Rainwater as a resource is mistrusted. It is something to get rid of as quickly as possible, hurried from roof and pavement into concrete pipes; suitable for gardens but not for domestic use.
In fact, research has shown that some rainwater, and even rainwater which has been stored in a tank, is as safe to drink as treated water provided through taps. More importantly, when rainwater is harvested it has the potential to significantly reduce the mains water needs of both domestic houses and commercial properties, lowering bills and putting less strain on drainage. It can also be used as a potential source of heating.
This article looks at a number of diverse but complementary UK research projects that have studied water quantity, quality and resource use considerations provided by a permeable pavement system (PPS).
A PPS provides a porous surface for rainwater; however it can also be constructed with a reservoir structure underneath which traps the water for rainwater harvesting. As a general rule, a total pavement depth of 500 mm can provide 1000 litres of water within 10sq m of paving. In practice, this resource is often used in both domestic and commercial schemes to provide the non-drinking water for a building. The volume of water that can be saved by PPS storage is very much dependent on the time of year, particularly where the PPS is linked to an outside tap for watering gardens and washing cars. As these uses for the water are unpredictable in terms of the volumes required and the regularity of demand for them compared with interior uses such as toilet flushing and the feed for washing machines, the savings to the user of a rainwater harvesting system vary.
The PPS can play an important part in limiting the demand for mains water, throughout the year in the case of interior use and during the months of peak demand during the summer. The financial incentive for rainwater harvesting also varies and payback on any investment is most efficient under a metering system. However, the extra cost to the user of a PPS in installing electric pumps and in-line filters, when already committed to using PPS as a driveway is minimal and may be less than £1000.
Storm water which is treated and stored in a PPS has to meet various quality standards in order to be considered for reuse as irrigation water. These standards will differ from the standards applicable to both simple disposals to a watercourse and for drinking water use. Irrigation water quality is determined based on the measurement of various parameters. Some of these include: Electrical Conductivity (EC), pH, total dissolved solids (TDS), elements in water (heavy or trace metals in water, sodium adsorption ratio (SAR), Carbonate (CO3) and Hydrogen Carbonate (HCO3), microbial water quality and total oil or/mineral oil in water. However, the characteristics of an irrigation water that seem to be most important in determining its suitability for irrigation include the total concentration of soluble salts, the relative proportion of sodium to other cations and the concentration of boron or other elements that may be toxic.
Our study simulated a worst case scenario of stormwater pollution for 10 weeks. 24ml per sq m of hydrocarbon was applied as a pollutant and a single dose of 17g of slow release nutrients was administered to the surface of PPS. The treated water stored underneath PPS models was used to irrigate tomato plants and rye grass for 10 weeks. Results showed that the concentrations of the selected elements were below irrigation, FAO irrigation water standards as well as the WHO heavy metals in drinking water standards. Concentrations of heavy metals obtained after the analysis of the soil 10 weeks later were below international limits. Analysis of plants after 10 weeks of irrigation indicated that the concentrations of heavy metals were within the normal ranges in plants. There was no significant difference between concentrations of heavy metals in tomato fruits irrigated with waters from PPS and tomato fruits obtained from a local supermarket.
Ground Source Heat Pumps (GSHP) are an established technology to provide low CO2 source heating for buildings. A set of double tube ground loops filled with ethylene glycol and water are placed in the soil and the liquids in the pipe are pumped around the loop extracting heat from water. The only energy required to power the GSHP is electricity for the heat pump. The system extracts heat energy from the wet ground and moves the heat via a pump to the building where, following contact with a compressor (which increases the temperature of the water), it is moved around the building, typically via underfloor heating. The energy available in the sub base of the PPS has been calculated to provide 1 kW of energy per 12-15sq m of paving.
In 2008, the GSHP PPS system was specified for a new three floor office development with around 7000sq m of total area in Bedford. The proposed design featured 6500sq m of car parking to go with the office and the GSHP paving was analysed for its suitability for the heating system. The drainage and heating system design were combined and the suppliers of GSHP equipment calculated that the PPS would provide sufficient heat to completely heat and cool the building. The design included infiltration of rainwater at the surface by the use of permeable block paving, storage and flow control of the infiltrated rainfall in the lined pavement sub base to a depth of 300 mm and recycling of roof water for non-drinking uses within the office development. On this site the storage element was not the permeable pavement but roof based tanks.
Together, these criteria prevented rapid runoff from the site and reduced both the rate and the volume of water discharging into the stormwater network. The plant room was established to hold the five heat pumps, which were 130 kW in capacity. The systems led to a typical 70% reduction on overall building carbon emissions, a reduction in utility bills of 50% and no local emissions or pollution. In addition there is a lower risk of fires and explosions than there is for gas boilers, no external equipment and noise units exceed NR35 and there are significant planning approval advantages.
Permeable Pavement Systems have enormous potential to provide added benefits to their role as a source control device without compromising efficiency in the long run, including provision of void spaces for storage, recycling of stormwater, serving as a source of irrigation water and a supply of renewable energy.