A multi-barrier process works on the principle of providing more than one means of removing any particular contaminant.  It means we do not rely on just one treatment process to do the job and a failure of one component will not compromise water quality.

Experts agree that a multiple barrier system employing appropriate treatment technologies is capable of reducing the concentrations of contaminants to such a low level that the risk becomes negligible.  This approach is supported by highly credible agencies such as the US Environmental Protection Authority and World Health Organisation in their guideline documents.

The first process the wastewater goes through is at the Wetalla Water Reclamation Plant, where the wastewater treatment plant combines several different processes to purify water. The physical process of screening removes all large particles and material that will not dissolve. A biological process follows, where a population of small organisms such as protozoa and bacteria feed on the dissolved organic material in the water. Oxygen is supplied at controlled rates to create zones ranging from high oxygen to none at all, to encourage specific types of organisms to grow.

The water then flows through large settling tanks where the organisms and their feed (organic matter) are separated out and returned to water at the beginning of this process to do their job all over again. The solid by-product, called biosolids, has excellent nutrient value and is used as a soil additive in agriculture. To ensure that there are no dangerous bacteria or viruses remaining, the water to be discharged down Gowrie Creek is disinfected to 3 star standard using chlorine or UV light. The water to be purified for potable reuse then continues to the next barrier.

Barrier 2 – Ultrafiltration

The next stop for the water is at the Advanced Water Treatment Plant where it will undergo ultrafiltration, or microfiltration. Ultrafiltration involves the use of polymer or ceramic membrane filters, with a pore size of 0.01 micron, to remove very fine colloidal (very small suspended) particles from water.

At this stage in the process parasites such as Giardia, Cryptosporidia and bacteria such as E coli (Escherichia coli), and any other disease-causing microbes that might have been present in the water are removed.

Barrier 3 – Reverse osmosis

Barrier three is known as reverse osmosis (RO). During the RO process, pressure is applied to squeeze the water across an extremely fine membrane filter. The pore size of the RO membrane is so small (0.0001 micron) that bacteria, viruses and chemicals cannot pass through.

The membrane traps small pollutants such as salts, drugs (e.g. chemotherapy drugs, anti-inflamatories, antibiotics and other pharmaceuticals), chemicals (e.g. personal care products), hormones, and importantly, viruses, while allowing the water molecules to pass right through.

Very small amounts of some organic chemicals may pass through a RO membrane, but these are safely destroyed at the next stage (or barrier 5) in the Water Futures – Toowoomba project.

It is important to note that not many hormones enter our water system to begin with. Hormones are large organic molecules (more than ten times the size of water molecules), making them relatively easy to remove through membrane filtration. Current Australian research shows that more than 90% of the hormones in raw sewage will be destroyed in the water reclamation plant (barrier 1).

Note:
Most contaminants are large when compared to a water molecule. However, how molecular weight compounds will pass through the membrane, but the combination of UV light and advanced oxidation (through the use of a chemical such as hydrogen peroxide) generates hydroxyl radicals that can oxidize the organic molecules that pass through. The advanced oxidation process does not lead to any additional chemicals being left in the water.

UV light also kills pathogens (mainly virus) by damaging the DNA to prevent replication.

Barriers 4 & 5 – Ultraviolet light disinfection and advanced oxidation

By this stage the water is already more than pure enough to drink. UV light disinfection and advanced oxidation provide additional layers of protection by passing the water through intense light, like the cleansing rays of the sun but hundreds of times stronger.

UV disinfection irradiates drinking water to kill pathogens such as bacteria and viruses, and inactivates Cryptosporidium and Giardia. Advanced oxidation will break down and destroy any remaining organic chemicals or contaminants.

Barrier 6 – Cooby Dam

The recycled water is then blended with Lake Cooby’s surface water.

The water from Cooby Dam is naturally hard and contains dissolved organic matter. In the days before Toowoomba found other water supplies hardness was a major water quality issue for Toowoomba. Hardness destroys soap and leaves a calcium carbonate buildup around tap fittings. Bore water and dam water both pick up minerals through the ground but water that has been processed through RO is soft. The water that that will come through the Advanced Water Treatment Plant will improve the quality of Cooby Dam’s water.

Barrier 7 – Mount Kynoch Water Treatment Plant

At the Mt Kynoch Water Treatment Plant, where our current drinking water is already processed, Toowoomba's water is treated to meet the health and aesthetic requirements of the National Health and Medical Research Council’s Australian Drinking Water Guidelines. This process does not change with the advent of Water Futures – Toowoomba, it is something we have always done.

Most "dirt" (organic material, micro organisms, minerals) present in the dam water is measured as "turbidity" and turbidity is settled out of the water in a large settling tank. Multi-media filters (crushed coal, sand and gravel) remove the remainder of the turbidity. To settle and filter small particles, a specialised treatment chemical (coagulant) is added that makes the particles larger (flocculate). The larger particles (flocs) containing the dirt settle and become sludge that is removed. The smaller flocs are trapped in the filters.

Under conditions where the raw dam water is relatively clean, the settling stage is not needed, and the plant can be operated in what is called “contact filtration”.

Nuisance minerals such as iron and manganese are removed through adding chlorine prior to filtration. Chlorine oxidises these metals, bringing the ions from solution into suspension so that they may then be filtered out. Adding chlorine in the treated water also destroys microorganisms, not trapped by the filters, and a sufficient quantity of chlorine is added to ensure some is present at the consumer tap, to prevent bacteriological re-growth in the pipes.