Deep-bed down-flow two-media filters were used in pilot plant studies with filtration of secondary settled wastewater. FeSO4 or FeCl5 was applied as a precipitation agent, and NaAc·3H2O was chosen as a carbon source when denitrification was desired. The concentration of PO4-P in the filtrate from the pilot plant study never exceeded 0.05 mg PO4-P/l when iron salts were dosed. The curves showing the concentration of P-tot and PO4-P in the filtrate as a function of the quotient between the dosage of iron and the concentration of PO4-P in the influent to the filter followed approximately an exponential relationship. The total nitrogen reduction over the filter bed increased from an average of 2.3 mg (NO3+NO2)-N/l at the beginning of each experiment to an average of 4.3 mg (NO3+NO2)-N/l towards the end of the test. When only secondary settled wastewater, suspended solids, primary settled wastewater, iron salts, or sodium acetate was added, at a hydraulic load of 10 m/h, the time before clogging became 100 h, 10–15 h, 20–40 h, respectively. Almost the entire pressure drop was located on the surface of the filter bed and 0.25 metre down in the expanded clay layer.

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