Seawater Desalination Feasibility Study
What is desalination?
Desalination is the removal of dissolved salts and other impurities from seawater. A desalination plant turns salt water into water that is fit to drink.
Why is desalination being investigated?
To secure Melbourne's water resources for the future, the Victorian Government developed the Central Region Sustainable Water Strategy (CRSWS), which was released last year. The strategy aims to ensure reliable and safe water supply for the future through measures such as further water saving strategies, recycling and large-scale augmentation. The CRSWS proposed that a feasibility of study of seawater desalination be undertaken.
Who is undertaking the desalination feasibility study?
The study is being undertaken by Melbourne Water using consultants with specialist international experience in desalination technologies, engineering, environmental assessment and socio/economic evaluation. In addition, the current experience gained in Perth, Sydney and the Gold Coast for desalination proposals is being taken into account.
What does the study involve?
The study is addressing a wide range of issues relating to the economic, environmental, technical and social aspects of a desalination plant for Melbourne. Some key issues that are being investigated are energy consumption and potential greenhouse impacts, the required capacity, life cycle cost and the siting of the plant. In relation to the siting of a plant, the potential environmental impacts of brine disposal and the seawater intake, social and amenity considerations and the proximity of potential locations to the existing water supply network are being examined.
The study is drawing on both national and international experience ensuring that the most suitable available technology will be adopted while minimising potential environmental impacts.
What is Reverse Osmosis?
Reverse osmosis is the technology most commonly used in modern desalination plants. The process involves using pressure to drive seawater through a semi-permeable membrane.
The membrane allows fresh water to pass through while it retains salts and other impurities, which are discharged back to the ocean in a manner which minimises the risk to the environment.
Overview of Treatment
Seawater is taken from the sea to the desalination plant using a submerged intake and connection pipeline. This is designed in such a way as to minimise the possibility of marine life being drawn into the pipeline.
For a reverse osmosis plant, the seawater first passes through a pre-treatment process. This process removes suspended solids and other matter that may harm the reverse osmosis membranes. This pre-treatment is typically achieved by filtration. The pre-treatment process also includes chemical dosing to prevent scaling and fouling of the reverse osmosis membrane.
The pre-treated seawater is then pumped through the reverse osmosis membranes to produce fresh desalinated water.
The desalinated seawater is then further treated to ensure the water meets all the required drinking water standards.
After treatment, the drinking water is pumped into the water distribution network.
Brine is discharged to the sea via a diffusing system, which ensures thorough mixing with seawater. Hydrodynamic modelling is carried out to assist in the design of the diffusing system to ensure that any impact on the marine environment and the overall amenity is minimised.
Desalinated seawater is commonly used as drinking water. It has been used for water supply on passenger vessels for over 50 years and in many countries such as Spain, Singapore and USA.
Development of Desalination
Thermal desalination (a technology using evaporation to produce drinking water) was historically employed to desalinate water, primarily in the Middle East region in the 1950s. Membrane technologies were developed in the ensuing decades, with reverse osmosis arriving in the 1970s. Reverse osmosis is currently considered the cheapest and most energy efficient option for seawater desalination in Melbourne.
Recent developments
Advancements in membrane technology and reliability have increased their use for desalination, leading to greater efficiencies, reduced energy consumption and lower cost.
A 130 million litre per day seawater desalination plant was recently constructed in Perth. It commenced operation on 18 of November 2006. Within Australia, large-scale desalination is also under consideration for augmenting existing water supplies for Sydney, Port Augusta and Gosford, with construction of a desalination plant recently having commenced on the Gold Coast.
Is desalinated water safe to drink?
A seawater desalination plant can achieve water quality that meets Australian Drinking Water Guidelines published by the National Health and Medical Research Council (NHMRC), which include requirements for taste, colour and odour in addition to health, and the Victorian Department of Human Services requirements.
Once treated to meet the Australian Drinking Water Guidelines and Victorian Department of Human Services health requirements, desalinated water would be safe to drink and therefore could be integrated into Melbourne's existing drinking water network.
How much energy would be used?
Energy consumption associated with desalination using reverse osmosis has declined with advances in technology. Reverse osmosis is currently considered the least energy intensive of all existing seawater desalination technology.
If a seawater desalination plant were to be built for Melbourne, consideration would need to be given to minimising greenhouse impacts.
What size would the plant be?
The feasibility study is investigating the required capacity of the plant and how this may integrate with a diversified mix of water supply options that are presently being considered.
How the plant would be monitored?
As with all other drinking water supplies in Victoria, the water quality from a desalination plant will be regulated by the Department of Human Services. The brine discharge to the ocean will be regulated by EPA Victoria.
Where could the plant be located?
A range of locations will be assessed through the feasibility study.
How much would a plant cost?
The life cycle cost of the total project varies depending on factors such as size, the proximity of the plant to a suitable water supply network connection and suitable seawater quality and the ease with which the plant inlet and outlet can be constructed.