Gold Coast Desalinated Water Tour

Transcription

1 Gold Coast Desalinated Water Tour Desalinated water Desalination is a technology that separates dissolved salts and other minerals from seawater to provide clean, fresh drinking water. This water is then blended with other drinking water supplies and distributed directly to homes, businesses and industries in South East Queensland. The Gold Coast Desalination Plant can produce up to 133 million litres of pure water a day, equivalent to about 50 Olympic size swimming pools. Desalinated water is a climate-resilient source of fresh water. It can continue to produce fresh drinking water, even during a drought, keeping everyone supplied with clean, safe drinking water through good times and bad. 1

2 The Desalination Process 1. Control room So much is happening at this plant that a complex computer system is needed to manage it. A Supervisory Control and Data Acquisition system, or SCADA system, monitors, controls and safeguards each stage of the desalination treatment process. 2. Sea water intake The desalination plant draws seawater through a small intake structure far out at sea. The seawater flows down the inlet, then along an underground tunnel, and is pumped from an inlet shaft into the plant. 3. Drum screen Seawater is pumped through a screen to remove any particles, such as algae, that may be present in the seawater. 4. Pre-treatment Seawater is pumped into large tanks for filtration. Here, small suspended particles in the seawater are trapped as they pass through sand filters at the bottom of each tank. The filtered seawater then flows to the reverse osmosis process. When the filter beds get too clogged up they are cleaned. 5. Cartridge filters and pumps Large pumps are needed to force the filtered seawater through reverse osmosis membranes. 6. Energy recovery An energy recovery device recovers energy that would otherwise be lost by the reverse osmosis process. This energy is re-used to reduce the power consumption of the plant. 7. Reverse osmosis Reverse osmosis works by forcing water through a special plastic, semi-permeable membrane sheet to remove salt and other minerals. 2

3 8. Remineralisation Pure desalinated water is so pure that minerals have to be added to it. Small amounts of lime and carbon dioxide are added. This makes it more like the water we get from dams. 9. Pure water tank Desalinated water is stored in a two large 15 million litre tanks before it is pumped into South East Queensland s Water Grid supply network. 10 Pure water pipeline Desalinated water is pumped to parts of the Gold Coast and Brisbane. It joins the Water Grid network and is blended with treated dam water to supply our homes and industries. 11 Residuals treatment All the water is carefully managed. Even backwash water from the pre-treatment filters is treated to remove suspended particles. The cleaned salt water is then returned to the sea. 12 Sea water outlet The left-over salty water is returned back to the ocean through an outlet tunnel, where it disperses in the ocean currents. 3

4 Control Room So much is happening at this plant that a complex computer system is needed to manage it. A Supervisory Control and Data Acquisition system, or SCADA system, monitors, controls and safeguards each stage of the desalination treatment process. A chlorine residual chemical is also added to prevent any algal growth in pipes or tanks. Almost everything at the plant can be operated from the control room, including water flow, pumps, valves and filters. Most of the processes are automatic. In fact the plant can be operated with only two staff on duty. Water quality or safety alarms will automatically shut down the process, and less urgent warnings allow the operator time to investigate and correct faults before water supply is interrupted. The desalinated water meets the Public Health Regulator standards and Australia's strict Drinking Water Quality Guidelines. Most people do not notice any difference in taste or colour. In fact, sometimes desalinated water is used to improve the taste and colour of drinking water supplied by dams. 4

5 Sea Water Intake To get to the Gold Coast desalination plant, seawater is drawn down through an inlet on the seabed and through a concrete-lined tunnel underground. The seawater is then pumped from a 70 metre deep inlet shaft at the plant and piped to the filtering and process buildings. It takes about one hour for the seawater to travel the length of the 2.2 kilometre inlet tunnel. The ocean intake structure rises four metres up from the seabed. Seawater enters a mushroom-shaped intake structure through bar screens and flows slowly along the tunnel, at a rate of about half a metre per second. At full capacity, over 340 million litres of seawater moves through the inlet tunnel each day. A special boring machine was used to construct the tunnel. The borer drilled 2.2 kilometres through hard rock, 70 metres deep, to the offshore intake. A 2.8 metre diameter concrete pipe lines the tunnel wall. Even with three curves in the tunnel route it met up precisely with the intake riser pipe constructed at the same time. To prevent biological growth on the tunnel surface, the seawater is chlorinated each day for about one hour. 5

6 Drum Screen After being piped in from the ocean, the water first goes through a drum screen process. The seawater passes through a 3 millimetre screen. Particles such as algae are removed from the seawater and collected in a wheelie bin. Very little material is collected because the intake is in deep water and the seawater enters the intake very slowly. The water is now ready for the pre-treatment process. Large pumps lift the seawater from the inlet shaft, through pipes up to the drum screens. From here it flows to the pre-treatment building. Since the intake is in deep water and two metres above the sea bed, very little material is collected on the screen. What is collected is transferred into a wheelie bin and weighed, then sent to landfill. 6

7 Pre-treatment The seawater needs to be filtered before the desalination process. Filtration works by mixing a coagulant, called ferric sulphate, with the seawater. The coagulant helps bind suspended particles into small clumps which sink and get trapped in the filter beds. Seawater is filtered in large tanks. At the bottom of each tank is a filter bed, made of a layer of coal and sand. The seawater flows through the filter and the suspended particles get trapped in the fine coal and sand. When the filter bed needs cleaning, the sediment is backwashed from the tanks and removed for treatment in another part of the plant. Seawater is pre-treated by adding sulphuric acid to reduce the ph, then ferric sulphate and a polymer are added. The seawater flows into four mixing tanks where the suspended particles bind together and form larger, heavier particles. This process is called 'flocculation' and makes the next filtration step more effective. From the flocculation tanks the water flows through 18 settling tanks which filter the seawater through 80 centimetres of coal and then an 80 centimetre layer of sand. These tanks remove the accumulated, clumped particles, decreasing the turbidity and lowering the "silt density index" (SDI) of the seawater. Only if the seawater turbidity is below 0.2NTU and SDI is below 4 is the water pumped to the next stage reverse osmosis process. The tanks are backwashed with air and water every 40 hours, or sooner if the filters become too dirty. The dirty water is then pumped to the residuals treatment area, where the dirt and coagulant are removed. The cleaned backwash water is returned to the ocean. 7

8 Cartridge Filters and Pumps Before pumping seawater through the reverse osmosis vessels, it first passes through cartridge filters to trap any remaining fine particles larger than 5 microns -- there are 1000 microns in a millimetre. Cartridge filters are very effective and back-up the pre-treatment filters. High pressure pumps and energy recovery devices then force the seawater though pipes and into racks of pressure vessels that contain reverse osmosis membranes. High pressure pumps and Energy Recovery Devices increase the seawater pressure to about 55 bar (5500 kpa) forcing seawater against the reverse osmosis membranes. The pumps have been designed with fine tolerances to ensure they achieve very high efficiency. To balance out the enormous forces each pump is symmetrical with two suction lines and one central discharge pipe. Optimal operation and efficiency is achieved though Variable Speed Drive electric motor controllers which ensure that the local power grid is not affected when the massive 4.5MW pumps start. 8

9 Energy Recovery The salty water, or brine, from the first pass of the reverse osmosis process still has most of its original pressure. This water pressure powers a piston system to feed filtered seawater into the reverse osmosis vessels. The device recovers about 94% of energy that would otherwise be lost. This means that the Gold Coast desalination plant needs less power and becomes much more energy efficient. The energy recovery system, called a Dual Work Exchange Energy Recovery or 'DWEER', harnesses the brine's water pressure to run a mechanical piston system. The recovered energy means that only 45 per cent of the filtered seawater needs to pass through the high pressure pumps. The remaining 55 per cent is equal to the brine flow and is powered by the DWEER system. Reverse Osmosis Reverse Osmosis works by forcing seawater through thousands of pressure vessels that contain reverse osmosis membranes. Rolls of membrane sheets are wound into cylinders and placed inside each pressure vessel. Seawater enters the vessel and flows over the membrane surface. 9

10 The membrane sheet is semi-permeable because it allows water to pass through - or permeate while preventing salt from passing through. Membrane sheets are sealed at three edges like a plastic bag. There is a mesh spacer on the inside and outside the bag which allows water to flow between the sheets. Filtered seawater flows across the outside of the membrane bags. Under the high pressure, H20 molecules squeeze through the membrane to the inside of the plastic bag which has an opening into a central collection tube. This water is called permeate. Water that does not permeate through the membrane becomes more concentrated in salt. This water is called brine. The permeate water flows out at both ends of the inner collection tube and the brine flows out around the outside, leaving at the opposite end of the pressure vessel to the seawater inlet. About 40 per cent of all seawater that flows through the plant becomes desalinated water. Seawater makes two passes through the pressure vessel system. The first pass consists of nine racks, with each rack containing 186 RO pressure vessels. There are eight membrane elements inside each RO pressure vessel (a total of 13,392 elements). About 45% of seawater from the first pass becomes permeate. The remaining 55% of the flow, which is brine, goes to the Energy Recovery Device and is then returned to sea. Of the 45% that is first pass permeate, about 1/5th from the front of the inner collection tubes is piped directly into the blending tank. The remaining 4/5ths from the rear goes through a second pass. The second pass consists of three racks, with each rack containing 144 RO pressure vessels - and a total of 3,456 membrane elements. The second pass has two stages that, in total, recover about 85 per cent as permeate. Permeate from the first and second pass is all high quality fresh water and flows to the blending tank to be mixed. Reverse osmosis membranes are regularly cleaned to remove any build up on the outside membrane surface, so that their performance can be maintained. Cleaning can be done to remove either bio-films or scale. 10

11 Remineralisation The pure permeate water has been stripped of all its minerals and has a low alkalinity. Minerals need to be added to stabilise the water. Small amounts of chlorine, fluoride, carbon dioxide and calcium in the form of lime are added. These minerals make the water closer to the type of water that we normally drink. Permeate water has so few minerals that it will readily dissolve salts that it comes into contact with. This can cause changes in ph so the water must be stabilised to ensure that it is suitable for drinking. Carbon dioxide is dissolved in water to form carbonic acid which, with the lime, forms bicarbonate to increase the water's alkalinity. This is similar to what happens naturally - rain collects carbon dioxide as it falls and absorbs calcium as it flows over rocks in rivers. Adding these minerals helps desalinated water to match more closely other water sources, such as treated dam water. As with regular drinking water plants, water is also chlorinated to ensure quality in the distribution system and Fluoride is added in accordance with Government policy to improve dental health. 11

12 Pure Water Tanks By the time the water reaches the Pure Water Tanks, the plant has recovered about 40% of the sea water as pure, fresh water. This superior quality water meets Public Health Regulation standards and the Australian Drinking Water Guidelines. It is regularly tested to ensure it is clean and safe. The plant can produce between 44 and 133 million litres of desalinated water a day. Daily water production may vary depending on demand and instructions by the South East Queensland Water Grid Manager. The plant can produce between 44 and 133 million litres of desalinated water a day. The storage tanks can be filled in about five hours at full production. In normal operation the tanks are held at about 80% full and the rate of pumping into the grid matches the rate of water flowing in from the reverse osmosis and remineralisation processes. 12

13 Pure Water Pipeline Desalinated water is pumped to the Robina Reservoir at Clover Hill, where it is mixed with treated water from the Hinze Dam. The blended water is distributed mostly in the southern section of the South East Queensland Water Grid, which includes much of the Gold Coast. It is also pumped north to Brisbane through the Southern Regional Water Pipeline. As the water leaves the desalination plant, seven critical water quality parameters are monitored. The automatic instruments are regularly checked to ensure they are reading correctly and additional water samples are tested at the laboratory on-site and by independent laboratories. A pipeline connects the plant to the Southern Regional Water Pipeline, which links to local water infrastructure across the Gold Coast, Logan and Ipswich region, before entering the Brisbane network. Pumps deliver water into the grid pipeline at a rate of up to 1.6 tonnes per second (1,600L/s) and with a total lift of 110 metres. The pipeline to Robina is 17 km long and contains 13,500 tonnes of water moving at about two metres per second at peak flow. Each pump comes with a 2,200 kg flywheel so that if the power supply to the pumps is interrupted, the pumps slow gradually and the massive column of water comes to a controlled stop. 13

14 Residuals Treatment Water from backwashing the pre-treatment filters is sent to a residuals treatment area where the mixture of seawater and solids mostly coagulant and fine dirt particles is cleaned. The dirty seawater is first thickened in a clarifier tank. Here, particles bind together in a lamella plate settling process and sink to the bottom of the tank, where they get removed. The solid waste is further separated from the seawater by a centrifuge machine. As the centrifuge spins, it forces seawater to move one way and solids to move the other. The left-over residue is sent to a landfill site and the clean seawater is returned to ocean. The residuals treatment system also receives residue from the lime preparation system, and neutralised chemicals which have been used to clean the reverse osmosis membranes. Depending on how clean the incoming seawater is, and the rate of water production, about 10 to 30 tonnes of solid waste is collected each day. The left-over waste is emptied into large bins, which are removed by trucks and sent to a licensed landfill site. The water removed in the residuals treatment process is continuously monitored for turbidity to ensure that it is clean and can be returned to sea with the brine. 14

15 Sea Water Outlet About 60 per cent of all the seawater that comes into the plant is returned to the sea. The left-over salty water, called brine, is very clean, having gone through the initial filtration process. The brine is returned to the ocean through an underground tunnel and released by a 200 metre long diffuser pipe system, over one kilometre out from Tugun beach. The mixing zone is large - covering an area of about 8 football fields. WaterSecure monitors the impact of the brine under strict environmental regulations set by the Queensland Department of Environment and Resource Management. Regular sampling is undertaken around the mixing zone to monitor the health of the marine environment. Since the plant opened in 2009, no negative impacts have been detected. The structures are attracting marine life and effectively creating an artificial reef. The brine is released along a 200 metre diffuser pipe system at a depth of approximately 19 metres. There are 14 diffuser outlets that mix the brine with the seawater. The brine contains approximately twice the level of salt as the surrounding waters and quickly mixes with the seawater. Locating the outlet in a high current mixing zone ensures there is sufficient dispersion and dilution to avoid any build-up of brine on the seabed. The mixing zone is 120 metres by 400 metres - about the size of 8 football fields. There is an ongoing monitoring program at the outlet shaft, measuring the quality of the seawater in the mixing zone, to ensure there is no adverse effect to marine life. Regular sampling is done to monitor the chemical, biological and physical aspects of the surrounding marine environment. The diffuser system attracts fish and other marine life. The centre is only 230 metres from the seawater intake where high salinity levels have not been detected. 15

16 SEQ Water Grid South East Queensland s Water Grid comprises over 530 kilometres of large pipelines, new dams and reservoirs, pumping stations, a desalination plant and three water purification plants. The Water Grid can move large amounts of water throughout the region to where it s needed most. This flexible approach allows the Water Grid to meet the demands of an increasing population and changing weather patterns. Desalinated water is now a crucial part of our new Water Grid. The Gold Coast Desalination Plant at Tugun provides a secure source of pure water for South East Queensland. Frequently Asked Questions What is desalinated water? Desalination is a technology that separates dissolved salts and other minerals from seawater or other salty water to provide clean drinking water. Modern desalination plants use a process called reverse osmosis, which involves the removal of salts and other minerals out of the water as it moves through a membrane process (moving through many thin sheets of material) under high pressure. Desalinated water is often used to supplement drinking water supplies in many countries, including some Australian cities and towns. Is desalinated water presently in use? Many areas of the Gold Coast and Brisbane receive a blend of dam and desalinated water, however daily supply (production) and demand will result in blend variations throughout different areas of the water supply network. Why is desalinated water being used? Water is a precious resource, yet less than 10 per cent of Australia's urban and industrial water is recycled. As South East Queensland grows in population, and to protect our water supply from future droughts, desalination is a socially, environmentally and economically viable solution to help preserve our drinking water supplies. Currently, almost 2.8 million people live in South East Queensland and this figure could increase to more than 6 million by Even with significant new efficiency measures to reduce water consumption, this sustained level of population growth is substantially increasing the region's demand for water. Desalinated water is expected to provide up to 30% of our water supply by How can desalinated water be used? Desalinated water from the Gold Coast Desalination Plant is already added directly to our water supply. All desalinated water passes through numerous safety checks and water quality treatment systems. Where does desalinated water come from? The Gold Coast Desalination Plant at Tugun began supplying water in February 2009 and connects to the South East Queensland Water Grid. The plant can supply up to 133 megalitres of desalinated water a day. 16

17 Is desalinated water safe to drink? Yes. Desalinated water undergoes high standards of treatment to ensure it is safe. There are standards and regulations that apply for its use. Regular monitoring and reporting is required to ensure the water being supplied is of the highest quality. Desalinated water meets the requirements of the Australian Drinking Water Guidelines. It is clear and odourless. Are there guidelines for desalinated water use? Yes. Desalinated water can be safely used for a variety of purposes appropriate to the level of treatment they have undergone, in accordance with Queensland's Water Supply (Quality and Reliability Act). How much power does the Desal plant use? Modern desalination plants use advanced technology and energy saving devices which result in being far less energy intensive than traditional plants. The plant's use of energy saving devices - which recover up to 94 per cent of the unused energy from the brine - combined with carbon offsets and reduced energy consumption put it among the best performers in terms of energy for desalination plants worldwide. The Gold Coast Desalination Plant has the capacity to produce 133 million litres of drinking water a day. To produce a million litres of water, the plant consumes about 3.2 megawatt hours (MWh) of energy. So at full production, the plant consumes about 412.3MWh of energy per day, making it one of the most energy-efficient desalination plants in the world. Is the water discharged from the desalination plant affecting the marine environment? No. The plant has been designed to minimise any potential impacts on marine ecology and considerable effort has been placed on ensuring the quality of the marine environment. The long-term independent marine monitoring program being undertaken at the Tugun Desalination Plant shows that the plant is operating in compliance with the environmental license conditions which have been developed to prevent environmental impacts. Small plants and animals that live on the ocean floor are key indicators of a healthy environment because they are long-term residents - rather than moving in and out as more mobile organisms such as fish do. Results show these small plants and animals to be thriving on and around the marine structures. Underwater footage shows that the marine structures are providing a habitat for a diverse variety of marine organisms, effectively creating an artificial reef. WaterSecure is meeting its environmental and regulatory responsibilities through its ongoing monitoring around the desalination plant's marine structures. Independent experts from Griffith University were instrumental in setting up the rigorous water quality testing regime and analysing data. The marine monitoring programs were designed in conjunction with the Department of Environment and Resource Management and an independent third party expert. The marine monitoring program is based on sound science and includes continuous, real-time monitoring of the quality of the brine discharged back into the ocean. The monitoring undertaken includes measurement of ph, chlorine, dissolved oxygen, temperature, turbidity, suspended solids and salinity. Further monitoring of the marine life and sediments on the seabed is also undertaken. This is done by sampling a selection of around 60 parcels of seabed, which are then taken back to a laboratory and analysed to determine health, numbers and diversity of seabed based marine organisms. 17

18 In conjunction with this scientific program, regular visual monitoring around the marine structures is undertaken and shows marine life is abundant and diverse indicating they are unaffected by brine discharged from the desalination plant. Underwater footage shows that the marine structures are providing a habitat for a diverse variety of marine organisms, effectively creating an artificial reef. 18