Landfill aeration may cause economic problems due to the amount of power consumed by blowers. Thus, this study proposes the use of an air amplifier to reduce the power consumption of air injection into landfills. The developed air amplifier is an aerodynamic device that induces a large amount of airflow using a small quantity of compressed air caused by the Coanda effect. Field experiment results demonstrated that the use of the air amplifier reduced power consumption by at least 90% and showed an air amplification effect of approximately 3.5 times compared with existing in-situ aeration systems. After aeration, the methane (CH<sub>4</sub>) reduction efficiency was 90.3%. The CH<sub>4</sub>/CO<sub>2</sub> ratio was 0.12 (0.06–0.25) on average, and the CH<sub>4</sub>/CO<sub>2</sub> ratio decreased as the oxygen concentration increased. Thus, the air amplifier is a low-cost solution for landfill aeration systems. In addition, aeration using existing leachate collection and drainage pipes was found to be more economical than air injection using air injection wells. However, despite the air injection, approximately 20% of organic carbon was decomposed anaerobically. The CH<sub>4</sub>/CO<sub>2</sub> ratio range of 0.56–0.90 was presented as a criterion for categorizing a landfill as semi-aerobic.