarbonate (HCO3-), and fluoride (F-)]
A process that removes ions from water using exchange resins (usually solid beads that are covered with fixed negative or positive charges),
A cation exchange resin has fixed negative charges that are neutralized by H+. When irrigation water is passed over resin, cations in water replace the H+ ions and are held on the resin.
Likewise, an anion exchange resin has fixed positive charges that are neutralized by hydroxide ions (OH-). When irrigation water is passed over resin, anions in the water replace the OH- ions and are held on the resin.
H+ and OH- ions released from resins and combine to form water.
Deionization unit will contain both anion and cation resins so that all salts are removed.
Deionization is very effective and produces a higher quality water than is generally needed in crop production.
Cost increases with amount of salts in water to be removed.
Higher salts content, frequently resins need to be regenerated or replaced.
Cost of deionized water is generally five to six times higher than that of water purified by RO.
If high-quality water is required (as for holding cut flowers) and the initial salts content of the water is high, RO can be used as an initial purification step and final quality be achieved by deionization. Final costs may actually be lower than with deionization alone.
Removing Individual Salts: Iron and Manganese
Iron and manganese in water become oxidized to insoluble forms that are responsible for black or brown stains on foliage of plants that are overhead irrigated. Iron concentrations of less than 0.3 ppm are required for micro-irrigation systems. Several ways to remove these elements:-
Precipitation in pond: Pump source water into a pond where insoluble iron and manganese compounds can precipitate and settle out.
Pump in as a spray for rapid oxidation of iron and manganese to insoluble forms.
Enough time must be allowed for iron and manganese to settle out, and pond must be large enough to accommodate irrigation volume needs without disturbing bottom layer of iron and manganese sediment.
Oxidation filters: oxidize iron and manganese to insoluble forms using air, potassium permanganate or chlorine.
The sediments are removed by filters that must be periodically cleaned, usually by backflushing.
Sand may also be used as a filter. If a chemical oxidant is used, this must be renewed as it is used up. Manganese is slower to oxidize and settle out of the water.
Efficient removal of manganese, chemical coagulation before sedimentation- filtering may be required.
Removing Individual Salts: Fluoride
Calcium and magnesium may be removed from hard water to eliminate salt deposits left on foliage by overhead irrigation.
This can be achieved by water softening- replacing the calcium and magnesium with potassium.
Usual water softening unit uses sodium, not potassium. High levels of sodium may be harmful to plants, and softening unit that uses potassium should be used instead.
Total salt content of water is not changed and the potassium is used by the plants.
Over fertilization with potassium may occur if the water is very hard.
Potassium chloride in the softening unit must be recharged.
Removing Individual Salts: Fluoride
Fluoride can be removed from irrigation water by adsorption using activated alumina or activated carbon.
When using activated alumina, pH of water is first adjusted to 5.5 before treatment.
The activated alumina unit can be regenerated with a strong base, such as sodium hydroxide, and reused. Water pH does not have to be adjusted before treatment with an activated carbon unit and the carbon is usually replaced when its adsorption capacity is used up.
Fluoride is not soluble above pH 6 so maintaining a media solution pH above this level will prevent most fluoride toxicity problems.
Removing Individual Salts: 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.
To increase the boron-removal efficiency of a reverse osmosis system, the pH of the water needs to be adjusted to be slightly alkaline (pH 7.5).
Thin film composite type membranes that are more tolerant of the higher pH should be used.
Blending with Rainwater and Other Non-Problem Water
Collected rainwater could be blended with problem waters such as with high alkalinity, high EC, or excess Na and Cl or to improve quality of recycled tailwater and industrial wastewaters with high nutrient content used for irrigation.
Other non-problem sources of water could also be used for blending. Rainwater has a natural pH of about 5.6 and a very low mineral content.
Acidic rainwater with a pH range of 4.0-5.0 is acceptable for irrigation; it’s poorly buffered and will have little effect on growing medium pH.
Water with a pH below 4.0 should not be used as it may injure seedlings and young transplants.0
Rainwater should be collected from clean, well-maintained structures free from mineral contaminants such as zinc and other metals.
Water should be tested for pH and minerals at least twice a year.
