Should anaerobic digestion plants be on the menu for food processors?

In the third part of our special edition on plant efficiency, FoodProductionDaily.com examines not only the benefits but also the feasibility of food processors tapping into AD as a way of cutting costs and boosting their green credentials.

Anaerobic Digestion (AD) is a biological process that happens naturally when bacteria breaks down organic matter such as food, in environments with little or no oxygen. It is a treatment that composts waste and produces a biogas that can be used to generate electricity and heat.

AD has been attracting huge attention and last year the UK Government made development of the technology a central part of its waste reduction strategy. But since then only a handful of subsidised projects have been realised.

However, last month Premier Foods, one of the UK’s largest food companies, announced that construction of a closed loop commercial-scale anaerobic digestion plant at its RF Brookes ready meals factory in Wales was under way.

The £5m-facility will convert food waste into energy to help power the factory, providing significant savings in waste disposal and energy bills and reducing carbon emissions by around 8,500 tonnes per year. The AD plant, which will be owned by Insource Energy, is scheduled for completion by the end of 2010 and is expected to be fully operational in early 2011.

The facility, which received a £0.5m subsidy from WRAP Crymu, will have the capacity to process around 10,000 tonnes of food waste a year, and is expected to generate 300 kW total energy. It will supply power for about six per cent of the food processor’s electrical consumption and around the same amount for its hot water usage, John Scott, InSource Energy managing director told FoodProductionDaily.com

This is believed to be the first example in Wales of a factory being partially powered by its own waste in this way and is one of only a handful of such closed loop facilities in the UK, he added.

Feasibility ​

While nobody disputes the environmental benefits of recycling food waste, the question for industry is whether anaerobic digestion plants are both technically feasible and economically viable.

Scott declared it to be technically possible as long as there was a ready and consistent source of solid or semi-solid food waste. The amount needed depends on the nature and concentration of organic material, he said. “Any food processing plant can use AD but whether it is economical viable will depend on whether the food source is of suitable quality or quantity.”​

AD can theoretically use any raw materials used in food processing. Feed stock comes from rejected foods that are not fit for use, trimmings from food preparation and food sludge in effluent collected in drains after wash downs.

While it is very difficult to generalise, the annual waste volume needed to make AD economically workable would be around 3,000 tonnes, said Scott. Almost inevitably, major industry players – with their larger capacities and economies of scale - were more likely to be able to generate the volumes needed.

“Any food processor should at least be thinking about this and whether it is doing enough with its waste,”​ he added. “The bigger companies should be considering the full scope of their options.”​

Practicalities​

While the size of an AD plant varies according to its capacity, Scott said “it’s not massive but it’s not small”​ – advising firms should factor in between a third to half an acre (1,300 – 2,000 sq meters).

The largest single item is the digestion tank where waste foodstuff is held for a sufficient time to allow the bacteria to break it down and release the energy. Typical digestion time is between 30-40 days, said Scott. An AD plant with a capacity to process three thousand tonnes of food annually would need a digestion tank to cope with of 10 tonnes of food waste a day over this period.

Scott added that several companies contributing waste to - and getting energy from - a single AD plant could be a practical solution where space is an issue. But he said that at present there appears to be a cultural concern among companies about doing this

“There does seem to be concern among food processors about bringing in waste from another site,”​ said Scott. “They are happy to use their own but there is a reluctance to use waste from other plants – even if they are part of the same firm. Food companies do not want to be seen as part of the waste management process.”​

This is a view that we hope will change because there is a big difference between waste food material from the processing stream and kerbside waste collected by councils.

“We hope that with time industry players will think more intelligently about it - it is not waste but fuel,”​ he concluded.