result from the passage of air through the filter and underdrain systen Where natural draft is used, the following needs to be included in the design Underdrains and collecting channels should be designed to flow no more than half full to provide a 2. Ventilating access ports with open grating types of covers should be installed at both ends of the central collection channel 3. Large-diameter filters should have branch collecting channels with ventilating manholes or vent stacks installed at the filter periphery 4. The open area of the slots in the top of the underdrain blocks should not be less than 15 percent of the area of the filter 5. One square meter gross area of open grating in ventilating manholes and vent stacks should be provided for each 23 m- of filter area. he use of forced- or induced-draft fans is recommended for trickling filter designs to provide a reliable supply of oxygen. The costs for a forced-draft air supply are minimal compared to the benefits. For a 3800 m/d wastewater treatment flow the estimated power requirement is only about 0.15 kW. As an approximation, an airflow of 0.3 m/m- min of filter area in either direction is recommended. A downflow direction has some advantage by providing contact time for treating odorous compounds released at the top of the filter and by providing a richer air supply where the oxygen demand is highest. Forced-air designs should provide multiple air distribution points by the use of fans around the periphery of the tower or the use of air headers below the packing material, as there is very little headloss through the filter packing to promote air distribution. For applications with extremely low air temperature it may be necessary to restrict the flow of air through the filter to keep it from freezing Settling Tanks. The function of sealing tanks that follow trickling filters is to produce a clarified effluent. They differ from activated-sludge settling tanks in that the clarifier has a much lower suspended sent to sludge-processing facilities or returned to the primary clarifiers to be settled with primary solf n solids content and sludge recirculation is not necessary. All the sludge from trickling filter settling tanks is Trickling filter performance has historically suffered from poor clarifier designs. The use of shallow clarifiers for trickling filter applications, with relatively high overflow rates, was recommended in previous versions of the " Ten States Standards".Unfortunately, the use of shallow clarifiers typically resulted in poor clarification efficiency. Clarifier overflow rates recommended currently in the"Ten States Standards"are more in line with those used for the activated -sludge process. Clarifier designs for trickling filters should be similar to designs used for activated-sludge process clarifiers, with appropriate feedwell size and depth, increased sidewater depth, and similar hydraulic overflow rates. With proper clarification designs, single-stage trickling filters can achieve a less than 20 mg/L concentration of BOD and TSS Process Design Considerations The trickling filter process appears simple, consisting of a bed of packing material through whic wastewater flows and an external clarifier. In reality, a trickling filter is a very complex system in terms of the characteristics of the attached growth and internal hydrodynamics. In view of these complexities, trickling filter designs are based mainly on empirical relationships derived from pilot-plant and full-scale plant experience. In this section trickling filter performance for BOD removal and nitrification features that affect performance, and commonly used process design approaches are reviewed Effluent Characteristics. Historically, trickling filters have been considered to have major advantages of using less energy than activated-sludge treatment and being easier to operate, but have disadvantages of more potential for odors and lower-quality effluent. Some of these shortcomings, however, have been due more to inadequate ventilation, poor clarifier design, inadequate protection from cold temperatures, and the dosing operation. With proper design, trickling filters have been used successfully in a number of applications. Typical applications, process loadings, and effluent quality are summarized in Table 8-4 Eluent quality Tab. 8-4 Trickling filter applications, loadings and ment kg BOD/m-d- 0.3-1. 0 Loading Criteria In the Combined BOD removal kg BOD/m-d 0.1-0.3 ctivated-sludge process, biodegradation Tertiary nitrification g NH N/mad 0.5-2.5 NH-N,mg/L05-3 iciency was shown to be related to the Partial BOD removal kg BD/m-d 1,540 BOD removal 40-70 average srt for the biomass or the F/M ratio. For both of these parameters, the solids or biomass can be sampled and reasonably well quantified. However, for trickling quantifying the biomass in the system is not possible, and only recently has progress been made to control the solids inventory to some degree by the dosing operation. The attached growth is not uniformly 898-9 result from the passage of air through the filter and underdrain system. Where natural draft is used, the following needs to be included in the design: 1. Underdrains and collecting channels should be designed to flow no more than half full to provide a passageway for the air. 2. Ventilating access ports with open grating types of covers should be installed at both ends of the central collection channel. 3. Large-diameter filters should have branch collecting channels with ventilating manholes or vent stacks installed at the filter periphery. 4. The open area of the slots in the top of the underdrain blocks should not be less than 15 percent of the area of the filter. 5. One square meter gross area of open grating in ventilating manholes and vent stacks should be provided for each 23 m2 of filter area. The use of forced- or induced-draft fans is recommended for trickling filter designs to provide a reliable supply of oxygen. The costs for a forced-draft air supply are minimal compared to the benefits. For a 3800 m3 /d wastewater treatment flow the estimated power requirement is only about 0.15 kW. As an approximation, an airflow of 0.3 m3 /m2·min of filter area in either direction is recommended. A downflow direction has some advantage by providing contact time for treating odorous compounds released at the top of the filter and by providing a richer air supply where the oxygen demand is highest. Forced-air designs should provide multiple air distribution points by the use of fans around the periphery of the tower or the use of air headers below the packing material, as there is very little headloss through the filter packing to promote air distribution. For applications with extremely low air temperature, it may be necessary to restrict the flow of air through the filter to keep it from freezing. Settling Tanks. The function of sealing tanks that follow trickling filters is to produce a clarified effluent. They differ from activated-sludge settling tanks in that the clarifier has a much lower suspended solids content and sludge recirculation is not necessary. All the sludge from trickling filter settling tanks is sent to sludge-processing facilities or returned to the primary clarifiers to be settled with primary solids. Trickling filter performance has historically suffered from poor clarifier designs. The use of shallow clarifiers for trickling filter applications, with relatively high overflow rates, was recommended in previous versions of the "Ten States Standards". Unfortunately, the use of shallow clarifiers typically resulted in poor clarification efficiency. Clarifier overflow rates recommended currently in the “Ten States Standards” are more in line with those used for the activated-sludge process. Clarifier designs for trickling filters should be similar to designs used for activated-sludge process clarifiers, with appropriate feedwell size and depth, increased sidewater depth, and similar hydraulic overflow rates. With proper clarification designs, single-stage trickling filters can achieve a less than 20 mg/L concentration of BOD and TSS. Process Design Considerations The trickling filter process appears simple, consisting of a bed of packing material through which wastewater flows and an external clarifier. In reality, a trickling filter is a very complex system in terms of the characteristics of the attached growth and internal hydrodynamics. In view of these complexities, trickling filter designs are based mainly on empirical relationships derived from pilot-plant and full-scale plant experience. In this section trickling filter performance for BOD removal and nitrification, features that affect performance, and commonly used process design approaches are reviewed. Effluent Characteristics. Historically, trickling filters have been considered to have major advantages of using less energy than activated-sludge treatment and being easier to operate, but have disadvantages of more potential for odors and lower-quality effluent. Some of these shortcomings, however, have been due more to inadequate ventilation, poor clarifier design, inadequate protection from cold temperatures, and the dosing operation. With proper design, trickling filters have been used successfully in a number of applications. Typical applications, process loadings, and effluent quality are summarized in Table 8-4. Loading Criteria. In the activated-sludge process, biodegradation efficiency was shown to be related to the average SRT for the biomass or the F/M ratio. For both of these parameters, the solids or biomass can be sampled and reasonably well quantified. However, for trickling filters quantifying the biomass in the system is not possible, and only recently has progress been made to control the solids inventory to some degree by the dosing operation. The attached growth is not uniformly Tab. 8-4 Trickling filter applications, loadings and effluent quanlity