Treatment Train Approach

Whilst it is clearly understood that permeable pavements are highly efficient control mechanisms for water quantity less is known about the water quality enhancement, which takes place as the water passes through the large volume of stone in the sub-base.

In extreme situations eg, where the water may be carrying a high nutrient load, a series of SuDS systems, in train, may be used to clean the water further.(polishing) The choice of, and the way in which the systems interact, will depend upon site circumstances and this site specific design is a common approach with SuDS design.

Permeable pavements have been installed and monitored for more than 20 years to demonstrate their performance in terms of water treatment. It is now well proven that within permeable pavements, microbes develop naturally in response to ongoing contamination and this treatment capability is further enhanced by filtration and aeration within the construction.

Permeable pavements also limit the concentration of pollutants by immediate localised interception and long term research has demonstrated that the following are effectively removed from stormwater run-off:

• Heavy metals
• Hydrocarbons
• Phosphorus
• Nitrogen

Hence permeable pavements can negate the need for costly traditional petrol/oil interceptors and can be used as a primary treatment facility in accordance with the Greater Dublin Sustainable Drainage Strategy (Volume 3 Chapter 6) often without the need for further polishing of the water.

The mechanisms by which permeable pavements intercept pollutants and provide water quality enhancement, are widely understood to be:

• Filtration/sedimentation and retention of solids
• Adsorption(pollutants attach or bond to elements within pavement)
• Biodegradation of organics such as petrol
• Tanking of systems preventing contamination of groundwater

As permeable pavements provide mechanisms that encourage these processes to occur, and designs ensure that flows are controlled to allow these processes to occur, permeable pavements are very effective in providing water quality enhancement. Information collated by CIRIA from worldwide research shows that between 50 and 95% of water borne pollutants are removed by permeable paved systems. This can be further enhanced by the treatment train approach.

Surely, when we stop to consider the existing pipe methods of stormwater drainage, where surface water and its contaminants are simply conveyed to the nearest watercourse, these systems offer a vast and much more environmentally friendly solution.