Water. desalination + reuse

May/June 2013

Water. Desalination + reuse

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irriGatiOn Desalination works for agriculture in the right circumstances _________ Olga Barron, Principal Research Scientist, CSIRO Land & Water, Australia ___ Editor's Note: Population growth, food security concerns, climate change impacts on agriculture, freshwater resources overuse and land degradation worldwide are forcing international scientific communities to look for alternative approaches to our current resource management approach. This includes all aspects of our water resources including desalination. OPPORTUNITIES TO generate cost-effective and potentially climateindependent water resources of controlled quality for agriculture could be linked with desalination technologies1. Desalination allows a widening utilisation of available resources by producing freshwater from saline or brackish natural water sources. As conventional water production costs have been rising in many parts of the world and costs of desalination have been declining over the years, desalination has become more economically attractive and competitive. By 2015, the costs of freshwater treatment, wastewater reuse and desalination are likely to be similar, at least in USA2.However, currently, desalinated water produced worldwide (77.4 million m3/d)3 still comprises less than 1% of total worldwide water use with only 2% of total desalinated water production is currently used for agriculture. Many countries are beginning to use desalinated water in agriculture, albeit at varying rates4.The highest proportion of desalinated water use in agriculture occur in Spain (where current installed capacity is 1.4 million m3/d and 22% is used in agriculture for irrigation of herbs, fruit, olives and vineyards) and Kuwait (current installed capacity is over 1 million m3/d and 13% is used for agriculture). Saudi Arabia, the world's largest single producer of desalinated water, accounting for about 30% of global capacity, uses only 0.5% of its desalination capacity for agricultural purposes. Other countries which use desalinated water for food production are Italy (desalination capacity 64,700 m3/d – 1.5% for agriculture), Bahrain (620,000 m3/d – 0.4%), Qatar (0.1%) and USA (1.3%). The wider application of desalination technologies for agriculture is limited by its relatively higher cost, as well as by the need for agriculture to be close to saline and brackish feedwater resources as well as a safe and cost effective disposal option for brines. National assessments of the applicability of desalination technologies to support agricultural water supply are currently under way in Chile, China and Australia. The overall efficiency of the combined production of water and food, as well as an opportunity for better soil management, should be the basis for an assessment of desalination's applicability to agriculture. Relatively high desalination costs can be offset by the adaptation of more efficient irrigation technologies (less water use per kg of crop production) and increases in agricultural productivity and its quality (greater crop production per unit of water used). Water prOductiOn There are a number of benefits for desalinated water use in the agricultural sector. The most obvious one is that the technology produces an additional water resource. However, costs are a major limitation, and less expensive options are likely to be more attractive for agriculture. Desalination plants used for agriculture are relatively small, and according to 'economy of scale' principles they produce water at a high cost. For agricultural purposes, seawater desalination using reverse osmosis (RO), considered to be one of the most efficient desalination technologies, is not normally used as the process is expensive due to high energy demands. Brackish water desalting is typically a | 22 | Desalination & Water Reuse | May-June 2013 third of the cost of desalinating seawater; however, inland brackish water is more often associated with groundwater, which is not an infinitive resource (as seawater is considered) and a clear definition of a sustainable groundwater yield is required to avoid aquifer depletion. Inland desalination is also challenged with the required disposal of desalination by-products (brine) with all currently available options adding a significant cost to water production, ranging from 10-20% to more than a five-fold increase in the cost of water production. High desalinated water recovery rates lead to a reduction in disposal cost per unit of water produced. In addition, proximity of feedwater sources to irrigated land can significantly reduce the cost of water supply as it minimises water distribution costs. There is also a need for plant simplicity with reduced requirements for labour, but including automation and safety measures to guarantee production. Water-use efficiency Opportunities for applying desalination technologies to agriculture go hand-inhand with the requirement to improve the irrigation practices efficiency, which can allow for significant reductions in water demand and is critical for future improvements in water-use efficiency whether using desalinated water or other water sources. Gregory summarised the challenges and opportunities in water-use efficiency

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