Moisture monitoring and control in gas-phase biofilters to achieve high ammonia removal efficiency and low nitrous oxide generation

Biofiltration is a relatively low-cost method for the treatment of large flow rates of livestock air emissions that usually contain low concentrations of ammonia. Moisture content is a key factor that determines the success of biofilters since low moisture content can reduce ammonia removal efficiency while high moisture content may increase production of nitrous oxide, which is a powerful greenhouse gas. Controlling moisture content at 45% to 55% (wet basis) is recommended to achieve high ammonia removal and low nitrous oxide generation. This study aims to develop an impedance-based moisture sensor, together with a well-designed water pump control strategy, to provide a real-time biofilter moisture content monitoring and control method with the purpose of maintaining reliable biofilter performance. A high-frequency large-format moisture sensor that is able to measure medium moisture content within a 30 cm x 30 cm x 15 cm (L x W x H) space was developed and tested. The sensor was sensitive to moisture content ranging from 15% to 63%. A water pump control strategy was designed to maintain the moisture contents within 44% to 49% in two identical bench-scale cylindrical (ID = 45 cm, H = 50 cm) biofilters packed with a commonly used medium. In a one-month test, the sensor readings were stable and sensitive to moisture changes. The moisture contents were successfully managed within 44% to 47%, and the overall moisture changes were as low as 1.08% and 0.30% for the two biofilters, respectively, according to a water balance calculation. Moisture contents measured by oven-drying were 5% to 10% higher than the sensor-measured results. We believe the differences displayed between the sensor-measured results and the oven-drying results are acceptable for biofilter applications. The two biofilters showed high ammonia removal efficiencies (82% to 92%) and low nitrous oxide concentrations (0 to 0.32 ppm), demonstrating the feasibility of the moisture monitoring and control method in bench-scale biofilters.