Making every drop count

The situation will only intensify as climate change speeds up and the demands of urbanisation and industrial development put more pressure on subterranean acquifers formed hundreds of thousands of years ago, now tapped almost dry. 

Almost 85 per cent of the water used in the MENA region is for irrigating agricultural land. And the food produced is insufficient. More than half the calories consumed in the region are from food imports, leaving the region vulnerable to global prices and supply chains. 

The balance between feeding the population and maintaining water resources is not a new conundrum. The practice of evaporating brackish or salt water and condensing vapour into freshwater for domestic and agricultural use can be traced to Alexandria and Palestine two thousand years ago. Modern desalination technologies are sophisticated but expensive and energy intensive.

Desalination – high quality water that comes with a price

Today there are more than 19,000 desalination plants worldwide, pumping 92.5 million cubic metres a day, according to the latest figures from the International Desalination Association (IDA). This is up from 88.6 million cubic metres a day in 2016, 52.8m in 2008 and five million in 1980. 

The growth of the market this year is attributed to demand from Gulf countries desalinating seawater. According to Dr Jauad El Kharraz at the Middle East Desalination Research Center in Oman, the global market is led by Saudi Arabia with a total cumulative capacity of 15,378,543 m3/day followed by the United States with 11,815,772 m3/day (though this is mostly from brackish water) and the United Arab Emirates (UAE) with 10,721,554 m3/day. Qatar and Kuwait are now rely wholly on desalinated water for domestic and industrial use. Desalination is no longer a marginal resource.

There are two types of desalination processes, thermal evaporation and membrane separation. These require a force to drive the separation process of pure water from brine. For thermal processes the force is the temperature difference. 

But conventional processes are energy hogs. 

In multi-stage flash technologies, where convective heating of seawater occurs within the tubes and evaporation takes place from a flow of brine ‘flashing’ in each stage to produce vapour, it takes the equivalent of 10-16 kilowatt hours of energy to produce one cubic metre of fresh water from seawater. This is assuming that the plant is located alongside a power plant — for a standalone unit it would be much higher. 

For multi-effect distillation, where evaporation occurs from a seawater film in contact with the heat transfer surface, it takes between 5.5 and 9 kilowatt hours per cubic metre. When driven by fossil fuels, the total cost for a cubic metre of fresh water is between 80 cents and $1.5 for MSF and 70 cents and $1.2 for MED.

For membrane separation the driving force is pressure, in which a salty feed is passed from one side to the other of a membrane that concentrates salts into waste. Advances in the membranes used in Reverse Osmosis has reduced this energy requirement to between 3 and 4 kilowatt hours per cubic metre, bringing the price to between 50 cents and $1.2 per cubic metre. 

The problem of scaling or ‘biofouling’ on membranes has plagued this technology, particularly in the Middle East because of the high level of salt in water from the Gulf and Red Sea. Hot weather also impacts the operation of these facilities, and research into by-products and pre-treatments is ongoing. 

The water produced through RO can be of high quality. In Singapore, where regulations are stringent, they are even bottling and selling RO water derived from waste water, in California they are using it to recharge acquifers. 

But how appropriate is this in agricultural settings, and where large volumes are needed? 

Scientists are looking at a host of alternative strategies to reduce costs using renewable energy resources such as solar and geothermal power. They are also developing technologies that are appropriate to different settings, including technical infrastructures available and the salinity of the feed water used for farming, which also increases as a consequence of deterioriating water quality from aquifers.

Membrane innovation

At the Water Desalination and Reuse Centre, King Abdullah University of Science and Technology (KAUST), researchers have developed innovations relevant to agricultural settings, whose energy consumption is less than 2 kilowatt hours per cubic metre.

Environmental engineer, Noreddine Ghaffour, works on membrane distillation, a technology in which feed solution is heated and brought into contact with a membrane that allows only the vapour to move through dry pores to condense on the other side.  Low feed temperature is enough to drive this process, meaning low grade waste heat or renewable energy is feasible. 

Ghaffour has developed special membrane modules and designed polymers to improve the amount of water. A multi-stage direct contact membrane distillation module that would be suitable for stand-alone small-scale desalination plants in remote areas is now underway. 

“The unit, which has high internal heat recovery and higher driving force, hence improved vapor transport at lower energy consumption, could be driven by solar energy, low-grade waste heat or low-enthalpy geothermal sources. The process is also resistant to heat source variation and can treat a wide variety of impaired water quality without impacting the product water quality. This enables the process to be used in different areas for different applications,” he says.

Fertiliser driven forward osmosis (FDFO) in hydroponics, a technique for growing plants without soil, is another of Ghaffour’s research areas in collaboration with Ho Kyong Shon of Sydney’s University of Technology (UTS).

“Forward osmosis membrane technology is driven by a difference of concentration [between saline feed water and draw solution] and not difference of temperature or pressure,” says Ghaffour. “Instead of having a commercial draw solution we use real fertiliser that has high concentration of the nutrients that plants need to grow. We need to dilute that fertiliser with water to use it in hydroponics anyway. So we are killing two birds with one stone: we are diluting the fertiliser, at the same time we are treating the impure water quality.” 

The less saline concentration, he says, the better, but seawater would be usable if combined with nanofiltration, or pressure assisted osmosis. Pilot studies undertaken with Sherub Phuntsho at UTS showed that FDFO is better able to meet the quality of water plants need, because it retains some of the minerals they need to grow, while removing difficult ones, such as boron. They even grew lettuces with it.

"Understanding the perspective of the farmers is critical for the success of the innovation."

The WDRC is also working on greenhouses using membranes, including a project using salt based liquid dessicants to capture water vapour and passed through hollow fibre membranes to recover fresh water. “We can treat high salinity feeds, as high as three times the brine reject of reverse osmosis, using the heat of the sun” says Ghaffour. 

Efforts are underway to improve efficiency and implement a pilot designed his colleagues TorOve Leiknes and Ryan Lefers. If it is successful on a commercial scale the liquid dessicant system could reduce the amount of water needed to grow 1 kilogram of tomatoes and lettuce from an average of 200 litres of fresh water to 1 litre. Alternatively, in salt tolerant crops, that water can be blended with seawater or ground water for greater quantity.

Ghaffour’s newest research, yet to be published, is with his counterpart in plant science, Mark Tester, on partial desalination: “Our latest studies focus on plants or crops with high salinity tolerance – that makes desalination cheaper. In reverse osmosis 40 per cent of the net cost is in energy, because of the high pressure required. Nanofiltration doesn’t require such pressure. The pores are quite big. It removes part of the salt  – mainly the divalents, but monovalents and other contaminants such as boron might be an issue for some plants. We are also exploring naturally driven membrane distillation or forward osmosis, as standalone or with nanofiltration, by blending products at a rate to meet specific crops’ water quality requirements.

Andrea Ghermandi, from the Department of Natural Resources and Environmental Management at the University of Haifa in Israel, has two pilot plants using nanofiltration of brackish water powered by solar power at farm scale, one in Israel (Hatzeva) and one in Jordan (Karama). 

Brackish saline water is available in these places for irrigation but using it has drawbacks: it needs more water, it makes the soil salty and only certain crops can grow. 

Results from Hatzeva show that quantities of potato, maize and millet could be grown with 25 per cent less water and fertiliser than with brackish water, and 10 per cent more sorghum could be harvested. Salt tolerant red beet was grown with the concentrate, providing another crop and avoiding the problems of a highly salty waste product. 

“When it comes to agriculture, nanofiltration has two crucial advantages compared to reverse osmosis: it uses a substantially lower amount of energy and it leaves a larger concentration of nutrients that are essential to the crops, such as calcium and magnesium in the permeate. This translates into economic and environmental benefits. Our research shows that the main expected downside of nanofiltration  — higher concentration of dissolved solids in the permeate — is perfectly compatible with the growth of crops, including salt-sensitive crops like strawberry,” said Ghermadi. 

Significantly, Ghermandi’s research also reflects the fact that farmers in Jordan and Israel are conservative, experienced, concerned about saving water in agriculture, and worried about increased water salinity. 

“Understanding the perspective of the farmers is critical for the success of the innovation. The surveys we have conducted have revealed some important and unexpected results."

Ghermadi says that, for instance, Israeli respondents are unsurprisingly primarily concerned about investment costs and operational costs. "When it comes to what they would do with high-quality freshwater if they had it available, a majority of them state that they would continue to grow the current crops, aiming for a higher yield, rather than switch to new cultivations, even if potentially more profitable. Similarly, generic financial innovation incentives would be preferred over larger incentives to switch to new cultivations.” 

Adsorption alternative

One of the most promising of new technologies is adsorption desalination (AD). Invented by Kim Choon Ng of WDRC, the technique is capable of treating direct seawater, brine water and other highly polluted industrial waste water using at less than1.2 kilowatt hours per cubic metre.

AD utilises the double bond surface forces that exist between water vapour and silica gel. It needs only low temperature heat of between 55-80 degrees to power the sorption cycle. Sea water is fed into an evaporator at its natural temperature. Silica beds packed around tube fin heat exchangers, suck the vapour. The silica is heated to release the vapour, which is then condensed.  Hot water is used to heat the exchangers is powered by solar power and recirculated through the silica beds in a closed loop. It produces high-quality water and a cooling effect, without significant scaling. Because it has no major moving components it is easy to operate. 

In Riyadh, Saudia Arabia, a fully automated AD chiller plant implemented by King Abdulaziz City of Science and Technology went operational last year, producing 100 cubic metres of fresh water a day. When hybridised with multi effect distillation the MEDAD plant will have a capacity of 700 cubic metres a day, turning rejected brine from RO processes into potable water using low grade heat from thermal solar panels. 

Plants like these do have large footprints and high initial investment costs compared to membrane systems. Nevertheless they could be advantageous in an agricultural setting. 

“In rural areas where we don’t always have highly skilled manpower to operate a membrane, these systems are simpler,” says Ghaffour. “More importantly, they can operate with intermittent energy supply and treat challenging feeds.”

Water reuse

Most commentators from science and industry believe that for irrigation it is important to look at water reuse.

“Water reclamation is part of the game,” says Ghaffour. “In the domestic arena a lot of waste water is treated and dumped into the sea. There are technologies that make this water as pure as drinking water. There are some companies making profit from treating industrial waste water, including municipal water. Huge amounts of water could be harvested this way.”

This is in line with international thinking. The focus of the UNESCO World Water Assessment Programme this year was about supporting renewables driven waste water reuse for agricultural needs. 

“This is a key solution for the future,” says IDA’s general secretary Shannon McCarthy, “We are in contact with organisations like the UN FAO on the use of water reuse technologies for irrigation moving forward in the future, coupled with renewable powered units.”

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