Grinding matter for the UK's first biogas plant

A biogas plant required a solution to keep its digestion process working efficiently, ensuring that food waste was reduced down to comply with regulations governing animal by-product disposal. Mono's TR Muncher fitted the bill, reducing the waste to an homogenous product.

A view of the Holsworthy Biogas Plant in Devon, UK.
A view of the Holsworthy Biogas Plant in Devon, UK.

The town of Holsworthy in Devon is home to the UK's first biogas plant, and takes in 75,000 tonnes of foodwaste and animal manure per year. It uses the gases produced by anaerobic digestion to generate electricity and pipe hot water, which is utilised by the surrounding community.

The digestion process requires the waste to be in a mostly liquid form when it is pumped into the digestion tank, in order to achieve maximum efficiency in gas production. The waste is pumped to the TR Muncher which macerates and reduces it to a homogenous product, ideal for digestion processes.

Mono's TR Muncher is a twin shaft, low speed, high torque grinder. When it is used, biodegradable waste passes through a series of cutters, which revolve at differential speeds to pull apart, crop and shear solids such as bones, flesh, offal and vegetable waste. Liquid passes through the cutter stacks, whilst the solids are trapped and macerated to a small, consistent particle size, to help comply with legislation and improve process efficiency.

The design of the Muncher incorporates an intelligent auto reverse sequence controller, which enables the Muncher to unblock itself and restart automatically, without the need for manual intervention. The cutter stacks are inclined at 25° from the vertical axis to allow any solids that are difficult to grind to drop clear of the cutters into the trash trap, on reversal of the cutter stacks. The trash trap is fitted with a large diameter access port to enable easy removal of any rejected debris. The design also saves on maintenance downtime and associated costs, as the cantilever drive shaft eliminates the need for bottom seals and bearings, significantly reducing wearing components and offering cost savings on replacement parts. The complete drive, gearbox and cutter assembly can be easily and quickly removed from the unit's casing without removing the main body from the pipework, saving time on routine maintenance procedures.

Ensuring correct particle size

The regulations regarding the disposal of animal by-products state that particle size cannot exceed 12 mm and the Muncher's ability to achieve this was a key consideration in the specification of the equipment.

Mono was able to reassure the plant engineers that the TR Muncher would stand up to the challenge. The Muncher supplied was fitted with 8.0 mm thick cutters, a primary consideration in controlling particle size. The cutters also have seven teeth around the circumference to further control the particle ‘length’ between the tooth cropping action. Lastly, in order to offer a consistent and effective ‘cut’ on fleshy and stringy material the two cutter shafts rotate at different speeds – tearing and ripping the fleshy material, combined with a scissor action across the opposing cutter faces.

“We have been very pleased with the performance of the TR Muncher,” said operations director Jake Prior. “Its capabilities reassure us that we are fully compliant with current regulations.