02.The recycling process
What exactly is recycled water?
Water recycling is the process of taking effluent (wastewater and sewage) and treating it to a level that’s appropriate for its intended use. For potable (drinkable) use, the recycled water has to be treated to a sufficiently high level that it’s suitable for human consumption.
The indirect potable reuse process being implemented in southeast Queensland, trialled in Perth and under discussion in Goulburn adds another step: highly treated recycled water is mixed with other water supplies above or below ground before it arrives at your tap (see Jargon buster).
There’s no standard 'off the shelf' project — each one is unique, with different specific technologies and very diverse natural water catchment characteristics — but generally speaking indirect potable reuse involves a number of steps.
- After going through micro filters, the water undergoes a reverse osmosis process, which involves forcing the water molecules across a dense plastic film. The water can pass through the film, but other molecules, including viruses, bacteria and even tiny salt molecules, can’t.
- As an added precaution the water undergoes oxidation and disinfection, using hydrogen peroxide and very strong ultraviolet light.
- The recycled water is then added to a reservoir or groundwater aquifer, where it can be stored and blended with the regular water supply.
- Before being put into the drinking water system, the blended recycled and regular water also undergoes the normal drinking water treatment process.
CAPTION: Effluent gets treated at existing wastewater treatment plants, before it reaches the recycling plant. The recycled water is then mixed with the natural water supply and undergoes existing drinking water treatment before arriving again at your tap.
There’s a difference between this kind of planned reuse — with advanced water treatment and risk management — and incidental reuse. In some river systems, towns upstream discharge their treated sewage into the river and towns further downstream draw water from the same river. For example, people living in towns that draw water from the Murrumbidgee below Canberra, and then down the Murray to Adelaide, are already partly using reuse water.
Even the city of London is located downstream from a number of wastewater treatment plants that discharge into the Thames, so part of its water supplies also reuse water. And Sydney’s water supply receives some treated sewage from Goulburn and Lithgow. This water goes through the regular water treatment system and is considered perfectly fit to drink.
With national guidelines on recycling water for drinking likely to be finalised in April, greater expansion of water recycling is anticipated over the longer term. The aim of the guidelines is to make sure recycling is done safely, without being sidetracked by whether recycling is a good or bad idea. The guidelines will explain what the risks are, and how to manage them. Decision makers and project managers should use the guidelines when assessing and carrying out projects.
Potable water: Water intended for human consumption — suitable on the basis of both health and aesthetic considerations for drinking or culinary purposes.
Indirect potable reuse: The discharge of recycled water into surface water or groundwater (called managed aquifer recharge) to supplement drinking water supply, rather than going directly from the treatment plant to your tap.
Recycled water: Water that’s been reclaimed from sewage, greywater or stormwater systems and treated to a standard that’s appropriate for its intended use.
Reverse osmosis: An advanced method of wastewater treatment that works by forcing water molecules across a semi-permeable membrane to separate it from impurities.
Sewage: Material from internal household and other building drains. It includes faecal waste and urine from toilets; shower and bath water; laundry water and kitchen water. (Sewerage is the network of pipes and infrastructure that transport the sewage.)
Water recycling: A generic term for water reclamation and reuse.