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Technology Focus: Pre-treatment technology


PROJECT SUCCESS AND FAILURE Gary Jones, Technical Director of Langage AD, outlines the importance of selecting the right pre-treatment technology for your AD operation.

“All the major AD technology providers are aware of the problems caused to equipment by abrasive grits, sediments and floating layers. But it is sometimes the case that, while they may mention this issue to prospective clients (always read the small print!), they don’t always impress on them the importance of it. Some providers have even been known to protect their liability by specifying an input feedstock which simply does not exist. However, operators should be in no doubt that all food waste contains contaminants which, if not removed, will lead to expensive repairs, disposal and operating costs. While screening technology in many UK plants has been woefully inadequate, fortunately, there are some great pre-treatment systems out there.

So, why do contaminants matter so much and at what points do they affect the digester and associated equipment? We must first accept how aggressive decomposing food waste is, with its low pH level, and how difficult abrasive grits are to move around the system. This understanding is essential when selecting equipment. After all, the bugs inside the digester don’t care if there is a floating layer; it will not stop the gas production. They are also indifferent to sediments accumulating on the digester floor. However, the resulting diminishing biological space in the digester will cause trouble: the bugs will become overfed, leading to rising acids and the eventual


demise of the digester. The correct pre-treatment technology can therefore bring huge rewards, but not just for the front-end of the system. All operators aspire to achieve the PAS 110 standard for digestate and careful selection of separating machinery is crucial in helping to meet these stringent requirements, too. A good understanding of your feedstock and the right engineering design will significantly enhance a plant’s chances of success. Waste differs in its composition regionally, as well as at point of origin, so this needs to be taken into

The Hybag separation hammer mill turns AD feedstock into a soup-like substrate, at least 99 per cent organic

consideration when designing an AD facility. Supermarket waste will present different problems to domestic waste, for example. With this in mind, it’s worth considering multi-stage clean up systems. The better systems generally have two or more processes for each of the problem points of sinking and floating layers. The best time to integrate these systems is in the design stage, but there are one or two successful retro-fit systems available – if you know where to look! Take time to research all the available options thoroughly; it will save you time and money in the long run.”

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