Getting to Grips with... Carbon Footprinting
Working out the carbon footprint of a project such as a drainage installation involves a number of variables, with material choice playing a key part, writes Stuart Crisp
by Stuart Crisp, Business Development Director, Concrete Pipelines Systems Association
A carbon footprint measures the total greenhouse gas emissions caused directly and indirectly by all activities within a defined set of boundaries. For drainage installations, emissions may cover the whole life of a drainage system, from the extraction of raw materials used in pipe manufacture to materials used to bed the pipe in the ground all the way through to what happens at the end of the pipeline’s life as a drain or sewer.
What gases contribute to an installation’s carbon footprint?
All six of the Kyoto Protocol greenhouse gases should be measured in an installation’s carbon footprint: Carbon dioxide (CO2), Methane (CH4), Nitrous oxide (N2O), Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs) and Sulphur hexafluoride (SF6).
A carbon footprint is usually measured in tonnes of carbon dioxide equivalent (tCO2e). The carbon dioxide equivalent (CO2e) allows the different greenhouse gases to be compared on a like-for-like basis relative to one unit of CO2. CO2e is calculated by multiplying the emissions of each of the six greenhouse gases by its 100-year global warming potential (GWP).
Users should be aware that when comparing pipeline material options, some carbon footprint databases are based on CO2 emissions alone, which can cause confusion and lead to inaccurate assessments. When British Precast compared the CO2-equivalent figure for concrete to the CO2-only figure, it showed a minimal increase in emissions of 2% for concrete pipes. For plastic pipes, however, the increase in emissions is at least 20%, if the CO2-equivalent figure is used instead of the CO2 only figure. The main reason for this is that the manufacture of plastics usually creates significant emissions of methane, which has an impact 25 times greater than carbon dioxide, according to IPCC 2007 data. The difference could be even more pronounced if the resin used in the plastics manufacturing process is sourced from further afield - such as Asia - due to higher transport emissions and more carbon-intensive grid electricity.
What should be included in the carbon footprint of a wastewater pipeline installation?
In the case of wastewater pipeline systems both the material of pipes and bedding material used for their installation need to be considered.
How useful are generic values in determining the carbon footprint of a drainage installation?
The problem with generic carbon footprints and drainage installations is that when the generic figures are adjusted to reflect the values of specific applications there can be a huge deviation from the standard value. In fact, the deviation from these generic values can be as high as 85% for plastic pipes and up to 53% for concrete pipes. This situation is discussed in an Institution of Civil Engineers Proceedings Paper: The Carbon Footprint of sewer pipes: risks of inconsistency. Where multiple deviations apply the figures could be higher still. (For the full research see Proceedings of the Institution of Civil Engineers, Volume 168, Issue ES1).
For the paper, British Precast took Bath University’s Inventory of Carbon and Energy, the most widely acknowledged UK source of carbon footprint data within the construction industry, as the default value for concrete and HDPE pipes. It then modelled various scenarios to help establish the importance of context on these generic values. The exercise showed that pipe bedding can increase the carbon footprint of a wastewater pipe by 14-48%, depending on the type of pipe, its size and the class of bedding.
Is it possible to reduce an installation’s carbon footprint by using less bedding?
Class S bedding is commonly used. It requires a pipe to be completely surrounded with granular material to effectively distribute loads and increase the load bearing capacity of the pipe. For rigid pipes such as concrete, bedding classes N,F and B all use considerably less granular material than Class S, but still enhance a pipe’s load-carrying capacity.
Bedding selection is particularly important in the case of flexible pipes because they have relatively little inherent strength and derive a significant proportion of their structural strength from the embedment at the sides of the pipeline. In reality, this means that most standard plastic sewer pipes are installed with Class S bedding with the surrounding embedment taking the majority of the flexible pipe’s design loading.
In the case of rigid pipes such as precast concrete, a significant proportion of the design strength is inherent in the pipe itself. Such pipes rely on bedding simply as a means of distributing loads and providing a supporting reaction under the pipe, which allows users to choose from a wider range of bedding options.
Class B uses far less granular material and has a lower carbon footprint than Class S. Class B is a common bedding option for rigid pipes. Modelling by the CPSA showed that switching to Class B bedding from Class S can reduce an installation’s embodied carbon by between 8% - 17%.
How does the pipe material itself affect the scheme’s carbon footprint?
In 2010 the concrete pipeline industry published a detailed study of the embodied carbon for precast concrete pipes, manhole rings and cover slabs. The study indicated that concrete pipes have up to 35% lower cradle-to-site carbon when compared to HDPE pipe using the same, Class S, bedding solution.
The plastic pipe industry responded to this study with an alternative set of carbon footprint values of its own. This was based on a much older, generic study, as opposed to one based specifically on a manufacturer’s own plastic pipe products. In fact, some have questioned whether the carbon footprint produced by the study is based on a recognised methodology and contemporary data. This study, unsurprisingly, indicated a lower carbon footprint for plastic pipes.
While the merits of each study can be debated, what this snapshot serves to highlight is that different accounting methods can lead to different results - something that it is worth remembering when making a comparison.
What are the implications of using the wrong value for a drainage installation?
The implications of the British Precast research is that if decisions are based on unadjusted, generic carbon footprint values, the water industry could be making inappropriate design and procurement decisions for specific installations. The water industry’s assets are currently estimated at 2.32 million tonnes of embodied carbon emissions, and any opportunity to minimise this should be grasped.
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