INDUSTRY FOCUS WATER & WASTE TREATMENT GAS PRODUCTION INCREASED BY 25-30%


A project carried out at one of Scottish Water’s Sludge Treatment Centres by Landia and Cambi is helping deliver an upswing in biogas production


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mprovements continue to be made at Scottish Water’s Sludge Treatment


Centre (STC) in Nigg, just south of Aberdeen. But at a site still approaching its full potential, 0.9~1.0MW per tonne of Dry Solids is already being consistently produced. Part of its route-map to lead the water


industry to net zero emissions and beyond by 2040 (five years ahead of the Scottish Government’s ambitious plan to become carbon neutral by 2045), the STC at Nigg is becoming a showpiece energy generation centre.


AIMING FOR THE TARGET In recognising some sweeping transformations that would have to happen over the next 25 years to achieve its target, Scottish Water’s refurbishment at Nigg has seen an upgrade of Cambi’s Thermal Hydrolysis Process (THP) – and the introduction of a new Digester Mixing System – from Landia. Scottish Water commissioned the fifth


ever Cambi THP plant in 2001. Over the past two years, Cambi has modernised outdated control and measurement


systems – and is currently upgrading the heat exchangers. The plant can treat up to 60 tonnes of dry solids per day, with improvements increasing this amount. Simon Wrigglesworth of Scottish Water


Services, Grampian, said: “We’ve still got work to do, to deliver the full suite of improvements – and will keep fine tuning – but since the refurbishment, taking everything into consideration, we are producing approximately a quarter to one third more biogas than we were previously.” He added: “We’ve come a long way


since the old compressor mixing system at Nigg, which failed regularly. This resulted in the effective digester volume being reduced, because the fouling up of vessels with an accumulation of solids lessened the amount of gas that we could produce.” When the two 4,000m3


digesters at


Nigg were cleaned out during the centre’s refurbishment, it was revealed that 45% of the volume had been lost due to grit and heavily compacted sludge. Central to the need to help make this STC electrically self-sufficient and reduce its


The externally mounted Landia Gas Mix at Nigg


carbon footprint was the important choice of a new mixing system. “We looked at various options,”


EXCHANGER IS A MORE ROBUST AND RELIABLE REPLACEMENT


HRS Exchangers recently supplied a G Series gas cooling heat exchanger to food waste AD plant operated by a UK waste management company. The anaerobic digestion plant transforms 45,000 tonnes of food waste from domestic and commercial


sources into renewable electricity and high value digestate biofertiliser. From the beginning, the plant had been designed to be as efficient as possible and included exhaust gas heat recovery. However, when the original heat exchanger fitted to the exhaust system failed after just a few years of operation, the client approached HRS to provide a more robust and reliable replacement. The HRS G is a complete stainless-steel multi- tube heat exchanger specifically designed for exhaust gas cooling and thermal recovery. The hot exhaust gases


flows through the interior tubes of the heat exchanger while the


service fluid, in this case water, flows though the surrounding shell. The use of stainless steel is a key factor in the longevity of the G Series as rapid corrosion of the


original carbon steel unit was one of the reasons for the original’s failure. Other design features include a bellows to allow for thermal expansion, and a drain in the header


to allow the acidic condensate, which forms during operation, to be removed, as well as a hatch to allow for manual cleaning and inspection. In operation the new G Series heat exchanger cools the


exhaust gas from ~530˚C to ~320˚C and the heat recaptured from the process is used in the AD facility, and to provide hot water around the site. Since installation, the HRS G Series heat exchanger has performed so well that HRS has been asked to quote for the replacement of another unit, which is used to treat the digested sludge from the plant, with an HRS DTI Series unit. HRS Heat Exchangers www.hrs-heatexchangers.com


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continued Wrigglesworth, “but as has been proven, Landia, who were confident that their system could handle the thickness of the sludge, did exactly what they said they were going to do. There was nothing showy; they gave us a good feeling right from the start and provided an impressive whole life cost analysis. They were clearly very determined to bring about the benefits we required.” Both digesters at Nigg are now fitted


with two diametrically-opposite Landia Chopper Pumps, which draw thick liquid from the bottom of the tank, where solids are chopped to accelerate the digestion process and prevent pipe clogging.


THE RIGHT MIX In the first stage of the mixing process, the liquid 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 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. With two 30kW systems on each digester, the installed power of the Landia mixing system equates to an economical 15 watts per cubic meter. When factoring in reduced running times, as low as 20 minutes in the hour, energy


NOVEMBER 2020 | PROCESS & CONTROL / PROCESS&CONTROL


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