Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/133796
IRRIGATION leads to reductions in water production cost when estimated per unit of water supplied, as well as reducing needs for chemicals and membrane replacements and potentially the number of personnel operating and maintaining a plant. This means that the cost of desalinated water can be reduced proportionally to the mixing factor. Mixing may also reduce the need for post-treatment which adds cost to produced water: permeate is commonly required to be remineralised and ionically balanced (as permeate has reduced levels of calcium and magnesium as well as being slightly acidic). Overhead irrigation sprinklers, Australia in irrigated agriculture as "Irrigated crops occupy about 15–20% of the total cropped area but contribute 33–40% of the production, so they are crucial to the world's food supply. Most irrigation is applied on the surface (84.5% of the total), with smaller amounts via sprinklers (13.5%) and localised systems (2%)5." Globally, almost two-thirds of the water delivered to irrigated crops is lost as drainage or runoff, or both. Those losses are associated with storage and conveyancing efficiencies (30%) as well as farm irrigation efficiency (37%). Gregory continues, "Until recently, most irrigation was scheduled on the basis of fully meeting crop water requirements, but, with sprinkler and localised drip systems, it has been possible to demonstrate that deficit irrigation strategies can not only sustain yields and profitability, but also reduce water use5." This suggests that it is possible to significantly reduce water use in irrigated agriculture. This also means that the cost of water which is effectively used by the plants is greater, when water losses are payed for, but not productively used. However, implementation of high-efficiency irrigation systems only comes after paying the cost of technological advancements in irrigation practice, and as such is also an expensive alternative to current costs. WATeR quAlITy RequIRed fOR IRRIGATION The majority of irrigated crops can tolerate water of relatively low quality (greater than 750 mg/L of total salt content, which can be higher in sandy soils). This provides opportunities to mix permeate with water of marginal quality to increase the volume of water available for agriculture. The lower water quality requirement INcReAse IN AGRIculTuRAl pROducTIvITy One of the identified benefits for desalinated water use in the agricultural sector is that it increases the productivity6 and quality of some agriculture products and at the same time leads to lower water consumption and recovery of salinityaffected soils. Irrigation of citric fruit plantations with desalinated water led to increases in production by 10 to 50% (depending on the water quality of irrigated water used prior to introduction of desalinated water), while water needs reduced by 20%. For a case of greenhouse production of bananas irrigated with desalinated wastewater, fertilizers and water use were reduced by 50% and 30% respectively, leading to an increase in banana production and the earlier maturation of plants. An increase in productivity is also related to the leaching of accumulated salts in the soil profile by the high quality desalinated water. In Spain, greenhouse products (horticultural, flowers and ornamental plants) provide greater added value per unit of irrigated water (€ 5.79/m3 on average), followed by vineyards and fruit trees (€ 1.08 and 0.68/m3 respectively), and cereal grains (€ 0.06/m3). An average of € 0.41/m3 was estimated for all products. These figures relate to high-value crops, for which the overall water cost may be marginal compared to total costs. At this stage it is unlikely that the production of cotton, rice or sugar can be effectively supported by water supplied from desalination plants. The above leads to the conclusions that the use of desalinated water for agriculture is most likely to be cost-effective in a tightly controlled environment, using agricultural practices with the mosteffective water use and crops with high productivity Such conditions are often associated with greenhouses and the | 24 | Desalination & Water Reuse | May-June 2013 production of high-value irrigated crops. This is the case in Spain; however it is also important to mention that the high level of financial support and subsidies provided to the agricultural sector in Spain and other EU countries make this option more viable. AusTRAlIA Australia is the world's driest inhabited continent. Over the last decade agriculture along with other sectors in Australia has had to deal with a reduced availability of water due to drought, and debates about water rights are highly sensitive and nationally important. Agriculture is the largest consumer of water in Australia, accounting for 55–65% of total national water consumption, which represents more than 10,000 million m3/year7. Most of this water is used for irrigation of crops and pastures. In 2008-09 the gross value of total Australian agricultural production was Aus$ 41.8 billion (US$ 42.8 billion). About two-thirds of total agricultural production is exported, but currently the high value of the Australian dollar greatly affects the competitiveness of its agricultural exports. Under such conditions, sufficient and reliable water supplies are a key strategic element in ensuring the viability of the Australian agricultural sector. To support the decision-making process, the National Centre for Excellence in Desalination Australia (NCED) has established a project looking into opportunities for desalination technologies' adaptation in agriculture. Currently there are six major seawater RO desalination plants in Australia with total water production capacity of 1,223,000 m3/d (ranging from 133,000 to 400,000 m3/d) with a unit cost ranging from Aus$ 1.00 to AU$ 2.27 (US$ 1.02-2.32) per m3. The high cost of desalination is attributed to high energy and labour costs. A series of small plants are also used by mining companies and in remote communities, where the cost of desalinated water production is much higher due to their locations. There are also examples of treated wastewater desalination and reuse. A national analysis undertaken within the NCED project has shown it unlikely that farmers are willing to pay more than Aus$ 1.00/m3 of water for current farms or for their expansion, and in many regions even this cost is considered to be unacceptably high (eg. common water costs in southwest Western Australia are Aus$ 0.18–0.50/m3 (US$ 0.18-0.51/