Waste not, want not

The UK produces around 15 million tonnes of household, commercial and industrial food waste every year. Most of it ends up in landfill to rot. Not only is this a huge environmental problem, as the rotting food generates greenhouse gases such as methane, it is also an expensive option. Landfill is taxed at a standard rate of £82.60 per tonne, which will increase from April 2016. By 2020, the UK will have to obey an EU rule put in place to reduce the amount of biodegradable waste, including food waste, ending up in landfill to 35% of 1995 levels. It has also been predicted the UK will run out of capacity in landfill sites within the next five years. What can be done to tackle this problem?

Currently, around 2 million tonnes of food waste per year is sent to anaerobic digestion (AD) plants. Here, the food is broken down by microorganisms in the absence of oxygen to produce fuel and biofertiliser. The fuel produced is a methane-rich biogas, which is used to produce electricity and heat; this prevents methane being released to the atmosphere and reduces the amount of fossil fuels needed. According to the Scottish government, energy produced from the 2 million tonnes of food waste could power a city the size of Dundee for six months. The AD process significantly reduces greenhouse gas emissions as for every tonne of food waste recycled, between 0.5 and 1 tonne of carbon dioxide is prevented from entering the atmosphere. Another environmental benefit is the biofertiliser can be used on farmland instead of fossil fuel-derived fertilisers.

What’s going on in the lab?

At the University of York, Andrew Hunt is one of the researchers working towards finding new ways to recycle food waste at the Green Chemistry Centre of Excellence. ‘There’s a huge amount of potential in waste that I think is under-utilised,’ he says. ‘We tend not to think of it as waste here at the Centre – to us, it’s a resource that we just don’t use at the moment.’

One of the projects at the Green Chemistry Centre of Excellence is extracting valuable chemicals from orange peel. Orange peel is a source of sugars, cellulose, hemicellulose, limonene (which is normally used in fragrances, food and medicines) and pectin (which is used as a gelling agent and thickener in food). The team found that limonene is a potential bio-derived replacement for toluene, a solvent used in many industrial processes, found in paints and coatings, and known to affect the central nervous system.

When extracting compounds from food waste, it can be difficult to separate the product from the solvent being used as bio-derived solvents typically have high boiling points. So, Andrew’s team extracts chemicals from food waste using supercritical carbon dioxide (CO₂) instead. A supercritical fluid is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It moves through solids like a gas and dissolves materials like a liquid. ‘The advantage of using CO₂ is that when you release the pressure, the CO₂ just goes away. You get a product with no solvent residues,’ says Andrew. This is particularly important if the product is going to be used in food products. A lot of food waste is plant-based and this contains phytosterols (steroid compounds similar to cholesterol). ‘They’re added to margarine to reduce blood cholesterol and they can be extracted quite nicely with supercritical CO₂,’ says Andrew. More good news for the environment is that the CO₂ is a recycled waste product from power generation and fermentation processes. ‘We can use it to extract a whole range of valuable chemicals including waxes, which can be used for coatings and polishes.’

In fact, Andrew’s team has been extracting waxes from maize (corn) for use in anti-foaming applications. ‘Anti-foaming agents are added to washing powder to stop your kitchen or utility room filling up with foam,’ he says. ‘We were looking for natural bio-derived alternatives to traditional anti-foaming agents. Wax from maize is very effective at reducing foam and doesn’t inhibit the cleaning of the clothes. That was a really nice result.’

Starbons are another product to come out of the University of York, developed by Vitaliy Budarin and James Clark, who heads the Green Chemistry Centre. Starbons are mesoporous carbon-containing materials extracted from starch, which can come from potato peelings. Mesopores are pores with diameters of between 2 and 50 nanometres. The starch is heated until it forms a gel; water is then removed from the gel to produce an expanded starch structure. The starch structure is converted to carbon by heating to produce a variety of structures with different properties, which can then be used for different applications. Starbons can be used for chromatography, as catalysts and as absorbents. They have also been used to separate mixtures of metals.

It’s not only food waste from the end of a food product’s lifetime that is being considered for recycling, a lot of useable waste is created during production processes too, such as crop harvesting. The Institute of Food Research (IFR), based in Norwich, UK, aims to minimise this wastage, or where this is not possible, to find ways of turning the waste into valuable products. They say it is estimated more than 400 billion litres of the vehicle fuel bioethanol, which is made by fermenting sugar, could be produced each year from crop wastage. Current processes to generate bioethanol from this waste are complex and inefficient because high temperatures and acidic conditions are needed to release the sugar from the waste. These conditions cause the waste to break down into the compounds furfural and hydroxymethylfurfural, which are toxic to the yeast that encourages fermentation. One way to avoid this is to use genetically modified yeast, but recently, a research team at the IFR found a natural yeast strain that is not affected by the conditions. It can replace genetically modified yeasts to turn agricultural by-products such as straw, sawdust and corncobs into bioethanol.

On a big scale

There is a huge amount of research into making use of food waste going on in laboratories around the world. The next step is to scale up the amounts of products produced, such as the bio-derived solvents, so they can be used on a large scale in industrial processes. The teams at the University of York begin this process by working with their own biorenewables development centre. This is a scale-up facility designed to increase the amount of product made, from small amounts in a round bottomed flask to kilos of product that can then be handed over to an industrial partner for further testing to ensure that scaling up is feasible.

Three years ago, the Green Chemistry Centre of Excellence started working with an industrial partner – an Australian company called Circa – to produce green replacements for toxic solvents, including N-methyl-2-pyrrolidone (NMP). One substitute the team discovered for NMP is cyrene (dihydrolevoglucosenone), which they synthesised from waste cellulose (a plant material that can come from food and forestry wastes). Cyrene has low toxicity and its properties are similar to those of NMP. ‘In the future, it is hoped this process will move to a full-scale production plant,’ says Andrew. Soon, industrial solvents will need to pass through REACH legislation, so there is an urgent need to replace toxic ones with non-toxic alternatives. REACH is a European Union (EU) regulation for the registration, evaluation, authorisation and restriction of chemicals and was put in place to protect human health and the environment.

Working with companies to cut down or make use of their food waste is something the Green Chemistry Centre of Excellence is familiar with as it has set up a scheme called WasteValor that advises small and medium-sized companies. WasteValor identifies opportunities for companies who create or process food waste to turn their waste into a source of income and minimise the amount going to landfill. For the companies that could use chemicals extracted from their food waste, WasteValor assesses opportunities to use these chemicals, which may be both financially and ethically advantageous to the companies, and advises on partnership opportunities with companies that generate waste.

Closing the loop

Food waste is a resource that is being considered as part of a ‘circular economy’ being introduced by the EU this autumn. Currently, the way food is used is linear – the food is produced, we buy the food, consume what we want, then bin the rest. Closing the loop is the aim of the circular economy plan. This means instead of food waste ending up in landfill, it is all recycled. For the EU, this could improve resource productivity by 30%, which would mean by 2030 there would be an increase of 1% in EU gross domestic product and 2 million new jobs created. For researchers, ‘the circular economy strategy will push them towards thinking about using waste as a feedstock rather than getting fresh material either from petrochemical sources or from mining,’ says Andrew. ‘In terms of funding, it could be very good for researchers working in the areas of green chemistry, environmental chemistry, materials recovery and metallurgy where they’re looking at new technologies to recover these things.’

Originally publlished in The Mole