Delivering United Utilities’ sludge balanced asset programme
United Utilities (UU) currently produces 202,300 tonnes dry solids (TDS) of raw sludge per year and this is estimated to increase to 243,000 tonnes by 2015 due to a combination of population growth and the introduction of stricter water quality legislation. UU has an incineration facility capable of burning 30% of this sludge (after digestion) at the Mersey Valley Processing Centre (MVPC) in Widnes. The remaining sludge is recycled to land in a ratio of approximately 60% digested sludge to 40% limed. This reliance on land as a significant proportion of the disposal strategy presents many risks to UU. The landbank available to receive sludge is decreasing under pressures such as changes in farming practices, public perception, and reduction in brown-field reclamation. There will become a point at which sludge production will equal the landbank availability. This has necessitated UU to revise their strategy to reduce their reliance on land.
In 2002, UU formulated a strategy reducing its reliance on land application from around 70% to circa 35% by the installation of additional incineration. The strategy was to be achieved by upgrading the existing capacity at the MVPC and building a new incineration plant in Lancashire to receive limed cake fromsurrounding wastewater treatment works (WwTW). These projects were supported by the economic regulator OFWAT. The MVPC extension project was successfully delivered in 2010.
A project team made up of experts within UU with support from partners MWH was set-up to design and deliver the Lancashire Processing Centre (LPC) to burn 52,000 TDSA of raw sludge. However, concerns grew within the team regarding the sustainability of a new incineration plant. Therefore workshops looking at many potential solutions were arranged and three options, including incineration, were identified. These were investigated in more detail by the project team with a focus on capital and operating costs, environmental impact, energy balance and carbon footprint.
Carbon calculation model
A highlight of the evaluations was the development of a pioneering carbon calculation model to provide a differentiator in the decision making process. This was the first time this had been done within UU and has subsequently become a valuable carbon assessment tool, utilised as “business as usual” on all capital projects within UU’s AMP5 programm.
The model was externally verified by The Scottish Institute of Sustainable Technology (SISTech), which stated the work “is an excellent example of a tool designed to provide an in-depth assessment of options for responding to a specific problem.” Results from the model, along with the other decision making parameters, demonstrated that, based on the knowledge and constraints at the time, it was very difficult to justify the raw sludge incineration project.
SBAP – a new solution
In September 2007 an alternative solution, which was more environmentally beneficial than the LPC, was approved by UU at board level. It was entitled Sludge Balanced Asset Programme (SBAP) to reflect the innovative approach which maximises the use of existing distributed assets by considering them as a holistic system.
SBAP involves the construction of an advanced sludge treatment plant at Davyhulme WwTW, Manchester, to treat both on-site sludges and imported sludges from 7 (No.) WwTWs predominantly based in Lancashire. The plant will reduce quantities for disposal and produce an enhanced quality, pathogen free product. The product will be suitable for recycling to grassland, arable land and cultivation, and the increased sludge quality will improve take up from farmers and growers.
This approach required a thermal hydrolysis process (THP), upstream of the existing digesters, which will improve sludge characteristics allowing increasing volumes to be pumped to the MVPC. This will give greater flexibility in dealing with increased sludge volumes. The SBAP sludge will have better burning characteristics, reducing the fossil-fuel requirement at the MVPC. The pre-treatment process also improves the digestibility of the sludge such that the amount of biogas produced is increased.
Utilising this biogas increases the amount of renewable electricity generated at the WwTW enabling the entire works to be self- sufficient thereby significantly reducing operating costs and ultimately benefiting customers. Surplus power can be exported to the grid. Planning permission has been granted for the proposed development and tender documents were issued in 2009 for a competitively tendered lump sum performance based specification. In November 2009, Black & Veatch were appointed as the main contractor to deliver the works.
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