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Bioenergy: how local food waste powers our homes

AD plant overviewIt takes five tankerloads of food waste daily to feed the two anaerobic digesters at Lower Reule Farm. The 'porridge' like material is initially piped into the holding tank in the foreground, before being fed to the two digesters behind.

In the UK we create an estimated 15 million tonnes of food waste each year, of which more than a third is disposed of in landfill or via the sewers. But, writes Robert Hine, a growing proportion is being diverted to a process known as anaerobic digestion (AD) to produce methane-rich biogas, which can be burned to generate heat and electricity. Hence AD is a source of renewable energy from what would otherwise be thrown away. Furthermore, the anaerobic (oxygen-free) nature of the process and methane capture help reduce greenhouse gas emissions compared to landfill, and the liquid byproduct yields a biofertiliser suitable for agriculture.

Each year some 30,000 tonnes of food waste is treated locally by Lower Reule Bioenergy, a family-run business based near the village of Gnosall west of Stafford. Manager Helen Franklin described the operation of the plant as she showed me round the Gnosall site. The project was the brainchild of her father, and starting operating in 2010, one of the first farm AD plants to come on stream in the UK. It now generates enough electricity to power 2500 homes each year.

'Depackaging' is the first step

Initial processing is carried out at the company's other site at Four Ashes near Gailey. Here the food waste is 'depackaged' , macerated, and blended to form a Biogas outlet'porridge' before being transported by tanker to the Gnosall site. Some 40% of the material is household food waste collected by local authorities, with the remainder classed as 'commercial waste', including out-of-date food from supermarkets, catering leftovers, and so forth. Recovered packaging is incinerated in the nearby 'energy from waste' plant at Four Ashes.

Biogas is continuously piped from the digester to be scrubbed clean of impurities
before being burnt in the combined heat and power (CHP) units.

Smooth operator

The AD plant at Lower Reule consists of two large cyclindrical digesters, a holding tank for the incoming porridge, equipment for turning the digestate - the spent 'porridge' - into biofertiliser, and two combined heat and power (CHP) units to generate electricity for the grid. The plant operates continuously 24/7, apart from downtime for maintenance, and all aspects are monitored closely to ensure smooth operation and safety.

The digestors are essentially large vats containing a mix of bacteria that are fed every hour with a balanced feed of the porridge, supplemented with mineral elements to ensure optimum fermentation. Inside the digesters, the temperature is maintained at around 35-40 degrees celsius, and the contents are mechanically stirred. The biogas Control roomdrawn off consists typically of 52-62% methane plus carbon dioxide and other gases. Measures are taken to remove hydrogen sulphide due to its highly corrosive properties.

Helen Franklin, manager of Lower Reule Bioenergy, in the control room. All aspects of the operation are monitored continually day and night, to ensure smooth running of the site.

Biogas is not the only product

The biogas is then combusted to produce heat and electricity. Output from each of the CHP units averages around 500-600 kW, giving a potential overall maximum yield of 1.3 megawatt (MW). Some of the excess heat is already used on site, and there are plans to install a multi-purpose dryer to utilize the energy for drying cereals and other crops.

Spent digestate emerges as a thick 'soup', from which fibrous material is separated, to leave a thin slurry. In order to comply with Animal By-Products regulations, this slurry is pasteurized to kill any harmful pathogens, before being pumped to storage lagoons for use as a nutrient-rich biofertiliser. It is injected directly into the soil surface, both at Lower Reule and on neighbouring farms. The fibrous leftover material is used as a soil improver in land reclamation schemes. Helen is proud of the fact that AD is a 'closed loop' system, in which everything can be put to good use.