Biofilters

Figure 1: Biofilter design. Photo: Department of Water, Government of Western Australia.

Biofilters are excavated basins or trenches that are filled with porous filter media and planted with vegetation. They use natural and physical processes to filter stormwater and remove pollutants from stormwater runoff (Department of Water, 2011). 

Biofilters can be integrated into urban design, ideally connected to the main stormwater system to manage runoff and filter pollutants before they enter waterways and oceans. 

Biofilters are an effective water sensitive design technique that work in a range of climate, soil and groundwater conditions. They can be fitted or retrofitted into any context and are effective in all stages of urban water management. They control water from the source by capturing rainfall and reducing pollutants such as coarse sediment, phosphorus, nitrogen and heavy metals. They control water runoff through retention, detention and conveyance. Additionally biofilters minimise erosion and are useful in flood management. 

A biofilter is made up of several layers (See Figure 1). At the bottom is a free draining layer and drainage (if required) that controls the water table. Above that, a transition layer of sand or fine gravel separates the filter later above from the drainage layer below. The filter layer can be a range of soil types that filters fine sediments and metals and aids the growth of the plants. Above that, stone mulch suppresses weeds and retains moisture. At the top, dense planting species should be selected based on their capacity to absorb pollutants and tolerate inundation (Department of Water, 2011). 

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Name of NbS

Bio filters

Type of NbS

Created or constructed living ecosystems

Location

Urban. Biofilters can be implemented in all Oceanic regions, especially ones that are affected by flooding and increased rainfall.

Case Study:

The Watershed Partnership Initiative (WPI): Biofilters Case Study

Figure 2: Samoan community builds and maintains its own raingarden/biofilter system. Photo NOAA

Relationship to Indigenous knowledge

Indigenous communities believe that we are the kaitiaki (guardians) of our environment, and that we have a responsibility to protect it. In fact, the United Nations report in 2018 noted that indigenous communities protect nearly 80% of the remaining biodiversity on the planet. 

Stormwater runoff often ends up in our oceans, where pollutants damage ecosystems. Many Oceanic nations rely on the health of the ocean for their livelihoods, as well as their spiritual and physical wellbeing. Nature based solutions to pollutants in our waterways, such as biofiltration, creates solutions that align with indigenous world views, and are able to be implemented by affected communities. 

Climate change benefits
  • Changes in rainfall
  • Flooding
  • Increased incidence / distribution of disease
  • Soil erosion and landslides
  • Reduced fresh water availability / quality

In Oceania, marine processes are critical factors in the region’s climate systems and ongoing changes in climate have already begun to show noticeable ecological effects. Shifts in rain belts, winds, storms and cyclones have caused increasing temperatures and increased rain. Sea temperatures are rising too, causing shifts in ocean chemistry. Pollutants and ocean acidification cause coral bleaching, damage ecosystems and leave coastlines less protected from storms (Pacific Islands Climate Adaptation Science Centre, 2024). 

Water resources are significantly affected by changes in rain patterns. When stormwater systems cannot keep up, overflow collects pollutants, rubbish, fertiliser, chemicals and sediment, which enters waterways and oceans. Flooding is becoming increasingly common in Oceania as weather patterns become increasingly erratic and unpredictable. Stormwater infrastructure struggles to keep up, proven by recent events in 2023 and 2024 in Aotearoa, Fiji, Samoa, Vanuatu and other Pacific Islands. Pollutants in our water increase incidences of water-bourne diseases and damage clean water resources. 

Biofilters are designed to filter pollutants from low intensity showers – capturing water and dispersing it evenly. Biofilters can capture 95% of stormwater runoff (Hornsby Shire Council, 2018). They can be implemented in all urban contexts because of their flexible design – they can be any shape or size. They require less maintenance and cost than typical stormwater systems and can be retrofitted to support existing systems where needed. 

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Societal / socio-cultural benefits
  • Fresh water security and quality
  • Waste management and hygiene

Protecting fresh water resources and removing pollutants from waterways and oceans is a vital part of securing resilience to climate change for Oceanic nations. Biofilters can control rainfall in normal rain events, and support stormwater runoff in extreme rain events. Natural filtration requires no harmful chemicals.

Rising sea levels and increased rainfall is also compromising the efficacy of coastal sewage systems and wastewater infrastructure, and contaminating groundwater (Wiegner, 2022). Biofilters can be implemented in coastal communities as nature-based solutions.  

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Ecological and biodiversity benefits
  • Aesthetic value
  • Climate regulation 
  • Disturbance prevention
  • Fresh water
  • Purification

The Islands that make up Oceania have fragile and distinctive ecosystems that are famed around the world for their beauty. That beauty is characterised by the natural world – lush landscapes, beaches and oceans. These islands are on the front lines of climate change, where effects are already being felt. Solutions need to be based on nature to honour the natural landscapes and be implemented by communities whose connection and care for the land runs deep. Biofilters are a way to deal with stormwater, flooding and increased rainfall in an aesthetic, natural way that local people will understand. 

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Technical requirements

There are specific types of soils required for each layer of a biofilter system. Each layer needs to be of a certain depth, and each soil needs to be of a particular permeability. Specific plant species that are particularly good at withstanding large amounts of rainfall or heat can be recommended, and if pipes are used for drainage, there will need to be a specialist involved.

Issues and Barriers

Implementation of biofilter systems requires initiation and planning. On a city scale, that might be a Council, or a local body determining where the biofilter system would be most effective and creating it. In public spaces, it will likely require permission from the Council and the resources to implement the system.

Creating biofilters requires knowledge of the systems, as well as the correct types of soils and planting to maximise the positive benefits. These are all easily overcome and can even be part of an education and training programme of volunteers.

Opportunities

There are opportunities in Oceania to train local communities how to build and maintain biofilters. As countries around the world fund climate change projects in the Pacific, there is a risk of applying a western lens of success and strategy onto projects. Hard infrastructure projects, such as sea walls, have been less successful in Oceanic regions, because local people are more attuned to nature-based solutions. Biofilters are a nature-based solution that aligns with indigenous world views of reciprocal relationships with the land. 

There has already been success in biofilter projects in Oceania. In Samoa, for example, they are being used to protect coral reefs, and in New Zealand they are increasingly being included in urban design projects.

Financial case

Biofilters are a relatively inexpensive method of managing stormwater, flooding and increased rainfall. Any initial cost is offset by the fact that they require minimal maintenance. Aside from the monetary cost, the benefits of biofilters on ecosystems are multiple and increasing stormwater resilience means reducing damage to infrastructure and community’s lives.

Figure 3: biofiltration in action. Photo: Save Tarrant Rainwater
References
  • Department of Water. (2024). Water sensitive urban design: Biofilters. Government of Western Australia. 
  • Hornsby Shire Council. (2024). Biofilters and Raingardens. Hornsby Shire Council.
  • Messina, A., Rice, S., Vargas-Angel, B., & Biggs, T. (2016). Baseline Assessment of Faga’alu Watershed: A Ridge to Reef Assessment in Support of Sediment Reduction Activities and Future Evaluation of Their Success.
  • National Oceanic and Atmospheric Administration (NOAA). (2015). Protecting American Samoa’s Coral Reefs One Rain Garden at a Time. NOAA Coral Reef Conservation Programme.
  • National Oceanic and Atmospheric Administration (NOAA). (2022). Coral Reef Task Force Graduates First Watershed from Watershed Partnership Initiative. NOAA Coral Reef Conservation Programme.
  • Planning and Urban Management Agency. (2018). Urban Design Standards: Apia Central Business District (CBD) and Waterfront Areas. https://www.mwti.gov.ws/wp-content/uploads/2022/09/Apia-Urban-Design-Standards_Final.pdf 
  • Renold, J. (2024). Indigenous Peoples hold Key Knowledge for Sustainable Water Solutions. If Not Us Then Who.
  • UNESCO World Water Assessment Programme. (2018). The United Nations world water development report 2018: nature-based solutions for water.
  • Wiegner, T. (2022). Predicting sea level rise impacts to coastal wastewater infrastructure and water quality for adaptive planning and increased coastal habitat resilience. Pacific Islands Climate Adaptation Science Centre.