Water & Wastewater Treatment

In the know Digging deeper: Energy and carbon management Carbon in Question Using generic rather than actual carbon footprint figures for drainage installations can result in some wide variations, calling into question some product specs and purchasing decisions T he carbon footprint of some drainage installations could be up to 85% higher, or lower, than standardised carbon values, according to research by British Precast. The findings show that when site-specific factors are included in standardised carbon footprint figures, the deviation from these generic values can be as high as 85% for plastic pipes and up to 53% for concrete pipes for a single factor. Where more than one deviation factor is applicable the variation could be considerably more. The findings are revealed in the Institution of Civil Engineers' Proceedings paper The Carbon Footprint of Sewer Pipes: Risks of Inconsistency. At a time when the water industry is under pressure to reduce its carbon footprint, the paper highlights the risk of using generic data, inappropriate results or calculation methods to justify specification and purchasing By Stuart CriSp Business Development Director, ConCrete PiPeline SyStemS ASSoCiAtion (CPSA) decisions in the sewerage sector. For the paper, British Precast took Bath University's Inventory of Carbon and Energy and, based on 1000mm lengths of large diameter concrete and high-density polyethylene (HDPE) sewerage pipe installations for 450mm, 1200mm and 2100mm diameter pipes, set out to align various elements to specific project scenarios. Bath's inventory is generally accepted to be the water industry's most widely acknowledged source of carbon footprint data. The baseline scenario used for the study was the two data sets published by Bath University as 'default' values for concrete and HDPE pipes. For concrete pipes, the Bath inventory data comes from a study by Carbon Clear for the Concrete Pipeline Systems Association, based on PAS2050 (2011). A 2005 study by TNO for Plastics Europe is used for the HDPE pipes. Fi–een scenarios were modelled to help establish the importance of context on these generic values. The scenarios included: concrete and plastic pipes with different classes of bedding – Class S and Class B; concrete and plastic pipes based on carbon dioxide emissions only; concrete pipes with no cement replacement and HDPE with 5% recycled HDPE resin content; concrete pipes with cement sourced from Germany and plastic pipes with HDPE resin sourced from both UK and from Asia; both types of pipes where construction was 750km from the pipe manufacturing plant; and concrete pipes manufactured using reinforcement made from recycled steel. The results of the bedding design modelling, for example, highlight the impact different bedding classes can have on an installation. The purpose of the bedding is to distribute the load so that the pipe remains structurally stable and within its design limits. Bedding Class B, for example, requires significantly less granular material than Class S which requires a pipe to be completely surrounded. HDPE sewer pipes generally require Class S bedding to help control a flattening (ovalisation) of the pipe. By contrast, the inherent strength of rigid concrete pipes frequently allows the use of Class B bedding for the same loading conditions, which requires significantly less granular bedding than Class S. The modelling showed that simply switching to Class B bedding can reduce an installation's embodied carbon by 8 – 17%. These scenarios also highlight the need to understand the impact of different emissions indicators. While most carbon footprint databases are based on the carbon contribution of all main greenhouse gas emissions to produce a footprint figure, some databases are based on CO2 emissions alone. When British Precast compared the carbon footprint figures for CO2-only and CO2-equivalent carbon footprints for all greenhouse gas emissions, it found that the CO2- equivalent showed a 2% increase over the CO2-only figure for concrete pipes and a massive 20% increase for plastic pipes with resin sourced from Europe. What is more, if the resin had been sourced from further afield, such as Asia, the difference would have been even more pronounced. Calculation methodologies and other assumptions can also impact 34 | JUly 2015 | WWt | www.wwtonline.co.uk

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