Abstract: Abstract: In situ methane enrichment method can increase the methane content in biogas, which is a cost-effective technology used for producing bio-methane compared with conventional biogas upgrading technologies. During the in situ methane enrichment process, liquid stream from the anaerobic digester is circulated through a CO2-desorption unit and then returned back to the digester. The most important step in this process is CO2 desorption. In this study we evaluated the effects of different CO2 desorption techniques. Three CO2 desorption methods including air stripping, vacuum extraction and ultrasonic-auxiliary air stripping were studied through experiments. The experiments were conducted under mesophilic (38℃) anaerobic digestion by using synthetic wastewater with glucose and supplemental nutrients. The experimental apparatus included one anaerobic digester with volume of 1 500 mL and two CO2-desorption bottles with volume of 500 mL used for air stripping and vacuum extraction respectively. The results showed that the average volume fraction of methane in biogas reached 76.4% and 78.3% for the air stripping groups (aeration fluxes of 205 mL/min and daily stripping time of 30min), and increased by 17% and 20% compared to the control (65.4%) with the daily recycle rates of liquid stream (DRR) at 0.2 L/(L·d) and 0.4 L/(L·d), respectively. The cumulative methane production is similar to controlled one for the group with DRR of 0.2 L/(L·d). But the cumulative methane production decreased by 15% compared with control one for the group with DRR of 0.4 L/(L·d). In situ methane enrichment effect of the group of CO2 vacuum extraction with 0.1 MPa vacuum maintaining 5 min was slightly lower than that of the air stripping group with aeration flux at 205 mL/min, daily stripping time 20min and DDR 0.2 L/(L·d). The methane volume fraction of biogas in the first group was decreased by 4.6% compared to that of the second group . However, the difference is not significant (P>0.05). The results of experiment of ultrasonic-auxiliary air stripping, with aeration flux of 100 mL/min, daily stripping time of 21 min and ultrasonic frequency of 40 kHz, show that ultrasonic wave can accelerate CO2 air stripping, especially when the concentration of CO2 in the liquid was high. During the initial 3 min of stripping the CO2 removing rate of ultrasonic-auxiliary air stripping group was advanced by 32% compared to air stripping group. The total free CO2 removal amount from the liquid stream was increased by 29.8% compared to air stripping group during the whole stripping time of 21 min. Furthermore, ultrasonic wave has potential advantages on CO2 removing by lowering pH of liquid and increasing anaerobic digestion efficiency through decomposing many complex organic compounds. Therefore, the effect of air stripping and vacuum extraction on methane enrichment has no significance difference, but vacuum extraction has no problem of oxygen inhibition, the control of which is vital for successful running of in situ methane enrichment process of air stripping. Ultrasonic-auxiliary air stripping method has advantages for CO2 desorption due to its unique characteristics of lowering pH, degrading complex compounds and increasing CO2 removal.