UK: Thirty thousand tonnes of potato waste converted into renewable energy

Local Generation Ltd packs potato and vegetable products for retailers on three sites in Cambridgeshire and Lincolnshire. The existing Fenmarc packing plant washes, sorts, grades and packages potatoes. Established in the 1960s, the site has expanded to the point where around 100,000 tonnes are now processed every year. Some of these don’t reach the standard required for sale and are rejected. Until now, most of this waste has been sent off site for disposal. But Local Generation saw a golden opportunity to combine their potato waste with other food waste and use it in a much more sustainable way.

They started by talking to Monsal. Monsal in turn recommended Wardell Armstrong for all the planning advice, permitting and application aspects. Local Generation director Nick Waterman said: “We found in Wardell Armstrong a partner who totally shared our view of waste as a highly valuable resource to be managed in the best possible way. Their experience and track record gave us huge confidence that we’d be able to see the project through successfully. They also got us thinking in a different way and helped us to make well informed decisions.”

An early first step was to go through all the issues and options, and identify any potentially problem areas before any formal applications were made. As part of this process, Wardell Armstrong suggested organic rather than chemical treatment of the waste water. They also brought in transport consultants who could advise on increased vehicle movements and new access, and managed the production of drawings and transport assessments.

Pre-application discussions with the local authority and permitting discussions with the Environment Agency helped to reduce the overall time taken from a typical twelve months down to just five. Even though a full environmental impact assessment was not needed, all the screening and scoping aspects were approached in the spirit of an EIA in order to ensure that all the issues were fully addressed. This ensured a comprehensive application - from the design and layout of plant to air dispersion, odour and noise modelling, ecological surveys, and assessments of flood risk, drainage and contamination.

Another critical aspect led by Wardell Armstrong was that of public consultation. Events as well as individual appointments were held to talk local people through the process, explain the environmental safeguards, and ensure that they could freely give their views. These were taken into account in the design and layout of plant. After the planning application had been submitted a local exhibition was arranged with drawings, renders and technical information on display. Again these were modified based on feedback. “Involving people in this way made a huge difference to the support and buy-in,” said Nick Waterman.

At the heart of Local Generation’s planned new facility is anaerobic digestion – an entirely natural biological process which stabilises organic waste in the absence of air, and transforms it into biogas (60% methane and 40% CO2) and biofertiliser. It consists of four stages - hydrolysis, acidification, acetogenesis and methanogenesis. It replicates the process that takes place in a landfill site, but in a controlled way so that biogas and biofertiliser can be extracted and put to sustainable use. Although AD has been used in elsewhere in Europe, notably Germany for some years, and in the water treatment industry in the UK since the 70s, Local Generation are early adopters of the technology for dealing with food waste.

As the organic waste first comes into the AD plant it’s fed into a shredder or macerator which chops it up into small pieces. From here it passes into a hopper via a magnetic separator to remove any ferrous metals – and from this point the waste is totally enclosed. It is then reduced to a fine slurry consisting of particles no larger than 2mm.

Any inorganic waste such as plastic packaging is skimmed off and separated. Heavy contaminants such a stones or pieces of glass sink to the bottom of the tank and are removed by a screw press.

The fine slurry is then mixed with re-circulated water from the AD process and water recycled by the organic treatment plant to ensure the correct consistency. From here it’s transferred to a buffer tank for storage, and then pumped at a controlled rate through a heat exchanger to raise the temperature and pasteurize it. It’s then passed back through the heat exchanger to reduce the temperature before going into the digester.

It’s within this tank that the process of digestion takes place. The slurry is continually mixed to prevent heavier sludge from settling and to maintain consistency. During this time it’s broken down by micro organisms - naturally occurring bacteria that regard the waste material as food. They release biogas which is drawn off into a storage bubble. The slurry remains in the digester until the digestion process is completed, gradually passing out into a buffer tank where it’s aerated by having air pumped through it to halt the production of methane.

From here the slurry goes into a centrifuge within the main building. Water is spun off using the same principle as a spin dryer, and the dried “cake” of digestate which falls out is sold as an organic soil conditioner to large local potato suppliers who are already delivering potatoes daily. By backhauling this material - estimated at 12,000 tonnes per year - they can save on transport costs as well as putting organic material back into the degraded fenland soils and reduce reliance on chemical fertilizer.

To ensure effective odour control, the main AD building has a negative pressure ventilation system which continually draws air out to keep the building at slightly lower pressure than its surroundings. The exhaust air passes through a biofilter – in simple terms a big box of bark chips where bugs eat the contaminants in the air. Fast-acting roller shutter doors automatically open when a vehicle approaches, and close behind the vehicles.

The biogas mixture which is produced by the digestion process bubbles to the top of the tank and is drawn off. After going through a water separation process to ensure that it’s dry, it’s stored in a twin membrane gas storage bubble before being drawn into one of three combined heat and power (CHP) units, each of which will have an electricity production of 500Kw for a theoretical combined output of 1.5Mw. Some of the electricity will be used within the potato packing plant, while any surplus will be exported to the electricity grid. Annual output is expected to be just over 1MW, enough for 1200 homes.

The CHP units also capture heat as part of the combustion process. This will be used within the existing packing plant for space and water heating as well as refrigeration, using absorption cooling, to keep storage areas at the right temperature. There’s also potential to use it for heating greenhouses and the organic water waste treatment plant during winter.

“We aren’t going to be greedy,” says Nick Waterman. “We plan to sell some of the electricity and heat we generate to the local community, so that local residents and business people can benefit from having access to new and competitively-priced green energy supplies.”

With planning permission given by Cambridgeshire County Council in September, Local Generation are now going full steam ahead and aiming to be fully operational before the end of 2010.

Source: www.ecofriendnews.com

Publication date: 11/24/2009

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