The feasibility of anaerobic co-digestion of waste activated sludge (WAS) and grass Egeria densa (Ed.) 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 VSadded 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 VSadded). 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 methane production, as evidenced by low Root Mean Square Prediction Error (rMSPE) and Akaike's Information Criterion (AIC), as well as high Pearson's correlation between the predicated and actual values. Additionally, the parameters analysis highlighted that the co-digestion with Ed. substantially promoted the hydrolysis rate (k(hyd)) and methanogenesis potential (f(d)) of sludge, further explaining the increased solids removal and output of methane. This study demonstrated the sustainability and attractiveness of anaerobic co-digestion of sludge and grass E.d., providing a sound basis for cost-efficient biomass stabilization and bioenergy recovery. (C) 2015 Elsevier Ltd. All rights reserved.