Feast on this: abrasive drying for food waste

A mechanical process has been developed which can deal with both food and packaging waste at the same time in a single batch. Ed Mant explains

Every year in the UK it is estimated that around 30% of food purchased from retail outlets is thrown away by consumers. Mostly these are items that have past their sell by date and often discarded still in their packaging. Recent studies have shown that as many as 30,000 more ready meals or chilled food items are being purchased per month than during the same month in 2005 - an increase of 15%.

The figure of 30% equates to a staggered 2.7kgs per person per week. Because of the make-up of this waste, much of it still has to go to landfill. This is due to the fact that the alternative disposal routes such as anaerobic digestion and composting are unable to cope with the food and packaging waste without separating it. Separation involves substantial costs being incurred, something which is undesirable to councils and businesses alike.

Packaging dilemma
To divert the organic part of this waste stream away from landfill, waste needs to be separated at a waste processing facility, after which the food waste would be sent to a technology such as composting. This still poses the question of what to do with the packaging waste.

Once the packaging has been in contact with the food, it is deemed to be contaminated. As such the only way that this can be sent to be recycled is by cleaning and washing the packaging before transporting it to a recycling plant. This again incurs additional costs, meaning there is a double impact on the overall cost of waste disposal.

In recent years, various initiatives have tried to address this problem - the main one being the introduction of protein and starch-based food packaging which is labelled as biodegradable. This, however, is not strictly true. Although the packaging is made from organic materials, it will not biodegrade back into a natural organic material, it will merely break down into small granules, leaving a plastic-like sand.

Over time this may biodegrade further, but this is a slow process and will not provide a rapid fix to the problem - something which is needed if the problem of food and packaging waste is to be reduced significantly in a short space of time.

On the technology front, biological processes such as anaerobic digestion (AD) and composting are perfectly viable solutions and well suited to the processing of purely organic waste streams. However, the problem of where to dispose of the packaging waste contaminated with food remains. Also, due to the biological nature of the processes they are somewhat limited in the wastes they can process.

Variation of waste streams
AD, for example, relies on natural enzymes to break down the waste at an accelerated rate inside a digestor. However, these enzymes favour a consistent waste stream on a day-to-day basis. Slight variations in waste stream will not pose a problem, but daily variations could cause the negative effect on the enzymes, meaning they die out, thus stopping the process working.

Should this occur, the digestor would need to be emptied and the loading process would have to be started again. This would mean there was a quantity of part-digested waste which would need to be disposed of elsewhere before it could be sent to land. This leads on to one of the other negative aspects of these processes - the by-products.

Both AD and composting result in a by-product, which is typically sent to land, effectively like land filling on top of the land. Although this by-product contains nutrients, changes in policy mean that the locations where this material can be disposed of are becoming fewer in numbers. The other by-product of both processes is methane - this is emitted during the decomposition stage of the process. In some cases this gas is channelled into energy recovery systems, but these systems will only produce small volumes of energy from the gas. In a lot of cases these facilities do not make use of the gas and it is vented to the atmosphere.

There is now a technology that has been developed which could put an end to the problem of food and non-recyclable packaging waste disposal. Inetec, an environmental technology innovator based in South Wales, has developed a novel technology for processing food and non-recyclable packaging waste in its raw form. This technology, called abrasive drying, is a purely mechanical process, meaning both the food and packaging waste can be processed at the same time in a single batch.

The mixed waste is loaded into the processing vessel where it is macerated against itself over a period of around 16 hours. During this time heat is applied to the base of the vessel. This raises the temperature of the waste leading to the vaporisation of, first surface level moisture, and then cellular level moisture. This vapour is drawn out of the chamber by a series of fans, after which it is condensed back into the liquid effluent suitable for draining to foul sewer.

What remains after this drying process is a dry, highly prepared biomass fuel. This fuel looks similar to coffee granules, but has a high energy value of between 20Mj to 25Mj per gram. This fuel can then be used to generate energy in a number of forms. For smaller installations the biomass fuel can be combusted and the thermal energy used to generate steam, hot water or for space heating purposes. On a larger scale, the fuel is fed through a gas conversion process and used to drive conventional gas engines which generate renewable electricity.

These large-scale operations are run under the name EnCycle, a subsidiary of Inetec. The aim of EnCycle is to design, build and operate a network of ten plants around the UK during the next five years. Each of these plants will be processing up to 500 tonnes of food and packaging waste a day and will generate around 24MW of renewable electricity.

The first of these centres, to be located in North East Lincolnshire, was recently granted planning permission, and construction is due to start in late 2007.

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