Scotland Plans Its Largest Ever Anaerobic Digestor

With UK landfill taxes set to rise in April 2011, as the emphasis grows on recycling and recovery, while new incentives encourage renewable energy from waste, and innovative new technologies come on stream, the opportunities for small and medium-sized contractors have never been greater.

Nowhere is this truer than in the area of commercial and industrial biomass wastes, where the volumes being generated in the UK are four to five times higher than municipal solid waste. And nowhere are the commercial and environmental pressures greater than in Scotland, where the recycling targets are the highest in Europe. 

So the recent news that planning permission has been given for Scotland’s largest ever anaerobic digestion facility, capable of handling around 105,000 tonnes of biomass a year, should be a real inspiration for smaller contractors looking to diversify into new resource streams and to take advantage of a whole new market.

Led by Banks Developments in partnership with recycling firm Scotwaste, the anaerobic digestion (AD) plant will be built as part of a £70 million (US$111 million) facility at Pond Green Energy Park near Bathgate, in between Scotland’s two largest cities, Glasgow and Edinburgh.

The facility as a whole will have a design capacity of 200,000 tonnes/year. Although the waste input will mainly be commercial, the site is well suited to serve the West Lothian municipal waste in addition to existing Scotwaste inputs. It will also generate enough electricity to power more than 7000 homes and enough heat to serve the equivalent of 9000 homes.

The location is well placed to serve densely populated areas that generate large volumes of organic waste. The site is jointly owned by Scotwaste and Banks Developments.

However, AD is relatively new in the UK — although there are around 30 plants at various stages of development, only three are operational and of a similar scale to the proposed Pond site.

An operational AD plant in the UK (Credit: Wardell Armstrong)

AD: Design for Life

Wardell Armstrong, an engineering and environmental consultancy, was called in by Banks Developments to provide early input on energy from waste, to advise on the various technological options for anaerobic digestion, and to prepare a financial model for the proposed facility. The firm was also briefed to carry out the design of the facility.

The anaerobic digestion system at Pond will use a two stage process in order to maximise control of the bacterial communities. Before reaching one of the three large anaerobic digestion tanks the slurry is treated in the buffer and hydrolysis tanks which act as a buffer between the ‘raw’ slurry and the digestion tanks in two ways. Firstly, they allow for the quality of waste passing through the digestion tanks to be accurately controlled to ensure the most efficient treatment of the slurry. Secondly, they have the effect of homogenising the chemical attributes of the incoming feedstock providing a natural balance for PH and other variables.

The prepared slurry is then pressurised, by retention for a minimum of one hour at 70°C, in order to immobilise pathogens and active seeds. Through the use of air coolers the temperature of the slurry is then reduced to 37°C to meet the requirements of the digesters. Once the correct temperature has been achieved the slurry is pumped into the digestion/fermentation tanks for anaerobic digestion. In order to prevent heavier sludge from settling and to maintain consistency, the digester is also equipped with a slowly rotating agitator.

The addition of fresh slurry into the bio-reactor and the simultaneous draining of the fermented liquid is a continuous process. Once in the reactor the slurry remains there for approximately 15—20 days. This is how long it takes to form a stable post-formation digestate material. Once the organic fraction has worked its way through the digestion tanks it passes into the strip tank to undergo aeration which effectively terminates the anaerobic digestion process and prevents the formation of biogas in the following stages.

The generated gas is transported into a low pressure gas storage facility via filters suitable for drying and purification — preventing them from being released to atmosphere and holding them ready for use in powering a combined heat and power plant on the site.

The remaining, non-digestible material which the bacteria cannot feed upon, along with any dead bacterial remains constitutes the solid digestate.

This digestate is dewatered and then heat dried to produce a stabilised odourless organic material.

Air quality assessment is also essential to ensure that emissions from the stack would be suitably dispersed.

‘This development drives us firmly forward into the twenty-first century,’ said Scotwaste director Stewart Melrose. ‘As a local business and employer we’ve built up a good customer base and a good reputation, but we wanted to go much further by using new technology to convert the waste streams in the area into a sustainable resource.’

Colin Anderson, managing director of Banks Property Development, said: ‘We hope to start construction during 2011, with the facility coming on line in 2012.’

Daniel Leaver is the senior waste & resource manager at Wardell Armstrong


Sidebar: MORE UK ENERGY FROM BIO WASTE DEVELOPMENTS

Sterecycle, the waste treatment and renewable power company, has recently been awarded planning permission by Essex County Council to develop a waste recycling and biomass Combined Heat and Power facility at Harlow in Essex.

The development will treat up to 240,000 tonnes per annum of municipal household and commercial waste from the local area.

Sterecycle Harlow will be developed to house the biomass CHP plant which will be used to generate electricity for export to the grid. The company is also expanding its facility in Rotherham, South Yorkshire in response to increasing demand from local authorities and commercial customers, it says. This development has planning permission to double the current capacity of 100,000 tonnes annually and construction is expected to be complete in mid-2011.

Meanwhile, one of the UK’s largest wood recyclers Growing Beds Recycling Services is to build a 2.6 MW biomass-fired power station, situated in a disused 1960s Ministry of Defence research and development building in Thurleigh, Bedfordshire. This project is also due for completion during summer 2011.

Under the terms of a joint venture agreement with waste to energy company Bioflame, specialist renewable energy investors the Ventus Venture Capital Trust Funds and B & W Waste Management Services, Growing Beds will divert 30,000 tonnes of low-grade wood waste as feedstock for Twinwoods Heat and Power Limited’s (THPL) system.

THPL has signed a contract with a supermarket giant, which will purchase the energy output under the terms of a long term contract.

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