Substrate inhibition and multiple states in a continuous nitrification process

Multiple steady states were observed in a continuous nitrification process in a laboratory activated sludge reactor operating at 18°C and subject to high ammonium concentration in feed (up to 300 mg/l as N): under the same operating conditions a state of either complete oxidation to nitrate or of partial oxidation to nitrite may be attained, depending on the initial conditions. A model incorporating Monod and Andrews kinetics for growth of the two species was developed and employed for parameter estimation. A steady-state solution, expressed in terms of the measured variables—total biomass concentration in the reactor and in the effluent—was derived and its parameters were estimated. The model was then analyzed to show domains of steady state multiplicity under two control strategies in which either the sludge age or the total biomass concentration is kept constant.