MINIMUM-COST BIOFILTERS FOR REDUCING ODORS AND OTHER AERIAL EMISSIONS FROM LIVESTOCK BUILDINGS .1. BASIC AIR-FLOW ASPECTS

The pressure drop per unit depth (Delta P Pa/m) across seven potential minimum-cost biofilter media was determined at different moisture contents for superficial air velocities (u) between 0.03 and 0.13 m/s in pilot scale biofilters, and fitted to the equation Delta P = au(b) where a and b are constants. Mean values of a ranged from around 200, for pure heather, to around 6000 for a peat-heather mixture. Mean values of b ranged from 1.20, for coir, to 1.69 for softwood chips of over 75 mm screen size. Estimates were made of particle specific surface area and void fraction, under the conditions of use, for some of the packing media, thus allowing calculation of the modified Reynolds numbers prevailing during the experiments. This showed that the values of b observed experimentally were within the range expected from the Kozeny-Carman theory. Based on the physical characteristics of the various packing media studied, soft-wood chips of over 75 mm screen size appeared to be the most promising because they gave one of the lowest pressure drops, the lowest coefficient of variation for air distribution across the packing, and were the least compressible. All of these three physical factors are important for efficient, minimum-cost operation of biofilters in agricultural buildings. (C) 1995 Silsoe Research Institute