Water & Wastewater Treatment Magazine
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In the know Getting to grips with... carbon footprinting 28 | FEBRUARY 2017 | WWT | www.wwtonline.co.uk 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. The inherent strength of a concrete pipeline means carbon and cost savings can be made on pipe bedding