The mathematical model of sludge particles settling in the water treatment plant (settler) is considered. In the case of the residence time of sludge particles in the settler the model leads to a nonlinear age-dependent transport- diffusion with a nonlocal additional condition. This problem is formulated as an identification/optimal control problem, where the sludge concentration is assumed to be a control. For the case of constant ("average'') velocity, as a characterization of the optimal control problem two necessary conditions are obtained. These conditions permit reducing the nonlinear coupled two-dimensional problem to the two-point boundary value problem for the second order nonlinear ordinary differential equation, and then, to a nonlinear equation, with respect to sludge concentration. For the solution of the problem an iteration algorithm is derived. Convergence of the iteration algorithm is analyzed theoretically, as well as on test examples. The numerical procedure for the considered problem is demonstrated on concrete examples. (C) 2008 Elsevier Ltd. All rights reserved.