Full-scale biofilter reduction efficiencies assessed using portable 24-hour sampling units

Portable 24-hr sampling units were used to collect air samples from eight biofilters on four animal feeding operations. The biofilters were located on a dairy, a swine nursery, and two swine finishing farms. Biofilter media characteristics (age, porosity, density, particle size, water absorption capacity, pressure drop) and ammonia (NH₃), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), methane (CH₄), and nitrous oxide (N₂O) reduction efficiencies of the biofilters were assessed. The deep bed biofilters at the dairy farm, which were in use for a few months, had the most porous media and lowest unit pressure drops. The average media porosity and density were 75% and 180 kg/m³, respectively. Reduction efficiencies of H₂S and NH₃ (biofilter 1: 64% NH₃, 76% H₂S; biofilter 2: 53% NH₃, 85% H₂S) were close to those reported for pilot-scale biofilters. No N₂O production was measured at the dairy farm. The highest H₂S, SO₂, NH₃, and CH₄ reduction efficiencies were measured from a flat-bed biofilter at the swine nursery farm. However, the highest N₂O generation (29.2%) was also measured from this biofilter. This flat-bed biofilter media was dense and had the lowest porosity. A garden sprinkler was used to add water to this biofilter, which may have filled media pores and caused N₂O production under anaerobic conditions. Concentrations of H₂S and NH₃ were determined using the portable 24-hr sampling units and compared to ones measured with a semicontinuous gas sampling system at one farm. Flat-bed biofilters at the swine finishing farms also produced low amounts of N₂O. The N₂O production rate of the newer media (2 years old) with higher porosity was lower than that of older media (3 years old) (P = 0.042). Implications: Media characteristics and gas reduction efficiencies of eight field-scale biofilters were evaluated. The media characteristics measured in this study indicated that media older than 3 years should be tested for high-pressure drops and nitrous oxide (N₂O) production. The results also indicated that regular water addition can improve gas reduction efficiencies of biofilters but adding excessive amounts of water can cause N₂O production. Uniform water distribution can be a challenge.