Intermediate- and High-Rate Filters. Hig gh-rate filters use either a rock or plastic packing. The filters are usually circular and flow is usually continuous. Recirculation of the filter effluent or final effluent permits higher organic loadings, provides higher dosing rates on the filter to improve the liquid distribution and better control of the slime layer thickness, provides more oxygen in the influent wastewater flow. and returns viable organisms. Recirculation also helps Fig 8-2 to prevent ponding in the filter and Typical trickling filter to reduce the nuisance from odors process flow diagrams: and flies. Intermediate-and high signed as single- or two-stage first two of each series. processes. Flow diagrams for S(+R)_J varIous filter 2. Two filte at the same hydraulic application perform as if they were one unit with the same total depth Roughing Filters. Roughing treat an organic load of 1.6 kg/m d and hydraulic loadings up to 190 m/m?.d In most cases, roughing filters are used to treat wastewater prior to secondary treatment. Most roughing filters designed using plastic packing One of the advantages of roughing filte is the loy requirement for BOD removal of higher strength wastewaters as compared to activated-sludge aeration. Because the energy required is only for pumping the recirculation flows. the amount of BOD removal per unit of energy input can increase as the wastewater strength increases until s Sludge return =-口 more recirculation is needed to R Recirculated flow Secondary clarifier dilute the influent wastewater concentration or to Increase wetting efficiency. The energy requirement for a roughing application may range from 2 to 4 kg BOd applied/kWh versus 1.2 to 2. 4 kg BOD/k Wh for activated-sludge treatment Two-Stage Filters. A two-stage filter system, with an intermediate clarifier to remove solids generated by the first filter, is most often used with high-strength waste-water(Fig. 8-2b). Two-stage systems are also used where nitrification is required. The first-stage filter and intermediate clarifier reduce carbonaceous BOD, and nitrification takes place in the second stage Nitrification. Both BOD removal and nitrification can be accomplished in rock or plastic packing trickling filters operated at low organic loadings. Heterotrophic bacteria, with higher yield coefficients and faster growth rates, are more competitive than nitrifying bacteria for space on the fixed-film packing. Thus, significant nitrification occurs only after the bOd concentration is appreciably reduced Bruce et al. (1975) demonstrated that the effluent bOd had to be less than 30 mg/l to initiate nitrification and less than 15 concentration less than 20 mg/L is needed to initiate nitrification. Nitrification can also be accomplIshed.? mg/L for complete nitrification. Harrem es(1982)considered the soluble BOD, and concluded that separate trickling filters following secondary treatment8-5 Intermediate- and High-Rate Filters. High-rate filters use either a rock or plastic packing. The filters are usually circular and flow is usually continuous. Recirculation of the filter effluent or final effluent permits higher organic loadings, provides higher dosing rates on the filter to improve the liquid distribution and better control of the slime layer thickness, provides more oxygen in the influent wastewater flow, and returns viable organisms. Recirculation also helps to prevent ponding in the filter and to reduce the nuisance from odors and flies. Intermediate-and high rate trickling filters may be designed as single- or two-stage processes. Flow diagrams for various trickling filter configurations are shown on Fig. 8-2. Two filters in series operating at the same hydraulic application rate (m3 /m2 .h) will typically perform as if they were one unit with the same total depth. Roughing Filters. Roughing filters are high-rate-type filters that treat an organic load of more than 1.6 kg/m3·d and hydraulic loadings up to 190 m3 /m2·d. In most cases, roughing filters are used to treat wastewater prior to secondary treatment. Most roughing filters are designed using plastic packing. One of the advantages of roughing filters is the low energy requirement for BOD removal of higher strength wastewaters as compared to activated-sludge aeration. Because the energy required is only for pumping the influent waste-water and recirculation flows, the amount of BOD removal per unit of energy input can increase as the wastewater strength increases until more recirculation is needed to dilute the influent wastewater concentration or to increase wetting efficiency. The energy requirement for a roughing application may range from 2 to 4 kg BOD applied/kWh versus 1.2 to 2.4 kg BOD/kWh for activated-sludge treatment. Two-Stage Filters. A two-stage filter system, with an intermediate clarifier to remove solids generated by the first filter, is most often used with high-strength waste-water (Fig. 8-2b). Two-stage systems are also used where nitrification is required. The first-stage filter and intermediate clarifier reduce carbonaceous BOD, and nitrification takes place in the second stage. Nitrification. Both BOD removal and nitrification can be accomplished in rock or plastic packing trickling filters operated at low organic loadings. Heterotrophic bacteria, with higher yield coefficients and faster growth rates, are more competitive than nitrifying bacteria for space on the fixed-film packing. Thus, significant nitrification occurs only after the BOD concentration is appreciably reduced. Bruce et al.(1975) demonstrated that the effluent BOD had to be less than 30 mg/L to initiate nitrification and less than 15 mg/L for complete nitrification. Harrem es (1982) considered the soluble BOD, and concluded that a concentration less than 20 mg/L is needed to initiate nitrification. Nitrification can also be accomplished in separate trickling filters following secondary treatment. Fig. 8-2