Biofilters

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CASE STUDIES //

A biofilter system with layered planted substrate treating urban stormwater runoff while integrating vegetation into the urban fabric in Aotearoa New Zealand.

Definition

Biofilters are engineered planted systems that treat stormwater runoff by filtering water through layered substrates and vegetation.

What this strategy does

  • Treats stormwater close to the source using soil, plants, and microbial processes
  • Reduces pollutant loads entering streams while integrating vegetation into the urban fabric

Context
In Aotearoa New Zealand, urban runoff is a major pressure on freshwater ecosystems. Biofilters are widely promoted within water-sensitive urban design to improve water quality and reduce downstream ecological impacts when designed and maintained appropriately.

Technical considerations

Design considerations

Layered substrate configuration
Use graded layers (e.g. sand, gravel, organic matter) sized to achieve hydraulic performance while supporting microbial processing of nutrients and metals.

Plant selection
Prioritise locally eco-sourced native wetland and riparian species tolerant of periodic inundation and drying to maintain function and reduce replacement risk.

Hydraulic integration
Size and locate biofilters to intercept runoff from impervious areas while maintaining design flow paths that avoid prolonged ponding or scour.

Implementation considerations

Design priority
Integrate biofilters early in site layout to secure adequate footprint, setbacks, and overflow connections.

Key constraint
Performance declines if maintenance access, sediment forebays, or inlet protection are omitted.

Relevant tools or standards
CRC for Water Sensitive Cities – Adoption Guidelines for Stormwater Biofiltration Systems. Local council WSUD or stormwater device standards (where applicable).

Issues and barriers

Space limitations
Dense urban sites may constrain footprint, requiring trade-offs between treatment performance and land use.

Maintenance dependency
Sediment accumulation and vegetation decline can reduce treatment efficiency if routine maintenance is not resourced.

Public concern
Perceived risks of mosquitoes or stagnant water can affect acceptance, particularly where surface ponding is visible.

Synergies and opportunities

  • Climate change – Attenuates peak flows and supports urban cooling through evapotranspiration
  • Human wellbeing – Contributes to greener streetscapes associated with mental health benefits
  • Disaster risk reduction – Reduces localised flooding and erosion pressures
  • Freshwater security – Improves downstream water quality supporting aquatic ecosystems
  • Waste and pollution management – Removes nutrients and metals before they reach waterways

Financial case

Ecosystem services and/or performance value

Value type
Reduced downstream treatment costs and avoided flood damage through decentralised stormwater management.

Cost-effectiveness

Investment logic
When integrated into streets or open space, biofilters can be more cost-effective over their lifecycle than expanding piped stormwater infrastructure.

Monitoring and evaluation metrics

Core metric
Reductions in nitrogen, phosphorus, suspended solids, and metals can be measured upstream/downstream of the system.

Advanced or long-term metric
Vegetation survival and condition as a proxy for ongoing treatment performance.

Case study

Filterra at Owairaka subdivision

Additional resources or tools

Te Ao Māori and Water Sensitive Urban Design (Activating WSUD)
Landcare Research guidance

NIWA – Constructed Wetlands and Stormwater Treatment
https://niwa.co.nz/freshwater/freshwater-mitigation-systems/constructed-wetlands/constructed-wetland-guidelines