Scientists at Argonne National Laboratory, US, analyzed the water footprint of various forest-based biofuels produced from feedstock grown in the southeastern US – examining green water (rainfall), blue water (irrigation and process water) and grey water (wastewater discharge, the sum of the volume of polluted water discharged to a stream and the additional water required to dilute the pollutant, in this case mainly nitrogen, to an acceptable concentration).

The researchers found that, for biofuel produced from wood residue, the blue water footprint is minimal – 2.4 litres per litre of fuel produced – and only results from the conversion process (which gasifies the feedstock mixture and synthesizes it into a blend of fuel alcohols), without irrigation inputs. Wood's nitrogen grey water footprint is also low, 25 litres per litre of fuel produced, on average, the lowest among existing biofuel feedstocks. Green water (rainfall) is the dominant water source input, at 400–443 litres per litre.

Led by May Wu, the Argonne team proposes an analysis method tailored to mixed-materials wood feedstock. The team analyzed available data to determine water footprints, identifying regional forest type (including hardwood, softwood, and short-rotation woody crops), water requirements for growth, and climate, and considered the breakdown of feedstock to (a) wood residue, (b) thinning, (c) round wood, and (d) short-rotation woody crops.

Tree species, and by extension, feedstocks, grown in various regions depend on climate and soil conditions, which lead to variations in the water footprint. "Our analysis shows that the feedstock mix plays a key role in determining the magnitude and spatial distribution of the water footprint in forest regions," said Wu.

This work was supported by the US Department of Energy and is part of a multi-institution collaboration involving Argonne, Oak Ridge National Laboratory, Idaho National Laboratory, and the National Renewable Energy Laboratory. For more information about biofuel water footprints, visit http://water.es.anl.gov.

The scientists reported their work in Environmental Research Letters as part of the ERL Focus on Second Generation Biofuels and Sustainability.

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