Case Study 14: The Creed Waste Management Facility, Isle of Lewis - Anaerobic digestion
Renewable device Anaerobic Digestion
Relevant section in Toolkit 4.4.2
Project Description and nature of group.
The Creed Anaerobic Digestor Plant on the Isle of Lewis is owned and run by Comhairle nan Eilean Siar (Western Isles Council), providing a integrated waste management facility for the islands chain. Work was awarded in early July 05 for the integrated facility based in the Creed Business Park on the outskirts of Stornoway. A further development in Uist at Market Stance was also advanced to provide Waste Transfer facilities for the Uists and Barra.
The two integrated waste management facilities act as the hub for the Council's municipal waste management service delivery, allowing it to not only to meet but exceed the challenging targets for recycling and landfill diversion that have been set by the Scottish Government. It also ensures that the Western Isles is able to play its part in helping Scotland meet the requirements of the European Landfill Directive.
At present the Integrated Waste Management Facility processes waste from the northern isles (Lewis & Harris) and the southern isles (the Uists, Benbecula & Barra) as follows:
- Organic waste from both the northern isles and the southern isles;
- Residual (or black bag) waste from the northern isles only. Residual waste from the southern isles is sent to landfill directly because of the limitations imposed by the in-vessel composter issues.
How it works
Residual waste is mechanically screened, firstly to separate out metals for recycling, but primarily to produce an organic-rich fraction. This is then treated in a series of HotRot composting vessels to produce a low-grade soil conditioner for use in landfill restoration. Both the AD and the In-Vessel composting system are fully compliant with the animal by-products regulations.
The main waste treatment facility was the first plant in the UK to incorporate anaerobic digestion of source-separated biowaste (food, paper and garden waste) on a commercial scale. The Linde dry-digestion technology lies at the heart of the process. The biogas produced is used to generate up to substantial electrical power annually for export to the local network, whilst the solid digestate is matured to produce high-quality compost for local use. The facility also houses the recycling of glass and baling of plastics and crushing of cans.
Additionally, there is an in-vessel aerobic composting process on site which reduces and stabilises some of the residual waste prior to being used as a lower grade soil improver product in landfill restoration. The plant currently employs twelve people directly and a number indirectly. It also hosts the CNES Waste Aware team.
The biogas powered CHP unit will provide electrical power and heat for the whole facility yielding a net surplus of electricity. A complementary Hydrogen Project, H2 SEED, will sit alongside this utilising the space capacity and provide the initial infrastructure for the Hydrogen developments. There are also possibly three wind turbines to be co-located at Creed, the power from which could also be utilised to generate Hydrogen. The biogas engine has been recently commissioned and the electrical energy produced is metered to benefit not only from unit sale of electricity but also the Renewable Obligation Certificates ( ROCs) value of the energy produced.
Performance data indicated that electrical energy produced by the bio-gas engine over the period November 07 to April 08 was 23,909kWh and the heat energy was 35,863kWh. Of this electrical energy 11,240kWh was exported to the grid and the remaining 12,669kWh was consumed internally. Over the same six month period the energy imported from the grid was 258,420kWh making the total energy consumed 271,089kWh.
In March 2005, following competitive tender, Earth Tech were awarded the contract by Comhairle nan Eilean Siar ( CnES) to design and build two integrated waste management facilities.
The commissioning of the Integrated Waste Management Facility began in October 2006 (approximately). As part of this process the biogas engine was commissioned on May 23 rd 2007. There is an ongoing programme of public education to promote source segregation of organic material and to maximise organic material recovery. As a consequence the anaerobic digester is not operating at its potential and certainly not at full (design) capacity as yet.
Issues and learning points
Since the facility opened, significant technical difficulties with the in-vessel composters have prevented their effective use. Indeed, since December 2007 this equipment has been taken out of commission pending resolution of the technical issues. During periods when the in-vessel composters have not been operational the residual waste stream has been diverted to landfill directly without undergoing processing.
The total quantity of waste handled at the Integrated Waste Management Facility is:
- Organic stream - 3281 tonnes between Jan-Dec 2007. 740 tonnes for quarter 1 of 2008.
- Residual stream - 6675 tonnes between Jan-Dec 2007. 1636 tonnes for quarter 1 of 2008. [Note: this does not include the southern isle residual waste.]
January - December 2007 - 1501 tonnes of the organic stream was rejected as oversize and sent to landfill. The remaining 1780 tonnes (a calculated quantity rather than a measured quantity) was input to the anaerobic digester.
January - March 2008 - 398 tonnes was rejected and landfilled. The remaining 342 tonnes was input to the anaerobic digester.
The anaerobic digester produced 616 tonnes of dry digestate between March and December 2007 - the amount of dry solids was too low for input to the dewatering plant in January & February 2007. In the first quarter of 2008, 221 tonnes were produced. The amount of liquid digestate is not recorded. At present, liquid digestate is sent to drain although small quantities are diverted, occasionally, as input to the anaerobic digester to improve the liquid content.
January - December 2007 - 2248 tonnes of processed material was rejected as oversize and sent to landfill together with 2693 tonnes of unprocessed material sent directly to landfill. Thus a total of 4941 tonnes of residual waste was landfilled. 1767 tonnes of processed material was input to the in-vessel composters.
Kenny John MacleodComhairle nan Eilean Siar
Isle of Lewiskjmacleod@cne-siar.gov.uk