In this paper, a model is developed to optimally manage the power flows between the different components of a grid-connected hybrid energy system supplying a marine outfall wastewater treatment plant. The system is equipped with photovoltaic/battery system and an energy recovery system generating power using pressurized wastewater supplied by the treatment plant. For simulation purpose, a case study of a marine outfall system situated in Cape Town is used to reveal the dynamic of the proposed setup and model. The aim of the developed model is to lessen the grid energy costs which operates under the variable time of use tariff. This can be achievable by capitalizing on the generated renewable power from both the photovoltaic and energy recovery systems, while optimizing the battery storage power flow. Based on the simulation results, the established model revealed a daily energy cost saving of around 75.5% that can be achieved from the proposed marine outfall’s operation, as compared to the use of utility grid as the only source of electricity used to meet the same load demand.