Chemical precipitation in wastewater stabilization ponds – in Scandinavia called fellings dams – has been in operation for more than 50 years. Slaked lime and aluminium salts are the most common chemicals used for precipitation. Long and narrow forms of the ponds and a detention time of at least 5 days have shown, even at a low water temperature and below an ice cover, to produce an average effluent quality of 70 mg CODCr/l, 0.2 mg Tot-P/l, 20 mg Tot-N/l (CODCr: chemical oxygen demand; Tot-P: total phosphorus; Tot-N: total nitrogen) and low levels of pathogenic bacteria. The systems use low amounts of energy and no wastewater is by-passed at the plants. Fellings dams have recently been tried to support overloaded wastewater collection systems.

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