Calculation of the Carbon Footprint of United Utilities’ Sludge Balanced Asset Programme

Barber, B., United Utilities PLC


United Utilities presently produces 202,300 tonnes raw sludge dry solids per year and this is
expected to increase to 243,000 tonnes by 2015 (based on current estimates) due to a
combination of population growth and introduction of stricter water quality legislation. United
Utilities has an incinerator capable of burning 20% of this sludge after digestion. The
remaining sludge goes to land in a ratio of approximately 60% digested sludge to 40% limed.
However, heavy reliance on land application tied to further potential restrictions, such as:
public perception, NVZs, phosphorous limitations and revisions to the proposed EU directive
had originally led United Utilities to devise a strategy to reduce reliance on land and exposure
to risk based on installation of further incineration capacity of 52,000 tonnes of sludge. Since
the original strategy, the situation has changed again with energy consumption, nutrient
recycling, carbon and sustainability playing a far greater role than previously envisaged. This
paper describes how evolving drivers resulted in a change in solution from a proposed raw
sludge incineration plant to a balanced solution involving advanced digestion. It specifically
presents results from a carbon footprint calculation model designed to determine carbon
footprints for existing (application of limed sludge to land), raw incineration and advanced
digestion scenarios. This is the first project within United Utilities to undertake carbon footprint
modelling and the work helped the project team to develop a more sustainable solution which
will enable United Utilities to reduce its carbon footprint by 32,000 tonnes CO2e/year which is
8% of its existing carbon footprint.

Advanced digestion, carbon footprint, incineration, sludge strategy, sustainability.

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