Objectives: Biofiltration is a proven method to reduce odor and gas emissions from swine buildings and manure storage units. Biofilter media selection and moisture content fluctuations significantly influence biofilter performance. Media with significant amounts of small particles have higher static pressure drops which lowers airflow through the biofilter. Inadequate moisture can reduce biological activity and decrease filter effectiveness. Too much moisture (following a watering or rainfall event) can plug media pores, restrict airflow through the media, produce anaerobic pockets and generate nitrous oxide (N2O). In this study, a naturally occurring adsorbent (diatomaceous shale) was used as an additive to manage biofilter media moisture content. Also, corn cobs were tested as an alternative biofilter media because wood chips are expected to be less available in near future due to emerging emerald ash borer disease. The objectives of the study were to 1) Measure water adsorption and desorption capacities of the diatomaceous shale, 2) Test the effectiveness of the corn cobs as biofilter media and diatomaceous shale as a biofilter media additive, and 3) Estimate the cost of using the diatomaceous shale as a biofilter media additive.
How research was conducted: The experiments were conducted at the University of Minnesota West Central Research and Outreach Center (WCROC, Morris, MN). Five 3.3 ft ×3.3 ft ×3.3 ft wooden biofilter cells were built to treat air from pit fans on a swine finishing barn. Four cells were filled with whole corn cobs. One of the four cells contained no diatomaceous shale and the other three cells contained 5, 10, or 15% (by volume) shale. The fifth cell was filled with wood chips. The media depths were- wood chips: 0.5 m, corn cobs: 3.3 ft, corn cobs with 5, 10, and 15% shale: 3 ft. Each cell had its own air blower that pulled air from a duct connected to two exhaust pit fans.
Media characteristics (water absorption capacity, porosity, density and pressure drops across the media) were measured as described in previous studies. A semi-continuous sampling system was used to measure biofilter cells’ inlet and outlet gas concentrations. Gas sampling started on September 12, 2013 and continued until November 22, 2013. The GSS sampled six sample lines and the ambient airline for 15 min 14 times a day and daily averages were calculated using 14 sample averages.
Discussion of the findings: Diatomaceous shale was found to have good water adsorption capacity but superior desorption capacity (desorbs most of the water it adsorbs when relative humidity of the environment is 50%) unlike other adsorbents (e.g., activated carbon, diatomaceous earth, zeolite) that can desorb less than 5% of the water they adsorb in 24 hours.
The corn cobs were less dense and more porous than the wood chips at all moisture levels tested. At wet conditions (60 and 74% moisture levels), wood chips had low porosity, which may result in anaerobic pocket formation and nitrous oxide generation. At any moisture level and mixing ratio corn cobs’ porosity values were above 25%. The cobs had more than 100% moisture absorption capacity at dry conditions (13.5%) while the absorption capacity of the wood chips was 57%.
Wood chips had the lowest percent gas reduction efficiencies; ability to remove gases from manure pit ventilation air. Corn cobs with 15% shale had the highest percent reduction efficiencies, followed by corn cobs with 10% shale and corn cobs with 5% shale. The average H2S and NH3 percent reduction efficiencies of corn cobs with 15% shale were much higher than those of corn cobs without shale (cobs with 15% shale- H2S: 86%, NH3: 50%; cobs without shale-H2S: 13%, NH3: 12%). The high percent reduction efficiency of the cobs with shale was probably due to shale’s good moisture adsorption and desorption capacity. Inlet and outlet N2O concentrations of the biofilter cells were close to each other and no N2O generation was observed even after water was added. The estimated cost of the corn cob media with 15% diatomaceous shale was $0.88 per cubic feet per minute (cfm) air treated. The corn cobs cost was less than $0.05 per cfm air treated.
What these findings mean to the industry: Corn cobs with 15% diatomaceous shale can be used as biofilter media to treat H2S and NH3 emissions from swine buildings. Corn cobs are another biofilter media option if wood chips become unavailable due to emerging emerald ash borer disease. Corn cobs would be a good alternative media with excellent moisture absorption capacity. Although using diatomaceous shale increases cost, it significantly improves gas reduction efficiencies and reduces the need of a watering system. Useful life of corn cobs as biofilter media still needs to be determined.
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