Sludge handling banished
Water companies looking for low-carbon systems should examine lagoon technology, insists Gurney's David OrmeFacultative treatment of domestic sewage has been largely ignored in the UK, but targets for reduced carbon footprint have prompted a resurgence of interest in this proven technology. Based on lagoons, facultative treatment has been labelled land-hungry and smelly in the past. It is a shame if these perceptions remain, as the reality can be very different. Scottish Water is about to take delivery of its third Aero-Fac plant at Bowmore on the Isle of Islay. The utility's existing plants benefit from reduced physical footprint, integral sludge digestion and odour-free operation. Using available wind energy, backed up by low power electrical motors, Aero-Fac is proven to be one of the lowest total-life cost sewage treatment systems.
Anglian Water acknowledged this by commissioning the first of a further three Aero-Fac plants at Sutton St James in Lincolnshire. The company conducted a thorough risk and value process, concluding that operational cost savings of up to 75% could be realised by opting for Aero-Fac over comparable treatment systems.
Anglian Water calculated the carbon footprint for Aero-Fac and two alternatives - local treatment using a package plant and pumpaway.
The study concluded that Aero-Fac provided the lowest overall carbon footprint option, right from the date of commissioning. It was estimated that over a predicted 40-year lifespan, Aero-Fac's carbon footprint was over six times smaller that the next best option.
Under low load conditions aeration is provided by surface absorption enhanced by gentle mixing using wind-power, when available, and small 500W backup motors under calm conditions.
When load increases, dissolved oxygen sensors initiate the operation of a diffused air system for just long enough to deal with the additional loading, minimising energy use.
The aeration systems ensures that any odorous gases produced by sludge digesting at the base of the primary cell in particular, are oxidised before they reach the surface. Sludge production too is negligible, which means there's no need for sludge handling, thickening, transport or disposal for the lifetime of the plant.
Construction too sticks to the low carbon footprint model. The treatment cells are formed on-site using cut and fill techniques. Adding a facultative primary stage in front of an overloaded traditional plant provides many of the same benefits, but also introduces automatic balancing of flow and load, elimination of the need for primary screening and a loading reduction of up to 80% on the existing plant.
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