COMPARING THE EFFECTIVENESS OF POSITIVE AND NEGATIVE AERATION IN CONTROLING EMISSIONS FROM COMPOSTING PROCESSES

Odor is released from composting processes with the discharge of process air and with fugitive emissions from bed surfaces. Untreated exhaust may cause off-site odor complaints. In the case of enclosed processes, inadequate air handling systems may contribute to corrosion and excessive occupational exposure to air toxics. The air handling challenges, as discussed in this paper, relate to both aerated static pile and agitated bed composting processes, treating biosolids, animal waste, or other organic waste materials.A portion of the total pollutant mass may be captured for treatment by drawing the process air into the pile process under negative pressure and discharging the exhaust through an odor control process such as a biofilter. However, the process aeration blowers must handle the hot corrosive exhaust air plus condensate, instead of handling clean air, resulting in higher operational costs than with positive aeration. It is not possible to capture all of the exhaust through negative aeration, due to the following losses: convective surface emissions and diffusion due to high bed temperatures; intermittent collection during on-off cycles of process aeration blowers; and material handling activities.This paper presents the results of full-scale experiments at two facilities in which the emissions from positive and negative aeration were compared under otherwise similar operating conditions, in order to evaluate the odor control benefit of negative aeration.Solid waste Authority of Palm Beach County – Enclosed agitated bed facilityCity of Columbus, Ohio – Outdoor aerated static pile facilityBoth facilities process 25 dry tons per day of undigested sludge. For both facilities surface emissions were sampled using isolation flux chambers and odor concentration was analyzed using ASTM Method 679–91. Some discrepancies in odor emissions results suggest that the isolation flux chamber may under-estimate the odor emissions from a compost pile surface.Although the odor emission rates at the two facilities differed significantly, it was concluded that negative aeration could capture 65 to 69 percent of pile surface emissions. When all odor sources are considered, negative aeration with treatment of process exhaust provides significant reduction in odor emissions as compared with no capture or treatment, but not as such reduction as total enclosure and treatment of all exhaust.