Water quality is a hot topic when it comes to designing drainage

Planners are careful to stop pollutants such as oils from contaminating the water supply. B&E hears from Hanson Formpave Researchg Manager Dr Stephen Coupe about the role of permeable paving in pollution prevention.

PERMEABLE paving systems (PPS) are most often urban surfaces that allow rainwater to infiltrate into them whilst retaining a structurally supportive role.

PPS should be multifunctional and can operate as car parking areas, pedestrian zones or roads subjected to light traffic amongst others. Crucially however, PPS can operate as the on-site drainage system, with attenuation of the rate of runoff and the total runoff volume. Also, improvements to the quality of discharged water relative to ordinary “non-Suds” techniques can be made, reducing the risk of downstream environmental harm.

The use of PPS as a tool within sustainable drainage systems (Suds) is now widely accepted as good practice in the UK.

The critical beneficial factor in the use of PPS is the permeability of the surfacing material. Most urban surfaces such as impermeable concrete and asphalt do not allow significant downward percolation of rainfall and therefore almost all the precipitation in a rainfall event rapidly runs off into sewers and streams. This brings with it a host of potential hazards. Firstly, as rainfall in winter is expected to become increasingly heavy and prolonged, the risk of floods due to extra runoff is higher than before. More UK urbanisation, particularly in the South-East will exacerbate this problem, with yet more runoff channelled into swollen rivers.

Picking up pollutants

This urban water picks up many different urban-derived pollutants whilst running off impermeable surfaces and transports them in either a dissolved form, bound to silt or as free product, into receiving systems. Hydrocarbons are discharged, including the carcinogenic polycyclic aromatic hydrocarbons as well as metals like zinc, copper, lead and chromium. These toxins are known to cause significant ecological harm and their permitted levels for discharge are being made progressively more stringent. The water framework directive is set to be a key piece of legislation in ensuring that good water quality is maintained in all aquatic systems and that any pollution, however diffuse, is considered a potential threat.

Permeable paving differs in several ways from the typical pavement or road construction. A material of prime importance for optimal retention and biodegradation of pollutants is the presence of a geotextile, ideally a non-woven random mat arrangement, typically of polypropylene/polyethylene compressed fibres. The geotextile layer is responsible for the majority of pollutant retention and functions as an appropriate substrate on which oil degrading micoorganisms grow and decontaminate the retained hydrocarbons and herbicides.

Examining oil retention

Early research at Coventry University in the mid 1990s was conducted on the problem of urban pollution and its mitigation by using Suds. This focused on a comparison between permeable paving and traditional impermeable surfaces such as asphalt and concrete. Results quickly showed that the permeable paving retained oils to a much greater extent by allowing the oil to percolate down into the sub structure rather than being remobilised by artificial rainfall.

Researchers progressively added oil to the three types of paving surface, and released contamination in discharged effluent after about 50 millilitres of added oil from asphalt and concrete systems. They found even after 170 millilitres of added oil the level of discharged oil in effluent was only just detectable at around 2 mg/l.

A much larger experiment over an extended time period, demonstrated the effectiveness of the PPS in retaining and degrading hydrocarbons by biological treatment. This experiment was deliberately contaminated by 100 times the estimated oil loading per square metre of urban paving (17.8 g /m2 week) to give a significant safety factor.
Less than 3% of the oil was released. Average concentrations in effluent released from the system were around 20 mg/l, significantly less than the stipulated concentration of 100 mg/l or less, the level needed to be accepted as a class two interception device. This retention capability was maintained for another three years with no significant decrease in efficiency. The total oil mass added over five years was 1150 grams.

Microbial action

Despite this excellent performance, concern grew about the possibility of a pulse of free product oil being released from the retention area if the storage capacity was breached. Closer examination showed that the stored oil was subjected to considerable biodegradation by microbial action. The resident microbial species already found on PPS materials were able to use the oil as a source of food by breaking down the carbon in the oil. This was a fully aerobic process, and as the geotextile was close to the surface of the PPS and close to the air, posed no realistic threat of anaerobic biodegradation, which could lead to the release of methane or oxides of nitrogen. The addition of the simple inorganic nutrients nitrogen, potassium and phosphorus was shown to further increase the biodegradation rate.

As the breakdown of oil by aerobic processes generates carbon dioxide, this can be measured to give a good idea of the rate of biodegradation. Simultaneously, the consumption of oxygen can be measured also gives an idea of the rate of treatment. The background concentration of CO2 of 0.03 %, the levels in the PPS atmosphere reached 0.47 %.

This was conclusive proof that the oil was being progressively removed from the system and the PPS was not just a trapping mechanism for pollution but an aerobic bioreactor capable of significantly improving discharged water quality.

Trapping contaminants

A final experiment in this series showed the fate of the added oil within a permeable paving to determine the area that hydrocarbons were retained. A PPS was broken open after a period of activity and the oil on all of the  discrete layers of the system was extracted chemically and the concentrations of oil determined instrumentally. From the added total of oil any not accounted for was considered to be degraded. The vast majority of oil was either trapped on the geotextiles or degraded meaning that the geotextile is pre-eminent in the trapping and decontamination of hydrocarbons. The purpose of the biological action is to maintain the retention function of the PPS, and prevent the build up of a reservoir of contaminant, that could have made the geotextile impermeable or a catastrophic pulse of released material.

Analysis of the biological and chemical properties of the system underwent extra refinement over the next few years and demonstrated the ability of the system to retain over 99 % of added zinc and copper and rapidly degrade commonly used herbicides. All the presented data is complemented by the fact that the surface infiltration rate through the permeable blocks is at least 4500 mm/hour when first installed demonstrating the attenuation rate
for flow achievable by the installation of permeable paving.

PPS are now the best validated Suds in the field of pollution prevention. Current research and development programmes include the use of PPS as underground water storage tanks for water recycling and reuse schemes and the use of PPS as areas in which to deploy ground source heat pumps. Both these schemes demonstrate the potential of linking Suds with other environmental schemes and the multiple benefits that can be provided within one site.

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