Blue, Green and Grey Infrastructure: what’s the difference – and where do they overlap?

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A pond which overflows into underground storage basin.

Proprietary Systems:  The Confluence between Green and Grey Infrastructure in Integrated Urban Water Management

All over the world  there is an increasing recognition that stormwater runoff and other wet-weather induced flows from urban catchments (eg. combined sewage overflows) are major sources of pollution into receiving watercourses resulting in water quality impairments and related adverse impacts on the ecology of streams and local receiving water bodies. These flow sources are also the primary cause of flooding and erosion.

For many years now, I have been based in the USA and research conducted in my adopted country has clearly highlighted that sediment laden stormwater runoff is the foremost cause of water quality impairments and that priority pollutants of concern such as nutrients and heavy metals tend to be associated with the finer sediment fractions.

In the US, It is the Clean Water Act that is directing progress towards achieving the desired good ecological status and those aspirations are just as passionately followed in member states of the European Union, via the EU Water Framework Directive.


In each case there’s an accepted need to improve the quality of wet-weather induced discharges from urban catchments, particularly from stormwater discharges, “Combined Sewage Outfalls and other “hot spots” such as industrial estates, garage forecourts, motorway services, recycling yards and heavily trafficked roads. The challenges of controlling and treating wet-weather induced discharges from highly urbanised catchments are not due to a lack of treatment techniques or methodologies; as a wide range of processes for water treatment are available to address the technical challenges of treating wet-weather flows to remove the pollutants of concern prior to their  discharge into the environment.

Grey, Green and Blue

These techniques range from what we might call Grey Infrastructure: conventional piped drainage and water treatment systems (ie. pipes, tanks, conventional treatment systems including energy-intensive water treatment systems and processes such as membranes and reverse osmosis);

Blue Infrastructure: the use of proprietary small footprint high-efficiency devices installed and retrofitted within existing collection systems; and

Green Infrastructure:  the emerging trend for systems such as Bio-filtration, ponds, wetlands, rain gardens and other natural land and plant based ecological treatment systems and processes.  “Green Infrastructure” is typically seen as being synonymous with LID (Low Impact Development) (as it is widely known in the US); SUDS (Sustainable Drainage Systems); and WSUDS (Water Sensitive Urban Design).

The real challenge however, particularly in the highly urbanised catchments, is that space is usually at a premium. So though “Green Infrastructure” is attractive and the direction to go in terms of more sustainable approaches; in the urban retrofit scene, the space requirements for “Green Infrastructure” are such that they are often not feasible solutions on their own.  

The Space Issue

Space constraints, particularly in the ultra-urban catchment setting, are now resulting in a re-evaluation of traditional LID and SUDS practices with an increasing recognition of the role that proprietary systems can (and have to) play in integrated urban water management. This is because of an identified need for compact (ie. small footprint), high efficiency, robust and low maintenance controls and treatment systems that can be used as part of a tool kit of integrated solutions to address the challenges of urban diffuse pollution and flooding from CSOs, Sanitary Sewer Overflows (SSOs) and stormwater runoff; in particular systems and devices that can be retrofitted into the existing urban drainage infrastructure and can also be deployed in conjunction with both Green and Grey Infrastructure to make the best of both.

This need for compact, high efficiency and low maintenance systems has long been recognised and as a result, a range of novel advanced proprietary systems based on non-conventional technologies have evolved to address these challenges. These technologies are generally characterised by being more compact and more efficient than their conventional technology surrogates. They are particularly well suited to retrofitting into existing “Grey Infrastructure” systems and are also effective in combination with “Green Infrastructure” either as pre-treatment systems to improve their efficacy and facilitate maintenance or provide space efficient treatment trains.

In future blogs I will describe case examples of the use of innovative proprietary systems here termed “Blue Infrastructure”, at the confluence of “Grey” and “Green” Infrastructure, to provide cost effective solutions to problems of flooding and pollution in a number of urban catchments; with typical cost savings of the order of 50%, being realised.  In this respect the US experience offers a number of interesting examples.

One example of “Blue Infrastructure” is a novel stormwater filtration technology that was initially developed through the Small Business Innovative Research program of the US Environmental Protection Agency (EPA) by researchers at The University of Alabama under the leadership of Dr. Robert Pitt with Richard Field of the US EPA as the project officer. The system has subsequently been commercialised as a modular proprietary stormwater treatment system that is being  put to work particularly in New Zealand and the USA, as an easy to maintain, cost-effective high-rate stormwater treatment system.


The system is an advanced, novel, passive, high capacity, upflow filtration device that incorporates multiple elements of a treatment train (screening, sedimentation and high-rate filtration) in a compact modular device. Flow in an upward direction counters gravitational forces to fluidise the media allowing the entire depth of the media bed to be utilised; resulting in greatly extended filter run times. These filters have been shown to be more efficient on a unit-area basis than traditional down flow or radial flow filters; offering a smaller footprint because of their higher flow-through capacity.

The Up-Flo™ Stormwater Filterr

Up-Flo™ Stormwater Filter


Proprietary systems and Blue Infrastructure provide a way to facilitate the maximum use of existing infrastructure and are proving to be a “middle way solution” and an effective “bridge” between the traditional “Grey Infrastructure” and the Emerging “Green Infrastructure”. They are an essential part of the comprehensive “toolbox” of appropriate solution sets that can be tailored to suit site specific requirements in integrated urban water management. These facilitate and provide a means of addressing the adverse impacts of urbanisation in a space and cost efficient manner, whilst providing a means of maximising the existing urban infrastructure.  

Photos Showing Up-Flo™ Filter Maintenance

Up-Flo™ Filter Maintenance - Cleaning out the Sump and Changing Filter Media


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