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Building the potential of green-blue walls

Rain gardens may bring greenery to dense cities, but green-blue walls are a new way of engineering that takes water into consideration, writes Hafizah Osman.

Australia is known to be one of the driest countries on earth and sprucing up its concrete jungles with greenery has been important aesthetically and to bring other benefits. But with water being a key concern when it comes to these green infrastructures, one researcher has taken great strides in alleviating this problem.

UNSW Pro Vice-Chancellor (Research) Professor Ana Deletic is passionate about green water infrastructure, also called water sensitive urban design (WSUD), and has worked extensively in that field. This involves the use of plants, soils and natural processes to treat water.

“These nature-based solutions are now implemented quite widely across Australian cities, especially Melbourne," Deletic said.

“My team has worked extensively on the development of rain gardens, which led to us creating the design guidelines, which underpin their adoption."

But rain gardens still take land and plants could suffer during dry weather spells, leading to system failures.

Creative experiment

In solving this problem, Deletic has been working on green-blue walls – vertical biofilter gardens in urban areas that use greywater or stormwater instead of potable water for their irrigation.

“These vertical landscapes are not only beautiful, they also provide multi-functional benefits like insulating buildings and improving micro-climates in dry and hot cities – plants pump water into air, achieving the same effect as evaporative coolers," she said.

“And instead of using potable water, they also filter greywater and stormwater, producing clean water for further use."

Deletic’s main concern is that Australia doesn’t have an abundance of water and shouldn’t be using whatever precious water available on green infrastructure.

“Imagine you live on a higher floor of a building. Every time you shower, your shower water gets collected and delivered to a green-blue wall installed at the outside of the lower floor.

"Your water then gets treated using the green wall media and plants, and is then collected at the bottom of the wall to be used for things like flushing toilets or garden irrigation,” she explained, describing one of the potential solutions.

Deletic added that nitrogen and phosphorus found in soaps, toothpastes and shampoos work as fertiliser for plants. Otherwise, they create algae that choke rivers and bays.

Although water treatment plants remove these nutrients from wastewater, Deletic said it is an energy-intensive process in comparison to this sort of biofilter.

Getting ahead

But research into green-blue walls is only in its infancy, having been around for less than five years and requiring far more time, exploration and investment to make the technology more robust.

“Although they are recognised as beautiful multi-functional urban infrastructure, green-blue walls are in their early days and are very different in their design, operation and maintenance from rain gardens, constructed wetlands or other WSUD forms.”

Deletic also said more work needs to be done on the engineered media as they need to not only support plants but also enable enough water flow and treat water to a high standard.

“It’s not easy to make it work, but it is feasible as it is low tech, low energy and has a low environmental footprint.”

Governance also has a crucial part to play in the rollout of green-blue walls and other WSUD infrastructure across Australia.

Deletic said its uptake has not been as successful as WSUD systems are multi-functional technologies requiring coordination of investments from state governments, councils, water corporations, developers and households as they are all benefiting from these systems.

“It is about who benefits and who pays. In Australia, since the governance of creating WSUD infrastructure is multi-layered, getting the investment from the many players can be tricky.”

Deletic also warned of companies “blindly” super imposing WSUD design guidelines developed in one climatic region to other regions.

“Countries like China are now focusing on ‘sponge city’ concepts, but our WSUD guidelines are being misused in such countries by some businesses," she said.

"These guidelines were created with Australian conditions in mind and shouldn’t be implemented without the relevant modification."

First published as 'Installing green-blue walls' in Current magazine May 2018.