Irrigation Water Treatment
Salt content in Irrigation Water – Can Be Remove Through Reverse Osmosis Plant
The excess of salts content is one of the major concerns with water used for irrigation. A high salt concentration present in the water and soil will negatively affect the crop yields, degrade the land and pollute groundwater.
The suitability of water reuse for irrigation with high salt content depends on the following factors:
– Salt tolerance of the type of crop
– Characteristics of the soil under irrigation
– Climate conditions. The quality of the irrigation water plays an essential role in arid areas affected by high evaporation rates and cause high concentrations of salt accumulating in the soil.
– Soil and water management practices
In general water reuse for irrigation purposes must have a low to medium salinity level (i.e. electrical conductivity of 0.6 to 1.7dS/m). (See table below).
Special account should be taken to coastal areas where the infiltration of sea water poses a high risk of salinity in the water that is then pump from wells to be used in irrigation. For example in Spain the overexploitation of groundwater resources for agriculture lowered the water table and as a consequence caused the seawater intrusion in the coastline.
Salinity with moderate content of salts can be used if moderate leaching occurs.
Water with high saline (ECi>1.5) and sodium (SAR>6) should not be used for water irrigation. Nevertheless, in some places with water shortage, water with high salinity concentration is used as a supplement for other sources and therefore a good management and control is essential and the salt tolerance of the plants must be considered.
If water with a very high salinity is used (extreme water shortage circumstances) the soil must be permeable, drainage must be adequate, water must be applied in excess to provide considerable leaching and salt-tolerance crops should be selected.
Real hazard!! A percentage of 21% of total irrigated land is estimated to be damaged by salt (see table below).
Salinization of soils on Irrigated Lands
Source: Adapted from F.Ghassemi, A.J.Jakeman, and H.A. Nix, salinisation of Land and Water Resources (Sydney: University of New South Wales Press, 1995)
If a farmer annually applies 10,000 tons of irrigation water to a Ha of crops, which is typical, between 2 and 5 tons of salt will be added to that land every year. Unless these salts are flushed out, enormous quantities can build up over the course of years or decades.
Salt tolerance of different crops
The yield of different crops in relation with the salinity content of the water used for irrigation, depends on the type of crop, soil and environmental conditions.
The most distinct signs of injury from salinity is reduced crop growth and loss of yield. Crops can tolerate salinity up to certain levels without a measurable loss in yield (salinity threshold). When the salinity level is bigger than the threshold, the crop yield reduces linearly as salinity increases.
Management Practices for Irrigating with Saline or Sodic Water
The following consideration should apply:
– Adequate internal drainage. This measure is intended to avoid free movement of water in the root area.
The appropriate leaching requirement depending on tolerance levels for specific crops should apply to avoid the accumulation of salt. For example if natural drainage is not adequate, a drainage system must be installed.
– Higher water availability in soil. At high salt concentrations plants will not absorb all the normally available water.
– Proper management and control of SAR and salinity controls. Ex. add soluble calcium such as gypsum (calcium sulphate) to decrease the SAR to a safe value.
Irrigation water quality
The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. For example, the physical and mechanical properties of the soil, ex. soil structure (stability of aggregates) and permeability, are very sensitive to the type of exchangeable ions present in irrigation waters.
Irrigation water quality can best be determined by chemical laboratory analysis. The most important factors to determine the suitability of water use in agriculture are the following:
– Salinity Hazard
– Sodium Hazard (Sodium Adsorption Ration or SAR)
– Carbonate and bicarbonates in relation with the Ca & Mg content
– Other trace elements
– Toxic anions
– Free chlorine
Parameters of reuse water with agronomic significance
|Parameter||Significance for irrigation with recycled water||Range in secondary and tertiary effluents||Treatment goal in recycled water|
|Total Suspended SolidsTurbidity||Measures of particles can be related to microbial pollution; it can interfere with disinfection; clogging of irrigation systems; deposition||5-50 mg/L||<5-35TSS/L|
|BOD5COD||Organic substrate for microbial growth; can bring bacterial re-growth in distribution systems and microbial fouling.||10-30mg/L||<5-45mgBOD/L|
|Total coliforms||Measure of risk of infection due to potential presence of pathogens; can bring bio-fouling of sprinklers and nozzles in irrigation systems||<10-107cfu/100mL||<1-200cfu/10mL|
|Heavy metals||Some dissolved minerals salts are identified as nutrients and are beneficial for the plant growth, while others may be phytotoxic or may become so at high concentrations. Specific elements (Cd, Ni, Hg, Zn, etc) are toxic to plants, and maximum concentration limits exist for irrigation||< 0.001mgHg/L<0.01mgCd/L<0.02-0.1mgNi/L|
|Inorganic||High salinity and boron are harmful for irrigation of some sensitive crops||<450-4000mgTDS/L<1mgB/L|
|Chlorine residual||Recommended to prevent bacterial re-growth; excessive amount of free Chlorine (>0.05mg/L) can damage some sensitive crops||0.5->5mgCl/L|
|Nitrogen||Fertilizer for irrigation; can contribute to algal growth and eutrophicationin storage reservoirs, corrosion (N-NH4), or scale formation (P)||10-30mgN/L||<10-15mgN/L|
Source of information: Valentina Lazarova Akiçca Bahri; Water Reuse for irrigation: agriculture, landscapes, and turf grass; CRC Press.
Menu of Options for Improving Irrigation Water productivity
|Category||Option or Measure|
|Technical||– Land leveling to apply water more uniformly- Surge irrigation to improve water distribution- Efficient sprinklers to apply water more uniformly
– Low energy precision application sprinklers to cut evaporation and wind drift losses
– Furrow diking to promote soil infiltration and reduce runoff
– Drip irrigation to cut evaporation and other water losses and to increase crop yields (see table below)
|Managerial||– Better irrigation scheduling- Improving canal operation for timely deliveries- Applying water when most crucial to a crop’s yield
– Water-conserving tillage and field preparation methods
– Better maintenance of canals and equipment
– Recycling drainage and tail water
|Institutional||– Establishing water user organizations for better involvement of farmers and collection of fees- Reducing irrigation subsidies and /or introducing conservation -oriented pricing- Establishing legal framework for efficient and equitable water markets
– Fostering rural infrastructure for private-sector dissemination of efficient technologies
– Better training and extension efforts
|Agronomic||– Selecting crop varieties with high yields per Liter of transpired water- Intercropping to maximize use of soil moisture- Better matching crops to climate conditions and the quality of water available
– Sequencing crops to maximize output under conditions of soil and water salinity
– Selecting drought-tolerant crops where water is scarce or unreliable
– Breeding water-efficient crop varieties
Sources: Amy L. Vickers, Handbook of Water Use and Conservation (Boca Raton, FL: Lewis Publishers, in press); J.S. Wallace and C.H. Batchelor, “Managing Water Resources for Crop Production”, “Philosophical Transactions of the Royal Society of London: Biological Science, vol. 352, pp.937-47 (1997)