Tread with care

The current publicity around this year’s water shortage in the UK brings sharply into focus a global issue that is set to become increasingly significant – that of water conservation. The global statistics are stark: by 2050 the world population is expected to reach 9 billion, competing for a finite fresh water supply.

At the same time, more people are living in urban and suburban areas where water consumption per capita is higher and the strain on the existing stock of fresh water will increase considerably. There are many ways to control water consumption, but industry must play its part and find ways to evaluate and reduce the amount of water used in manufacturing.

The water footprint concept was introduced in 2002 by A Y Hoekstra from UNESCO as an indicator of water use. Estimates have been produced to express the water footprint of an individual (the average global water footprint is 1,385m3 per year), while various industries are also debating ways to calculate the water footprint (or embodied water) of specific products.

There is no internationally agreed method for this type of calculation, although the International Standards Organisation (ISO) is working towards water footprint standard ISO 14046 that would sit within the ISO 14000 environmental management series.

Until this ISO standard is agreed, there is room for considerable interpretation and subjectivity in the claims made by manufacturers. The CPSA represents manufacturers of precast concrete wastewater pipeline systems and is making considerable investment in research to provide robust and independently assessed measures of the environmental impact of its products – with embodied water being one of these.

One of the difficulties associated with assessing water use is to establish the boundaries of the activity being considered and ensuring that any comparisons between different materials are using the same starting point.

For example, an independently accredited study to PAS 2050 (Publicly Available Specification for assessing the life cycle of greenhouse gas emissions of goods and services) completed by CPSA shows that the amount of water used at precast concrete pipeline factories is 76.3l of borehole and mains supplied water per tonne of product produced. However, this figure only considers factory use. Clearly if all upstream life-cycle stages are included (as they should be) the figure will rise.

A study carried out at the Building Research Establishment (BRE) for the Precast Flooring Federation (PFF) shows that the average cradle-to-gate embodied water for precast flooring is 800l per tonne. This figure assumed a precast factory water consumption rate of 121 to 136 litres per tonne produced. Although the factory usage is higher than for our own pipeline products, we nevertheless feel that the similarity in the precast material means that this figure is an appropriate measure.

One of the reasons for producing a calculation for embodied water is to enable specifiers to make informed choices between products, so it is an obvious next step to consider the embodied water of equivalent plastic pipe products. Life-cycle assessment studies carried out by Plastics Europe (2005 and 2006) suggest that the embodied water per tonne of product ranges from 58,000l for HDPE Pipe to 80,000l for uPVC Pipe.

However, it is more appropriate to consider the comparative figures per metre length of pipe rather than per tonne of material.

When taking into account the weight differential, we still find that a concrete sewer pipe has a cradle-to-gate water footprint between 2.6 and 6.7 times lower than an equivalent size plastic pipe. For example, an HDPE Pipe at DN 1200 would have a water footprint of between 3,886 and 5,200 litres per metre depending on the strength grade, with an equivalent size concrete pipe showing a footprint of 1,072l per metre.

These calculations consider the cradle-to-gate water footprint (as opposed to factory consumption) and this distinction is very significant. The water footprint of plastic is significantly externalised to other parts of the world as PP and HDPE for plastic pipes manufactured in the UK use resins sourced from overseas locations, many of which are in water stressed regions.

By contrast, concrete is locally sourced and over 88% of concrete produced in this country is responsibly sourced to the BRE standard BES 6001 – Framework Standard for the Responsible Sourcing of Construction Products. This is higher than any other construction material sector.

With a new and detailed ISO standard it will be easier to understand the boundaries and implications of water footprints. For now, the industry needs to use the best reference information available for calculating the water footprints and these must be LCA studies based on primary data rather than secondary sources.