Total nitrogen removal in two-step biofiltration

A one-year study was carried out on two semi-industrial submerged up-flow biofilters - a predenitrification (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 m(3) of reactor per day-, obtained with high TSS and carbon loading rates, at a water velocity up to 21.5 m(3)/m(2).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).