We propose a novel methodology to accomplish the optimal performance of a raceway: an open-channel pond with a rotating paddlewheel, where circulating wastewater is used for the cultivation of algae that will be employed as source for bioenergy production. Algal productivity maximization is addressed by means of optimal control techniques for partial differential equations, within a simulation-based optimization framework. So, we introduce a rigorously detailed mathematical formulation of the optimal control problem (where the state system couples Navier-Stokes equations on a free surface domain with a large system of nonlinear convection-diffusion-reaction equations for the evolution of algae, nutrients and oxygen, where the design variables are the initial height of water and the velocity of the paddlewheel, and where the objective function corresponds to the maximization of algal concentration at the process final time -under some geometric and technological constraints). Then, we suggest a numerical algorithm for its computational resolution where, within an ALE strategy, we use finite element techniques for the simulation step and a gradient-free algorithm for the maximization one. Finally, we show some preliminary results related to the numerical optimization of the problem.