Water & Wastewater Treatment

WWT PUMP AND VALVE SUPPLEMENT 2019 5 Again, this is such a huge advantage over systems stuck at the base of the tank, that over time get covered in the very grit that they are meant to be suspending. Even with extra recircula- tion pumps to try and prevent the bubbles taking the same old route – compared to a system specifically designed to mix the whole tank – they struggle to produce decent amounts of energy. As soon as a bubble diffuser is switched off, the grit falls down – which is totally different, for example, to the new system for Scottish Water at Nigg, where in the first stage of the mixing process, liquid 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 continues to mix a-er the pumps are switched off thanks to the rising gas bubbles. Each digester at Nigg has been 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 of pipes and nozzles. With 2x 30kW systems on each 4000m 3 digester, the installed power equates to 15 watts per cubic meter. When factoring in reduced running times (as low as 20 minutes in the hour), energy savings are substantial. In addition, reduced running times means that pump-wear components last significantly longer. Down on the island of Jersey, at the Bellozanne STW, there's no sign of a 'mixing system' that only skims the base of the tank. When weighing up Bello- zanne's drive towards a highly robust plant where downtime would be a thing of the past, Doosan Enpure were also busy sourcing kit that would meet key requirements. As well as trying to maximise gas yields, the requirement was for low maintenance, with all pipework and moving parts for the three new anaerobic digesters, to be on the outside of the tank. Building a new plant presented the perfect opportunity to engineer out old unwanted problems, especially the very costly maintenance for compressors. These went down even if there was just a small amount of foam in the plant's old gas mixing system. Prior to the upgrade, a digester had to be taken out of service, with costly tankers brought in to remove two to three metres of grit/silt and rag. With new-build there are of course more options at the design stage, so at Jersey, it was decided to include an isolation valve between digesters and the actuated valve so that an entire digester didn't have to be decommissioned if ever there was a problem with a valve. Operating times also came into the equation – rejecting systems that run flat out, so in effect, giving an operator the flexibility and confidence to fine-tune the mixing for maximum gas yields. From a process point of view, the superior "Make no mistake, good mixing is critical. And as well as greatly im- proved gas yields, the additional benefits of externally-mounted mixing equipment are there to be gained." PAUL DAVIES An externally mounted digester mixing system at Bellozanne STW, Jersey.

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