Treating Desalinated Water

Prof. Ori Lahav from the Faculty of Civil & Environmental Engineering has developed an elegant method for adding magnesium to desalinated seawater - making it fit for drinking and agricultural use

Prof. Ori Lahav

By Noam Bercovitz

About three years ago Prof. Ori Lahav of the Environmental, Water and Agricultural Engineering Division of the Faculty of Civil and Environmental Engineering was invited to a meeting at the Water Authority that focused on examining proposals for adding magnesium ions to desalinated water for agricultural purposes. While listening to the many suggestions proposed by engineers and other inventors, Lahav came up with an idea of how to solve the problem cheaply and simply. He waited until the presenters had finished and then quickly told the committee members about his idea for solving the problem.

At the end of the discussion Prof. Lahav returned to his lab in order to develop the idea into a structured process. He proposed to Ms. Liat Birnhack, who was about to complete her Master's degree in a similar area, that she take the research on as a subject for a direct track doctorate. A year after this, Prof. Lahav and Birnhack published a number of papers, and in parallel, the Technion took out a patent on the process. Today, the process developed as a result of this patent is being marketed around the globe. Lahav relates that now, at the end of the development stage, the process has remained faithful to the principles of his first idea that came to him during the committee's discussions. What had been needed since then were mainly fine-tuning and optimizations of the process in addition to its adjustment to existing different conditions in desalination plants already working. At the same time the research has been extended to additional practical and theoretical issues and to the development of competitive processes.

The need for a supplementary process

In the near future, desalinated water will become a significant source of the domestic and agricultural water supply in Israel and in many other places in the world. In the first phase of the desalination process salts are removed from seawater using different processes such as reverse osmosis, and almost completely distilled water is produced, with very low concentrates of sodium and chloride ions remaining in the water. This type of water cannot be used and requires a supplementary or completion process, termed in the professional jargon "post treatment". Desalinated water that does not undergo post treatment is aggressive to concrete structures, accelerates corrosion of metals, is very likely to dissolve iron oxides that have deposited onto the walls of old metal pipes and thereby cause the phenomenon of red water, is not palatable when drunk, lacks the necessary minerals humans need to get from water, is not suitable for agriculture and is not suitable for use as a source of treated wastewater after being used as drinking water.

During the post treatment process, various salts are reinserted into desalinated water, which ensure that the water has several features such as alkalinity (the ability to neutralize acids), calcium ion concentration (a mineral important for human health that also enables specific sediments to deposit onto the walls of transport pipes in order to minimize corrosion), a pH balance and more. The exact composition of desalinated water has been set by new Israeli standards, the first in the world that were developed especially for desalinated water, and which were drawn up by Lahav, Birnhack and others for the Israeli Ministry of Health.

Why Magnesium?

The need for returning magnesium to desalinated water arose for several reasons. First, it turned out that desalinated water from the Ashkelon facility that was used for agriculture damaged crops because of the absence of magnesium. Later on, the World Health Organization published recommendations that defined the minimum concentrations of magnesium and calcium ions that drinking water must have because of their importance for general public health. In addition, the presence of calcium ions gives water the hardness that it needs to create the internal sediment in metal water-carrying pipes, which diminishes the corrosion rate and thereby reduces the pipes' wear over time.

How Does It Work?

Magnesium appears in water as a divalent cation - that is, an ion with a positive double charge, denoted Mg²⁺. Calcium is similar in its chemical properties and is denoted Ca²⁺. The two ions are found predominantly in seawater, with the concentration level of magnesium being five times greater than calcium (in eq/l units). The core of the idea is to use the seawater, which is at any rate is being delivered to the desalination plant after preliminary treatment, as a magnesium source, and to exchange it with calcium, which is in any case supplied by the desalination plant during the water hardening process in the post treatment stage. The hinge on which the process turns is cation exchange, a method that is broadly used in chemical processes for different purposes.

The cation exchanger in Lahav and Birnhack's facility is a resin that has a selective adsorption property for divalent cations only. First, seawater is pumped through the resin so that the resin is charged with magnesium and calcium ions at a 4:1 ratio. In the next stage, desalinated water with Ca²⁺ ions is passed through the resin. There is an excess of Ca²⁺ concentration in this water as a result of the water hardening process in which calcium is introduced through the dissolution of quarry material called calcite (calcium carbonate). In order to maintain a chemical balance, cations are exchanged between the solution and the resin, the result of which means that magnesium ions are released into the water and an equal amount of calcium ions are adsorbed into the resin. The process is time dependent and the final magnesium and calcium concentrations in the water can be controlled through the amount of time the water with the excess calcium concentration is pumped through the resin. At the end of the process the resin is charged mainly with calcium ions, and when the system switches back to seawater the calcium ions are exchanged by magnesium ions, etc.

Manufacturing and Production

The advantage in the new method is in its simplicity, the relatively low investment in infrastructure it requires and the low cost of the process due to its use of raw material that is anyway part of the desalination process at the facility. Another great advantage is that the process does not create any waste. After the patent had been registered, the Technion Technology Transfer unit chose a private company with which to partner and establish a jointly owned company called Renewed Water Minerals, which today manufactures and markets the device. The company has to date invested a significant amount of money in development and it is reasonable to assume that in the near future the process will be integrated in desalination plants in Israel.

Links:
A presentation of the full process - http://www.renew-minerals.com/rwm-preso/02.htm