The biological purification of municipal wastewater as an organic waste with a low carbon (C)/nitrogen (N) ratio was explored in the presence of a carbon-rich lignocellulose substrate with a high carbon/nitrogen ratio in this study. The composition of the substrate was optimised to obtain the best biogas yield while maintaining reactor stability. The best composition was modelled and determined using response surface methods and mixture design. The findings revealed that anaerobic co-digestion of municipal wastewater with high-carbon/nitrogen-ratio substrates such as the sugarcane plant and wasted black tea improves biogas generation. This behaviour was observed in reactors with more than 70% (w/w) cane waste during testing in this study. Spent black tea as a co-substrate is suitable for codigestion with municipal wastewater. However, due to the antibacterial properties of polyphenol and tannins in it, the presence of this substance at a high percentage in combination leads to the loss of useful microorganisms of anaerobic digestion and reduces the biogas production yield. The best substrate composition contains 25% (w/w) lignocellulose waste of sugarcane, 34% (w/w) spent black tea and 41% (w/w) municipal wastewater, which produced 239 ml/g volatile solids of biogas.
