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11-4 MULTIPASS (RECIRCULATING)'PACKED-BED FILTERS 743 TABLE 11-13 Nitrogen removal by recirculating filters Medium Loading..:..- - -•----- size, rate, Total nitrogen,mg/L Percent Location mm gal/W-d' Influent Effluent removal Oregon(Ronayne, 1984) 1.2 1.5 58 32 45 Paradise,California(Nolte and Associates, 1992)• 3.0 4.3 63 35 44 Paradise, California(Nolte and Associates, 1992) 3.0 2.5 57 26 54 Florida(Sandy et al., 1988) 2.4 3.8 55 9.6 82 '(9aVft2-d)x•40 = mmld 1 - Table 11-13. Nitrification and denitrification are responsible for the removal. In- crease- 'riitr`ogen removal can be achieved by using"flooded underdrains to-enhahcef denitrification, following the filter with an anaerobic filter, or addin • a supple- -mental carbon source, If the ammonia level is high (over 60 mg/L), and alkalinity levels are low, nitrification can reduce the pH excessively. As ammonia is nitrified; 7.14 mg of alkalinity, expressed as calcium carbonate (CaCO3), is destroyed for every 1 mg of ammonia oxidized to nitrate (see Example 2-9 in Chap. 2). If the community water supply is a soft,- ow-alkalinity water, there may be insufficient alkalinity to buffer the pH during nitrification.If the pH is reduced through the filter, the overall performance of the bacteria for removal of BOD and ammonia can be impaired. A source of alkalinity,such as sodium or calcium carbonate,can be added. Alternatively, the denitrification process, which restores about half of the alkalinity lost in nitrification, can be maximized. Alternatives to maximize nitrogen removal . with recirculating sand filters are discussed in Sec. 11-6. Design Considerations , Important factors in the design of RPBFs include type and size of filter medium, depth of filter bed, hydraulic loading rate, organic loading rate, dosing frequency and rate, distributed dosing, and modular design. These design considerations are l examined in the following material. Typical design criteria for recirculating filters are presented in Table 11-14.The design of an RPBF is illustrated in Example 11-6. Type.,and size of.-filter media. The types of filter media used for RPBFs range from coarse sand to bottom ash to fine gravel.The finest medium reported was a sand with an effective size of 0.3 mm(Belicek, 1986).Excessive clogging and short filter runs were observed with this sand size, and the investigators recommended an increased sand size of 0.6 mm. Screened bottom ash has been used in sizes ranging from 0.9 to 2.4 mm. The range of the medium sizes for gravel filters is from 1 to 6 mm. A typical value for a fine gravel size is 2.5 mm.When fine gravel larger than 3 mm is used, a sharp drop in the degree of nitrification has been observed(Ball and Denn, 1997). i