World first for full scale autoclaving with wet AD

Paul Boughton
A sustainable waste treatment facility in the UK is set to become the first in the world to apply full scale autoclave processing to wet advanced anaerobic digestion. Stephen Barnes reports.

Handling up to 75,000 tonnes of municipal and commercial/industrial waste per year, state-of-the-art technology at a waste treatment facility near Plymouth, Devon, UK, will make the anaerobic digestion (AD) process up to four times faster than conventional systems.

Biogas production rates will also be significantly enhanced, enabling the facility to generate up to 3.2MW of renewable electricity and to export around 26,000MWh 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. Engineering and environmental consultancy Wardell Armstrong played a key role in helping to secure planning permission, and worked with the operator to put safeguards. [Page Break]

Autoclave processing technology

The £15 million Lee Moor facility is being developed at the Imerys Minerals china clay works by AAD (South West) Limited, an Aero Thermal Group company. Their innovative autoclave processing technology upstream of the AD plant will employ high temperatures (160°C) and pressures to break down lignin and cellulose structures within paper, packaging, cardboard and woody plant wastes, making them suitable for anaerobic digestion.

This means 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. Materials removed after autoclaving and at the screening stage, such as metals, plastics, glass and textiles, are also effectively cleaned and sterilised, improving the quality of these recyclates.

The Lee Moor 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 prepared by Wardell Armstrong which helped to secure planning permission from Devon County Council on 30 November 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.

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, Environment Agency and Natural England. It showed that emissions will easily meet all statutory air quality standards, and that deposition rates on nature sites of less than 1 one per cent of target levels will have no significant impact on the environment. [Page Break]

Noise assesment

The noise assessment included continuous surveys over four days to assess the existing background day and night time noise levels in the vicinity of the site. Computer modelling using SoundPLAN was then undertaken to predict the noise levels at local residential dwellings 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).

The water quality assessment studied the impact of potential water discharges from the site on the nearby 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 a clean and high quality water that's 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.

The consultancy also prepared the environmental permit application which is currently being determined by the Environment Agency. [Page Break]

Unlocking potential

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 160degC 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 3.2MW of renewable electricity and 3.8MW 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 per cent dry solids. It will then be used in the restoration scheme for 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. [Page Break]

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 advanced anaerobic digestion facility will bring big benefits in terms of diverting waste from landfill, reducing greenhouse gas emissions, generating green energy and producing a high quality soil improver.

"It will unlock both the energy and fertilising potential of municipal solid waste," said Tristan Lloyd-Baker, managing director of AAD (South West) Ltd. "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."

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