Proceedings

Advanced Digestion at Cardiff and Afan; Dwr Cymru Welsh Water drive for lowest sustainable cost of sludge treatment and 15% reduction in Carbon Footprint

Oliver, B., Evans, B., Bowen, A., Evans, R., Dwr Cymru Welsh Water

(free)

Abstract: 
Dwr Cymru Welsh Water has developed an innovative sludge strategy for AMP5, moving away from energy intensive thermal drying and lime stabilization to Advanced Digestion with a programme to process 75% of its sludge production across four key sludge treatment centres.
The core of this strategy is the development and delivery of Advanced Digestion plants at both Cardiff and Afan. These sites will process 50,000 tDS/y using Cambi thermal hydrolysis plants and new concrete digesters, prior to belt press dewatering, storage and recycling enhanced sludge cake to agriculture. This sustainable approach to sludge treatment has been encouraged by Welsh Assembly Government, Regulators, local planning departments and local communities, with no objections to planning proposals.
Detailed design and delivery of the projects has progressed smoothly through DCWW’s capital delivery partners Imtech Process and Morgan Sindall. The projects have been delivered ahead of programme, within budget and will generate more than 4.5 MW of renewable power, reduce operating costs by over £7M/y and reduce DCWWs operational Carbon footprint by approximately 15%.
Design and delivery experience together with early commissioning experience is presented in this paper. The next goal is power self sufficient wastewater service at Cardiff.
Keywords: Advanced Digestion, Cambi Thermal Hydrolysis, High Efficiency CHP, enhanced sludge quality, power self sufficient operation, operational savings, and sustainability
Introduction: 
DCWW sewage sludge treatment and recycling management plans have been progressively developed over previous AMP periods to ensure sewage sludge is effectively treated and recycled to agriculture with the original main emphasis being consistent outlet.
With the potential onset of new legislation early in AMP3 aimed at tightening controls on the agricultural recycling, the sludge strategy at that time focused on rationalising the number of sludge treatment centres and establishing effective pathogen reductions processes at strategic locations throughout Wales. It was confirmed that agricultural recycling was the most sustainable outlet and therefore sewage sludge was treated to meet the standards agreed in the Safe Sludge Matrix. The following sludge treatment processes were then established as part of the sludge management plan.
• Mesophilic Anaerobic Digestion (MAD) followed by batch storage achieving the ‘conventionally’ treated standard.
• Lime Stabilisation by either hydrated lime or quick lime dosing achieving an ‘enhanced’ treated standard.
• Thermal drying of sewage sludge at high temperatures to produce an ‘enhanced’ treated product with a very high Dry Solids (DS) content.
The AMP4 sludge management plan maintained the serviceability of the established sludge treatment assets however in the early years of AMP4 the following factors highlighted the need for a more significant strategic change to the sludge management plan for future years.
• Likely significant increases in energy prices
• Climate change and the need to reduce greenhouse gas emissions
• Reductions in sludge volumes for transport and recycling
• Opportunities for the production of ‘Green Energy’
• Higher than frontier sludge treatment and recycling costs.
Notwithstanding the need for longer term consistent outlet routes, the opportunities to recover energy from sewage sludge by maximising the generation of green power was identified as a major opportunity for DCWW to invest in sustainable sludge treatment processes. This would address all the above factors and importantly significantly reduce operating costs and carbon emissions for sewage sludge processing and recycling.
The benefits of this approach were demonstrated by the comparison of current and projected DCWW operating costs for sludge treatment and recycling. This was presented in a Roadmap format to show a ‘do nothing’ scenario compared with an investment in energy recovery processes, see chart below.

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