ACWA: The key to efficient RO desalination

Quarry pool, Jersey, where seawater is stored before treatment.
Quarry pool, Jersey, where seawater is stored before treatment.

Fresh, safe and plentiful, water is something we take for granted in most of Europe. But in other parts of the world, acute water shortage is a very real problem. Rapid world population growth tripled during the 20th century and continues to expand, is adding further pressure. Estimates are that 4 billion people in 52 countries are likely to suffer from water shortages by 2025.

But it's not only places like North Africa, the Middle East and other arid regions where water shortage is a problem. Closer to home, shifting weather patterns with increasingly prolonged dry spells, are also affecting water security in more places than you would think. London and the South East of England is a water stressed area for example. Hose pipe bans have been all too familiar and island communities like the UK Channel Islands are also at risk as rainfall patterns change.

Changing consumption habits, and more importantly recycling water, will have to increase across the globe as demand for fresh water grows. But, with 97% of the world's water in the oceans, desalination plants are a major part of the solution, able to provide alternative fresh water resources, especially for coastal communities, or to relieve stress on conventional water supplies.

Indeed there are now thousands of desalination plants worldwide with heavy concentrations in North America's West Coast and the Middle East. Beckton in East London is the site of a standby desalination facility operated by Thames Water, ready to provide some 150 million litres of drinking water each day if water restrictions are enforced. The likelihood is that the number of UK desalination plants will increase by 2050 as climate change and population growth further impact water stressed areas.

Until recent years desalination technology was dominated by energy intensive thermal or distillation processes where seawater or brackish water from underground sources is heated under pressure to condense out purified water. Typical processes include Multi-Stage Flash distillation (MSF) and Multiple Effect Distillation (MED). These have been largely superseded by membrane-based solutions including Electro-Dialysis Reversal (EDR) and more commonly Reverse Osmosis (RO), the preferred solution at ACWA Group. Here, salt water under pressure, up to 80 bar for seawater, is forced across a semi-permeable membrane to separate the salts, leaving concentrated brine on one side and freshwater for downstream consumption on the other.

Continuing technological advances in the thin-film composite polyamide membranes which are predominantly spiral wound into individual pressure vessels or canisters, and the manufacturing ability to cram more membrane into each canister to dramatically increase the working surface area, have seen steady efficiency gains. Another advantage is that the plant footprint is significantly smaller than earlier thermal processes.

With a growing range of spiral wound RO membrane manufacturers, each with different characteristics in terms of operating pressure and temperature, selecting the right membrane for each environment is also becoming more important.

Seawater intake pumping station.

Upgrades at La Rosière, Jersey  

On the UK island of Jersey for example, ACWA is currently engaged on a major upgrade and modernisation programme for Jersey Water's La Rosière desalination plant. Jersey is one of the Channel Island communities at risk from reduced rainfall and rising demand. The purpose is to improve security of supply during periods of low rainfall, while increasing capacity and importantly reducing running costs.

The island is dependent on surface water for its public water supply and with only 120 days storage capacity in its reservoirs La Rosière provides an essential standby facility during prolonged dry periods. The first desalination plant in the UK, Jersey Water last upgraded La Rosière in 1999.

The latest modernisation works will bring the plant right up to date, providing a highly resilient and efficient fresh water solution. At the same time, the facility will almost double in capacity to take account of changing climate conditions, population growth and summer demand from agriculture and increasingly, tourism.

The turnkey solution comprising design and procurement through to installation and commissioning will see the complete redesign and expansion of the existing seawater reverse osmosis (SWRO) process streams. It includes new pressure vessels, an additional dual media filter, next generation high capacity RO membranes, pre-treatment systems and associated equipment. To keep the plant footprint small and to satisfy demanding island planning authority constraints, all new technology and equipment will be contained within the existing facility.

The new plant will take fresh water production when the plant is online, from the current capacity of 6 million litres per day, up to 10.8 million litres. ACWA's design allows for potential future expansion to 15 million litres per day, ensuring additional longevity for the plant should demand grow further.

Reverse Osmosis uses hydraulic pressure, typically up to 80 bar, and semi-permeable RO membranes to diffuse pure water, referred to as permeate, from seawater. The new solution is designed to recover pure water at a rate of 45% from the seawater. The solutes in the seawater feed system are concentrated as a result of the process and are returned to the sea as concentrated brine.

To achieve this, two streams with 72 individual pressure vessels each containing 6 high pressure reverse osmosis membranes, 864 in total, will be deployed at La Rosière.

A recovery ratio of 40% to 50% is typical for seawater solutions but depending on the application and the salinity of the feed water this ratio can vary significantly, especially for brackish water which contains less dissolved salts and therefore requires less energy.

Commenting at the start of project, which is due for completion mid 2016 Helier Smith, chief executive at Jersey Water acknowledges that: “ACWA's track record and international experience with desalination and advanced water treatment technology, makes them a perfect partner for this critical water industry infrastructure project on Jersey. With the current plant at the end of its serviceable life, ACWA's solution will increase capacity and resilience for the Island's public water supply in times of low rainfall and built-in energy recovery will ensure that running costs are lower when the plant is online.”

ACWA technology at the Palm desalination plant, Dubai.

Additional DMF vessel.

Energy recovery  

Despite the efficiency of modern RO desalination technology producing fresh water from seawater remains an energy intensive process requiring large amounts of electricity. Operating plants off peak and at night when electricity demand is low can help. But the latest plants are also using advanced energy recovery systems to reduce energy usage.

For example at La Rosière ACWA is specifying a state of the art energy recovery system utilising a PX Pressure Exchanger to recover hydraulic energy from the reject stream of concentrated brine which is designed to provide significant energy savings.

Membrane contamination  

Membrane contamination and fouling is a key consideration for the design and engineering team to ensure efficiency and longevity for RO desalination plants and it is crucial that all chemical, mechanical and electrical engineering disciplines are fully embedded on a project.

Pre-treatments including particulate screening and backwashing systems are important. Membrane scaling as salts precipitate out is another issue with a direct impact on output. Calcium carbonate and calcium sulphate in particular need to be managed and further pre-treatments and dosing systems are normally required to de-alkalise the feed water. Here, sulphuric acid and other chemical antiscalants are typically used.

At the 64 MLD Palm Jumeirah Reverse Osmosis desalination plant in Dubai for example, also developed by the ACWA Group, a range of ultrafiltration and chemical dosing systems for ferric chloride, chlorine dioxide, sulphuric acid, antiscalants, sodium hypochlorite, sodium hydroxide and sodium bisulphate are employed as part of the solution. The plant was commissioned in 2007.

In most cases post-treatment is also required to restore the pH and mineral content, which may have been stripped by other processes.

Of course, excessive chemical intervention is expensive, so a design which balances all these factors will help to both optimise recovery rates for fresh water and control long term plant operating costs.

Fresh water will continue to be a global issue, but the good news is that desalination projects incorporating Reverse Osmosis are already playing a key part in the solution. As water stress increases in other parts of the world, top-up desalination facilities to complement conventional water supplies, as opposed to new reservoir and pipeline infrastructure to pipe in water from other areas, will look increasingly attractive. The technology is already mature with low risk to investors and almost daily improvements in filtration, membrane and energy recovery technology will see continuing efficiency gains.