Approach to steady state in completely mixed flow reactors

Environmental engineers frequently employ the assumption that steady-state conditions are approximated after three hydraulic residence times. In this paper, the conditions under which this assumption is valid are elucidated. For well-mixed systems, the time to approach steady state depends on the kinetic order of removal mechanisms, the initial pollutant concentration in the control volume (=C-0), and the kinetic rate constant. The assumption of three hydraulic residence times to steady state was found to be conservative if C-0 = 0. However, if C-0 equals the influent concentration, then more than three hydraulic residence times may be required to approach steady-state conditions with some zero-order reactions.