Gradual shift to reverse osmosis

13 April 2015

Governments are expanding desalination capacity to meet rising water demand, and are increasingly focusing on reverse osmosis

Rising per capita consumption and population growth rates as high as 9.3 per cent a year in Qatar are driving up demand for water across the GCC.

Peak water consumption in the region is projected to grow from 14.2 million cubic metres a day (cm/d) in 2013 to 23.4 million cm/d by 2020, according to MEED research.

To meet this demand, governments are planning to build 14 million cm/d of new desalination capacity, to supplement the 19 million cm/d already installed.

Desalination reliance

Due to extremely low rainfall and depleting groundwater resources, the GCC relies heavily on desalination, for up to 79 per cent of water supplies in Bahrain and 75 per cent in Qatar, according to the Qatar Environment & Energy Research Institute (QEERI).

“Consumption is always rising, but adding desalination plants is costly, environmentally unfriendly and unsustainable as it relies on fossil fuels,” Mohamed Darwish, principal investigator for desalination technologies at QEERI, told the MEED Qatar Projects 2015 conference earlier this year. “We have to look for a sustainable solution.”

Up to 79 per cent of existing desalination capacity in the Gulf uses thermal desalination methods, predominantly multi-stage flash (MSF), with a smaller proportion of multi-effect distillation (MED). Thermal desalination capacity is co-located with power plants, taking advantage of heat and electricity produced in cogeneration facilities.

But a gradual shift is under way to reverse osmosis (RO) desalination, where water is instead filtered through a semi-permeable membrane; the majority of planned capacity is set to use the technology, reflecting an increasing concern with efficiency.

The GCC was slow to adopt RO technology due to the characteristics of the Gulf’s waters and the abundance of cheap energy resources, and these still pose an obstacle.

Key fact

In Saudi Arabia, 79.8 per cent of planned capacity will use reverse osmosis technology

Source: MEED

Government clients still prioritise cost and reliability, but of late are increasingly focusing on slowing the exponential growth in domestic fuel consumption, and it is this that is encouraging the uptake of RO technology.

In Saudi Arabia, 79.8 per cent of planned capacity will use RO technology, while the remainder, to be built in the 2020s, is undecided. Similarly, more than 90 per cent of planned capacity in Oman will utilise RO.

Qatar is the last GCC country to move away from thermal methods, awarding a contract for its first hybrid utility-scale desalination plant in March. Japan’s Mitsubishi will develop the Ras Abu Fontas A3 desalination plant, with 318,000-410,000 cm/d of thermal capacity and 182,000-273,000 cm/d of RO capacity.

Energy efficiency

“The general trend seems to move towards the more energy-efficient RO,” says Titia de Mes, industry leader for water at Dutch consultancy Arcadis. “The speed of this depends on how rapidly power plants are replaced. So it will be very slow as the existing cogeneration power and water plants will remain active for a while.”

While RO makes up almost 60 per cent of the desalination capacity planned in the region, as opposed to 20.8 per cent of existing capacity, there are still financial and environmental disadvantages to its large-scale use.

The benefits of the technology are not decisive and may depend on specific conditions at each site. Membrane fouling is a recurrent operational issue.

“RO plants have a lower capital cost, but higher operational costs than thermal plants, making thermal slightly cheaper over 25 years,” says Saad Alani, an independent water consultant leading the process design of the industrial/municipal wastewater and desalination plants for Jizan Economic City in Saudi Arabia. “Also, up to 65 per cent of water intake is rejected as brine for RO, compared with 10 per cent for thermal technologies.”

Higher costs

Although RO plants require no thermal energy, they are still energy intensive. QEERI estimates it takes a total of 5-6 kilowatt hours of electricity to produce a cubic metre of desalinated water. Costs are higher in the Gulf due to the warm, shallow, highly saline water, which has a high content of fats and oils.

“Seawater RO is less efficient with highly saline water, as found in the Gulf, as more energy is needed for pumping,” says Alani. “Salinity in the Gulf has already increased by 2 per cent due to brine discharge, and this is affecting its environment and fish stocks.”

The largest operational cost for thermal desalination in most regions is fuel and energy requirements. This is reduced in the Gulf by the abundance of fossil fuels and subsidised feedstock for the power and water sector.

A QEERI figure of at least $3 a cubic metre for thermal desalination in Qatar is probably an underestimate. The opportunity cost from selling valuable hydrocarbons resources at a subsidised rate on the domestic market often goes uncounted, yet reduces the motivation to seek efficiencies.

Preferring RO

But RO is widely expected to be the most commonly selected technology in future for a variety of reasons.

“The general long-term development will be an uncoupling of power and water [facilities], as future ways of power generation will not have the possibility to provide for thermal desalination to assure cogeneration: for example nuclear, solar and wind energy,” says De Mes. “The number of seawater RO plants in the GCC is gradually increasing as the process becomes more energy efficient and membrane prices decrease.”

New developments around RO, itself a decades-old technology, are increasing efficiency and lowering costs.

Improvements in seawater pre-treatment for RO became a priority following the 2008-09 ‘Red Tide’, when algae blooms turned the Gulf crimson for eight months. Aside from environmental damage, it forced the shutdown of several desalination plants.

Ultra filtration

RO plants at the time used just a sand filter, but intake is now commonly treated via dissolved air flotation (DAF), which removes suspended matter.

An ultra-filtration membrane before RO also removes many particles and works as a barrier, protecting more expensive membranes. This was installed at the 218,000-cm/d Addur II independent water and power project (IWPP) in Bahrain, commissioned in 2012. Both techniques reduce RO maintenance costs by prolonging the useful life of membranes.

In addition, improvements in pumping, energy recovery and two-pass methods are reducing operating costs and volumes of reject water, but are also raising capital expenditure.

The ultimate achievement in sustainable desalination would be utility-scale renewable desalination. But while small-scale solar desalination is tried and tested, direct solar desalination is currently only used in remote rural areas to produce small quantities of water, or in pilot plants.

Qatar, Abu Dhabi and, most intensively, Saudi Arabia are studying the utility-scale application of solar-powered desalination. However, the cost of utility-scale plants is prohibitive, and is unlikely to be feasible in the near future.

Abu Dhabi Future Energy Company (Masdar) selected companies for four projects to pilot different solar desalination techniques in 2014; the firms were France’s Suez Environnement and Veolia, Spain’s Abengoa and the US’ Trevsi. Construction of the facilities is nearing completion and preliminary results should be available in mid-2015.

Renewable desalination

Saudi Arabia, meanwhile, is constructing the world’s first utility-scale renewable desalination plant.

The local Advanced Water Technology - a newly formed subsidiary of Taqnia, the technology arm of the Saudi Public Investment Fund - is building the plant along with Abengoa. The consortium will use 16MW of photovoltaic (PV) solar panels to produce 60,000 cm/d of water using RO technology, which will supply the northeastern city of Al-Khafi.

But there are serious drawbacks to renewable desalination.

“Solar is a popular option, but it needs large amounts of land and skilled labour for operation and maintenance, which raises costs,” says Alani. “Also, the power requirements for desalination are still too high for solar to replace conventional power sources. Nuclear will be the future.”

QEERI’s pilot projects have focused on various types of concentrated solar power (CSP) formations producing steam and RO as the most cost-efficient techniques. However, costs remain far above even the least efficient conventional desalination methods, especially when research and development costs are considered.

“The only thing that stops us from doing solar desalination is the cost, especially if you use direct solar desalination,” said Darwish at the MEED Qatar conference.

“You have to have a solar power plant that reduces the amount of conventional energy needed to run the most efficient desalination technologies.”

The technologies remain at the testing stage and will not be commercially viable in the foreseeable future.

Water cycle

With the costs and inefficiencies of existing technology unlikely to be dramatically reduced in the near future, efforts to improve sustainability and cost efficiency of desalination in the GCC will need to focus on refining existing processes and managing the entire water cycle.

“Utility subsidies hamper the encouragement of good housekeeping, and wastewater treatment plants often just discharge treated effluent into the sea,” says Alani. “The different authorities for desalination, groundwater, wastewater and irrigation should be brought together to create a strategic approach to total cycle management and zero-waste policies.”

These would include the commercialisation of the salt produced as a byproduct of desalination, and treating wastewater, which has a much lower salinity and is therefore cheaper to process. Treated sewage effluent (TSE) reuse can be incentivised for irrigation and district cooling purposes, reducing desalinated water demand.

Demand-side management is difficult when consumer prices do not reflect costs. Steps are being taken in this regard, however. Abu Dhabi’s decision in 2014 to raise water prices by up to 170 per cent was aimed at reducing escalating demand growth and followed Dubai’s successful hike of costs for expatriates in 2011.

Subsidised water rates also discourage the uptake of TSE. Although GCC governments are promoting TSE reuse, only Dubai has achieved 100 per cent reuse rates. Along with higher water rates, this has stabilised demand and has saved millions in new capacity investments.

Enhanced reverse osmosis techniques

? Forward osmosis - uses an osmotic agent rather than pressure to induce osmosis, and is claimed to reduce membrane fouling and energy consumption. UK firm Modern Water began operation at a test plant in Al-Khaluf in Oman in 2010;

? Dewvaporation - uses humidification (below boiling point) to distill water. Piloted in the US;

? Nano-engineered membranes - membranes engineered at the near-molecular level to remove certain particles that could reduce fouling and increase efficiency. Currently difficult to scale up to utility level;

? Membrane distillation - hybrid thermal-membrane technique suitable for direct solar applications. Only vapour passes through the membrane;

? Capacitive deionisation - removes ions via electrodes. Efficient but primarily applied to brackish water;

? Electro-deionisation - a post-RO treatment using membranes and ion-exchange rather than chemicals;

? Self-cleaning fluidised bed heat exchanger - more efficient and higher temperature heat transfer, usually for multi-stage flash, without fouling.

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