When Grey Meets Green - A Hybrid Treatment Story

Whilst water sensitive design (WSD) and Green Infrastructure are effective stormwater management solutions, proven to mitigate the effects of impervious development and urbanisation, they tend to occupy a large footprint. Additionally, due to the size of device required for effective treatment, the construction and maintenance costs can be significant and ongoing.

With the focus on urban intensification the cost of land has increased and the struggle to dedicate high-value, prime building land to green treatment devices or WSD is increasingly difficult to justify. Consequently, designers are inclined to revert to grey infrastructure and innovative proprietary solutions. However, in some situations grey infrastructure is the best practicable option to achieve effective treatment where space is constrained.

Grey infrastructure is an ‘engineered’ solution to a problem that, in terms of public preference, is better managed by a more natural solution. Often stormwater treatment for a site is designed as either grey or green infrastructure, however, there is opportunity to better incorporate the best of both grey and green to overcome the space and cost constraints. The benefits of using hybrid system includes reduced maintenance costs, aesthetically pleasing, reduced land requirement, improved treatment efficiency to name a few. There is also an opportunity to design each device, in a hybrid treatment train, to target different contaminants for challenging sites or retrofitting devices on existing site.

This paper will present case studies of projects where hybrid treatment systems has been applied. It will provide further discussion in regards to the benefits of using a hybrid design. In particular, performance, space constraints and costs.