World First: Autoclaving for ADvanced Digestion

The environmental impact assessment for the prepared as part of the planning application considered a number of aspects, including the site's visual impact on the landscape A sustainable waste treatment facility near Plymouth in the south west of England is set to become the first in the world to apply full scale autoclave processing to wet advanced anaerobic digestion. by Stephen Barnes Sited on a former china clay refinery owned by mineral extraction and processing giant, Imerys Minerals, the £15 million Lee Moor advanced Anaerobic Digestion (AD) facility is set to become the first of its type in the world. The autoclave processing technology being installed upstream of the AD plant will employ temperatures of 160°C and high pressures to break down lignin and cellulose structures within paper, packaging, cardboard and woody plant wastes, making them suitable for digestion. The facility is being developed by AAD (South West) – part of the Dorset based AeroThermal Group of small specialist companies which provide turnkey engineering solutions. The firm has also developed the autoclaving technology being used on the project. Handling up to 75,000 tonnes of municipal, commercial and industrial waste per year, the state-of-the-art technology at the facility is expected to make the AD process up to four times faster than conventional systems. Biogas production rates are also expected to be significantly enhanced, enabling the facility to generate up to 3.2 MW of renewable electricity and to export around 26,000 MWh per year to the national grid. Up to 20,000 tonnes of CO2 emissions will be saved annually, while nutrient-rich digestate will also be produced for use as a soil conditioner in restoration works at a china clay works on the same site. One of the big advantages of the autoclave technology being used at the Lee Moor site is that totally unsorted municipal 'black bag' wastes and high organic fraction supermarket and kitchen food waste can be processed simultaneously at the site, at a faster rate than normal and with more renewable energy being produced from a given amount of waste. In addition, materials removed at the screening stage after autoclaving, such as metals, plastics, glass and textiles, will have been effectively cleaned and sterilised - improving the quality of the recyclates. Model planning For the first decade of its operation digestate from the plant is likely to be delivered to the Imerys Minerals' controlled sites in the locality for restoration of their china clay workings The facility will incorporate a high level of site controls, process technology and abatement equipment to ensure that both the public and the environment are protected. These played a key part in the planning application and supporting environmental impact assessment which was prepared by engineering and environmental consultancy Wardell Armstrong, and helped to secure planning permission from Devon County Council in late 2011. Areas assessed in detail included land use and soils, hydrology and hydrogeology, traffic and access, air quality, noise, ecology and wildlife, landscape and visual impacts, and socio-economic effects. The consultancy also prepared the environmental permit application which is currently being determined by the Environment Agency. Detailed air dispersion modelling was carried out to assess any potential impacts from the site on residential dwellings, local businesses, surrounding land and nature sites. Using the latest version of AERMOD, a quantitative model based on the Gaussian theory of plume dispersion, the methodology took in a range of input data including the characteristics of the release (rate, temperature, velocity, height, location), the terrain, meteorological data and the locations of buildings and tanks adjacent to the proposed emission points. It then predicted the concentration of substances in the air, as well as the long term mean and short term peak ground level concentrations over the modelled area. Gases such as oxides of nitrogen, sulphur dioxide, carbon monoxide, particulates, ammonia and odour were included. The modelling was reviewed in detail by the local authority, the Environment Agency and Natural England - an executive non-departmental public body responsible to the secretary of state for environment, food and rural affairs. The model showed that emissions will easily meet all statutory air quality standards, and that the projected deposition rates on nature sites of less than 1% of target levels will have no significant impact on the environment. The noise assessment included continuous surveys over four days to assess the existing background noise levels in the vicinity of the site, both during the day and at night time. Computer modelling using SoundPLAN was then undertaken to predict the noise levels at local residential dwellings which would likely to be generated by the operational activities (including traffic movements) associated with the new facility. The modelling demonstrated that the noise levels likely to be generated by the site during the daytime, night time, weekdays and weekends are less than background. Noise levels from the site at all residential dwellings will be very low and less than 35dB(A). Further to this, the water quality assessment studied the impact of potential water discharges from the site on the nearby watercourse of Wotter Brook. Water is used in the AD process, but will largely be treated and reused. As any excess may need to be discharged to the Wotter Brook, it will be treated in an advanced dissolved air flotation and biomembrane system to produce clean, high quality water which is suitable for discharge to this surface watercourse. Extensive air dispersion modelling, noise calculations, water quality assessments and risk assessments have all combined to ensure that the highest standards of waste management technology and strict controls will be used to ensure that there is no harm to the public or the environment. Unlocking potential AAD (South West) is using autoclaving technology developed by its parent company, AeroThermal Group at the facility The Lee Moor facility will be made up of two autoclave plants, screening and separation equipment, anaerobic digestion plant with associated buffer and digestate storage tanks, dewatering plant and a combined heat and power (CHP) plant. The two autoclaves will operate in parallel, each treating mixed municipal wastes in ten tonne batches at temperatures of approximately 160°C for 45 minutes at a pressure of seven bar. After being autoclaved, the waste will then be conveyed to screening equipment to separate the organic and inorganic fractions. Metals and other inorganics will be removed for recycling, while the organic fraction will be transferred to the AD plant for biogas and digestate production. The biogas will be combusted in the CHP plant to produce up to 3.2 MW of renewable electricity and 3.8 MW of heat. The electricity will be exported to the national grid, while the heat will be passed to a boiler to raise steam for use in the autoclaves and to provide heat for the AD tanks. The technology enables the steam to be recycled between the two autoclaves, significantly reducing the amount of energy needed by the system. The digestate will be dewatered in a centrifuge plant to approximately 25% dry solids. It will then be used for a restoration scheme at the Lee Moor china clay pits which are located nearby. This will reduce the need to import restoration materials from other facilities which might be located significant distances away, further reducing costs and environmental impact. As a stabilised and sanitised organic rich soil conditioner and fertiliser, the digestate will contain nutrients such as nitrogen, potassium and phosphorus that are essential for plant growth. A large proportion of these nutrients will be held in organic form and released slowly over a number of years as the material breaks down. This will allow for better synchronisation with the demands of plants than is normally possible with inorganic nutrient fertilisers. Applying the digestate will also improve soil structure and water retention capacity, encouraging the growth of grassland and other plants used in the restoration scheme. The autoclaves and screening equipment will be located in enclosed buildings, fitted with fast action doors and surrounding air curtains to prevent any fugitive emissions of odour. Air within the buildings will be drawn by extraction fans through sealed pipes to odour control equipment consisting of a high tech scrubber and a biofilter. The air will be cleaned and discharged to atmosphere via a dedicated stack. Waste will be tipped, stored and treated entirely in enclosed buildings and tanks. Emissions from the odour control stacks and the CHP plant will meet strict emission standards set and regulated by the Environment Agency. Once fully operational in April 2013, the Lee Moor facility will divert waste from landfill, reduce greenhouse gas emissions, generate energy and produce a high quality soil improver. "It will unlock both the energy and fertilising potential of municipal solid waste," explains Tristan Lloyd-Baker, managing director of AAD (South West). "With enhanced recycling rates and increased renewable electricity production it will pioneer commercial scale autoclaving to AD or advanced AD and launch it into the 21st century. The local production of significant quantities of 'compost' will also significantly speed up the restoration of Dartmoor's china clay quarries." Stephen Barnes is an associate director at Wardell Armstrong Shanks to Use Autoclave at 230,000 TPA Wakefield Plant In a separate project Shanks Group has been awarded a contract to develop a residual waste treatment facility at South Kirkby, Yorkshire which will also use autoclave technology as part of an advanced anaerobic digestion system. The Milton Keynes, UK based waste and recycling company says that under the 25 year PFI contract the facility will process up to 230,000 tonnes per annum of municipal solid waste (MSW) from the Wakefield District, helping to increase the local authority's landfill diversion rate towards 90%. Technology According to Shanks a variety of materials will be segregated from incoming wastes for recycling, at the residual waste treatment facility. It will also produce a refuse derived fuel (RDF) for processing at a multi-fuel plant being built at the Ferrybridge Power Station. A separate onsite materials recycling facility (MRF) will sort the clean mixed recyclate materials for use by specialist markets. The remaining organic waste will be treated using a state-of-the-art autoclave, which the company says has already been built and rigorously tested. The autoclave process will sterilise the remaining material before it is fed into a 65,000 tonnes per annum anaerobic digestion (AD) plant, where it will be converted into biogas for renewable energy generation. The energy generated will be used both to power the plant and for export to the grid. The company estimates that it will generate sufficient energy to power approximately 3000 homes. According to Shanks the autoclave technology has already been built and extensively tested and will sterilise material prior to being fed into the anaerobic digestion system For the design of the AD plant, Shanks has awarded a £10 million contract to a joint venture between equipment manufacturer Ros Roca and technical services provider, Imtech (RRIJV). According to RRIJV it will complete the initial project design by March 2013, while the civil work will be undertaken by construction and civil engineering firm, Kier Infrastructure and Overseas. The residue from the AD process will be used as a nutrient-rich soil conditioner. Leeds based 4R Recycling, which specialises in the recycling of by-products from industry and utility companies for use on agricultural land and restoration sites has signed a 25 year contract with Shanks to manage this residue. This contract will see 4R handle up to 40,000 tonnes of digestate annually when the facility becomes operational. The digestate will be transported by 4R and used in land restoration across Wakefield and throughout the former Yorkshire and Nottinghamshire coalfields. Green waste delivered to the residual waste treatment facility will be processed via an enclosed air controlled composting plant. Funding The residual waste treatment facility has been funded by the UK Green Investment Bank (GIB) and a group of international banks including the UK's Barclays, Germany's BayernLB and Japan's Sumitomo Mitsui Banking Corporation. The GIB says that it will provide up to £30.4 million of senior debt to the project on commercial terms alongside the banking syndicate, which together are providing a total of £121.7 million. The contract will see approximately 250 people recruited to work on the construction of the new facilities and a further 60 permanent positions created, making a significant contribution to the local economy. "The UK Green Investment Bank is committed to reducing the amount of waste which goes to landfill, supporting the UK in its transition to a low carbon economy, whilst driving a commercial return for the bank," comments Lord Smith of Kelvin, chair of the UK Green Investment Bank. "Each year the UK generates approximately 190 million tonnes of waste, which causes environmental damage and costs businesses and consumers money," he added. A long time coming According to Peel Hunt, an independent full-service broking and advisory house with an exclusive focus on UK mid and small caps, the contract has been a long time coming. The broker explains that VT Group had previously been named as preferred bidder back in November 2007 when financial close had been hoped for summer 2008. It was subsequently postponed until late 2010. In October 2011 an agreement was signed for a contract completion date in February 2012. However, the VT Group was acquired by Babcock Group plc in July 2010 and Babcock became the lead partner. According to Wakefield Council, following a review of the former VT Group business, Babcock strengthened its preferred bidder consortium with the appointment of Shanks Group in February 2011 - with Shanks taking the lead role. "It has been a long journey and at times challenging, but throughout the negotiations we have never lost sight of the need for additional finance to deliver a waste management system for the future," explains Joanne Roney OBE, chief executive of Wakefield Council. "This is a big step forward in how a key public service is delivered in the District. The agreement means investment in household waste collection and recycling and more jobs for the District," she adds. More Waste Management World Articles Waste Management World Issue Archives