Innovative Mixed Biogas Digesters

Online Editor

Comprehensive mixing of biogas digesters is playing a crucial role at one of the most dynamic food-waste-to-energy facilities in Europe

Typically operating at more than 90% efficiency, Eco Sustainable Solutions’ plant near Dorchester in southwest England, produces biogas at an average of 140m3 per tonne of food waste, which is sourced from 60,000 domestic kerbside collections in Dorset.

Processing 42,000 tonnes of food waste per annum (posting its best performance results since opening in 2014), Eco’s two 2,600m3 digesters are each fitted with Landia’s externally-mounted mixing system, comprising 18.5kW chopper pumps (invented by Landia in 1950) and Venturi nozzles.

How does the biogas system work?

Running for just 20 minutes per hour, the mixing system’s chopper pumps benefit from a unique knife design that prevents solids from entering the casing.

“We much prefer a mixing system that is outside the tank”, says Eco’s operations manager, Kieran Purkis. “I’m really not sure why some biogas systems still require mixers to be lifted out of a tank. We are health and safety led, so it’s a no for us. It involves far too many risks. It also doesn’t make sense financially. In addition to helping us improve our biogas yields by more than 10%, another benefit of the Landia digester mixing system is that it keeps our team safe”.

The initial upswing in biogas came in 2014, when Landia was chosen for a new, second digester. As well as greatly reducing the health and safety risks, the boost in producing more biogas with consistent concentration of methane circa 58-62% saw Eco move to upgrade the equipment on its first, existing digester. Here, a rotary submerged system had been in operation, but the considerable downside of downtime became very apparent when an internal mixer broke, meaning that the digester had to be drained down to retrieve it. During days of lost production, this type of scenario can cost a biogas plant a loss of in the region of £250,000. As with all biogas digesters – even when very well managed – draining down to remove accumulated grit every two to three years is a very costly exercise in going offline.

“Since then, we have continuously improved our processes, and today keep fine-tuning to reach optimum levels. We have outstanding operators, whose tremendous work and pride in the plant makes it one of the very best in the business,” says Purkiss.

He continues: “With our onsite labs, we are constantly testing, carefully balancing and maintaining levels; knowing the whole time that our digesters are being comprehensively mixed by a safe and very robust system.”

Compared with mixers that have to run 24/7, the Landia units make energy savings, and at only 20 minutes per hour operation, means far less wear and tear, greater longevity and less requirement for spare parts. Landia has also worked with Eco on modifying the inlet flange into the pumps so that the food waste feedstock goes straight to the macerating blades of the chopper pumps at the lower half of the digesters; improving the infeed with specially adapted pipework to enhance the process.

In what has become a real science at innovative Eco, half of the clean, renewable energy (12,000MW per annum) is utilised by the neighbouring Dorchester Feed Mill; (owned by the Mole Valley farmers co-operative) the first feed mill in the UK to be powered completely by renewable power. The other 50% from the 2.5 acres site is sold to the National Grid. From its nutrient-rich digestate, Eco also produces and supplies a diverse range of sustainable landscaping products that return goodness back into soils.

Ed Johnson, Eco’s AD manager, says: “Even when you are running a plant with relatively low solids (4-6%) as we do, effective mixing is very important. Even a 5% rise in our production can mean an extra £200,000 in PPA (purchase power agreements). As part of our ongoing drive to maximising production and be one of the best performing plants in Europe, we have upgraded our screening, but having both digesters mixed properly is crucial. The Landia system is very good and robust. It doesn’t require rebuilds. We also have no issues in obtaining spares.”

Biogas plant boasts no issues in eight years

Eco’s site supervisor, Antonio Rodrigues, has been at the site near Dorchester since day one. He says: “In eight years, the Landia digester mixing system has been brilliant. It’s a great idea. Apart from replacing a couple of solenoids, we’ve had no issues in eight years. We know that our digesters benefit from being completely mixed.”

As such a successful operation that has already recycled over 4,000,000 tonnes of organic material, it is no surprise that Eco Sustainable Solutions is looking to expand with a new biogas facility to add to its plant near Dorchester.

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