AD plant makes important new upgrade with new mixers.

When Ross Mingotti joined AMPGREEN Estates’ Greengill Farm AD Plant in December 2021, the plant was already grappling with significant mixing challenges in its final storage tank. By February 2022, just two months after his arrival, one of the paddle mixers in the secondary tank failed.

The plant, located near Penrith, Cumbria, in the far north of England, had used these mixers successfully since their installation in 2017. However, with the failure of one mixer and the difficulty of accessing them without emptying the 3-million-gallon digestate tank, it quickly became clear that addressing the issue would be no small task.

“The final storage tank had already been offline for a year,” Ross recalls, “but at least we were still able to run our primary digester, which is part of the site’s unique tank-within-a-tank design.”

Jamie Seggie, who was appointed AD Plant Manager in February 2022, soon found himself facing a much more complex situation than he had anticipated. Matters were further complicated by the invasion of Ukraine, which led to delays in receiving replacement mixers. As a 2.4MWh plant, any downtime was costly, both in lost income and the difficulty of maintaining consistent grid power supply.

“After waiting for three months with no updates on the delivery of parts,” Ross adds, “we decided to explore other options. Our final storage tank had become heavily crusted, but we managed to get it moving temporarily with a makeshift propeller mixer mounted on a tractor. We attended the ADBA’s World Biogas Expo in Birmingham to explore potential solutions.”

At the expo, Ross and Jamie evaluated several new mixing systems, and soon after, suppliers visited the plant to assess the situation. They quickly recognised the severity of the problem but, more importantly, Ross and Jamie were determined to choose a partner who would not only provide the necessary equipment but also offer reliable ongoing support.

“There were a lot of suppliers keen to sell us solutions,” says Ross. “But we didn’t want to be upsold on unnecessary extras. We needed a supplier who would understand our needs and be committed to providing long-term support. The previous design of the paddle mixers had created significant maintenance challenges—servicing them required emptying the tank, which was time-consuming, expensive, and disruptive. Plus, after a tank was emptied, it could take up to 40 days to get the gas quality back to optimal levels.”

Jamie Seggie, Plant Manager, explains: “As we struggled with poor mixing, we saw increased hydrogen sulphide levels and fluctuations in gas quality. This meant spending less time focusing on the digesters and more on engine protection. There were also knock-on effects, like more frequent oil changes and faster wear on downstream equipment. Additionally, with our thermophilic system, the costs to heat the tank to maintain a temperature above 50°C were rising.”

A New Approach to Mixing
Ross and Jamie knew they needed a more reliable mixing system that would not only minimise downtime but also be manageable in terms of ongoing maintenance. After careful deliberation, they chose mixers from Landia, a supplier with a proven track record in the AD industry.

“We chose Landia because they had a strong reputation, and importantly, they have a local presence and spare parts readily available in the UK. From the very beginning, they demonstrated a genuine commitment to helping us succeed. They weren’t just trying to sell us equipment—they were focused on providing a solution that would work for us long term. They also had a flexible approach, unlike some suppliers who focus narrowly on one part of the process.”

Paul Broadhurst at Landia proposed a tailored solution for the Greengill Farm plant. For the second digester (which has a 30m diameter and 8m height), Landia provided four 22kW submersible mixers, while for the final storage tank (46m by 8m), they installed six mixers—four 11kW and two 22kW—positioned at different heights and running at different speeds and times to create multiple mixing patterns. This approach was designed to ensure thorough mixing with minimal power consumption.

“With the new Landia mixers, we now have both tanks mixed properly,” says Ross. “The power consumption is significantly lower than we expected, and the reliability is much higher. Everything is running more smoothly now.”

The plant uses a variety of feedstocks, including maize, grass silage, and occasionally rye or wheat, amounting to around 104 tons per day. The second digester operates at around 8% total solids, while the final storage tank has a slightly lower solids content of 5%. To optimise the digestion process, AMPGREEN now ensures all feedstocks are chopped to no more than 5mm, promoting better breakdown and more efficient gas production.

Despite using around 15% of the power generated by the AD plant for its own operations, an impressive 85% is sent directly to the grid, contributing significantly to renewable energy generation.

Ongoing Support and Preventative Maintenance
In the six months following the installation of the Landia mixers, AMPGREEN Estates faced a final twist. Just as they had moved on to a more reliable solution, they were informed that the parts they had originally ordered from their former supplier were now ready—too late to matter.

“By the time we received the new parts from the original supplier, we had already moved forward with Landia’s solution,” says Ross. “But we were in a much better position. We now have a maintenance contract with Landia, who service the mixers every six months. They didn’t pressure us into signing an inflexible deal; instead, they were supportive, offering to train our own team if needed. The Landia technicians who come here are outstanding—they take a lot of pride in their work and genuinely care about our success.”

“The transition to the new mixers was stressful at times, as these projects often are,” Ross admits. “But throughout the process, we had constant communication with Paul Broadhurst at Landia. He and the Landia team worked alongside us to find the best solutions, and we always felt supported. We now have peace of mind knowing that if we need help, Landia is always there for us.”

With a robust mixing solution in place and a trusted partner in Landia, the Greengill Farm AD Plant is now operating more efficiently than ever, with reduced maintenance costs, better gas quality, and a more stable operation overall.