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AD contamination issues are still being ignored says Titech

Companies investing in the development of anaerobic digestion (AD) facilities are ignoring the issue of contamination claims TITECH, the leading manufacturer of sensor based sorting systems. TITECH’s concerns are founded on its experience of helping waste companies in mainland Europe to improve AD efficiency and productivity.

Jonathan Clarke, Managing Director of TITECH UK explains: "The UK is putting a lot of emphasis on AD as a means of treating organic waste and generating renewable energy. It’s a great technology with a lot of potential: but in order to deliver optimum results, the infeed material needs to be as pure as possible. Separate collections of food waste can help enormously, but there will still be packaging and other inert materials mixed in. These non-organic materials build up in the system and can cause serious damage and compromise productivity."

The Anaerobic Digestion and Biogas Association (ADBA) has already drawn attention to the issue of non biodegradable packaging materials, particularly plastics, which can become entangled in pump mechanisms and reduce the quality of the final, compost-like material. However, TITECH says that contamination can also take the form of inert materials like glass, which can accumulate in the tanks over time and compromise AD plant performance. Says Clarke: "It’s important to remember that AD facilities are likely to take infeed from a number of sources, including farms and commercial food processing plants as well as domestic food collections. The opportunity for inert materials to be mixed in with organic waste is therefore significant."

As an example, the company cites SUFI S.A, which runs a materials recovery facility in La Rioja, Spain, and handles up to 130,000 tonnes of municipal solid waste per year. The operation not only focuses on the recovery of dry recyclables but also isolates organic waste for further processing in an anaerobic digester. The facility found that high levels of contamination from inert impurities in the organic infeed were reducing the effectiveness of the digester. Smaller inert fragments were causing damage to the pumps, pipes and screw presses, whilst the heavier inert material settled to the bottom of the digester tank and reduced its overall capacity and efficiency. SUFI S.A found that these contamination problems were resulting in lengthy shutdowns, costing the company significant amounts of time and money.

A TITECH x-tract sorting system was introduced to target the inert materials in the organic infeed stream. This system uses x-ray transmission image processing to separate materials based on their atomic density. The accuracy of the x-tract system is therefore not affected by colour, shape or moisture levels, making it ideal for use in wet or dry AD applications.

The x-tract system now recovers around 90 per cent of all inert material before it enters the digester. As a result, the Rioja AD plant now regularly achieves organic infeed purity rates of around 95 per cent, which has resulted in a 19.6% increase in its methane production. This in turn equates to an increase in electricity production and the plant now produces up to 250-300 kWh/t of organic waste.

In conclusion, Jonathan Clarke says: "SUFI S.A’s experience clearly demonstrates that contamination is one of the key issues that can hinder the AD process. The UK has a prime opportunity to optimise AD plant design and avoid the problems that European plants are experiencing with contamination. Most waste companies see sorting systems as the last stage in materials recovery; but this example from Spain clearly demonstrates that TITECH technology can be used at the front end of the AD process to bring benefits including less downtime and a purer infeed, resulting in improved electricity production."

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