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The feasibility of anaerobic co-digestion of waste activated sludge (WAS) and grass Egeria densa (E.d.) with four different WAS: E.d. ratios was evaluated for the first time under mesophilic conditions. First-order kinetic, modified Gompertz, and Cone models were employed to reveal the principle kinetics of methane-rich bioenergy production from co-digestion. The results showed that the addition of E.d. could greatly upgrade the sludge methane production, and the highest methane yield averaged 198.32 ± 2.61 mL/g VS_added for WAS: E.d. ratio of 0.7: 0.3, 18.72 ± 0.14% higher respective to that of WAS alone (about 176.36 mL/g VS_added). E.d. improved the solubilization of co-substrates and avoided the build-up of volatile fatty acids (VFAs) and free ammonia ([NH₃]) and subsequent inhibition, inducing a stable digestion environment. Model simulation indicated Cone model best fitted the actual evolution of …