New Biogas Production Landmark Achieved for Scottish Water’s Sludge Facility at Nigg

New Biogas Production Landmark Achieved for Scottish Water’s Sludge Facility at Nigg

Since introducing Landia’s digester mixing system and upgrading Cambi’s THP, carbon dioxide emissions at Nigg have more than halved.

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If we took a short trip forward in time to 2040, and then looked back at how some organizations in the water industry successfully achieved carbon net zero, it’s likely you’ll see The Nigg Wastewater Treatment Works in Scotland.

This Scottish Water treatment center in Aberdeen is currently leading the way as a center of excellence for biogas production (from sludge), with its dedicated team tirelessly focused on improving efficiencies. It’s a place that believes in “better.” It is also the first wastewater treatment plant in the United Kingdom to successfully co-digest.

The Nigg Wastewater Treatment Works, which was built in 2002, has made some significant steps forward since Scottish Water took over ownership and operation in December 2018, working with Cambi to upgrade the existing thermal hydrolysis process (THP) and, importantly, installing a new digester mixing system from Landia.

From initially producing 250 cubic meters of gas per ton of dry solids, levels have increased; first to 350, and now to an impressive 400 to 420.  

The 400-plus figure is also being achieved without unlimited, costly power. Typically, if standard propellers were in use, around 17 watts per cubic meter would be required, but with the Landia system running at what amounts to less than 12 hours in a 24-hour period, only 7 watts per cubic meter are needed for the anaerobic digester mixing.

These latest numbers (at a site that has a small footprint for its energy output), are very much the result of fine-tuning and regular dialogue, as Nigg’s operation team lead, Ewan Dickie, explains.

“It is imperative that we understand the equipment and processes in front of us,” he says. “And two years on since the latest improvements began at Nigg, Landia is always there to answer any questions we have about enhancing the mixing of our digesters.”

Nigg’s operation team lead Ewan Dickie (left) with Paul Davies from Landia.
Nigg’s operation team lead Ewan Dickie (left) with Paul Davies from Landia.

Prior to the upgrade, the facility had switched off the old compressor mixing system that had all too regularly caused problematic pipe blockages. The clean-out of the digesters for the upgrade revealed that 45% of the volume had been lost to grit and heavily compacted sludge.

Robust mixing system

“Now, we have an extremely robust mixing system that comprehensively mixes our two 4,000 cubic-meter digesters,” says Dickie. “We don’t get any problems. The mixers just sit in the background, working away. The equipment is also so much easier to maintain, because it’s externally mounted.” 

Further evidence of Nigg proving how much energy from sludge can be produced at a wastewater treatment facility is its use of the biogas and heat from the combined heat and power units (CHPs) for the site’s new boiler. More than $500,000 annually is being saved on fuel oil. Two years ago, the original target set for utilizing the CHPs (supplied by Clarke Energy) was 56%. It now stands at 73%. The CHPs were producing around 80% of the power required to run the site at Nigg. That figure has improved to the point where, most of the time, Nigg is now exporting surplus energy to the national grid.

In addition to the intake from a local population of around 300,000, Nigg imports sludge cake from a further six Scottish Water wastewater treatment centers, and has commercial waste delivered from approximately 30 tankers per day.

Since the introduction of the Landia digester mixing system and the upgrade of the Cambi THP, carbon dioxide emissions have more than halved. The amount of biogas being produced has risen by more than 30%. In the first stage of the digester mixing process, the liquid (or sludge) is injected into the upper half of the tank, while biogas is aspirated from the top of the tank and mixed into the liquid. 

This not only has the benefit of reducing buoyancy at the surface of the liquid, but also sees the rising gas bubbles continue to mix after the pumps are switched off.

Cambi’s THP exposes sewage sludge at Nigg to high temperatures (160 degrees C to 180 degrees C) and pressures (about 6 bar), typically for 20 to 30 minutes for each batch to ensure pathogen kill. Sterilized and hydrolyzed sludge is passed to the flash tank, which operates at atmospheric pressure. The sudden pressure drop leads to substantial cell destruction of the organic matter in the sewage sludge, which is then cooled to its optimum temperature before it is fed to the digesters.

Developing more facilities

Based on the ongoing success of Nigg’s biogas production, Scottish Water is considering the development of a further six sludge-to-energy facilities, which would represent a giant leap towards achieving the net zero target by 2040; five years earlier than the goal set by the Scottish Government.

The Nigg facility is full of innovation, with constant, gradual progress, according to Simon Wrigglesworth, operations director for Scottish Water Grampian.

“We’re all for genuine innovation,” he says. “But sometimes it’s not just about new technology; it can be about doing something differently. Cambi’s THP has been around for over 20 years, and the chopper pump, which is a key part of the digester mixing system, was invented by Landia back in 1950.”

Part of Nigg’s innovation has also been its willingness to undertake a three-month trial to co-digest residues from local distilleries and breweries with its domestic sewage. Not only were 58 tons of carbon saved, but the plant saw methane levels increase from 60% to 64%. This first for the industry in the United Kingdom — which was led by the publicly owned water company’s commercial subsidiary, Scottish Water Horizons — could be just the start of a move that could have a major positive impact to a more circular and sustainable economy.

Keeping an ever-keen eye on all the production figures at Nigg is process engineer Relina Dusa. During the past two years, she has worked on optimizing the mixing with different run times, from 45 minutes per hour, down to 15. Operation now rests at 28 minutes per hour.

A need for proper mixing

“We had a need for proper mixing and now we have it,” says Relina. “It is important that we have experimented with different run times — and now I’m very happy with the gas yield of between 400 and 420 (cubes of gas per ton of dry solids) that we’ve now reached. Over the past two years I have worked closely with Tony Challinor at Landia. We never have to wait for answers and feedback. There haven’t been any issues with the mixing system, and though it might sound like a small point, the Landia service engineers always clean up properly after themselves and leave everything tidy. We work well together.”

From what has been learned and continues to be learned at Nigg, the prospect of Scottish Water developing six additional similar sites will be a landmark for the water industry in what can be achieved with sludge when optimizing biogas production.

“We’ve come a long way with Nigg already,” says Wrigglesworth. “The water industry rightly has to focus on achieving regulatory compliance, but we’re always looking at how we can maximize the generation of green energy in order to meet our commitment to reducing and ultimately eliminating carbon.”


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