A one-year study was carried out on two semi-industrial submerged up-flow biofilters - a pre-denitrification (pre-DN) step followed by a nitrifying (N) step, designed to treat the primary settled municipal wastewater of a full scale plant for both carbon and nitrogen removal. Performances of the pre-DN step have been investigated showing high removal denitrifying capacities – 1 to 1.2 kg NO3–N removed per m3 of reactor per day–, obtained with high TSS and carbon loading rates, at a water velocity up to 21.5 m3/ m2.h. The complete treatment plant – primary settlement + pre-DN/N loop – has demonstrated a nitrogen removal efficiency ranging from 70% without any additional carbon, up to 85% with methanol. COD and TSS removal rates were 91% and 97% respectively.

The above results show that two submerged biofilters in series form a treatment line suitable for nitrogen removal. This technical choice leads to very compact full scale plants such as Ahlen WWTP (Germany, 85 000 PE).

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