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Digestion tanks at Welsh Water waste water treatment plant

The water from the sewers spends less than 8 minutes on its journey through the plant - being filtered, having the bacteria eaten by microbes and then being blasted by UV lamps to kill everything off.
However, the solid organic matter ends up spending tens of days in these digestion tanks being turned into a compost-like low grade fertilizer. The fertilizer is primarily used on forest land, having fallen out of favour with food producers.
The spherical tank holds the methane byproduct which is burned to provide heat and some of the electricity to run the facility.