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in n 11-2 REMOVAL MECHANISMS IN PACKED-BED FILTERS 709. i (a) t dose/d (b) 4 doses/d (c) 24 doses/d Applied liquid dose fills the Applied liquid dose partially Applied liquid dose flows interstices of filter medium fills the interstices of the around filter medium in a I allowing some influent particles filter medium. thin film. Most interstices to pass through the filter are open. F untreated. FIGURE 11-6 Effect of hydraulic application rate on flow through filter: (a) 1 dose/d, (b) 4 doses/d,' and (c) 24 doses/d. i microorganisms can pass through the filter untreated (Fig. 11-6a). As the volume applied per dose is reduced, partially unsaturated flow occurs (Fig. 11-6b). If the volume applied per dose is reduced further, the liquid will flow around the filter medium in a thin film (Fig. 11-6c). When film flow occurs, the soluble and colloidal . I material in the wastewater is absorbed, and oxygen from the air in the open intersti- tial space is transferred across the thin liquid film to the aerobic bacteria responsible for oxidation of the organic carbonaceous materials. It should be noted that the con- centration - ; I centration of oxygen in air is about.250 mg/1- at 20°C. Because absorption and the high oxygen concentration in air play such an important part in the operation of both IPBFs and RPBFs,it is imperative that the applied liquid flow over the filter medium; in a thin film,-if the sin pass filter is to function most effectively. E Wh6d.ihe�flow,is in a.,thin film,_the-oxidation of carbonaceous materials will occur.tn.the upper portions,of,the filter.bed: Simultaneously, ammonia will be con- verted to nitrate.(nitrificatio_n). In turn, the nitrate will be converted to nitrogen gas (denitrification) in anoxic.microsites within the filter. Biological denitrification has f, been shown to occur under anoxic conditions and,for certain bacteria,under aerobic conditions (Robertson and Kuenen, 1990). The organisms responsible for denitrifi- cation utilize adsorbed.carbon in particulate matter as the energy source. To sustain the performance of the filter, the microorganisms in the filter must be maintained ' in the endogenous growth rate. if too much organic material is applied, the bacte— rial growth rate will increase and the accumulation of material can occur, ultimately leading to failure, as described later. Flow in a thin film is especially important if viruses are to be removed.In a recent study,it has been demonstrated that the formation of a uniform dense bacterial film, produced by increasing the number of doses per day, will have a significant effect on virus removal(Emerick et al.;A997a).Although sintered glass was used as the filter- ing medium,a similar effect was observed with sand,as shown in Fig. 1 l-7(Emerick i