Paving the way for clean water savings
University
of South Australia researchers are developing high strength porous
pavers that not only clean stormwater runoff, but harvest it for reuse.
This research represents a significant step forward in urban stormwater catchment design that integrates ecology with urban living, according to UniSA’s Professor of Sustainable Water Resources Engineering, Simon Beecham.
“Pavers being researched are made from inexpensive cast concrete and a special bonding material that adds structural durability to the open aggregate pavers, allowing stormwater runoff to filter through, with water treatment capabilities included that can be tailored to meet specific requirements,” Prof Beecham said.
“We can now design pavements that can biodegrade oils from cars and trucks, precipitate out heavy metals or strip nutrients from urban stormwater runoff by placing chemical additives in the concrete or substrate.
“The pavers enable rainwater to infiltrate the soil, to decrease urban heating, replenish groundwater and reduce flash flooding,” Prof Beecham said.
“A further benefit”, often overlooked according to Prof Beecham, “is that porous pavements allow tree roots to breathe. Roots need air as well as water, which is why so many street trees appear stunted. Breathable roads, car parks and pedestrian pavements give us the opportunity to bring ecology back into our towns and cities. That has to be better than the concrete deserts we’re building today.
“It will present a revolutionary way to build cities. Road transport is causing us to build more impermeable pavements and deal with more vehicle oils that we spill every day and more heavy metals from brake linings and tyre wear. Some of these metals are very toxic and need to be removed before the stormwater reaches groundwater. Our research will transform the environmental function of the pavement and enable us to build sustainable cities,” Prof Beecham said.
Treatments that remove pollutants can be engineered into the paver design during production, and can be optimised for a particular problem in a catchment. Ferric hydroxide added to the pavers precipitates out heavy metals such as lead, zinc and cadmium, while granulated activated carbon in the substrate ionically attracts dissolved organic matter such as that produced by biodegraded leaf litter from trees and the high dissolved organic carbon loads from effluent produced by wood pulping operations.
“These additives are effective over the lifetime of the pavers, with accelerated testing showing that they remove pollutants for 30 years in a controlled laboratory environment. This is a conservative estimate because, in reality, bacteria remove organic matter that can clog up the system, enabling additives to remain effective for two or three times longer than the test results,” Prof Beecham said.
UniSA’s research is heading in two areas – enhancing water quality treatment and designing tank systems to harvest and reuse water filtering through pavements.
Water storage is facilitated by excavating to half a metre below the pavement level, about 20 cm deeper than excavation for traditional paving methods. This area is then lined with a waterproof membrane and filled with coarse gravel, resulting in larger spaces that store more water than the finer gravel used under pavers. A thermally expanded clay, which holds onto nutrients, can be mixed with the gravel to provide a fertile environment for trees that will work for many years. Alternatively, if harvesting the water, the same clay can strip nutrients from the harvested water.
“What we end up with is an enormous underground rainwater tank covered by pavers. Water can be accessed using a small pump for irrigation but if we use a solar pump, the only energy used comes from the sun.
“If we could imagine having porous pavements on driveways and patios with water storage underneath all of the paved areas, the capacity to store water would be phenomenal, and much greater than could be achieved by installing large above-ground tanks.”
And the benefits don’t end there. Because there is no light, there is no algal development and the treated water is almost of drinking quality. The water quality treatment and harvesting potential of porous pavements is being investigated by PhD student Baden Myers under the supervision of Prof Beecham in the Water Science and Systems Centre, which is an SA Water and UniSA jointly funded research centre.
Prof Beecham believes the potential of this technology for collecting and reusing water is enormous. While not generally suitable for highly trafficked roadways, the porous pavers are ideal for car parks and have been used widely in supermarkets where turning occurs at low speeds.
“They would be perfect for all of our footpaths. Just in Adelaide alone the volume of water that could be collected across the catchment from footpaths, people’s driveways, and shopping centres would increase our storage by many hundreds of times and provide enough water for most outside water usages.”
Prof Beecham would prefer to see passive systems in place, where trees could be planted in the gravel-filled tanks and suck up the stored water without ever having to be watered.
“It’s an ideal system. There’s no energy because we’re not switching on pumps and the trees water themselves. These are simple, cheap solutions,” Prof Beecham said.
“We’ve even used them at UniSA’s Mawson Lakes campus where porous paving around the mathematics building discharges water into adjacent wetlands.
Prof Beecham has been working with researchers Yan Zhuge and David Pezzaniti at UniSA’s Water Science and Systems Centre to develop water saving solutions for porous pavements. They have recently been commissioned by the Concrete Masonry Association of Australia to develop software for porous pavement designs.
The researchers have been studying porous pavers made by HydroCon in Sydney, as well as locally produced solid concrete Boral pavers that enable water to filter through cut-out side sections.
While the initial cost of the porous pavers is slightly higher than traditional paving, excavation costs are about the same so, overall, the cost increase is between 10 and 25 per cent, but the water savings and environmental solutions are significant.