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Health and Safety: Vital for Survival in Anaerobic Digester Cleanouts

27 November 2017

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Why is it dangerous?

The Health and Safety Executive define a confined space as any space of enclosed nature where there may be a lack of oxygen and/or a risk of injury or death. These spaces not only include drains, sewers and storage tanks but also open-topped chambers, combustion chambers in furnaces and poorly ventilated rooms.  Anaerobic digesters most definitely fall within this remit and more awareness needs to be raised of the potentially fatal outcomes of unsafe cleanouts.  Slurry or biomethane related deaths are happening across the world.  Newspapers from India, to Ireland have reported on farm or slurry related deaths.  However, these deaths and injuries aren’t limited to work with slurry tanks.  AD tanks, containing the same toxic gases can be deadly if dealt with in an incorrect manner.   For example, Let’s Recycle reported on the devastating death of a 29-year old man in Dorset, UK due to intoxication by hydrogen sulphide gas.  Airing out the AD tank does not always make the facility safe.  Some of the gases produced are heavier than air and therefore can still cause asphyxiation.

 So why clean-out?

Surely, with these dangers in mind, there has to be a reason why clean-outs of AD tanks must take place.  Grit is the main culprit. Grit is an umbrella term for all kinds of inert high density materials.  These can include sand, stone, bones, eggshells and lime- to name only a few. These materials are highly abrasive at full-flow and cause wear on PD pumps, rotors and decanters.  Not only that, but when the grit settles it significantly reduces the capacity in the digester, meaning a significantly lower biogas yield can be produced.  Therefore, AD providers clean out their tanks in order to increase digestion and prolong the life of the sludge pumps.

If you have to clean-out, clean-out safely

So if you have to clean-out, do it safely. Many forums suggest that this should not be undertaken lightly and that if the knowledge on site of how to do this is limited, seek the help of a professional.  The institution of Chemical Engineers (IChemE) highlight the need for more understanding of past incidents within Anaerobic Digestion to “minimise the probability of recurrences” however the information available to the public domain remains limited in this area. It is clear that a wider dissemination of “near misses”, or past incidents is needed.

 What if you didn’t have to clean-out at all?

Clean-outs of AD tanks can often require the digester to be shut down for up to 40 days at a time.  It can take 10 days to stop the process, 5-20 days to clean-out and another 10 days to get back online.  Not only is there a significant loss in biogas production, but the clean-outs themselves can be costly.   The best solution is to not shut down at all, removing both the cost and the risk to health and safety.  In-line technology exists that creates a ‘purge loop’ where material from the digester is continually processed to remove grit.  This means the digester can continue to function and produce biogas, while the highest capacity is maintained.  Not only that, dangers and costs associated with clean-outs are also removed. 

To find out more about grit removal technology, contact us: