Desalination, Small Is Beautiful Say ACWA

Peter Ripley


Desalination, Small Is Beautiful Say ACWA

Desalination is increasingly being used in areas of the world where climate change is biting – the Middle East, the southern states of the US, the Mediterranean rim, Australia and the Caribbean, for example – but also for a number of other reasons. These include treatment of polluted industrial wastewaters from oil and gas production, preparation of high-purity waters for the food, drink and pharmaceuticals industries, and as part of a treatment train to treat difficult compounds such as recalcitrant organics.

Salinisation of groundwaters is also an increasing problem, ACWA believes, particularly in coastal areas where over-abstraction from inland aquifers has enabled saline waters to move in, and in some cases make the remaining but vital groundwaters unusable. In such instances, RO is increasingly being used as a relatively cost-effective means of allowing the continued use of valued water sources.

Offshore brackish groundwaters, which are found in many places – South America, the Gulf and South East Asia in particular – are also being considered as potable water sources as the world’s appetite for water increases. These groundwaters would have to be conveyed to shore by pipeline, like oil and gas, and as the waters are brackish they would require fewer membranes for treatment, which brings the costs down significantly.

This trend encompasses the proposed Thames Water desalination plant on the Thames Estuary near the Beckton wastewater treatment works. This plant will desalinate the brackish waters of the tidal Thames, and is intended to provide up to 140MLD of water to the UK’s capital city during droughts.

Particularly in highly-developed countries, ACWA is seeing that increasingly stringent discharge requirements for industry are driving the use of end-of-pipe desalination. In Australia, which is a boom area for desalination, brackish waste groundwaters from industrial processes such as mining are being treated using desalination to ensure they do not pollute the inland areas that surround the extraction projects.

Australia is increasingly using reverse osmosis (RO) to provide high-quality, reusable wastewaters both from industrial and municipal plants – this has the advantage of reducing micro-organisms and organics as well as salinity.

The resultant recycled waters are used by industries and power stations and the country is also considering mixing these highly-treated waters in reservoirs for onward consumption. Massive mining corporations are also constructing desalination plants both to provide water for the significant number of workers that such projects require and for the mining processes themselves.

Aquifer recharge and storage (ASR) is also driving RO use, for instance in Orange County, California, where highly-treated wastewaters are returned to the aquifer, thus ensuring that the aquifer is replenished and free from the threat of salinisation.

Trends also include efficiency – units are increasingly becoming smaller and more productive so costs are coming down and membranes are now providing 20 times more water for the same area as 20 years ago. ACWA is finding that reuse and recycling are on the increase, as is RO use in aquifer storage and recharge (ASR).

Technology trends

Reverse osmosis (RO) technologies are increasingly attracting attention, even in the Middle East where the low-tech but reliable multi-stage flash (MSF) and multi-effect distillation (MED) thermal technologies have been in use for decades, and where combinations of electricity generation and thermal desalination are the norm.

Unfortunately, these plants are energy-intensive and expensive to run so this consideration – and an increasing drive for ‘green’ cities such as Masdar in Abu Dhabi – has seen the region turn increasingly towards the newer technology.

The traditional energy/thermal desal combination also becomes problematic if there is a mismatch between energy and water demand, which is another reason why the more flexible RO systems are being increasingly chosen, ACWA believes – they do not rely on a continuous supply of steam from a large power plant and the initial investment costs are either similar or lower.

The trend towards separating power and desalination is illustrated by Abu Dhabi’s recent decision to designate the proposed Shuweihat 3 plant as an independent power project, moving the desalination plant that would normally have been teamed with it to Taweelah – a site much closer to the capital, Abu Dhabi city, and therefore much more convenient in terms of ongoing operational costs. ACWA predicts that this move may well signal that RO is being seen as a serious contender for the main desalination technology at the new plant.

In the Middle East, RO is also being installed in some situations as a pre-treatment for MSF or MED plants, ensuring that the quality of feedwater is more consistent and providing significant reductions in energy consumption that are very attractive in an era of increasing energy awareness.

Desalination is an incredibly dynamic technology that is evolving at a remarkable rate – twenty years ago, plants used twice as much energy as their modern counterparts, and were around ten times larger. This is particularly true for RO membrane technologies, where the rate of innovation has been extraordinary. The developments are ongoing – for instance, UCLA has been working on incorporating carbon nanotubes into the membrane film and this promising technology could double throughput.

ACWA has seen the price of the technologies continuing to fall, as does their energy demand, which is particularly true of RO systems.

Unfortunately the increasing cost of energy has to an extent masked this notable improvement but it is certainly the case that were it not for the remarkable technological advances that have taken place, RO would still be viewed as an expensive option whereas it is now increasingly being seen as the technology of choice in particular situations.

New twists on the RO process and potential new desalination technologies are at various stages of development and trial – combinations of distillation and membranes, for instance, among a raft of other exciting possibilities. Another example is PUB, Singapore’s water utility, which is trialling a forward osmosis concept using an osmotic membrane bioreactor that reportedly could reduce operational costs by 20%. ACWA is monitoring all of these developments with great interest.

Challenges

Despite the improvement in energy efficiency, energy usage is still one of the major desalination challenges. Resolving this critical issue has given rise to trials or consideration of solar, nuclear and wind energy among others.

In Australia, for many of the new RO plants that have been or are being installed to serve nearly every major city, the norm is to ensure that the plants are ‘energy neutral’ – that is, that either the plant operator purchases renewable energy to the equivalent of the amount used by the plant, or a renewable source such as a wind farm directly provides the plant’s energy. Solar power has also been used as a source of power in Spain, Australia and California – in such systems, high-tech solar thermal collectors generate energy that is converted using steam turbines into electricity.

Energy recovery devices such as turbines or pressure exchange systems are being increasingly used to recoup energy from the desalination process, generating considerable energy and operational cost savings – at Israel’s massive Ashkelon desalination plant and ACWA’s prestigious Palm Jumeirah desalination plant in Dubai, energy from the brine stream is recovered to provide significant cost benefits.

In the Gulf, nuclear energy is also being considered as a power source, and pressure-retarded RO, which converts the energy potential that exists between two water streams of different salinity into kinetic energy to drive turbines, is also being trialled in Europe. For the Thames Water desalination plant, the proposal is to run the plant entirely on renewable resources – biodiesel and possibly reprocessed cooking fat.

In some instances extremely low-energy solutions such as ‘seawater greenhouses’ (for example in the Sahara Forest project) are being proposed. This unusual quasi-desalination technology would use seawater to cool and humidify the air inside a greenhouse array, with potable-quality water being condensed from the resultant water vapour. Given the vast acreage of greenhouses around the world, this is a low-tech option with considerable potential, ACWA believes. A demonstration centre is planned to provide the data needed to optimise a full-scale version.

Pretreatment is also a vital element in reducing energy costs – robust and effective pre-treatment systems, such as UF and NF (ultrafiltration and nanofiltration) and rapid sand filtration, tailored to the requirements for each individual project, provide effective and sustainable solutions that remove the fouling constituents sufficiently to ensure the reverse osmosis (RO) membranes are operating at peak efficiency, and therefore optimal energy use and recovery rates.

ACWA Emirates’ high-profile, high-end project to build two identical seawater desalination plants for Dubai’s Palm Jumeirah project perfectly illustrates this approach. For this prestigious development, the treatment train involved screening, ultrafiltration, RO, second stage RO polishing, disinfection and remineralisation.

The process design had to overcome a number of challenges. In the early years of operation it was anticipated that ongoing construction activities at Palm Jumeirah could make the raw water of variable quality, so the design incorporated specially-chosen water quality monitors in the intake structure to give early warning of potential issues so that the necessary operational adjustments could be made.

RO is central to the treatment process, and here UF was seen as the ideal partner pre-treatment system, offering clear benefits over conventional technologies. The UF system protects the RO membranes, ensuring the seawater’s characteristics are suitable for RO treatment. As indicated above, an energy recovery system transfers energy at high efficiency from the RO plant brine stream to the low-pressure UF filtrate feed pumps. ACWA has designed and installed RO plants for water authorities and private enterprises throughout the UK and Middle East, particularly in areas where potable water is in short supply.

In the Gulf region, where marine algal blooms have caused significant problems, options such as construction of giant water storage reservoirs teamed with UV treatment to prevent bacterial growth are being used ahead of desalination to buffer against the worst effects.

Rising chemical prices are also an issue for RO desalination, and pre-treatment can again be a good solution to this cost conundrum, as membranes require less cleaning if proper pre-treatment is installed. There is a delicate equation that has to be balanced in each situation to optimise the inevitable cost of pre-treatment against onward operational costs, which companies such as ACWA are ideally placed to resolve.

Brine stream disposal is becoming an increasing challenge, particularly as environmental considerations come to the fore, including in the Middle East where the impact on coral reefs is being considered. Outlet diffusers can be used to reduce environmental impacts where brine is discharged back into the marine environment – research suggests that temperature and salinity return to normal seawater levels within around 75m of the discharge system.

Sometimes strict legislative requirements drive brine solutions – the Sand City desalination plant, which discharges into the vital Monterrey Bay ecosystem in California, had to guarantee that the brine stream would be diluted to match the background salinity in the bay. The solution was to opt for a low-recovery system and discharge the residual beneath the bay’s sands, which ensures that by the time the waste stream percolates back into the ocean it has no environmental impact.

The ‘zero liquid discharge’ concept is also being researched – this contemplates using water softening to reduce fouling and scaling, teamed with ion exchange to further decrease the burden on the RO plant, and ACWA understands that in theory this could enable recoveries in excess of 95%.

Environmental issues have also pushed moves such as low-speed abstraction of seawaters to minimise the impact on marine life, as used in Sydney’s desalination plant. Crystallisation and evaporation technologies can also be used to produce a salt crystal product that is highly saleable. This ability to view the brine waste as a resource is an encouraging new development – already, at some sites in Australia, the brine salts from the desalination process are being sold.

Scale is another significant issue for RO membranes, and this is another challenge that the industry is concentrating on resolving. In wastewaters, a high phosphorus content mean that insoluble calcium phosphate is deposited as scale on the membranes, so new and improved scale inhibitors are being developed to counter this. Low-P chemicals are unfortunately more expensive than their P-containing counterparts, so ACWA sees this is an ongoing challenge for the industry.

ACWA believes that these trends – towards increasing energy efficiency, lower operational costs, smaller and more effective technologies – will continue, particularly with respect to RO. Given the world’s soaring need for high-quality water and wastewaters, set against dwindling water reserves, desalination is fast becoming not only an attractive option, but a vital one.

For further information please email ACWA Services Ltd

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