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The right AD mix benefits everyone

plant operation ad

While food waste anaerobic digestion (AD) plants often rely on waste management operators to provide feedstock, the variable nature of the loads sup­plied can have a negative impact on the delicate biology which is integral to the smooth running of the plant. This, in turn, leads to low gas yields and reduced profit.

Equally, waste companies could benefit more from the arrangement if their choice of partner was more judicious. The advantages of sending food waste to AD are many. In addition to low gate fees and a strong positive environmen­tal impact, AD offers the chance to apply the waste hierarchy to full advantage – but only when material is processed to deliver its full potential.

For AD operators, a secure feedstock supply is crucial. The efficient running of an AD plant depends on maintaining good biology, which is built on consistency. No farmer would feed their cows sugar one week and high protein the next. Yet, thanks to a perceived shortage of available food waste, for many of our AD plants such variation is the norm. As a result, a large proportion of plants are not fulfilling their potential when it comes to gas yields.

One way to overcome this problem is to balance poor feedstocks with co-products to produce a more effective combination. Alter­natively, AD operators may choose to feed a nutritionally balanced feedstock ‘soup’ that provides consistent biogas levels and low maintenance requirements. As part of the larger AB Agri group, Amur is also focused on nutrition and feeds its 3MWe plant in South Milford, North Yorkshire, solely on Ch4rger – food waste blended to a fixed specification.

So why is quality feedstock in such short sup­ply, and how can waste managers and AD oper­ators work together more effectively to improve the situation?

Of the 10 million tonnes of commercial food waste produced each year, only 2.5 million tonnes are processed through AD. Meanwhile, Wales and Scotland are leading the way with separate household food waste collections but, across England, just 14% of household food waste is collected separately.

While there has been a great deal of talk about food waste in the waste hierarchy, the debate has largely focused on reduction and redistribution rather than residual waste. AD sits above both recovery (incineration with energy recovery), composting and landfill in the hierarchy.

For AD operators, making links with food waste producers can be challenging. In recent months, Amur has seen a large rise in demand for its Bullet biological biomethane potential (BMP) test, which suggests that increasing numbers of plants are being presented with one-off feedstock opportunities. But what is actually needed are regular supply deals that offer feedstock security and consistency.

Food manufacturers and local authorities are usually aware of AD as a way of making money out of food waste. However, they sometimes fail to understand the ways in which commingled collections can limit the value of food waste destined for AD. Although separate food waste collections are becoming more common, the standard offering is still a mixed waste or total waste management solution.

The cost of managing food waste effectively is also often forgotten. Processing residual food waste requires serious investment. For exam­ple, building an AD plant can cost more than £3m per MW of thermal energy produced, and a great deal of expertise is needed to maintain the right balance of nutrition and feeding of the plant so as not to upset the fragile biology.

Despite this, AD still offers the most cost-effective option for disposal of food waste. Since combustion plants tend to command higher gate fees, AD proves economical even once food waste has been separated from mixed loads.

Amur applies the same data trail require­ments and pre-acceptance tests to food waste as are employed for material destined for its animal nutrition business. Reporting on meth­ane levels and gas yields positions food waste as a genuine resource, helps businesses with envi­ronmental reporting and gives feedback on the productivity of waste.

In recent years, the AD landscape has changed. Many operators chase the real income of gas revenue over gate fees, and choose to pay a premium for high gas-yielding feedstocks. This policy also comes with the bonuses of lower processing and digestate recycling costs.

The advantages of separate food waste collections affect the entire supply chain. Work­ing with the right partners, waste management firms can benefit from lower gate fees com­bined with a more modern, secure and professional service that helps meet circular economy targets, while underpinning the UK’s renewable energy base.

Nigel Lee is general manager at Amur. Amur is part of animal nutrition firm AB Agri, which is, in turn, part of Associated British Foods. ABF’s grocery division includes Silver Spoon, Twinings and Ryvita

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