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Getting more out of anaerobic digestion

Nigel Lee, General Manager at Amur, says that anaerobic digestion plants need to work harder to improve gas yields if they are to achieve their budgeted income

Plants must get more out of their AD
Over the last 10 years, anaerobic digestion (AD) has taken the country by storm. There are now enough AD plants in the UK to power over a million homes but, even as technology and know-how improve, many are not fulfilling their potential. Failing to deliver on gas results in reduced income and missed targets – in effect, the plant operates as an expensive waste management solution rather than an effective renewable energy business. So why has this situation arisen, and what can operators do to improve performance?

When the UK’s first food waste AD, Greenfinch, began trials in 2006, it was met with wonder by a waste management industry just starting to contemplate the prospect of household food waste collections. AD was already in use by the water and sewage industries, but for food waste it represented a panacea. Today, many food manufacturers and local authorities specify AD for residual food waste, but show little interest in how effective plants are.

The best measure for performance is load factor, or running time. Recent years have witnessed phenomenal improvements – even six years ago, average load factors sat at just 46 per cent, compared with 73 per cent in 2016. However, with the most efficient operators reporting levels of 98 per cent, many in the industry still have a long way to go.

The biggest barrier is feedstock. Despite murmurs from central government on separate food waste collections for all households in England – in line with Wales and Scotland – collections currently lie at just over 50 per cent. Similarly, the bulk of commercial food waste collected does not make it to AD either.

However responsibly food is prepared, an element of food waste will always remain, and once reduction and redistribution have been addressed, waste management firms need to actively encourage separate collections and build relationships with AD businesses to ensure food waste meets its potential.

Better reporting can help. Typically, food waste producers receive a paltry ‘tonnage recycled’ figure for their material; reporting on methane levels and gas yields helps to position food waste as a genuine resource, and also make operators reflect on performance.

On the ground, improved feeding regimes have a direct impact on gas produced. AD is all about maintaining good biology, and good biology relies on consistency. This means sticking to one type of feedstock or, where that is not possible, giving the bacteria time to adapt.

Getting more out of AD
According to the Anaerobic Digestion and Bioresources Association’s (ADBA) 2017 Anaerobic Digestion Market & Policy Report, plants typically run at 80 per cent capacity; without sufficient feedstock available, they are forced to chop and change in line with what is on offer. Adversely, a common mistake is to over-feed a plant, thinking that greater volumes of feedstock will result in more gas.

Smaller plants may not have access to teams of biologists, but they are able to employ an external lab to monitor performance, and to seek expert advice on balancing feedstocks for better performance. Biological support for AD is a growing area and specialists can recommend everything from helpful additives to a full change in diet. The key is to find a partner that will look for patterns over time and work to anticipate changes before problems occur.

Recent changes to renewables incentives mean that agricultural plants coming onto the market will need to include a fraction of food waste in their ration. Based on the current climate, it seems likely that unless waste managers and local authorities do more to drive material to AD, feedstock shortages, and under-performance, will become increasingly acute.

Nigel Lee is General Manager at anaerobic digestion services provider Amur Energy

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