Biofiltration is commonly used for the treatment of gases containing low concentrations of ammonia. One very important biofilter operational parameter is the media moisture. High media moisture content usually favors ammonia removal, but leads to production of nitrous oxide - a powerful greenhouse gas. Monitoring and controlling moisture is a technical challenge, since the distribution of moisture in a biofilter is rarely even and the media particle size is often varied and includes some very large particles. The present study develops a high-frequency large format moisture sensor that is able to monitor media moisture changes within a 30cmx30cmx30cm (LxWxH) space. As a part of the sensor development, a moisture control strategy was designed to manage moistures in two identical bench-scale cylindrical (ID= 45cm, H= 50cm) biofilters packed with woodchip media. In a one-month test, the moistures were successfully maintained within a narrow range of 44-47% (wet basis), and the overall moisture change was as low as 1.08% and 0.30% for the two biofilters, respectively. The sensor outputs were stable and sensitive to moisture changes. At the controlled moisture range, the two biofilters showed high ammonia removal efficiencies (82-92%) and very low produced nitrous oxide concentrations (0-0.32 ppm). Overall, the results indicate that an unprecedented biofilter performance could be achieved with a precise moisture monitoring and control system.