The study considers the issue of optimal control of the start-up mode of the membrane distillation process. The aim of the work is to increase the efficiency of controlling the process of concentrating solutions in a contact membrane distillation unit, which will contribute to reducing the cost and increasing the level of energy saving of the process with prior uncertainty and changes in the permeability of the membrane over time. An analysis of various publications has shown that no single approach has been proposed to control the start-up mode of the membrane module of the process. For control purposes, a mathematical model of the dynamics of the membrane distillation process is proposed. The written mathematical model of the contact membrane distillation process is nonlinear with respect to the temperature of the solution at the outlet of the membrane module, which is also included in the equations that take into account the vapor flow through the membrane. With an increase in the temperature of the solution at the inlet of the membrane module, the temperatures of the solution and distillate at the outlet of the membrane module increase nonlinearly. The optimality criterion was the minimum process start-up time in the presence of restrictions on the final solution temperature. The penalty method and the gradient procedure on the second interval were used to change the control in order to reach the specified regime. The solution depends on the value of the weight coefficients of the penalty functions, which allowed reaching the specified regime in the minimum time.