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Recycling household food waste - The Low Carbon approach
30 August 2007
Food waste presents a new challenge for local authorities for two reasons: first, food waste makes up about 25% of household waste and is 100% biodegradable; second, alternate weekly collections of bins containing food waste is not popular with householders. There is an emerging solution to this challenge which has the enticing benefit that it also involves low-carbon technology.
Anaerobic digestion (AD) is a natural biological process (like composting) which recycles organic materials into biofertiliser. The key differences between AD and composting are: first, AD is sealed from the air; second, AD is suited to high-moisture materials (such as food waste); and third, AD produces renewable energy in the form of biogas.
The first product of AD, biogas, comprises about 60% methane with the balance being carbon dioxide. Biogas can be used in a conventional boiler to produce hot water, or, more usually, in a combined heat and power unit (CHP) to produce electricity and heat. In Sweden it is normal practice to upgrade biogas to pipeline quality, when it becomes known as biomethane, and to use this as a vehicle fuel. The recently published Energy White Paper plans to give electricity produced from AD double the value of renewable obligation certificates (ROCs), which significantly improves the economics.
The second product of AD, biofertiliser, can be in either liquid or solid form. Because AD is an enclosed process all the nutrients in the food waste are recycled and made more available for crops. The Government is currently preparing a Digestate Standard and Protocol which will enable quality digestate to be classified as a product rather than a waste.
What makes AD a "low-carbon technology" are the benefits of: avoided methane emissions from the uncontrolled decomposition of food waste; the displacement of mineral fertilisers, which require fossil fuels for their manufacture; the production of renewable energy; and the potential for reduced waste miles through the adoption of local solutions.
The UK's first food waste AD plant is operating in Ludlow. This is a Defra demonstrator, co-funded by Advantage West Midlands. The project is the culmination of a long-term partnership between South Shropshire District Council and Greenfinch Ltd, a specialist AD technology company based in Ludlow. Greenfinch has been researching the AD of food waste through a series of pilot projects since 1996 (when the cost of landfill was £8 per tonne).
The Ludlow Biodigester was commissioned in March 2006 and for the first 9 months of operation recycled the contents of 240-litre green wheel bins for food waste and garden waste collected on alternate weeks. Very important lessons were learned from this period: first, that little food waste was captured using this waste collection method; second, there was a high level of contamination from plastics, metals glass and stones in the wheel bins. The result was that there was reduced diversion of biodegradable waste from landfill, reduced energy yield and increased energy consumption. The low capture of food waste was supported by the Eunomia report prepared for WRAP in January 2007.
Local Authorities need to be careful when implementing new collection methods. Food waste was in the household waste stream in 1995, the baseline date for the Landfill Allowance Trading Scheme (LATS) whereas garden waste was not significant then. To try and capture the two together may result in underperformance with regard to LATS and will lead to the requirement for enhanced treatment for the garden waste under the Animal By-Product Regulations (ABPR).
Since January 2007 the Ludlow Biodigester has been processing only food waste, with some municipal grass cuttings. Householders are issued with a kitchen caddy, a roll of compostable corn-starch bags and a small bin for the kerbside. With this collection method there is a very high capture of food waste, more than 3 to 4kg per participating household per week. Furthermore there is very little contamination. The lesson is that the appropriate collection method leads to successful participation.
The weekly collection of food waste from households and its processing by anaerobic digestion is supported in the Government's Waste Strategy which was published earlier this year. WRAP has implemented a number of collection trials across England, which will inform on rates of capture and the types of collection methods.
The throughput of the biodigester is currently 75 tonnes per week, i.e. the food waste from about 20,000 households (the design figure is 95 tonnes). Biogas yield is 165m3 per tonne of food waste. This equates to the production of 310 kWh of electricity per tonne, of which 15% (45 kWh) is used for the process, giving net electricity of 265 kWh per tonne of food waste. There is also a significant amount of surplus heat produced which is to be used in a district heating scheme. The biofertiliser, which has been pasteurised at 70oC, is being used successfully in local agriculture.
As a Defra demonstrator the project is being independently monitored by the University of Southampton, who plan to publish their final report in September 2008. This will include an assessment of mass balance, energy balance, nutrient balance and operating costs. An important output from the publicly funded project is an analysis of the economics of the process. The facility includes a visitor centre for local authorities, and visits can be arranged.
Our interim view on the economics, given "double ROCs", is that a plant with a capacity of 10,000 tonnes per year processing food waste in corn-starch bags, with a net electricity output of 300kW, will be able to charge a gate fee of less than £50 per tonne and make an economic return. The food waste does of course not need to be only from households - WRAP's estimate is that households produce 250kg of food waste per year and that there is the same amount of food waste in total produced from commercial catering and shops. Therefore a 10,000 tonne per year AD plant could support a community of 30,000 households, based on a capture of two thirds of the available food waste.
There are already a number of AD plants being planned in the UK to process food waste. These are either being procured by local authorities or are being built as merchant plants in strategic locations, charging a gate fee, and co-digesting household food waste with commercial catering waste and food waste from processing factories.
For more details please contact biogas@greenfinch.co.uk