Critical Point Analysis for Toxic Waste Treatment

For design and operation of activated sludge processes treating toxic or inhibitory wastes, it is necessary to use an inhibition function such as the Haldane equation for relating growth rate μ to substrate concentration S, even when the sludge has been completely acclimated. The Haldane function differs from the Monod equation, which approaches a maximum growth rate asymptotically with increasing substrate concentration, in that growth rate increases with increasing concentration of an inhibitory substrate, reaches a maximum, and then decreases with further increases in substrate concentration. This behavior has important consequences for the treatment of toxic wastes. Once the growth rate of an activated sludge reactor attains the peak that is given by the inhibition function, the reactor is susceptible to sudden effluent deterioration and wash‐out. In this work, the critical, or peak, growth rate is quantified in terms of the biokinetic constants, the influent substrate concentration, and three selectable engineering control constants: Dilution rate D (reciprocal of detention time, $\tilde{t}$); recycle flow ratio, α; and recycle sludge concentration, $X_R\text{.}$ Design and operational strategies to avoid operation in the proximity of the critical growth rate are outlined. These strategies are illustrated by utilizing critical point curves which give the operational location of the critical growth rate in terms of the influent waste concentration and selectable engineering control constants.