Landia, Cambi boost biogas production at Scottish Water facility
South of Aberdeen, Scotland, improvements continue to be made at Scottish Water’s Sludge Treatment Centre (STC). Although the site has not yet reached its full potential, almost 1 MW per tonne of dry solids is being consistently produced.
Refurbishment of the STC has seen an upgrade of Cambi’s Thermal Hydrolysis Process (THP) and the introduction of a new digester mixing system from Landia.
In 2001, Scottish Water commissioned the fifth ever THP plant. Over the past two years, Cambi has modernised outdated control and measurement systems – and is currently upgrading the heat exchangers.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, but further improvements could increase this ‘considerably’.
Simon Wrigglesworth of Scottish Water Services in Grampian said: “We’ve still got work to do, to deliver the full suite of improvement – 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.
“We’ve come a long way since the old compressor mixing system at the STC, 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 4,000 m3 digesters at the STC were cleaned out during the facility’s refurbishment, it was revealed that 45% of the volume had been lost due to grit and heavily-compacted sludge. A new mixing system was central to helping to make the STC self-sufficient and significantly reduce its carbon footprint.
“We looked at various options,” said 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 the STC 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 clogging.
Cambi’s THP exposes sewage sludge at the plant to high temperatures and pressures, typically for 20-30 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.