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Putting Recycling into the Water Cycle

A guide through the urban water cycle and why we need to rethink the system.

Although 70 percent of the world's surface is covered by water, most of it--97.5 percent--is salty. Only about 2.5 percent of all water is fresh, but almost three quarters of this is frozen in ice caps at the poles, and most of the remaining one quarter is underground. Only a tiny fraction of the Earth's water is fresh and easily accessible.

The amount of water on Earth does not change. The amount of water that evaporates each year and the amount that falls back to the ground are consistent. If we have a drought, it is not because the world's rainfall for the year is less, but because rain is falling somewhere else and not on us.

When water evaporates, it leaves behind the other molecules it was associated with and is purified. When the world's population was smaller, our freshwater supplies were unstressed. Water was used, became wastewater and nature recycled it.

These days our limited water resources are facing pressure from increasing population at the same time as our climate is changing. We are using our precious water supplies at a faster rate than they are being replenished. We are seeing, as predicted, rainfall events that are less frequent and more violent and intense. In Australia, our main cities are all experiencing prolonged and serious drought. For example, Brisbane has been on level six restrictions for many months, with no outside watering allowed at all. Wivenhoe Dam that supplies Brisbane's water was reduced to 15 percent of its capacity before the last wet season.

The Urban Water Cycle

So where does our water come from and what happens to it once we pull the plug?

Supply Source: Rivers provide much of the water supplied for urban use. Often we build a dam in the watercourse so that the water is stored for drier times. The barrier we create stops the flow of nutrients and energy down the river. The river is degraded, and the habitats of the organisms that depend on the river are damaged. Recent years have seen a marked decrease in rainfall and the amount of runoff filling our dams--they are letting us down.

Water Treatment Plant: At the water treatment plant, water is treated so that it is safe to drink. The amount of treatment required (and cost) depends on raw water quality.

Water at Work: The water we use inside the house is mostly for washing and cleaning. Once used, it finds its way through the pipes of the sewerage system to the wastewater treatment plant.

The wastewater from industry may contain some chemicals, such as strong disinfectants that kill the bacteria in the treatment plant, or chemicals in high concentrations that are hard to treat. This trade waste is pretreated before being discharged. Another problem at the treatment plant comes from fats and oils, which should not be poured down drains.

Wastewater Treatment Plant: At the wastewater treatment plant, the water (effluent) is separated from the dirt (biosolids). Mother Nature's natural biological processes are used, but they are speeded up. In the reactor tank bacteria digest the carbon-containing molecules. The amount of treatment wastewater receives depends on how the effluent is to be managed. It is usually discharged into a waterway or the ocean. We are becoming very aware of the problems that insufficiently treated effluent, especially the nutrients in it, causes to the environment.

The current urban water cycle is not a cycle at all; it is a straight line from dam to disposal with a shortage of water at one end of the pipeline and waste and pollution at the other. The way we currently manage water is not sustainable. We can no longer afford to use water once and throw it "away." We need to change the way we think about and manage water.

Sustainability

We can use the water we have available more efficiently. Many of our old drinking water reticulation pipes have serious leaks--leakage reduction is an obvious way to save water. Education, pricing incentives and restrictions all have their place in helping us make water go further.

If our water management is to be sustainable, we need to seek the security of a diversity of sources of water that are not climate dependent. We must reduce our reliance on dams. In some parts of the world, rainwater tanks provide the only water available, and their use for domestic water supplies can considerably help lessen the demand on mains water. We should be encouraging decentralized systems and self-sufficiency in water management.

At least half of the water we use becomes available for recycling. Advanced recycling treatments include constructed wetlands, ozone and activated carbon, UV radiation and filtration, both conventional and through a range of membranes. Many think that all water used for non-potable purposes should be provided by recycled water, but in order to be affordable and to have impact on our water supply shortage, recycled water has to be included in the regular supply.

We have been recycling water for decades, where wastewater from a town or city upstream forms part of the water supply for a town or city downstream. We have relied on natural processes and dilution to reduce the concentration of impurities in the water and sophisticated treatment at the water treatment plant to ensure that the water is safe. This type of "unplanned recycling" is well accepted.

With modern reverse osmosis membrane technology, we can now clean up the wastewater so that it is cleaner than our current drinking supplies. The purified water is blended into the water from more conventional sources before it is delivered to our homes. This is called "planned recycling" and, although the water is purer than that in "unplanned recycling," the idea of drinking recycled water is often greeted with alarm because of the community's lack of knowledge and understanding of water quality and treatment. There are now several examples of reverse osmosis membranes being used to recycle water back into the drinking supply, such as Orange County in California and Singapore.

The same reverse osmosis membranes can be used to desalinate seawater. Seawater has 100 times more pollutants than effluent, so it is a more energy-expensive option than recycling. Although expensive, desalination is a sustainable option that diversifies the source of water.

The way we manage water depends on our local situation, but whatever the circumstances, some basic principles should be applied. We must reduce demand and improve water-use efficiency, recycle all the water that is available for recycling and, when that is no longer enough, desalinate seawater. Dams are no longer the way to provide water for our cities.

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