The variation of daily methane concentration in CPE-fixed zeolite bioreactor and the control under different OLR and HRT are shown in Fig. 7b. During the first HRT (35 d) at an OLR of 0.12 g-DOCadded/l-reactor/d, the CPE-fixed zeolite bioreactor compared with the control achieved higher and more stable methane concentration. When increasing the OLR to 0.51 g-DOCadded/l-reactor/d under subsequent HRT (14 d), the CPE-fixed zeolite bioreactor remained stable methane concentration around 90%, while that SLx-2119 in the control exhibited a slight decrease and fluctuation. Further increasing the OLR to 1.01 g-DOCadded/l-reactor/d in the third HRT of 7 d, methane concentration in the control decreased dramatically, whereas that in the CPE-fixed zeolite bioreactor remained as high as 80–90%. The CPE-fixed zeolite bioreactor obtained higher and more stable methane concentration, even under short HRT, high OLR and NH4+-N conditions. The result showed that CPE-fixed zeolite system could effectively enhance methane concentration in the bioreactor, and palisade enhancement depended on the specific immobilization mode of zeolite by CPE bed material.