Bioretention – Learning from the US Experience

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By Bob Andoh, Chief Technology Officer, Hydro International

Have you ever noticed a wasted opportunity in the islands, trees and plants at the side of our roads and car parks?   You see a tree pit in a car park, and close by, a surface water drainage channel.  They are completely unconnected to each other, and the surface water drain is discharging water with little or no treatment into the sewer network.    Combine the two, and you have a more sustainable drainage solution that treats pollutants and sediments, as well as minimising the amount of surface water runoff heading for end-of-pipe systems.

The missing link is bioretention.   Bioretention was first pioneered in the US – in Prince George’s County, Maryland to be precise, – by forward-thinking municipal engineers motivated by the principles of Low Impact Development (LID) in the late 1990s.  LID was a comprehensive land planning and engineering design approach that presented a new paradigm for stormwater management.  The LID movement has since grown in popularity and strength, leading to a much greater use of both engineered and natural bioretention technologies in the US.

LID’s aim is to mimic the natural pre-development site hydrology using a variety of techniques. LID shuns conventional end-of-pipe or in-the-pipe structural methods of drainage, and integrates stormwater controls throughout the urban landscape.  It sounds very similar to British SUDS principles, but a key difference with LID is a concept that favours distributed at-source landscape features over centralised high-impact ponds and swales.

In response to LID, Bioretention became more popular as a means of filtering stormwater runoff through an aerobic plant, soil and microbe complex to capture, remove and cycle pollutants through a variety of physical, chemical and biological processes.

Traditional bioretention and biofilter systems have tended to be landscaped drainage systems, planted with vegetation, trees and grasses.   Because a mix of physical, chemical and biological processes are all happening simultaneously in the soil and plant complex, bioretention is able to treat bacteria, nitrogen, phosphorous, heavy metals and organics such as oils and grease.

Engineering in nature’s way is often about combining ‘soft’ natural drainage processes with enabling engineered technologies that help to  mimic as closely as possible the natural hydrology of an area.

Conventional bioretention systems  tend to be space hungry, limited by soil conditions and prone to compaction e.g. by vehicles driving over them.    But new engineered technologies work in a more compact space, with predictable soils, are more easy to link to drainage inlet and outlet systems, easier to design against flow control and treatment parameters and perform consistently whatever the site characteristics.

One example is the Filterra® [link to product page] bioretention and biofiltration system, which is currently being introduced by Hydro International in the UK.  From the surface, the system looks like a normal tree box, with suitable plants, shrubs or trees protruding through a decorative grating in a typical concrete slab at pavement level.  Underneath, it comprises a concrete container with a 75 mm mulch, 500 – 1000 mm of a unique soil filter medium, an observation/cleanout pipe and an under drain system.

Engineered bioretention systems open up a new SUDS horizons for highways and drainage engineers to combine infiltration with storm water treatment whilst improving the landscaping and amenity value.   I believe they will have an important part to play in storm water management in future.

Download the e-guide: Bioretention – Thinking Outside the Box

Learn more about Hydro Filterra®

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