Constructed wetlands are a series of shallow, densely-planted, man-made ponds that help filter water through physical and biological processes. They provide a natural way to treat and remove pollutants from stormwater before it enters our creeks, rivers and oceans.
How wetlands work
Constructed wetlands typically have three parts that work together to help filter stormwater and protect it from flooding:
inlet zone – a sediment basin that removes coarse sediment
macrophyte zone – a shallow area densely planted with aquatic plants and the main part of the wetland, which removes fine particles and dissolved pollutants
high flow bypass channel – lets excess water flow around the wetland without damaging the plants
These work on three levels: physical, biological and chemical uptake, and pollutant transformation.
Plants are essential parts of a wetland, and are used to capture fine particles and trap a high proportion of absorbed pollutants attached to sediment.
They also slow and filter water, which encourages fine sediment particles to settle, and reduce the chance of sediment being scoured or re-suspended in the water during a large storm.
Biological and chemical uptake
Slimy microorganisms called biofilms grow on the surface of plants, absorbing and trapping pollutants through enhancing sedimentation and adhesion to fine suspended particles.
Wetlands transform the pollutants in stormwater in a number of ways:
their regular wetting and drying cycle stabilises and fixes contaminants, such as phosphorus and metals, in the underlying soil
microbial processes convert pollutants like ammonium and nitrate into nitrogen gas, which is released into the atmosphere (nitrification/denitrification)
ultraviolet (UV) treatment in open water areas provides some disinfection
Designing a wetland
Wetlands are almost always part of a series of treatments, called a treatment train. Other treatments, such as gross pollutant traps and sediment ponds in the inlet zone, protect the wetland by removing coarse sediment and rubbish. The wetland then removes fine particles and dissolved pollutants like nutrients and heavy metals.
Wetlands should be designed to:
promote settling of sediment larger than 125µm within the inlet zone
direct most of the flow volume through the planted macrophyte zone to enhance sedimentation, filtration, adhesion and biological uptake
ensure flows through the wetland are detained for 72 hours, using the outlet to control flow rates
ensure the macrophyte zone is protected from scouring and damage to the plants, by keeping water velocity under 0.5m/s and bypassing flows once the maximum extended detention depth is reached
ensure the plants are not inundated to excessive depths too frequently, as this may kill the plants and reduce the effectiveness of the treatment
When to use a wetland
Wetlands are usually used close to a catchment outlet or within a reserve where there is plenty of space. They are best built in land subject to flooding, but outside the main waterway channel.
Using a wetland versus a raingarden
Many developers and councils prefer wetlands because they provide amenity for the community. A wetland is generally preferred to a raingarden when:
the catchment is relatively large with high flows
there is plenty of space in a park or reserve and a water feature is desired
the site is flat, as wetlands require little difference in water level between the inlet and outlet - they can be used on sites where there is not enough difference in the water level for a raingarden
sediment loads, especially fine clay sediments, are high and may clog a raingarden
the area has low rainfall and long dry spells
Tips and advice
Wetlands may have open water ponds at the inlet and outlet, but should otherwise be planted so densely that the water is not easily visible. This avoids problems such as algal growth and sediment re-suspension through wind and waves.
Wetland plants must be protected from overly-high flow velocities and being frequently inundated with water for long periods of time.
An inlet zone sediment basin should be used to protect wetlands from sediment loads. In new developments, a swale or other treatments may be used instead.
Subdivisions should be designed so that wetlands provide stormwater treatment before water enters the main channel or waterway. Wetlands treating flows within or diverted from a waterway require a much greater level of design and analysis to make sure the inlet, outlet and bypass design provide suitable flow and inundation patterns for the plants.
The following resources provide further guidance for designing and building constructed wetlands:
Case studies and checklists – checklists identify hold points where checks must be made
Constructed Wetlands Design Manual – guidance for people designing and building wetlands for Melbourne Water
Wetland hydrologic analysis tool – MUSIC Auditor – check plant heights and inundation depths
WSUD Engineering Procedures – CSIRO publishing – Chapter 9, 'Constructed wetlands', has additional information