COMMITTEE ON AGRICULTURE
Rome, 13-16 April 2005
Water Desalination for Agricultural Applications
1. The availability of water resources of marginal quality such as saline groundwater, drainage water and treated wastewater has become an important issue, especially for irrigated agriculture in the arid and semi-arid zones of water scarce countries. Water desalination is a well-established technology for urban water supply and is the main source of potable water in the Gulf countries as their associated energy costs are low. Desalted brackish water and sea water are also being applied in certain countries for high-value crops.
2. Water desalination can also be crucial in emergency situations, where water supply has been affected by salinity, as in cases of sea water intrusion as occurred, dramatically, during the recent tsunamis in South and Southeast Asia. In such instances, water desalination may be key in the provision of potable water for the domestic use of affected populations.
3. The purpose of this paper is to inform the Committee on Agriculture (COAG) on water desalination technologies and their costs, as well as their environmental constraints and perspectives for application in agriculture, specifically for irrigation. This document is based on the conclusions and recommendations of an Expert Consultation on Water Desalination for Agricultural Applications, organized by FAO in Rome on 26 and 27 April 2004.1
4. The objectives of the Expert Consultation were to understand and analyse the current situation of water desalination technologies and their associated costs; environmental impacts and externalities; the economic feasibility for agricultural applications; institutional financial arrangements; and make a cost/benefit comparison between water desalination and treated wastewater re-use for irrigation.
5. Five external experts participated in the Consultation from areas where water desalination technology has been applied, mainly in the United States of America, North Africa, Spain and the Near East Gulf States.
6. A summary of the findings of the experts and recommendations on the five subjects are reported below.
State-of-the-Art Water Desalination Technology and Costs
7. The most widely applied desalination technologies are thermal distillation -which treats large volumes of water (55 000 m3/d)- and membrane technologies, reverse electro-dialysis (EDR) and reverse osmosis (RO). The treatment capacity using membrane technology can be down-scaled to the required use (large plants are considered to be above 5000 m³/d, medium plants range from 500 to 5000 m³/d, and small plants with maximum of 500 m³/d capacity).
8. Thermal distillation converts saline water into steam which is then condensed to form desalted water. In the EDR process, salts are separated from water by means of an electric load application. Finally, RO requires pressure applied to the intake water which is forced through a semi-permeable membrane capturing most of the salts, and higher pressure is required for higher salt concentrations of the intake water. The membrane technologies are applied to desalt brackish water with salt concentrations less than 10 g/l; whereas, RO and thermal distillation are applied for desalting sea water with concentrations over 30 g/l.
9. The costs of the various technologies are dependent on sea or brackish water salt content. They are in the following ranges:
10. Although current trends show that thermal distillation costs for large plants are falling because of economy of scale, the RO costs are decreasing at an accelerated rate due to new technology developments, competition, as well as economy of scale. The experts recommended that each specific case must be studied and evaluated carefully before selecting the most appropriate technology.
Environmental Impacts and Externalities Associated with Water Desalination Technology
11. Water desalination has both positive and negative impacts on the environment. The main direct positive impact is an increase in water availability. The indirect positive environmental impact is related to reduced soil salinization from irrigation with desalted water as compared to brackish water. Negative environmental impacts may result from the following: disposal of brine and residues from desalination; chemical additives used for anti-fouling and anti-corrosion; visual impact on the landscape; noise; emission of greenhouse gases from energy consumed, etc. In addition, disposal of brine in coastal and inland areas has different implications, where the complexity is greater for the latter but direct coastal discharge has marine ecological impacts.
12. Although technology and management options to reduce impacts are available with limited guidelines from UNEP, standards and complete Environmental Impact Assessments (EIA) as well as Life-Cycle Analysis for the technologies are much needed. Continuous monitoring of effluents and research on brine disposal are also required and indirect impacts from green house emission from energy consumption need to be factored in the assessments. EIAs still have yet to be integrated in the management policies of countries already implementing this technology.
Economic and Environmental Feasibility of Water Desalination for Agricultural Applications
13. Desalted water is not affordable for most crops; it may be affordable only for high-value crops, especially when capital costs are subsidized. Since brackish water is less saline than seawater, it is preferred, where available, over sea water for desalination for agriculture production. Furthermore, desalination facilities near the point of use are preferred to minimize transfer costs; however, in terms of operation and maintenance, small to medium plants tend to be less cost-effective than large ones.
14. Desalination programmes should be integrated with water resources management, with application of best water management practices (leaching requirements, better irrigation methods) and selection of appropriate salt tolerant crops. Optimum size and site of facilities should be studied, and better operating management of smaller plants is needed (automatic plant operations; farmer knowledge on operational processes; training of operators).
15. There are various financial arrangements in relation to partnerships between the government and the private sector for water desalination. Progress has been made towards private sector participation and investment, with guarantees from the government in most instances for desalination for drinking water. Contract models are evolving from Build Own Operate (BOO) to Build Operate Transfer (BOT), and others. However, institutional issues have emerged when incorporating such models into existing policy frameworks. The experts recommended Design Build Operate (DBO) as a new contractual model with many of its associated advantages, in particular, in relieving capital burden, transferring construction and operational risks to the private sector, and attracting technological innovations.
Comparison between Wastewater Treatment and Desalination in Agriculture
16. Both wastewater treatment and water desalination constitute potential sources of water for agriculture and other uses. There is commonality in treatment technologies with regard to tertiary wastewater treatment and water desalination. Generally, wastewater re-use in agriculture is less expensive than desalinated water. However, treated wastewater re-use also has problems in terms of public perception and potential health and environmental risks. Therefore, programmes should be introduced to inform the public on the benefits of re-use of treated wastewater.
17. Hybrid solutions, a blend of wastewater plants coupled with desalination plants, may have a place in urban and peri-urban agriculture. However, setting standards for the outflow quality of wastewater treatment plants and the associated effluent monitoring is very important.
18. Although WHO and FAO have developed health guidelines for treated wastewater re-use, no common standards have been set due to non-systematic implementation in countries around the world with different scales of cost/benefit valuation on treatment. For the reasons indicated above, due consideration should be given to both problems and benefits of wastewater re-use and water desalination.
19. The group of experts concluded that:
20. The Expert Consultation has raised awareness on the role of water desalination in agriculture and has contributed to updating information on this topic. To assist in this area, FAO proposes to:
1 The output of the Expert Consultation (a Technical Summary with the main conclusions and recommendations) and the experts’ papers will be published during the first quarter of 2005. In the meantime, a note on the Expert Consultation was released on the FAO Land and Water Electronic Newsletter No. 57, issued in June 2004 (Land-and-Water-L@mailserver.fao.org).