Douglas-fir bark, 3/4 inch and 1 1/2 inch in size, was tested in recirculating trickling filters 1, 2, and 3 feet in depth, hydraulically loaded at 2.27, 4.54, and 9.08 gallons per minute per square foot (gpm/sq.ft.) with 2 and 4 percent poultry manure slurry at 70° F. Preliminary tests...