Stormwater Recycling
Background
About 90% of the rainfall in Perth is absorbed into groundwater through soak wells or directly through the soil. These groundwater resources are used for a variety of purposes including environmental recharge, industry, agriculture, drinking water, garden bores and watering of public open space such as ovals, parks and golf courses.
Water sensitive urban design, naturally recycles stormwater to the environment.
In Perth, unlike most other cities in Australia, less than 10% of stormwater is transported by constructed drains to rivers, local waterways, wetlands and the ocean.
Piping stormwater away from the built environment in this way has traditionally been undertaken to prevent flooding and address waterlogging which precluded land development. These drains can carry pollutants and nutrients, such as nitrogen and phosphorous from garden fertilisers, into natural water bodies. Moving stormwater also prevents local recharge and use.
Most drains only run during winter and in times of summer storms. There are a handful of large drains which run all year round. These drains help to prevent flooding and generally flow to the Swan and Canning river system where the water is used to maintain river ecosystems.
The opportunity for stormwater recycling in Perth is predominantly from drains that discharge to the ocean.
Stormwater recycling is climate dependent. For example, the volume of water discharged from the Herdsman Main drain to the ocean all year round, has halved over the past 10 years (from around 12 gigalitres to less than 6 gigalitres a year). This is due to lower rainfall and reduced external watering by residents.
Stormwater collected through the drainage system will generally require treatment for pollutants from garden and agriculture fertilisers, and chemicals from industry and motor vehicles. Not only is this expensive, these water quality issues generally preclude its use for public drinking water supply.
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| A constructed swale in Ascot Water estate, Perth | 1970s sump in Perth suburb of City Beach, at the lowest point of the street. |
Source: Designs for stormwater disposal in public open space –
an ecological assessment of current practices in WA (Grose and Hedgcock)
Current Situation
The Department of Water are currently completing a study into the amount of stormwater that may be available for recycling in the Perth metropolitan area.
Increasingly, stormwater is being left to recharge local aquifers and for local use. For example at a street level, some roads are now being designed without traditional kerbing and associated drains to direct water to garden beds, swales and parks as well as infiltrating groundwater.
Some councils are also removing the bottoms of existing drainage system storage tanks so that the stormwater is used to recharge the shallow aquifer. Demonstration projects are building our capacity to recycle stormwater.
In Cottesloe, there is a project to remove 10 stormwater ocean outfalls and install
underground stormwater treatment, storage and recharge tanks. The filtered stormwater will then be used to replenish the Cottesloe groundwater aquifer, used to water public open space and parks and gardens. The project is a partnership between the Australian Government and the Town of Cottesloe.
The six major councils in the Western Suburbs have agreed to take the bottom out of the many sumps in their network. This will increase the recharge of the shallow groundwater aquifer and is an inexpensive solution to improving ground water recharge while still maintaining flood protection. The Australian Government is developing national guidelines to manage the health and environmental aspects of stormwater recycling.
The Future
Recycling stormwater collected in drains will be guided by the outcomes of the Department of Water Study, demonstration projects and the emerging regulatory framework.
Due to water quality issues, recycled stormwater will not be used for scheme water substitution. There may be opportunities to reuse some small volumes of water that currently discharge to the ocean. Stormwater recycling is climate dependent.