SUBSTRATE-INHIBITION AND MULTIPLE STATES IN A CONTINUOUS NITRIFICATION PROCESS

Multiple steady states were observed in a continuous nitrification pro cess in a laboratory activated sludge reactor operating at 18 degrees C and subject;to high ammonium concentration in feed (up to 300 mg/l a s N): under the same operating conditions a state of either complete o xidation 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 empl oyed for parameter estimation. A steady-state solution, expressed in t erms of the measured variables-total biomass concentration in the reac tor and in the effluent-was derived and its parameters were estimated. The model was then analyzed to show domains of steady state multiplic ity under two control strategies in which either the sludge age or the total biomass concentration is kept constant.