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WATER & WASTE TREATMENT LEADING THE WAY TOWARDS NET ZERO


A Landia digester mixing system is helping Scottish Water fine tune its processes


successfully achieved Carbon Net Zero, it’s likely we’ll hear about NIGG. This Scottish Water treatment centre in


I


Aberdeen is currently leading the way as a centre of excellence for biogas production (from sludge). It is also the first wastewater treatment plant in the UK to successfully co-digest. The Nigg Wastewater Treatment Works has


made some significant steps forward since Scottish Water took over ownership and operation in December 2018; working with Cambi to upgrade the existing THP (thermal hydrolysis process) and installing a new digester mixing system, from Landia. From initially producing 250 cubic metres of


gas per tonne of dry solids, levels have increased; first to 350, and now to 400 – 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 metre 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 metre are needed for the anaerobic digester mixing. These latest numbers 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 said. “And two years on since the latest


improvements began at Nigg, Landia are always there to answer any questions we have about enhancing the mixing of our digesters.” Prior to the upgrade, the facility had switched


off the old compressor mixing system that had all too regularly caused problematical 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. “Now,” added Dickie, “we have an extremely


robust mixing system that comprehensively mixes our two 4000m3


digesters. 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 & Power units (CHPs) for the site’s new boiler. £0.5M pa is now being saved on fuel oil. Two years ago, the original target set for utilising 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. That


f we took a short trip forward in time to 2040, and then looked back at how some organisations in the water industry


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 centres, and has commercial waste delivered from around 30 tankers per day. Since the introduction of the


Landia digester mixing system and the upgrade of the Cambi THP, CO2 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, whilst biogas is aspirated from the top of the tank and mixed into the liquid. This reduces buoyancy at the surface of the liquid, and 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˚C to 180˚C) and pressures (about 6 bar), typically for 20 to 30 minutes for each batch to ensure pathogen kill. Sterilised and hydrolysed 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. Based on the ongoing success at Nigg,


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. Simon Wrigglesworth, Operations Director for


Scottish Water Grampian, said: “We’re all for genuine innovation”, he said, “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.” He added: “It’s fair to say that at first,


investing in the Landia mixing system seemed a bold move, because others were seen as a much ‘cheaper’ capital cost. But in an industry that promotes TOTEX, Landia were unmatched in providing meaningful numbers for the total


cost of ownership, which showed that in the long term, they were in fact the least expensive option.” Part of Nigg’s genuine innovation has also


been its willingness to undertake a 3-month trial to co-digest residues from local distilleries and breweries with its domestic sewage. Not only were 58 tonnes of carbon saved, but the plant saw methane levels increased from 60% to 64%. This first for the industry in the UK, 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. Over the past two years, process engineer,


Relina Dusa, has worked on optimising the mixing with different run times, from 45 minutes per hour, down to 15. Operation now rests at 28 minutes per hour. “We had a need for proper mixing and


now we have it,” said Dusa. “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 tonne 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.”


Landia www.landia.co.uk


NOVEMBER 2022 | PROCESS & CONTROL 27


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