Beating the drought with low-pressure membranes

Despite extreme drought conditions and a rapidly growing population, the Western Australia city of Perth is getting water to its residents due to a groundbreaking desalination plant which uses low-pressure membranes and renewable energy to achieve its results.

The RO membrane system at work in a seawater desalination plant inKwinana, 40 km south of Perth, Australia. Photo courtesy of the Water Corporation of Western Australia.
The RO membrane system at work in a seawater desalination plant inKwinana, 40 km south of Perth, Australia. Photo courtesy of the Water Corporation of Western Australia.

In 2005, after years of declining rainfall and depleting reservoirs, the Western Australia Water Corporation was determined to find solutions to increase the area’s water supply. Exacerbating the problem was a rapidly growing population in the coastal region of Perth. The Water Corporation initiated a strategy known as ‘Security through Diversity’ that included water conservation initiatives such as recycling and sprinkling restrictions to help ensure an adequate amount of water for the future for Perth’s 1.5 million residents.                                            

 Another major component in the plan was the construction of a seawater desalination plant, which today supplies 17% of the city’s potable water. This desalination plant has proven to be cost-effective through the use of state-of-the-art and energy-efficient reverse osmosis membranes to filter the salt from the water, and a combination of energy recovery devices and wind power to achieve optimal energy efficiency. 

“We were looking for a reliable method of obtaining water that could meet our needs now and in the coming years,” said Water Corporation Manager Desalination, Sjoerd Sibma. “After studying desalination, it became clear that recent developments in membrane technology had made the process more affordable and lowered energy constraints, so that with the right components and design, a desalination plant could help our community without straining our resources.”

The Perth plant became fully operational in April 2007 and now produces 144,000 m3/day of water. Located in the industrial area of Kwinana, 40 km south of Perth, the project is the result of a successful collaboration between the Water Corporation and proAlliance, a Joint Venture of Degremont and Multiplex that built the plant after Degremont won the tender in 2005. A subsidiary, Degremont Australia Pty Limited, was chosen in an Alliance with the Water Corporation to operate the plant on a 25-year contract. For the reverse osmosis (RO) process, Degremont selected FILMTEC membrane elements from Dow Water Solutions to comprise the two-pass RO system for seawater and brackish water.

Prepping water for desalination

Located off the coast of Western Australia, Cockburn Sound provides the feed water for the plant.

First, water enters the plant through an open intake catchment located in the Sound, roughly 200 m offshore and 6 m below sea level. The water collected for desalination amounts to 0.02% of the water in the Sound. Coarse screens fill the intake tower followed by band screens onshore so that large floating objects cannot get through and damage the feed pumps.

To prevent contaminants from harming the RO membranes, the facility’s planners knew that water entering the RO system must undergo proper pretreatment. Fouling and scaling on the RO membranes can lead to the need for frequent cleaning and downtime that raises operation costs. In order to find the best pretreatment design for the operation, the planners conducted a pilot study during a four-month period in 2004. The pilot study tested for filtration velocity, most effective media type and size, and water conditioning when varying temperatures and water qualities are taken into account.

The results of the pilot determined that a single-stage filtration process would work best. Raw water is pumped into the system and static mixers condition the water before it undergoes pH correction with sulphuric acid and coagulation using ferric sulphate and a coagulant aid. Next, the water enters the filtration process where 24 pressurized dual media filters of sand and anthracite remove contaminants and particulate matter. Based on a 24-hour filtration cycle, the filters are backwashed with air and water at regular intervals. Fourteen cartridge filters of 5 microns act as safety filtration to ensure a quality permeate for the RO membranes.  

Low-pressure membrane technology

After filtration, the two-pass reverse osmosis treatment does the main work of desalination. To keep energy and operating costs in check, the plant needed low-energy membranes that would be robust enough to ensure long service life. At the heart of the RO treatment are nearly 18,000 membrane elements from Dow. The Filmtec elements are eight inches in diameter with an active area of 400 ft2. The process entails a first-pass seawater reverse osmosis (SWRO) stage with a 45% recovery rate followed by a second-pass brackish water reverse osmosis (BWRO) stage with a 90% recovery rate.

Twelve trains with Filmtec SW30HRLE-400 elements produce 160,000 m3/day of water that then passes through six BWRO trains with brackish water Filmtec LE-400 elements for further polishing to produce 144,000 m3/day of water. The low-energy brackish water LE-400 elements feature an increased element efficiency that produces higher and more consistent flow rates at lower feed pressures. At the same time, the staged system allows energy recovery devices (ERDs) to deliver pressure energy to the SWRO membrane feed by recovering it from the brine reject stream, reducing the energy needs of the process by 15.6 MW (megawatts).

Kai-Uwe Hoehn, Dow Water Solutions large projects manager for Australia and New Zealand, headed up the membrane installation at the plant. “The Filmtec spiral wound membranes were chosen primarily for their efficiency and durability,” he said. “Our membranes had been used in other large projects, so there was a track record of durability and proven performance over time.”

Each RO membrane is made of a thin-film composite material that features three critical layers: a polyester support web, a microporous polysulphone interlayer and an ultra thin polyamide barrier layer. The polyester support web provides the major structural support for the element and features a hard, smooth surface. The microporous layer is cast onto the web to act as a substrate for the polyamide barrier layer, allowing it to withstand high pressures and chemical degradation. In an RO element, each leaf is made of two membrane sheets that are glued together with a permeate spacer in-between that aids in membrane cleaning and reduces fouling. The membranes feature a higher number of shorter leaves and are manufactured in a fully automated process that ensures consistency.

“With proper treatment, the RO element can last five to eight years before needing replacement,” says Hoehn. “Since the plant opened, our data has shown consistent performance for these membranes. It pretty much looks like a flat line, which means it’s constant in terms of flow and salt removal.”  

Reducing energy intake

A main component of the Water Corporation’s strategy was to make the desalination plant a model of sustainability and to meet the pressure needs of an RO system. High pressure must be exerted on the membrane so that water can pass through the semi-permeable membrane. It also has to overcome internal osmotic pressure from the saltwater. Low energy membranes help offset pressure needs as do energy recovery devices. At Perth, Energy Recovery, Inc provided ERDs to help lower the plant’s energy consumption. The first-pass SWRO component has a specific energy consumption (SEC) of 2.4 kWh/m3, the lowest amount on record for a large-scale SWRO desalination plant.* Along with other energy-reducing measures, the device contributes to a total plant SEC of less than 3.6 kWh/m3, which translates into an energy savings of 15% to 20%. Data shows that the energy recovery devices maintain efficiencies over 94%.

The Water Corporation also purchases its energy from a renewable energy generator, in this case the Emu Downs Wind Farm, in WA’s mid west region 30 km east of the coastal town of Cervantes.

Ongoing monitoring

Extensive instrumentation at the plant keeps operations running smoothly. Analytical panels were installed on seawater, first-pass RO feed and potable water lines.

“Making sure that the plant did not negatively affect the ecosystem of the Sound was a primary concern,” said Hoehn.

Before the reverse osmosis brine is released back into the sea, it is mixed with clarified backwash water and dispatched through diffusers that help lower the salinity level of the discharge. The Water Corporation also established an extensive ocean monitoring program to measure the quality of the Sound.

Safe for drinking

With the final goal of securing safe drinking water for Perth residents, The Water Corporation developed standards in keeping with the Australian Drinking Water Guidelines. They required total dissolved solids (TDS) of 200 mg/L, bromide maximal concentration of 100 ug/L and alkalinity above 50 mg/L as CaCO3 (calcium carbonate) in the end product.

To make sure the desalinated water is suitable for municipal purposes, it is remineralized at the plant with a carbon dioxide injection and lime, along with an injection of fluoride and chlorine.                 

In November 2006, the first drinking water was produced at the plant. The treated water is held in an onsite tank before it is pumped to the Thompson Reservoir, 12 km east of the plant. The water is then blended with other sources to become part of the Integrated Drinking Water Supply Scheme, providing Perth with 45 gigalitres of drinking water per year.  

Perth’s achievement 

In 2007, the Perth facility was named Desalination Plant of the Year by Global Water Intelligence. Currently, it provides 17% of Perth’s municipal water needs. Australia now has six large desalination projects in various phases of implementation and Hoehn thinks the trend will continue.

“Advances in membrane technology will help produce more water and better water with less energy in the coming years,” predicts Hoehn. “The success of the Perth plant is a lesson for water-stressed communities that desalination can be an affordable, sustainable and highly effective component of a responsible water strategy.”

*from ERI PX® case study