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The right AD diet is a recipe for success

anaerobic digestion

Anaerobic digestion (AD), which employs bacteria to turn food waste into energy, has shifted from alchemical innovation to established technology in a relatively short time. There are now enough digesters in the UK to power more than a million homes, but oper­ators still struggle to source feedstocks and, sometimes, to produce optimum levels of gas.

Of the 10 million tonnes of food waste pro­duced each year, just 2.5 million tonnes is pro­cessed through AD. On average, facilities run at 80% capacity, so AD operators need to work with waste management partners to source greater volumes and a better quality feedstock.

For smaller facilities which do not have easy access to microbiologists, managing those stocks effectively can also be problematic. AD relies on consistent feeding and, with a short­age of material, operators find they have to change the ‘diet’ too quickly and risk upsetting the finely balanced bacteria.

Ollie More, head of policy at the Anaerobic Digestion & Bioresources Association (ADBA), says optimum performance is crucial for eco­nomic success. “The simplest metric for meas­uring performance for electricity plants is load factor. Plants have done well to increase levels to 73% during the past five years, but with the best operators reaching 98%, we still have a way to go. Businesses need to be productive, and those running at 70% are losing out on a fifth of their potential income.”

One way to raise awareness of feedstock quality is to treat food waste as a valued resource. For example, instead of simply giving customers figures for tonnages recycled, AD provider and operational consultancy Amur also provides waste producers with methane readings, gas yields and carbon figures. These all help with environmental reporting and give genuine feedback on waste productivity.

Amur owns a 5MW gas-to-grid plant in Yorkshire. Its parent company AB Agri has 36 years’ experience in blending feed for livestock and, in 2016, Amur launched Ch4rger, the first food waste ‘soup’ to be blended to the same specification each time.

Nigel Lee, Amur general manager, says the benefits of feeding a stable, consistent material will always override the temptation of cheaper, untested feedstocks. He said: “Operators who are focused on maximising returns understand the value of feedstocks and are careful about what they feed their plants. Introducing a product or feedstock which guarantees gas yields brings a valuable financial stability.”

Contamination of feedstock is also a concern, with ADBA’s 2017 Anaerobic Digestion Market & Policy Report showing that more than 90% of operators rate it as a significant issue. Along­side WRAP, ADBA is working on the Quality Action Plan to address contamination in diges­tate. The aim is to reduce or eliminate contam­ination, largely from plastic packaging, which finds its way into digestate, making it unsuitable for spreading on land.

Lee said that although food waste AD plants typically include a depackaging line, this tends to limit their options: “An alternative is to work in partnership. For example, Amur located its plant adjacent to Maltings Organic, a waste management firm which is able to depackage every type of waste. This results in Ch4rger delivering lower contamination levels, a higher rate of recycling and greater brand security for commercial food waste producers.”

He added that it is in the interests of waste managers to choose their AD partner carefully. “A well-operated plant will offer a consistent solution, whereas a plant that regularly has downtime will result in waste producers or brokers having to find an alternative at short notice. This can be expensive, at best requiring additional resource and haulage and, at worst, they may find themselves having to pay for liquid landfill.”

As AD businesses search for new sources of feedstock, shrewd waste firms are well placed to benefit. At around £29 per tonne for local authority waste and £22 per tonne for com­mercial waste, gate fees for AD are notoriously low. This is obviously problematic for operators that based their business model on 2015 rates, when local authority fees stood at £40 per tonne.

But More points out that for waste manage­ment firms and food waste producers, AD gate fees could act as an incentive: “Combustion plants command higher gate fees so, hopefully, even after separating out food waste from mixed loads, AD will often represent the most cost-effective option.”

Lee concludes: “We hope to see much stronger links between waste management and AD firms. Aside from benefiting from lower gate fees and strong environmental credentials, there is real scope to work together to use the waste hierarchy to full advantage, giving detailed reporting and Duty of Care, secure depackaging and a long-term contribution to UK energy security.”

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