The following is the water treatment process described by SEQ on their website:
Water stored in Wivenhoe Dam is released as required into the Brisbane River where it makes its way to the Mt Crosby Weir to begin treatment at our water treatment plants to become drinking water.
Pumping and screening
There are two pumping systems at the Westbank Treatment Plant. The first system lifts the untreated water after it flows through a trash screen that is made up of 25mm mesh from the river to the treatment plant.
Three pumps are located in pump wells 34 metres deep, each with a capacity to pump 125 million litres per day. This water is pumped in from the same level as the bottom of the Brisbane River. Only two of these pumps operate at one time with a third being a back-up pump.
The second pumping system lifts the treated water from the Westbank plant to large 90 megalitre pure water storages at Cameron’s Hill after treatment has taken place.
Coagulation
All raw water contains suspended particles which need to be removed.
The first process after the water is pumped from the Brisbane River involves adding alum (aluminium sulphate) to the water. Flash mixers mix the alum into the water for approximately 20 – 30 seconds, to destabilize the particles.
Water enters with a PH level of between 7.7-8.2. Alum lowers the PH to 6.8-7.2 where it causes flocculation in the water to occur.
Flocculation
Flocculation is the clumping together of the suspended particles that have been destabilised, to form heavier visible particles called “floc”. The “floc” remains in suspension as the water is flowing at high velocity through the flocculators.
Slaked lime and small quantities of sodium may also be added to promote coagulation and flocculation.
Sedimentation
Sedimentation is the oldest known method of water purification and has been used over thousands of years. The water flows slowly into two large settling sedimentation tanks that resemble swimming pools. These pools can hold 6.5 million litres of water and are 4.5 metres deep.
As “the floc” particles bind together, they sink to the bottom of the tank to form sludge. The sludge is regularly vacuumed out and travels to a waste sludge pool.
Dissolved Air Flotation
The Westbank Treatment Plant uses the Dissolved Air Filtration (DAF) process.
As chemically flocculated water enters the filtration chambers, a pressurized stream of water saturated with air is injected, causing the large formation of very fine bubbles to rise up from the floor. These bubbles become attached to “the floc” particles and float them to the surface. The air-floc particles accumulate to form a floating sludge blanket that is removed at regular intervals.
Clarified water is then passed through rapid gravity sand filter beds of finely graded silica sand and pebble layers. The remaining suspended particles are removed at this stage.
Sludge removal
All accumulated sludge that has been collected is processed via a centrifuge that removes the heavy particles and returns the water to a lagoon where it can be treated. The waste sludge is removed and used as landfill. It can also be used for the making of bricks.
Chemical disinfection
Large chemical silos contain the necessary chemicals for the treatment process. Each silo holds 340,000 litres of chemicals. The Lime silo holds 78 tonnes of lime which is mixed with the water as it flows into the storage reservoirs.
As a result of the filtration process, the water appears as clean as it does from our taps at home. It now has to undergo PH correction and disinfection.
Any water leaving the purification works is disinfected with chlorine to kill micro-organisms, bacteria and any viruses that may be present in the water.
Lime is also added to correct the PH for human consumption and ensure that it is not too acidic.
The water is then pumped to reservoirs at Cameron’s Hill. Fluoride is also added at Holt’s Hill as a protective dental health measure as mandated by the Queensland Governments $35 million program to provide fluoride all eligible Queensland water supplies by 2013.
Although chlorine is an excellent disinfectant it does not remain active for much longer than 6 to 8 hours. As the water finally leaves Cameron’s Hill Reservoir, chlorine and ammonia are added to form chloramine, a long lasting disinfectant, which ensures that any harmful micro-organisms are destroyed.
Internet Resource:
Toxic Tap Brisbane 