News & Views item - November 2007

 

Desalinated Water Quality and Agriculture: A Science Policy Form. (November 13, 2007)

The six authors of  the two page article in Science's November 9, 2007 Policy Forum, (subscription required) U. Yermiyahu,1 A. Tal,2,* A. Ben-Gal,1 A. Bar-Tal,3 J. Tarchitzky,4 O. Lahav5 have Israeli affiliations.

 

They put before their readers matters that would seem to be worth careful consideration for those designing desalination facilities in Australia, particularly if desalinated sea water is to be considered to be used for agriculture. Below are excerpts from the paper.

 

With 69% of the global water supply going to irrigation, present freshwater resources may soon be insufficient to meet the growing demand for food. A recent report concludes that, although the costs of desalination remain prohibitively expensive for full use by irrigated agriculture, for high-value cash crops like greenhouse vegetables and flowers, its use may be economically feasible.

 

In December 2005, a new seawater desalination plant was opened in Ashkelon, on Israel's southern Mediterranean coast. Its 100,000,000 m3/year production makes it the largest reverse-osmosis (RO) desalination facility presently in operation worldwide.

 

Recent evaluation of the effect of the plant's desalinized water on agriculture, however, produced some surprising, negative results. Changing these outcomes will require modifying future water management orientation and revision of desalination standards.

 

The table below is taken from the Science paper and indicates in part what is causing concern.

 

 

Boron (B) concentration in seawater averages 4.5 mg/liter and is slightly higher in the Mediterranean Sea. At these concentrations, B does not constitute a threat to human health but is highly toxic to many crops. Boron in neutral and acidic environments readily passes through the RO membranes. Without additional treatment, B in Mediterranean seawater after RO will reach 2 mg/liter, which is toxic for all but the most tolerant crops

 

The cost of desalinating 1 m3 of seawater at the Ashkelon plant was US$0.55 in 2006; in smaller facilities, the cost using the same technology could reach US$1/m3. This cost includes B removal and addition of SO42-, Ca2+, and alkalinity by means of a calcite-dissolution post-treatment process. Additional enrichment of the desalinated water with Mg2+ would raise the price further.

 

Desalinated irrigation water in Israel is often blended with other water sources. As a result, the quality of the final water actually delivered to farmers is unreliable. RO water is low in dissolved substances, with little buffering capacity relative to that of freshwater. Low buffering capacity increases risks of corrosion to metal distribution pipes. It also can have a profound impact on pH (and agricultural productivity) when the water is mixed with other sources.

 

Post desalination treatment of the water produced through reverse osmosis is both essential and an addition cost when considering its production and use.

 

The authors conclude:

 

If desalinized water was destined for agricultural use alone, simple blending strategies would be the most probable economical strategy, providing stable and high water quality. Yet, in more typical cases, where water supplies both municipal and agricultural uses, economic efficiency requires a balancing of treatment costs, drinking-water quality, and agricultural benefits.

 

Desalination facilities built today will be in place for decades, making planning now essential for long-term increased economic prosperity and agricultural productivity.

 


1Agricultural Research Organization, Gilat Research Center, Mobile Post Negev 2, 85280 Israel.

2Mitrani Department of Desert Ecology, Blaustein Institutes of Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990 Israel.

3Institute of Soil, Water, and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Post Office Box 6, Bet Dagan, 50250 Israel.

4Extension Service, Ministry of Agriculture, Post Office Box 25, Bet Dagan, 50250, Israel.

5Faculty of Civil and Environmental Engineering, Technion, Haifa, 32000 Israel.

6Author for correspondence: E-mail alontal@bgu.ac.il