The successful implementation and optimisation of advanced digestion for Dwr Cymru Welsh Water

Wilson, S.1, Oliver, B.2 and Merry, J.2, 1Dwr Cymru Welsh Water, 2Imtech, UK



Dwr Cymru Welsh Water’s strategy for AMP 5 was to install new CAMBI Advanced Digestion plants at Cardiff and Afan. These schemes complement an Enzymic Advanced Digestion plant at Eign and a new Anaerobic Digestion plant at Five Fords in Wrexham. This paper reviews the benefits of the strategy, experience in construction and the performance achieved over the first year of service. In summary, this strategy has helped Welsh Water half its costs for treatment and recycling of sludge to agriculture, operational savings are on track to exceed £7 million/year and an operational carbon saving of over 50,000 tonnes CO₂ has been achieved. Challenges discussed in this paper include dealing with variations in sludge production, unexpected volatile acid production, controlling digester foaming, optimising digested sludge dewatering and the impact of high strength return liquors on the existing wastewater treatment works. Industry leading performance standards have been achieved, particularly at Cardiff where the level of volatile solids destruction has consistently exceeded 50% processing mainly secondary sludge.

Key words:

Advanced Digestion, Cambi Thermal Hydrolysis, Carbon reduction, High Efficiency CHP, enhanced sludge quality, power self sufficient operation, operational savings, and sustainability,


In 2006 Dwr Cymru Welsh Water carried out a strategic review of its sludge treatment and recycling operations. Sludge treatment processes included thermal drying, lime stabilisation and mesophilic anaerobic digestion followed by batch storage. Although all treatment plants complied with the Safe Sludge Matrix, this study identified the need to address:

• 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.

This study also considered recent developments in Advanced Digestion whereby enzymic and thermal pre-treatment technologies had helped to maximise solids destruction, biogas production, and hence maximise renewable power generation and allow high quality enhanced sludge cake to be recycled to agriculture. Advanced Digestion presented opportunities to recover energy from sewage sludge by maximising the generation of green power and move to more sustainable sludge treatment processes. This would address all the above factors and 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 figure one below.

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