Facultative waste stabilization ponds in the UK, loaded at 80 kg BOD/ha day, produce effluents which comply with the European Urban Waste Water Treatment Directive (i.e., ≤25 mg filtered BOD/l and ≤150 mg SS/l). However, the Environment Agency of England and Wales typically requires a higher effluent quality of ≤40 mg/l unfiltered BOD and ≤60 mg/l SS, both on a 95-percentile basis. An ammonium-nitrogen requirement might also be applied. Traditionally, maturation ponds and reedbeds have been used to upgrade facultative pond effluents, requiring large land areas. This paper describes and compares aerated and unaerated rock filter performance for BOD, SS, nitrogen and faecal coliform removals, and highlights the land-saving opportunities as maturation ponds and reedbeds become redundant.

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