Thirst quencher

But utility companies are finding that reverse osmosis (RO) desalination technology can provide a quick and cost-effective solution to these problems, and as a result this method of desalination is gaining market share in the region.

In the RO desalination process, high pressure is used to push saline water through semi-permeable membranes to remove the dissolved salt content.

It does not require heat, unlike the thermal distillation methods, such as multi-stage flash (MSF) and multi-effect distillation (MED), which vaporise the water to remove the salts. This means the facilities can be built independently from power plants and be connected to the electrical grid instead.

Bad image

RO is not a new technology; it has been around for more than 30 years. For sometime, though, water firms in the Middle East were wary of experimenting with RO desalination because one of the early projects constructed in Bahrain during the 1970s famously suffered many complications as a result of its inadequate pretreatment system.

Other plants dating from the same era also experienced operational problems and some continue to suffer, as Paul Buijs director of Global Membrains consultancy explains: “RO requires a good pre-treatment. In the UAE and other areas in the Gulf there are some installations that are facing problems. You can bring most of the problems back to the fact that the buyer has gone for the lowest possible investment cost instead of the lowest cost of operation.

“If you apply a very rudimentary system of filtration you will get the particles out, but you won’t get the bugs out, and they start fouling and clogging up the membranes, reducing their efficiency. With sufficient pretreatment, especially when applying ultra-filtration, you will have a trouble-free RO system. Normally manufacturers would recommend a cleaning once every three months, however, we are involved in a system in Belgium which is cleaned once every year and even then it is a very limited cleaning.

It shows that with a good pre-treatment you get a very stable situation. And you can do that on any type of water – there are even ROs for industrial applications that are run on municipal effluent, which obviously has a very high biofouling tendency. With a good pretreatment there is no problem with RO.

A growing understanding of the chemistry behind the technology and the need for daily pH control, along with improvements in membrane design have enabled RO desalination to shed its bad image in the region over the past few years.

The fall in the cost of RO desalination has also helped it to gain in popularity, as Aqualyng’s Eric Jankel explains: “The economics of the technologies now favours RO, when all other things are equal.

The price for the end-user has come down, hitting a bottom about four or five years ago. The technology has also improved and the membranes are more productive. If you spend US $500 for an eight inch by forty inch element you are producing more water from that element today than you would have done five or ten years ago.”

Energy recovery

Another important driver in the shift towards this technology has been the improvement in the efficiency of energy recovery devices, which have significantly reduced the cost of operating of RO desalination units.

“Energy is about 25% of the cost of producing water on a lifecycle basis,” states Jankel. “If you save a marginal amount on energy you can make significant savings on the cost of production.”

The most energy-intensive part of the RO process is the initial pressurisation of the feedwater.

Thermal desalination plants are designed for maximum energy recovery because the heat they put in is recovered mainly from the power plant’s waste stream,” says Buijs, explaining the technology. “They also recover energy by cooling the outgoing water with the incoming water. But an RO uses pressure to make the water pass through the membrane.

The concentrate stream that leaves the membrane still contains a lot of energy. You can let that go to waste or you can use that energy to drive a device that pressurises the water coming into the RO.

The net energy requirement for a desalination plant can be reduced from 7-8 kWh per m3 to 3.5-4 kWh per m3 by using an RO plant fitted with a state-of-the-art recovery device, according to Jankel.

This can lead to considerable economies of scale and has triggered a growing tendency to build larger plants.

“Ten years ago the largest seawater desalination plant was probably in the range of 10 000-30 000 m3 per day, no0w the largest one is 350 000 m3 per day. And Australia is talking about building one at 450 000 m3 per day using RO,” Jankel adds.

Fitting energy recovery devices also means that a smaller pump can be used to generate the pressure, which again brings cost savings. Buijs believes this is the reason why there is still interest in using this equipment in regions where energy is still relatively cheap. “One of the main reasons to use an energy recovery device in RO is to save on investment costs because the money you spend on the recovery device you gain by not having to install a very big pump. So it pays for itself. The suppliers of these systems are really booming,” he says.

Tried and tested

But this does not mean that thermal desalination has had its day.

“It is not one method versus the other,” says Jankel. “RO will not take over. Thermal is very robust and proven. In certain applications, industrial cogeneration with a power station will continue to be utilised for the foreseeable future. But as a percentage of installed capacity RO will continue to gain because many new plants will not be beside a power plant or in an industrial zone.”

Buijs agrees: “The trend is towards RO, but where there is cogeneration of power, obviously, it is worthwhile to explore the traditional techniques, MSF and MED. But RO has the advantage in that it needs less energy, you use less chemicals and the brine you discharge is actually at the same temperature as the seawater you subtract so you don’t have any thermal effects upon discharge.

The other main advantage with RO is that the units are modular, which brings flexibility in their size and where they can be located. It also enables extra trains to be easily added as requirements grow.

Hybrid plants

As a result of the improvements in RO technology and the reduction in costs associated with it, many combined water and power projects are today designed to include thermal and membrane technologies. These are known as hybrid plants.

“There is a drive to create more efficient configurations using both technologies in these large plants,” says Jankel. “You have a base-load plant that is running all the time and in some configuration of plants throughout the grid you are meeting your base requirement. The problem is that in the summer they hit their peak demand.

On that day you need all of the power output that you can find and some of the high-pressure steam that could be used to make electricity is being used to make water. It is only about 3%, but there is a big debate on how you value that part. So in order to offset this parasitic loss they are building hybrid plants.

The Abu Dhabi Water and Electricity Authority’s two power and water plants in Fujairah, in the UAE, are hybrid plants: one combines MSF and RO technologies, while the second, which is still under construction, will use MED and RO techniques.

“In the UAE, the winter time base load for power is about 50% of the peak summer demand and this is a huge variant, whereas water demand is relatively stable for the year,” continues Jankel.

“The operational cycles are out of sync and RO can be used either in these hybrid configurations in the same plant or to diversify the production capacity in order to absorb some of that load and improve the economics of the power and the interaction with the thermal,” he adds.

Continual evolution

RO desalination technology has come along way since it was first commercialised in the 1970s. Major gains in salt removal capability, energy efficiency and membrane life, as well as cost reductions, have made it a viable and effective solution to meet the increasing demand for water in the Middle East. Great strides have also been taken in addressing the potential environmental impact of the plants, by designing better intake systems and outfall pipes.

Prices may not be able to fall any lower, but RO technology will continue to evolve. Today, there are many studies around the world working to alter and improve designs, for example, by looking at the potential of nanotechnologies and membrane distillation. Other projects aim to reduce the carbon emissions from RO plants by generating electricity from solar, wind, wave and other renewable energy sources or from nuclear power.

Another area under investigation is the use of floating RO desalination plants to produce backup supply when demand peaks. A 50 000 m3 per day floating desalination unit on the Red Sea is helping to alleviate a water shortfall in Jeddah, Saudi Arabia. These barges have the added advantage of enabling brine to be discharged deep at sea.

There is also interest in developing salt recovery techniques for industrial use, particularly in China. The concept of zero-liquid discharge is technically feasible, although drying salt requires a lot of energy. Nevertheless, this could become a realistic resource in the future because as economies expand demand for salt and salt-related products, such as caustic soda and chlorine will also continue to rise.

What is certain, though, is that desalination will continue to play a crucial part in addressing the worsening shortage of freshwater in the world. Whereas five or ten years ago desalination was often considered a last resort, it is now becoming a necessity in many regions. Furthermore, the most rapid industrial growth is taking place in regions where water scarcity is already is a very real and apparent problem, such as in China, India and the Middle East.

Within the field of desalination itself, RO technology is best placed to meet the new challenges posed by developments in the 21st century. “As places grow and they do these island and coastal developments the original concept of having power and water concentrated in one site no longer works,” comments Jankel.

When there is a lack of room to expand capacity, it makes more sense to distribute production out to where the demand is, especially as the development projects move away from the original urbanised centres.

“You can then also take advantage of that to recycle and reuse water to irrigate golf courses and so on there. The economics of the water balance favour a distributed system when you reach a certain point of development,” he adds

But, of course, building more RO desalination plants cannot solve the water shortage alone, as Buijs concludes: “Any measure you take is fruitless if you don’t change people’s attitude towards water.

“Raising consumer awareness will have a much more striking effect than all the technology you can think of.