Abstract
Northumbrian Water’s latest Advanced Digestion plant at Howdon Waste water Treatment Works has now been completed and is in full operation, and generating more than 100 MWh/d of renewable power. This is a key part of NW’s strategy to process all sludge through Advanced Digestion in order to maximise renewable power generation and move towards power selfsufficient wastewater service.
The new Advanced Digestion plant has been successfully delivered by GTM (a joint venture between Galliford Try and Imtech). This plant builds on the improvements achieved with recent projects and lessons learned from installation of an earlier plant at Bran Sands.
The new plant will process up to 40,000 tDS/y of sewage sludge and includes raw sludge screening, centrifuge dewatering, cake reception, Cambi thermal hydrolysis, sludge cooling, three 7000m3 digesters, three high efficiency 2MWe CHP units, and three composite steam boilers. Enhanced quality treated sludge is dewatered and recycled to local farm land.
Process commissioning of the new plant commenced on time in May 2012, and process start-up progressed smoothly. The plant has now been operated reliably under full-load conditions and early results show higher than expected biogas production with a best recorded efficiency level of 1.2 MWh/tDS. Some innovative features have been incorporated in order maximise re-use of available heat and provide a fully integrated and optimised solution.
The team is now supporting further optimisation of the Cambi Advanced Digestion plants at both Howdon and Bran Sands.
Northumbrian Water (NW) are considering further improvements including biogas into grid and co-digestion of food waste to maximise renewable power generation. This programme puts NW at the forefront of renewable power generation and sustainable wastewater service.
Keywords Advanced Anaerobic Digestion, Thermal Hydrolysis, Operational experience, Carbon reduction, Fertiliser value of enhanced sludge quality, power self-sufficient operation.
Introduction
NW constructed the UK’s largest thermal sludge drying facility at Bran Sands in 1995, and for 10 years sludge treatment was focussed on sludge drying at Bran Sands, Middlesbrough, and dewatering with lime stabilisation at Howdon, Newcastle.
In 2006 a strategic review identified that the installation of Thermal Hydrolysis (TH) Advanced Anaerobic Digestion (AAD) at both of these sites was a much more sustainable solution that would generate multi-million savings in annual operating costs and significant environmental benefits including:
- 2,000,000m³ of liquid sludge reduced down to 150,000m³ of sludge cake,
- Sludge cake being a Class A biosolid,
- 50% reduction in solids requiring disposal,
- 10 MW of renewable power from biogas production,
- 20% of NW’s total electricity demand being met by renewable sources,
- All of NW sewage sludge being used to produce renewable electricity,
- The facilities include cake imports to reduce the impact of sludge transportation,
- 20% reduction in carbon footprint,
- Provides NW with a more sustainable strategic sludge management solution,
- Existing assets used where possible, and
- Bran Sands driers and the ability to lime stabilise to remain as a strategic contingency
This strategy was agreed and two guaranteed performance design and construct contracts (totalling over £60m) were awarded to:
- Aker Solutions (now Jacobs E&C) to install AAD at Bran Sands in 2007
- GTM (a joint venture between Galliford Try and Imtech Process) to install AAD at Howdon STW in 2010.
The plant at Bran Sands has been operating successfully since August 2009.
