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Biofilters using media of crop residue materials have been shown effective in removing odorants from air exhausted from livestock buildings, especially swine facilities. Adequate media moisture is critical in maintaining biological growth in filters, and most biofilter installations use either portable or permanently installed equipment for adding water to the media. A reliable method is needed for continuously evaluating media moisture, to determine when water should be added to the filter to replenish moisture levels deep within the filter.

This project tested five different types of moisture meters in a typical biofilter medium: Lincoln Irrigation soil moisture meter, Farmex HMT-3 digital hay moisture meter, Campbell Scientific Hydrosense digital soil moisture meter, Vaisala Hummiter50Y relative humidity probe, and a site-built radio-frequency large-area capacitive plate sensor.

None of the soil and hay moisture meters performed adequately in predicting biofilter media moisture content. The relative humidity probe did, however, produce a calibration curve with a viable sensitivity in the media moisture range of interest, and was subsequently tested for a period of months in a field application. The large-area capacitive plate sensor showed nearly linear response of system capacitance with moisture content on laboratory tests at media moisture contents of 10 to 70% (wet basis) over energized frequencies of 300 kHz to 15 MHz.