Bio-filtration System in Action at Silvermist

A Bio-filtration system is as the name suggests is a living filter, employing microbes and wetland plants, gravel and sands, to filter the stormwater and urban runoff. Pollutants, increased nutrients, sediments and faecal coliforms in the stormwater negatively impact the health of the natural creek system, contributing to weed growth and exacerbate existing problems.

The filter consists of three layers underneath; gravel, sand and a carbon source with piping underneath to let the filtered water flow out. The native plants at the surface also play a role absorbing contaminants and nutrients and slowing the water flows down for absorption.

This image has an empty alt attribute; its file name is resized_4546.jpgSlowing the water down ensures it can soak into the filters and surrounding soil, this recharging of groundwater mimics the natural hydrology and moderates the rapid flow of stormwater from the hard, impervious surfaces above. This is an important balance to ensure it captures as much pollutants as possible.


The water will flow out of this pipe quickly so to slow it down rocks are concreted onto the ramp to diffuse water. The sediment basin does this too, reducing velocity and catching sediment that precipitate as a result of the slack water. It is important to catch this sediment where it can be cleaned out, before it continues downstream.

From the ramp there is a pool made from large boulders followed by a ramp of rocks to ensure there is oxygen added to the water and to slow the flow down. This picture shows the gradient is only very slight to slow the water down.

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Rock sills are built to stabilise the gradient, further reducing velocity and maximising water penetration.

Riffle zones made from smaller rocks help to oxygenate the water by increasing its exposure to the air, aiding oxygen absorption.

Most endemic species prefer low nutrients, whilst invasive weedy species enjoy the high nutrient urban runoff, so it is important to remove the excess nutrients from the stormwater before it enters the natural environment.

Blechnum ferns were used on the edges as the root systems of these naturally occurring ferns have excellent holding capacity and appear in the creek line systems sounding this site.

The steps involved for this project to happen include:

  •  the woody weeds were removed by contractors
  • a machine to shape the bed and for large rock placement
  • geotextile fabric and gravel, sand and piping added then jute mat and coir logs added below  the structure
  • rocks sills built and planting in and around structure

Each site is different, with different constraints as well as possibilities. Site assessment and careful design are important when planning a similar project. The fundamentals are pretty standard from site to site, and can be applied to most situations.

Photo credits from Tanya Mein. Article by Ed Bayliss Hack and Erin Hall