In this paper, we address the optimal control of carbon and nitrogen removal from wastewaters in the presence of inhibition by substrates and products by Anaerobic Digestion Model n degrees 1 (ADM1). The main objective is the optimal control of the dilution in order to enhance the concentration of methane gas. The ADM1 was previously calibrated using experimental data from batch experiments from a fish processing industry in order to guarantee the simultaneous carbon and nitrogen removal. The maximum concentration of methane gas is achieved by extremum seeking control based on periodic perturbation signals. The results show that the proposed extremum seeking method is effective in controlling the systems dilution and obtaining the maximum concentration of methane. The numerical simulation also verifies that the method provides adequate increase in production, improvement rate and settling time. A new extremum feedback design based on q-trigonometry is introduced to achieve faster closed-loop responses when compared to the classical approach. (C) 2019 Elsevier Ltd. All rights reserved.