CHESHIRE, UK, Oct. 23, 2007 -- Earth Tech, in conjunction with its technology partners Linde and HotRot has developed an integrated waste treatment facility, valued at £9.8 million for the Western Isles Council off the north-west coast of Scotland. The hybrid waste treatment facility is unlike any other currently operating in the UK.
The Western Isles -- A Unique Environment
The Western Isles is a truly unique environment. Whilst around one half of the islands' 28,000 population is concentrated in and around Stornoway on the Isle of Lewis, the remainder are spread over 3000 sq km of rugged and windswept terrain, on a group of islands stretching over 230 km from end to end. A journey from the top of Lewis to the southern tip of Barra involves a full day's drive along often single-track roads and includes two ferry crossings. The mainland of Scotland is 55 km away at its closest point.
In part due to the above, the Western Isles is equally unique in the way it has to manage its waste. Whilst the EU Landfill Directive and the Scottish recycling targets apply equally here as they do throughout the rest of Scotland, the logistics of waste collection, recycling, treatment and disposal do not.
Furthermore, being remote from the mainland, there is little potential for joint working with neighboring waste authorities. The Western Isles therefore had to find a solution that allowed it to be almost totally self-dependent in the way that it treated and disposed of its waste.
The Council decided that it needed to develop new waste management infrastructure to help it meet its waste management needs for the next 20 years and beyond. It also needed to satisfy the shorter-term recycling and landfill-diversion targets during the early years whilst the new collection systems were being developed and optimized. Innovation and flexibility in the way the new waste facility would be able to operate, was therefore crucial to the overall success of the project. Earth Tech, as the main contractor, in partnership with the Western Isles Council, as end client, worked jointly and with innovation to develop a solution by adopting a combination and configuration of process technologies never previously used in the UK.
Aims/Objectives
The key challenges that had to be met in the development of the project were:
• Long collection routes around the Isles meant that source-segregated collection was impractical for many households. The ability to accept and achieve recycling and/or recovery from both sorted and unsorted waste streams from within a single treatment facility was therefore essential to the success of the project.
• Lack of any on-island reprocessing capacity for recyclates, together with the high cost of exporting materials to the mainland meant the new facility needed to treat as many components of the waste stream as possible. Off-island reprocessing of recyclables is not viable for most materials due to the high cost of transport to the mainland. In particular, no on-island outlets exist for recycled paper.
• The island's source-separated collection system was expected to take a number of years to achieve maximum level of capture, therefore in addition to the long-term optimisation, the plant also needed to have the ability to maximize recovery and recycling during this "interim" phase.
• As the Western Isles relies very heavily on tourism, and in turn its positive environmental credentials, a non-incineration form of waste treatment was preferred. Accordingly, the client identified anaerobic digestion as a likely alternative.
• Aesthetic design of the new facility had to fit into a rural and largely non-industrial landscape.
• The Council had a limited financial budget for the scheme, and meeting this was also a challenge.
• In addition to all of the above, the fundamental driver was to satisfactorily meet the landfill-diversion targets allocated by the Scottish Executive, as part of its overall requirement to meet the EU Landfill Directive.
• Finally, the scheme had to assure compliance with the national recycling targets set throughout Scotland.
Method
Following competitive tender, Earth Tech UK was awarded the contract to design and build a solution for the Western Isles Council. Working with the client and their technical advisers, the project team identified all the key objectives and drivers for the success of the project, and worked in a true partnership to develop the overall most effective but affordable solution. In essence, a one-off response to a one-off problem. Earth Tech, using a unique combination of proprietary process technologies, designed a hybrid waste treatment facility that could operate in two distinct modes in order to meet both the short and long term needs.
Long-term Operation
Once the segregated biowaste collections becomes established and is achieving full capture, the source-separated biowaste (kitchen waste, paper/card and garden waste) would be anaerobically-digested to produce "green" electricity as well as a high-grade compost. Independently, the organic fraction from the residual waste is now aerobically composted in enclosed vessels to form a low-grade soil conditioner suitable for landfill restoration works. Both processes are fed via a common front-end reception and pre-treatment stage, which will feed the AD for 2 hours per day, and the IVC for the remaining 6 hours of the working day.
(see Table)
Interim Period
In the early period of plant operation, during which time the amounts of segregated biowaste collected would feasibly be much lower, the plant has the flexibility to operate in a fundamentally different mode. If so required, the organic fraction screened from the residual waste, together with the lower amounts of segregated biowaste, can be co-mingled and anaerobically-digested as a single feedstock. Although this would sacrifice the opportunity to produce a high-grade compost (since some of the organics come from residual waste and are therefore considered "contaminated") this approach does nevertheless maximize the production of biogas and as a consequence, renewable electricity. Furthermore, because the in-vessel composting units are otherwise redundant in this mode, they can now be utilized to compost the digestate products from the AD stage, to produce a low-grade soil conditioner suitable for landfill restoration.
In its ultimate operating configuration, the plant will treat about 20,000 tons per annum of biowaste and residual waste, delivered separately. As well as the extraction of metals, the generation of renewable electricity, and the production of high and low grade compost materials, the plant will consistently achieve an overall diversion from landfill of over 60%.
Furthermore, and whilst not part of the core scope of supply, the plant has been designed in such a way as to be able in future to receive and digest fish waste, one of the largest commercial waste streams on the Western Isles, and one which currently is being exported off the islands for treatment and ultimate disposal. The design of the new plant has incorporated this additional capability and extra capacity for potential incorporation into the facility in the future, should economics allow.
Project Outcomes
This is the first commercial-scale waste treatment facility in the UK to generate renewable electricity by the dry anaerobic digestion of source-separated kitchen and other organic waste. In so doing, the plant is not only totally energy self-sufficient, but generates an excess available for export to grid.
Whilst it has been designed predominantly for the long-term, the plant incorporates the flexibility to operate in a completely different configuration in the early years until source-separated collections achieve full capture. By maximizing levels of recycling, recovery and landfill diversion during these interim years, the facility allows the Western Isles to meet all of its environmental targets.
In seeking to ensure sustainability in the way the Isles manages its waste in the future, consideration has been given within the design of the plant layout. The plant is therefore able to accept other "difficult" commercial biowastes (e.g., fish waste) generated on the islands that are currently being exported off-island for treatment or disposal.
The introduction of a brand new and imposing waste treatment facility into the very rural landscape of the Western Isles was always going to be a challenge. Careful attention to detail in the form of the building and architectural aspect was incorporated into the overall design of the plant at a very early stage. On completion of the external works, the level of success achieved in blending the building form into the surroundings is there for all to see. Testimony to this is the complete absence of any adverse public reaction to the new plant, something that is almost unheard of for this type of development. Indeed, as waste is now beginning to be received at the facility (which includes a brand new HWRC for the public to bring their bulky and/or hazardous wastes), the islanders are now able to see the environmental benefits of the scheme in which they themselves are a key component.
Blending into a Rural Landscape
The project completed in December 2006 and was developed to program and within budget. The collection system for source-separated organics is now being rolled out to residents throughout the Isles, who are responding well and are enthusiastic about the scheme as a whole. Participation in sorting waste at source is improving week-on-week.
The plant is receiving and treating waste. Residual waste is being processed, recyclables are being extracted, and a low-grade soil conditioner is being produced for landfill restoration. Inoculation of the digester commenced, and shortly after that, the biowaste fed into the system. As a result of that procedure, heat and electrical energy are now being produced to power the plant. At the end of the commissioning phase, the new facility now allows the Western Isles to fully meet its waste management and environmental needs, as follows:
• Exceeding landfill diversion targets to such an extent that the council will be able to generate significant revenues by trading its surplus allowances under the landfill allowance trading scheme (LATS);
• Meeting the national recycling targets set by the Scottish Executive;
• Satisfying shorter-term recycling and diversion targets during the early years of the new collection system;
• Being an overall net producer of electricity from renewable sources;
• Being a producer of both high-grade and low-grade compost materials for use locally on the Isles, thereby reducing total waste management costs;
• Recycling glass into decorative landscaping products and recycled aggregate;
• Recycling plastics and metals for reprocessing on the mainland;
• Overall, making a significant contribution to the environmental sustainability of the Isles.
Tony Lewis is business development manager at Earth Tech.
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