secondary clarification. Their unique advantage is the small footprint with an area requirement that is a fraction(one-fifth to one-third)of that needed for activated-sludge treatment. Though they are more compact, their capital costs are generally higher than that for activated-sludge treatment In addition to BOD removal, submerged attached growth processes have also been used for tertiary nitrification and denitrification following suspended or attached growth nitrification Downflow and upflow packed-bed reactors, fluidized-bed reactors, and submerged RBCs can be used for postanoxic denitrification. Trickling filters and upflow packed-bed reactors are also used for preanoxic denitrification Mass Transfer Limitations A significant process feature of attached growth processes in contrast to activated-sludge treatment is the fact that the performance of biofilm processes is often diffusion-limited. Substrate removal and electron donor utilization occur within the depth of the attached growth biofilm and subsequently the overall removal rates are a function of diffusion rates and the electron donor and electron acceptor concentrations at various locations in the biofilm. By comparison, the process kinetics for the activated-sludge process are generally characterized by the bulk liquid concentrations The diffusion-limited concept is especially important when considering the measurable bulk liquid dO concentrations on attached growth process biological reaction rates. Where a DO concentration of 2 to 3 mg/L is generally considered satisfactory for most suspended growth aerobic processes, such low DO concentrations can be limiting for attached growth processes For uninhibited nitrification in the biofilm, a higher DO equired depending on the ammonia concentratio The concept of diffusion rates and the ability to develop anaerobic layer within the biofilm may be exploited to accomplish both growth processes with positive bulk liquid DO concentrations wh rock replaced by random plastic packing, (e) intermediate depth trickling filter converted to tower trickling filter, no design ypical examples of trickling filters: (a) conventional shallow-depth rock trickling filter, b) seven-sided trickling filter older Fig 8-I J one of four tower trickling filters 10 m high and 50 m in diameter with plastic packing. Blowers, used to provide air for ogical treatment, are located in the enclosures shown at the bottom lef and right-hand side of the tower filter. See Fig.9-4 amples of the rotary distributors used to apply wastewater to the top of the filter packing 8-2 Trickling Filters Trickling filters have been used to provide biological wastewater treatment of municipal and industrial wastewaters for nearly 100 years. As noted above, the trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continuously Treatment occurs as the liquid flows over the attached biofilm. The depth of the rock packing ranges from 0.9 to 2.5 m and averages 1. 8 m. Rock filter beds are usually circular, and the liquid waste Water is distributed over the top of the bed by a rotary distributor Many conventional trickling filters using rock as the packing material have been converted to plastic packing to increase treatment capacity. Virtually all new trickling filters are now constructed with plasti packing Trickling filters that use plastic packing have been built in round, square, and other shapes with depths arying from 4 to 12 m. In addition to the packing, other components of the trickling filter include a wastewater dosing or application system, an underdrain, and a structure to contain the packing The underdrain system is important both for collecting the trickling filter effluent liquid and as a porous structure through which air can circulate. The collected liquid is passed to a sedimentation tank where the solids are separated from the treated wastewater. In practice, a portion of the liquid collected in the underdrain system or the settled effluent is recycled to the trickling filter feed flow, usually to dilute the strength of the incoming wastewater and to maintain enough wetting to keep the biological slime lay 8-28-2 secondary clarification. Their unique advantage is the small footprint with an area requirement that is a fraction (one-fifth to one-third) of that needed for activated-sludge treatment. Though they are more compact, their capital costs are generally higher than that for activated-sludge treatment. In addition to BOD removal, submerged attached growth processes have also been used for tertiary nitrification and denitrification following suspended or attached growth nitrification. Downflow and upflow packed-bed reactors, fluidized-bed reactors, and submerged RBCs can be used for postanoxic denitrification. Trickling filters and upflow packed-bed reactors are also used for preanoxic denitrification. Mass Transfer Limitations A significant process feature of attached growth processes in contrast to activated-sludge treatment is the fact that the performance of biofilm processes is often diffusion-limited. Substrate removal and electron donor utilization occur within the depth of the attached growth biofilm and subsequently the overall removal rates are a function of diffusion rates and the electron donor and electron acceptor concentrations at various locations in the biofilm. By comparison, the process kinetics for the activated-sludge process are generally characterized by the bulk liquid concentrations. The diffusion-limited concept is especially important when considering the measurable bulk liquid DO concentrations on attached growth process biological reaction rates. Where a DO concentration of 2 to 3 mg/L is generally considered satisfactory for most suspended growth aerobic processes, such low DO concentrations can be limiting for attached growth processes. For uninhibited nitrification in the biofilm, a much higher DO concentration may be required depending on the ammonia concentration. The concept of diffusion limitations on nitrification rates and the ability to develop anaerobic layers within the biofilm may be exploited to accomplish both nitrification and denitrification in attached growth processes with positive bulk liquid DO concentrations. 8-2 Trickling Filters Trickling filters have been used to provide biological wastewater treatment of municipal and industrial wastewaters for nearly 100 years. As noted above, the tricklings filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continuously. Treatment occurs as the liquid flows over the attached biofilm. The depth of the rock packing ranges from 0.9 to 2.5 m and averages 1.8 m. Rock filter beds are usually circular, and the liquid waste Water is distributed over the top of the bed by a rotary distributor. Many conventional trickling filters using rock as the packing material have been converted to plastic packing to increase treatment capacity. Virtually all new trickling filters are now constructed with plastic packing. Trickling filters that use plastic packing have been built in round, square, and other shapes with depths varying from 4 to 12 m. In addition to the packing, other components of the trickling filter include a wastewater dosing or application system, an underdrain, and a structure to contain the packing. The underdrain system is important both for collecting the trickling filter effluent liquid and as a porous structure through which air can circulate. The collected liquid is passed to a sedimentation tank where the solids are separated from the treated wastewater. In practice, a portion of the liquid collected in the underdrain system or the settled effluent is recycled to the trickling filter feed flow, usually to dilute the strength of the incoming wastewater and to maintain enough wetting to keep the biological slime layer Fig. 8-1