Wales has the power
Dŵr Cymru Welsh Water has delivered advanced digestion plants early and is moving towards energy self-sufficient wastewater treatment, says Barry Oliver, technical director at Imtech Process
DCWW's sludge strategy for AMP5 moves away from energy intensive thermal drying and lime stabilisation to AD, with a programme to process 75% of its sludge production across four key sludge treatment centres. It has already achieved power self-sufficient service at Eign in Hereford, and is in the final commissioning stages with two schemes in South Wales, at Cardiff and Afan, where renewable power generation is anticipated to be 6.5MW continuous. The core of this strategy is the development and delivery of AD plants at both Cardiff and Afan. These sites alone will process 50,000 tDS/y using Cambi thermal hydrolysis (TH) plants and new digesters. This sustainable approach to sludge treatment has been encouraged by Welsh Assembly Government, regulators, local planning departments and local communities.
Design and construction
Full construction and delivery of the Cardiff and Afan schemes was required by April 2011 to maximise the ROC benefit and minimise long term costs to customers. Imtech Process was selected for the delivery of the process and M&E engineering work, working collaboratively with Morgan Sindall the civil design and construction partner.
A full technical and commercial assessment of available technologies was undertaken with DCWW. The most economic solution for these applications was Cambi TH due to the very high levels of secondary sludges. An integrated design team was used for both the Cardiff and Afan projects working closely with key suppliers and operations and standardising design wherever possible. The project team was co-located on-site at the earliest opportunity. Detailed design activity schedules were developed, integrated with procurement activities and programmed to ensure timely delivery.
A challenging design and delivery programme was developed with the key target of completing both projects six months early. This was reinforced through active management including a proactive approach to risk management developing risk mitigation plans wherever required and fully resourcing the project to allow effective expediting of key technology packages during off-site manufacture.
This included factory construction, pre-assembly and testing of both Cambi plants before disassembly and transport to site, allowing mechanical installation to be completed within one week of delivery. The key technology suppliers were incentivised to achieve the accelerated project programme with a focus on right first time. There was a continued challenge of best practice, time, cost and risk management, with the primary focus on safe delivery.
The commissioning strategy for the Cardiff and Afan AD plants benefitted from the lessons learnt at other full-scale plants. The steam boiler plant was initially proved using natural gas, allowing the plant to be tested and proved using water.
Similarly, high efficiency combined heat & power (CHP) units were initially operated using natural gas in order to prove the overall system. Special tests were planned to allow early commissioning of high risk areas such as sludge cake handling, dilution and pumping plant, including the proving of innovative, new instrumentation and control facilities.
A particular risk associated with similar plants has been process start up. Issues have included severe foaming of the digesters on start up and delays to allow process acclimatisation and the onset of reliable gas production. In order to minimise this risk, a series of bench-scale tests were carried out in order to identify the optimum seeding plan and start up rate. The final agreed digester start up plan was to transport digested sludge cake from the Cambi advanced digestion plant at Cotton Valley, Milton Keynes down to Cardiff, dilute with water and add sodium carbonate to increase alkalinity. Start-up of the digesters proceeded, as planned, right first time and without incident at both Cardiff and Afan.
CHP units were quickly operated on biogas and full Renewables Obligation Certificate (ROC) accreditation was achieved within the first month of process commissioning. The ramp-up rate was particularly impressive, allowing the first drier stream to be taken out of service within two weeks of starting the Cambi plant and the complete drier installation to be taken out of service within one month.
During the development and detailed design of the Cardiff and Afan AD plant, the operational and maintenance (O&M) costs of the existing sludge treatment plants were monitored. The O&M costs of the new treatment facility were reviewed and agreed with the local operational team, supported by information from key technical suppliers and actual costs at other full-scale plants.
The agreed operational savings are approximately ¡Ì7M/year. Imtech Process will continue to support DCWW to optimise plant performance and maximise savings over the first two years of operation.
Carbon modelling of the existing thermal drying plant and the new AD plant has been undertaken, including fuel and power requirements and emissions associated with transport operations. The carbon benefits of the AD plant include significantly reduced natural gas usage, reduced power consumption and renewable power generation.
Overall, the operational carbon saving from AD at both Cardiff and Afan is 35,000 tonnes CO©ü equivalent, which represents an operational carbon saving for DCWW of 15%.
Power self-sufficient wastewater service has been achieved at other AD plants, including Kings Lynn, Great Billing and Eign. However, achieving power self -sufficient service at Cardiff is particularly challenging due to high power requirements to lift the sewage into the inlet works, for aeration of the SBRs and pump transfer of final effluent to sea.
Although the performance of the existing SBRs has been optimised to minimise the power required for aeration, the average power use is approximately 100MWh/d which exceeds the expected power output from the AD plant. Further optimisation of the overall works will continue in order to drive towards power self-sufficient service.
The strategic development and implementation of the AD programme has been an excellent example of purposeful collaborative working with the delivery of both the Cardiff and Afan projects safely, within a challenging time-scale, and right first time through learning from previous experience at other sites. Detailed design and delivery of the projects has progressed smoothly through DCWW's capital delivery partners - Imtech Process and Morgan Sindall. And, both the Cardiff and Afan Advanced Digestion projects have been designed, constructed, and commissioned within two years of formal award in January 2009. The projects have been delivered ahead of programme, within budget and will generate more than 4.5MW of renewable power.
They will reduce operating costs by over ¡Ì7M/y and reduce DCWW's operational carbon footprint by approximately 15%.