Articles & Case Studies

Cut costs of up to £100,000? That settles (gr)it!

Posted: Monday 13th February 2017

Cut costs of up to £100,000? That settles (gr)it!

The process of cleaning the sludge out of an anaerobic digester is time consuming, messy and expensive, but it is vital to maintaining operational efficiency and reducing required maintenance.

The accumulated levels of settled grit in an anaerobic digester can be as high as a quarter to a third of its height by the time companies can no longer put up with the efficiency loss being caused and help is called in.

The digester then needs to be taken off line, the heating turned off, the feeding stopped and then there's the wait for the gas generation to wane down sufficiently for the digester to be safely vented and accessed.

Experts such as AndiDrain then commence the draining process, letting liquid sludge to go through a processer, for instance the CDEnviro D:Max, at around 100m3/day at first. This gradually gets slower as the head drops and the material gets thicker.

The CDEnviro D:Max will separate any rag and the grit in two separate streams, allowing the residual liquid sludge (<64microns) to be recycled.

At this stage, from the day that the digester is turned off to the time they've drained it far enough for the grit layer to be exposed, it could be as long as 15-20 working days.

"But now the fun really begins," according to Andrew Little, MD of AndiDrain. "It's all hands-on deck to manually jet the settled grit into a slop so that this can be sucked out by vacuum tanker."

The time taken here is variable depending upon the density of the grit deposit but it could take another 20-30 days to complete the cleanout job. This process is applicable to all digesters but in cases where the cover can be lifted, a small Bobcat type motorised shovel can be deployed by crane after the initial draining phase and the settlement removed in this way.

"The cost varies between jobs, but ballpark, a job like this could cost around £25,000 to £30,000 to execute," said Andrew. "Depending on geographical location and logistics, you can add disposal fees which could be as high as £145 per tonne to that. With transport at around £5 per tonne as well, that's £150 per tonne. Given an average density of 1.5 tonnes/m3, disposal of 300m3 of irrecoverable grit to landfill works out at 300 x 1.5 x £145. That's £67,500!�

In summary, a typical 1,000m3 digester cleanout could command a spend of up to £100,000. On the other hand, with more favourable odds, Andrew says that there have been jobs done for as little as £50,000!

Chris Zammit, Business Development Manager, at CDEnviro, says, "Treating the source material before feeding could cut down on the need to clean out the digester. CDEnviro data has shown the frequency could go down from every seven years to 23 years. Put like that, can you afford not to de-grit the source material before feeding?"

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August 2021

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