Biological nitrification kinetics in a fixed-film reactor

The kinetic parameters for combined carbon/nitrogen removal in a four-stage aerated submerged fixed-film (ASFF) bio-reactor of 50 l volume were evaluated using a synthetic wastewater at 20°C. Influent COD concentrations were in the range of 0•057-1•8 g COD l−1, and influent NH3-N concentrations were in the range of 0•0152-0•48 g NH3-N l−1 with hydraulic loading ranging from 0•028 to 0•112 m3 m−2 day−1. Good nitrification was achieved even when high mass loadings were applied. The substrate (COD or ammonia) removal in the reactor was described by a substrate-limiting phenomenon (reaction or mass transport). Substrate-removal rate followed Monod kinetics in the first stage of the reactor but was better described by first-order kinetics in the second to the fourth stages as the substrate concentration became much lower. The order of the reaction was apparently a function of the hydraulic loading, indicating the influence of mass transfer on the ‘apparent’ order of the reaction.