A Coupled Wetland Biofilter: The Best of Both Worlds

This paper describes an innovative stormwater treatment solution that combines the low head loss, large catchment properties of a treatment wetland with the high treatment efficiencies of a biofilter to achieve a reduced footprint solution.

The future of our cities will see development encroaching into more and more constrained areas as they regenerate and intensify. The application of the solution proposed in this paper could be of considerable interest for highly urbanised areas where there are challenges of high contaminant loads from catchments with high imperviousness combined with limited space for stormwater infrastructure. The paper describes the design philosophy of the system and systems being constructed. It then sets out the design, assessment and consenting phases of a New Zealand brownfield application to demonstrate how the solution could be applied in a local context.

The treatment system includes forebays and wetlands to provide some treatment as well as volume above the wetland that increases the temporary storage volume that can pass through biofilters. This also allows the biofilters to be located higher, thus, potentially allowing filtration treatment to be achieved in areas that do not have enough hydraulic grade for a more conventional filtration approach. This aspect is particularly helpful when considering a future where climate change induced sea level rise is reducing the head available for treatment in coastal areas. The other advantage of having biofilters above and separated from the wetlands is that it prevents them from being subject to constant low flows that would see algae growth reducing infiltration rates. This can provide for better long-term performance and lower maintenance requirements for future asset operators.

The New Zealand application presented is a retrofit designed to treat 600 hectares of commercial development discharging to the Māngere Inlet in Auckland. According to the MUSIC modelling undertaken, the coupled wetland and biofilter devices can achieve 75%TSS removal in in an area equivalent to 0.9% of the contributing catchment. This is less than half the space required for a conventional wetland treatment system using current Auckland guidelines. Further, the devices can treat a range of other contaminants, including intercepted leachate from adjacent landfills and they have been designed to integrate into a landscaped environment. The proposed application was designed as part of the NZ Transport Agency’s East West Link project in collaboration with Auckland Council. It was part of a successful application for resource consents with the Board of Inquiry decision noting that the stormwater measures incorporated for the wider catchment would be “expected to enhance the mauri of this water body and help to restore the mana of the wider area”.