Urban water recycling – nature does it anyway

June 5, 2009
By: David Walker, Executive Officer

The continued buyback of water entitlements from irrigators shows that water and water shortage are still very much in the news. Demands from cities to get a share of the water that is taken away from food production starts me thinking about how cities could perhaps make better use of the water that they have, perhaps by making more use of water recycling.

The first point to make is that all water is recycled. It is almost certain that the some of the water in the cup of coffee you are drinking as you read today’s paper has been through someone’s kidneys at some stage.

One of the key drivers of our ability to live on Planet Earth is the water cycle, the process where water is evaporated from the water (and soil) surfaces, is moved around the globe by weather systems, and returns to the earth’s surface by way of rain, hail or snow. Hopefully a reasonable amount of it will fall on the Liverpool Plains, so that it can allow plants to grow, providing us with our food, and our farmers with their income. 

The water that is not immediately intercepted and used by plants moves across the landscape in rivers or under the landscape through our groundwater systems. Surface water can move away quite quickly, so we build dams to make sure that we keep a supply available. Both sources are widely used for irrigated agriculture, and to provide industrial and urban supplies. 

Much of the world’s surface water ends up in oceans, where evaporation and then precipitation will ensure the cycle continues. The water cycle ensures that water is used over and over again.

Nature has many mechanisms that help to clean water as it passes across or through the landscape. Well-grassed areas beside watercourses filter landscape runoff. Wetlands remove sediments and nutrients form water. All sorts of creatures from dung beetles to blowflies clean up various types of organic matter. 

It is where our output of organic wastes is greater than nature’s ability to digest it, that the trouble starts. It is what happens to water after it is put to our more intensive uses that can cause problems. Once water has been used for irrigation, we have to be careful that off-site contamination with agricultural pesticides and fertilisers is prevented. That’s why used irrigation water is retained on farms and later re-used for subsequent irrigations.

In the case of town supplies that end up in the town sewerage system, the water must be treated to required standards before it can be released to further uses. At the most basic, it is left in evaporation ponds to re-enter the water cycle. Where volumes are too great for evaporation to remove it, it is passed through Sewage Treatment Plants (STPs) and then either used locally, often for irrigation, or returned to streams to move further down the catchment. Many towns in inland Australia have river water supplies that include the returned water from towns upstream. 

So the use to which ‘reclaimed’ water can be put depends on the quality of the water that comes out of STPs, which depends on the type of treatment process used. The components of wastewater treatment are generally described as the preliminary, primary, secondary and tertiary stages, with advanced treatment being added to the third stage when the effluent is to be returned to potable (drinking) or near potable standard. The extent of treatment is determined by what is necessary to reliably reclaim the effluent to make it suitable for its eventual use or discharge.

Membrane processes including microfiltration and reverse osmosis are increasingly being adopted for the most advanced tertiary treatment processes beyond conventional filtration, due to their ability to remove very fine particles from the effluent stream. In essence, the pores in the membrane are large enough to allow water molecules to pass through, but too small to permit the passage of salt, other minerals and large organic molecules.

These technologies can produce water that is more pure than the rainwater from our roof that many of us drink. Indeed some people complain that, like distilled water, it has no taste. 

In a world where we expect to have less reliable rainfall, and where there is escalating population pressure on existing supplies, we certainly need to get smarter at managing the water cycle. It is just like understanding how our soils work or how we need to manage plants. If we understand the processes, we can make sure our activities help them, rather than work against them.