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Science & Technology
Desalination made easy
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Issue Date: Jul 31, 2012
Graphite and copper can make brackish water potable
AS drinking water sources become scarce, the dependence on groundwater increases. But groundwater is brackish in many places and the technologies to make salty water potable are high on energy consumption, inputs and maintenance costs.
Scientists have now found a cost- effective device to desalinate water. They have taken their cue from one of the guiding principles of water purification—removing salt ions from water.
To make the device, a team led by Maarten Biesheuvel from Wageningen University in the Netherlands took two thin, inexpensive graphite rods or wires, which are highly conductive. The outer surface of the wires was coated with a porous carbon electrode layer. This turned one wire into anode (positively charged electrode) and the other into cathode (negatively charged electrode). Each wire was then pressed against a copper strip and clamped onto a plastic holder. The wire pairs were then dipped into saline water and a voltage between one to two volts applied.
(a) Wires adsorb salt ions under an applied voltage (b) The ions get stored at oppositely charged electrodes (c) Stored ions are then released in wastewater (Source: S. PORADA, ET AL)
This causes the negatively charged chlorine ions to get stored at the positively charged anode end and the positively charged sodium ions to get stored at the negatively charged cathode end without undergoing any chemical change. The wire pair was then inserted into wastewater and current removed. This allows the stored ions to be released into wastewater. By repeating this cycle about eight times, the researchers found that the salt concentration of the original brackish water of 20 millimolars (mM) was reduced to about 7 mM. Potable water is considered fit for drinking if it has salinity less than 15 mM.
When asked if this method can be used to treat water with 100 mM of salt, Biesheuvel said, “Higher the salinity, the more you need to shuttle the wires back and forth, or have more of those wires working together, so 100 mM would be the upper limit.” This technique is inexpensive and since the voltage required is low, it is perfectly compatible with solar panels and can be used at off-grid or remote locations, he adds.
The method also does away with the need to mix brine with the water being treated as in other desalination processes. It limits the efficiency of other techniques as brine in itself is salty.
The study was published in the May 29 issue of Journal of Physical Chemistry Letters.
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Good post on Desalination.
Salt water is desalinated to produce fresh water suitable for human consumption or irrigation. One potential byproduct of desalination is salt. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use. Along with recycled wastewater, this is one of the few rainfall-independent water sources.
Large-scale desalination typically uses large amounts of energy and specialized, expensive infrastructure, making it more expensive than fresh water from conventional sources, such as rivers or groundwater.
Desalination is particularly relevant to countries such as Australia, which traditionally have relied on collecting rainfall behind dams to provide their drinking water supplies.
According to the International Desalination Association, in 2009, 14,451 desalination plants operated worldwide, producing 59.9e6 cubic metres (2.12×109 cu ft) per day, a year-on-year increase of 12.3%.[4] It was 68 million m3 in 2010, and expected to hit 120 million m3 by 2020; some 40 million m3 is planned for the Middle East. The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates.
The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, energy is saved. Multistage, a leading method, accounted for 85% of production worldwide in 2004.
The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology. Membrane processes use semipermeable membranes and pressure to separate salts from water. Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology
The Indian water desalination market is expected to register a compound annual growth rate (CAGR) of 22 per cent for next five years on the back of increasing governmental support, industrialization and rising demand for fresh water, a new research report published by TechSci Research has said.
The anticipated market size for India water desalination industry is $0.63 billion by 2014. The desalination market in India has witnessed phenomenal growth in last three years and has now emerged a cost-effective solution to the need to provide water.
India, which currently has 182 desalination plants operating in different parts, is expected to have over 500 desalination plants by 2017, according to the report “Indian Water Desalination Plants Market Forecast & Opportunities, 2017”
According to the report, membrane-based technologies are considered 23 per cent cheaper for generating desalinated water when compared with thermal-based ones due to which 85 per cent plants in India are membrane based plants. The report singled out two Indian states -- Gujarat and Tamil Nadu -- for registering the highest growth in desalination capacity.
DrA.Jagadeesh Nellore(AP),India
Dr.A.Jagadeesh
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