Reducing the environmental effects of drainage systems
All materials affect the environment to some extent, but the environmental effects of drainage systems comprised of pipes and underground chambers are a complex mix of factors. There's the impact of materials extraction and processing, manufacturing, transport and distribution, installation and maintenance, and recycling/disposal at end of life. Dwarfing all of these, though, can be the environmental effects of drainage pipe and underground chamber performance over a drainage system's lifetime.
Besides having lower defect rates when underground, plastic pipes have comparable performance to other materials, but with lower total costs of installation and maintenance. Weight is about 7% that of concrete and 14% of clay. Handling is easier and doesn't need heavy lifting equipment. Plastic pipes don't have the resistance to deformation of rigid pipes, but their flexibility means they can give rather than break when under pressure. Pumping costs are lower because plastic pipes are generally smoother. Fewer deposits mean fewer blockages and less cleaning with no need for very high-pressure washers.
Defect rates of different pipe materialsPipes don't just transport stormwater and waste flows, but are also an important barrier to contamination and public health risks. Many countries are suffering an aging drainage infrastructure with corroded underground pipes risking serious water quality issues. The largest potential problems come from leaks and breakages in pipes and underground chambers.
A country-wide German, Swedish and Dutch sewage system study in 2003 compared long-term defect rates of flexible (plastic) versus rigid (mainly concrete and clay) pipes. Plastic pipes showed considerably fewer defects, although the results must be treated carefully. Only pipes up to 30 years old were included, but as plastic has been increasingly specified over that period, the plastic pipes in the study may have been on average younger.
The study however gave interesting detail on the type and frequency of failures. The study found more than seven times as many defects causing exfiltration (as opposed to infiltration) events in both plastic and rigid pipes. Around half of defects came from connections and displaced joints, with the other half made up of fissures and breaks, surface damage, obstacles and deformations.
The study also showed a major cause of failure to be errors in installation and supervision. That is an easy way of improving watertightness, so reducing environmental effects. Particularly in pipes carrying hazardous chemicals and sewage, even a small leak can cause major problems. Animal faeces or a dead carcass seeping into a tank can lead to microbial parasites like cryptosporidium contaminating drinking water for tens of thousands of people.
The study also used fuzzy logic to summarize the effects of the various sources, paths and effects of leaks (Figure 1).
Performance characteristics compared
There have been many studies comparing the wider environmental effects of the different underground materials. An independent and peer-reviewed Life Cycle Analysis earlier this year found plastic pipes (PVC pipes) in North America showed lower life cycle and carbon footprint impacts. The Sustainable Solutions Corporation (SSC) study was performed to ISO standards, and was based on a 100-year cradle-to-grave assessment. It found that PVC had "lower embodied energy, lower use-phase energy and longer life attributes compared to other pipe materials". It covered 4" to 60" (100 to 1500mm) pipes for drinking water, stormwater and sanitary sewers.
The study highlighted several aspects where PVC outperforms traditional materials, and found its results confirmed by LCA studies from around the world. It highlighted corrosion as a major cause of breaks and failures of rigid pipes in the US. This can be accelerated by sewage, wastewater or other fluids or gases inside the pipe. Bacteria that metabolize sulfur compounds to acidic chemicals are a particular problem for concrete, steel and iron since local colonies can grow to dissolve or corrode pipes.
The study found that plastic pipes made from PVC achieve the longest service life of all pipe materials "including brick, clay, concrete, fiberglass, polymer concrete, polyethylene, cast iron, ductile iron and steel". And because it does not corrode or roughen, the original installation design properties are maintained over its lifetime (Figure 2).
The study also found that PVC pipes do not emit toxic substances in their manufacture, or leach them into water being transported when underground. They have the lowest carbon footprint, and lowest life cycle costs because of low installation, operating and replacement costs. Overall, they had the lowest 100-year lifecycle embodied energy, with significant long-term pumping energy savings. Indeed, a smaller diameter PVC gravity pipe can often transport as much as larger traditional pipes, because there is less internal friction and because nominal diameter is not compromised by the build-up of deposits or corrosion products.
The study showed excellent behaviour by thermoplastics because of plastic's ability to withstand huge strains. It found no failures or stability problems over the four-year monitoring, which is in line with previous long-term studies of buried plastic pipes.
TEPPFA stressed the importance of having multiple data sets to evaluate the current and future methods of predicting pipe performance in accordance with actual behaviour. It remarked that the plastics pipes industry as a whole has not yet thoroughly discussed pipe design and installation aspects.
However, environmental aspects can be compared using Environdec's Environmental Product Declarations (EPDs), which give a standard way of communicating the results of life-cycle assessments, with criteria similar to those shown in Figure 3. They simplify assessments of a pipe system's environmental impact, and allow comparisons with alternative products at the system level. The declarations are based on ISO 14025 and EN 15804. Environdec's online database has more than 700 EPDs for a range of product categories by companies in 36 countries. They are an important step towards a unified approach to comparing environmental performance of different products. For an example, see this EPD page.
Read more? Download the white paper.