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11-5 OTHER PACKED-BED FILTERS 751 TABLE 11-15 Comparison of the performance of the Waterloo biofilter under conditions of natural convection and forced-air ventilation* Constituent Unit Influent Effluent Reduction Natural convection 130137 mg/L 234 14.4 93.5 NH4-N rnglL 8.1 10.2 NO,-N mgfL 0.5 24.7 Total coliform C1=U1100 mL 2.8 X 108 3.1 X 104 99-985 Forced ventilation BOD, mg/L 110 1.7 98.4 TSS mg/L 115 5.0 93.9 NH,-N mg/L 2.3 0.04 NO3-N mg/L 0.4 33.8 Total coliform CFU1100 mL 7.3 x 109 1.3 x 104 99"998 *From Jowett and McMaster(1994). The wastewater can flow vertically without clogging the medium while allowing air to flow in a separate pathway through the medium.When water moves through an absorbent medium, it.uses the interiors of the particles rather than the surrounding voids and is transferred from particle to particle across the points of contact. The large interparticle voids remain open for air circulation.Air flow around the particles occurs and oxygen is transferred directly to the bacterial film, without having to diffuse through water. Although the Waterloo biofilter is still-in the development stage, it represents a future trend in optimizing the size and loading rates for biological filters. In col- umn studies, the filter was loaded at 20 gal/ft2-d (800 LJm2-d), based on forward flow, which is 4 times higher than a recirculating gravel filter and 20 times higher P than typical rates for ISFs. A-comparison of the performance of the.Waterloo biofil- ter under conditions of natural convection and forced ventilation is presented in Table-11-15. Field units for treatment of septic tank effluent were tested in Ontario.The units consisted of 155 ft3 (4.4 M3)polyethylene or concrete tanks,with a filter surface area of about 35 ft2 (3.3 m2). The septic tank cffluent is pumped through perforated dis- tribution pipes that are set above the filter medium, flows slowly down and through the medium,and is discharged out the bottom.A-typical filter is shown in Fig. 11-27. Textile Packed-Bed Filter The textile filter (also known as the textile bioreactor) was developed as an alter- native to sand and gravel filters. By utilizing nonwoven textile chips (i.e., small pieces of cut textile, sometimes called squares or coupons) instead of a granular medium, hydraulic loading rates can be increased significantly, thus reducing space requirements for the filter. The textile filter can be operated as a single-pass filter or a recirculating filter, depending on flow and design considerations. The applied