A new phosphorus removal process, that permits smaller sludge production, low operating costs, and stable phosphorus removal has been operated on an experimental basis at a pilot plant. In this process, secondary effluent containing dissolved oxygen is dosed with ferrous salt solution and then contacts with filter media, so that Fe(II) oxidizes and sticks to the filter media surface and phosphorus is removed simultaneously. Study of the experiment results verified the following points. (1) When the influent phosphorus concentration is 3.5 mg/ℓ, the Fe/P mole ratio is 2, and space velocity is 6 per hour, the effluent phosphorus concentration can be reduced to the level of 0.1 to 0.3 mg/ℓ and the removal efficiency can be upgraded to 90% or more. (2) Phosphorus removal progresses rapidly so that phosphorus can be removed to the same extent as that in effluent at the 0 to 60 cm positions (at filtration rate of 200 m/D) in the upper part of the filter media. (3) It is presumed that the substance formed on the filter media surface works as a catalyst for ferrous and phosphorus removal. Therefore, influent pH level must be approximately 6.5 for constant treatment efficiency. (4) 40 to 50% of removed phosphorus adheres to the filter media.

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