Leading the way - desalination in Perth

Engineers Australia
February, 2007 Page: 32

Climate independent water sources are starting to be introduced to secure public water supplies in Australia. One such facility recently coming on stream is the Perth Seawater Desalination Plant commissioned by the Water Corporation of Western Australia. Gold Coast City, Sydney, Adelaide, Brisbane and Gosford- Wyong are among other coastal Australian cities that are considering seawater desalination as an option.

In the face of the driest winter on record, the Perth Seawater Desalination Plant is on track to deliver 45GL/a of drinking water into the Perth Integrated Water Supply Scheme (IWSS).

The plant at Cockburn Sound could be regarded as a world-leading model for future sustainable seawater desalination plants. A six month commissioning program commenced in October, with first water entering into the system in November 2006. At a peak capacity of 144ML/d, the $387 million plant, including the integration works, will be the largest seawater desalination plant outside the Middle East, and Australia's first large scale desalination facility.

It will also hold the title of the largest desalination plant in the southern hemisphere and be the biggest single water source feeding into the IWSS. The plant will provide 17% of Perth's water supplies which is significant given the vast reduction in rainfall over the last 30 years particularly in the southwest of the state and the resultant decline in run-off to the dams supplying the IWSS.

The plant was built by Multiplex-Degremont Joint Venture, in alliance with the Water Corporation and will be operated for 25 years by Degremont in alliance with the Water Corporation. It incorporates two separate streams each capable of producing 72ML/d of drinking water via a two pass seawater reverse-osmosis system. At peak production, the environmentally friendly submerged seawater intake, in about 10m of water 200m offshore, will gravitate 364ML/d of seawater via screens to a wet well. From this point two streams of feed water are pumped through the pretreatment facility, incorporating multimedia pressure filters and cartridge filters, by means of variable speed drive feed pumps.

The seawater is then pressurised to between 6MPa and 6.4MPa via six high pressure pumps and passes through 12 first pass seawater reverse-osmosis membrane racks. The permeate from the first pass system is then pumped through six second pass brackish water reverse-osmosis racks to produce high quality permeate. For every 10L of incoming seawater, the plant will produce 4L of drinking water and 6L of brine discharge at slightly less than twice the salinity of seawater. This fresh water is then passed through a post-treatment system, adding minerals to condition the water prior to being forwarded to a 12ML drinking water tank from which it is pumped into the IWSS.

The seawater concentrate is discharged 470m offshore via a 160m long, 40-port diffuser to ensure comprehensive mixing within 50m of the diffuser. Extensive modelling and monitoring has and will occur to ensure that there are no adverse affects resulting from this discharge.

An associated package of works includes 26km of large diameter pipelines and a 250ML/d pump station to transport water to the distribution network. The plant will also be greenhouse neutral as its energy requirements will be met from an 82MW wind farm north of Perth, from which the Water Corporation will purchase the 185GWh/a required to operate the plant.

The Emu Downs wind farm, developed by Griffin Energy and Stanwell Corporation, is Western Australia's second biggest wind farm and is located 30km east of Cervantes. It consists of 48 wind turbines and is part of the state program to meet the 6% renewable energy target for WA'S main electricity grid by 2010.

Sourcing the energy from the wind farm will make the Perth Seawater Desalination Plant the world's largest desalination plant using renewable energy. The Water Corporation said that coupling this energy with the low specific energy consumption achieved from the plant's novel design, incorporating isobaric energy recovery devices supplied by ERI, ensures that the plant is environmentally friendly.

The 10 processes of desalination:

1. Seawater intake - An 8m-diameter cylindrical structure positioned on the seabed 11m below sea level, 220m offshore in Cockburn Sound.
2. Chemical dosing - The seawater is shock-dosed at the intake with: sodium hypochlorite, sodium metabisulphite, ferric chloride and sulphuric acid coagulant aid.
3. Pretreatment - The pretreatment uses 24 horizontal pressure filters. Each filter has a surface area of 52m2 and has a nominal flow rate of 15ML/d.
4. Reverse-osmosis - first pass - The reverse-osmosis system consists of two passes, which means that all or some of the desalinated product from the first pass is relayed to a second pass for further desalination to ensure that the desired drinking water quality is achieved. The average total dissolved salts (TDs) achieved from the first pass is less than 300mg/L.
5. Reverse-osmosis - second pass - The average TDS achieved from the second pass is less than 40mg/L.
6. Remineralisation and disinfection - A stabilisation dosing system is essentially a chemical mixing system for lime water and carbon dioxide. Lime dosing involves 6t/d of lime while about 2t/d of CO will be injected into the system.
7. Drinking water storage and water pumping - The drinking water storage tank holds 12.5ML of water and the associated pumping station contains systems to transfer water 11km to Thomson Reservoir.
8. Process wastewater treatment - The seawater desalination plant has several wastewater streams handled through processes and disposal means that are environmentally acceptable.
9. Liquid wastewater disposal - The plant discharges to the ocean through an 1800mm pipeline extending 470m into Cockburn Sound. The final 160m of the pipeline incorporate 40 diffuser nozzles.
10. Integrated water supply - The drinking water is transferred 11km to Thomson Reservoir through a 1200mm-diameter mild steel concrete-lined pipeline.