should be proportional to the radius from the center. Headloss through the distributor is in the range of 0.6 to 1.5 m. Important features that should be considered in selecting a distributor are the ruggedness of construction,ease of cleaning, ability to handle large variations in flowrate while maintaining adequate rotational speed, and corrosion resistance of the material and its coating system In the past, fixed nozzle distribution systems were used for shallow rock filters(see Fig. 8-4b). Fixed nozzle distribution systems consist of a series of spray nozzles located at the points of equilateral triangles covering the filter bed. a system of pipes placed in the filter is used to distribute the wastewater uniformly to the nozzles. Special nozzles having a flat spray pattern are used, and the pressure is varied systematically so that the spray falls first at a maximum distance from the nozzle and then at a decreasing distance as the head slowly drops. In this way, a uniform dose is applied over the whole area of the bed Half-spray nozzles are used along the sides of the filter. In current practice, fixed nozzle systems are seldom used Figure 9-5 Fiberglass Vitrified clay block Underdrains. The wastewater Filter stone collection system in a trickling filter consists of underdrains that catch the filtered wastewater and oy block solids discharged from the filter final sedimentation tank. Underdrain trough underdrain system for a filter usually has precast blocks of vitrified clay or fiberglass grating laid on a reinforced-concrete subfloor(see Fig. &-5). The floor and underdrains must have sufficient strength to support the packing, slime growth, and the wastewater. The floor and underdrain block slope to a central or peripheral drainage channel at a l to 5 percent grade. The effluent channels are sized to produce a minimum velocity of 0.6 m/s at the average daily flowrate Underdrains may be open at both ends, so that they may be inspected easily and flushed out if they become plugged. The underdrains also allow ventilation of the filter, providing the air for the microorganisms that live in the filter slime. The underdrains should be open to a circumferential channel for ventilation at the wall as well as to the central collection channel The underdrain and support system for plastic packing consists of either a beam and column or a grating A typical underdrain system for a tower filter is shown on Fig. 8-6. The beam and column system typically has precast-concrete beams supported by columns or posts. The plastic packing is placed over the beams, which have channels in their tops to ensure free flow of wastewater and air. All underdrain systems should be designed so that forced-air ventilation can be added at a later date if filter operating conditions should change Fig 8-6 Typical under-drain system for Airflow. An adequate flow of air is undamental importance to the successful operation of a trickling filter to provide efficient treatment and to prevent odors. Natural draft has historically been the primary means of providing airflow, but it is not Ventilation always adequate and forced ventilation using low-pressure fans provides more reliable and In the case of natural draft the driving force for airflow is the temperature difference Precast concrete between the ambient air and the air inside the pores. If the wastewater is colder than the ambient air, the pore air will be cold and the direction of flow will be downward. If the ambient air is colder than the wastewater, the flow will be upward. The latter is less desirable from a mass transfer poir of view because the partial pressure of oxygen(and thus the oxygen transfer rate) is lowest in the region of highest oxygen demand. In many areas of the country, there are periods, especially during the summer, when essentially no airflow occurs through the trickling filter because temperature differentials are negligible The volumetric air flowrate may be estimated by setting the draft equal to the sum of the head losses that8-8 should be proportional to the radius from the center. Headloss through the distributor is in the range of 0.6 to 1.5 m. Important features that should be considered in selecting a distributor are the ruggedness of construction, ease of cleaning, ability to handle large variations in flowrate while maintaining adequate rotational speed, and corrosion resistance of the material and its coating system. In the past, fixed nozzle distribution systems were used for shallow rock filters (see Fig. 8-4b). Fixed nozzle distribution systems consist of a series of spray nozzles located at the points of equilateral triangles covering the filter bed. A system of pipes placed in the filter is used to distribute the wastewater uniformly to the nozzles. Special nozzles having a flat spray pattern are used, and the pressure is varied systematically so that the spray falls first at a maximum distance from the nozzle and then at a decreasing distance as the head slowly drops. In this way, a uniform dose is applied over the whole area of the bed. Half-spray nozzles are used along the sides of the filter. In current practice, fixed nozzle systems are seldom used. Underdrains. The wastewater collection system in a trickling filter consists of underdrains that catch the filtered wastewater and solids discharged from the filter packing for conveyance to the final sedimentation tank. The underdrain system for a rock filter usually has precast blocks of vitrified clay or fiberglass grating laid on a reinforced-concrete subfloor (see Fig. 8-5). The floor and underdrains must have sufficient strength to support the packing, slime growth, and the wastewater. The floor and underdrain block slope to a central or peripheral drainage channel at a 1 to 5 percent grade. The effluent channels are sized to produce a minimum velocity of 0.6 m/s at the average daily flowrate. Underdrains may be open at both ends, so that they may be inspected easily and flushed out if they become plugged. The underdrains also allow ventilation of the filter, providing the air for the microorganisms that live in the filter slime. The underdrains should be open to a circumferential channel for ventilation at the wall as well as to the central collection channel. The underdrain and support system for plastic packing consists of either a beam and column or a grating. A typical underdrain system for a tower filter is shown on Fig. 8-6. The beam and column system typically has precast-concrete beams supported by columns or posts. The plastic packing is placed over the beams, which have channels in their tops to ensure free flow of wastewater and air. All underdrain systems should be designed so that forced-air ventilation can be added at a later date if filter operating conditions should change. Airflow. An adequate flow of air is of fundamental importance to the successful operation of a trickling filter to provide efficient treatment and to prevent odors. Natural draft has historically been the primary means of providing airflow, but it is not always adequate and forced ventilation using low-pressure fans provides more reliable and controlled airflow. In the case of natural draft, the driving force for airflow is the temperature difference between the ambient air and the air inside the pores. If the wastewater is colder than the ambient air, the pore air will be cold and the direction of flow will be downward. If the ambient air is colder than the wastewater, the flow will be upward. The latter is less desirable from a mass transfer point of view because the partial pressure of oxygen (and thus the oxygen transfer rate) is lowest in the region of highest oxygen demand. In many areas of the country, there are periods, especially during the summer, when essentially no airflow occurs through the trickling filter because temperature differentials are negligible. The volumetric air flowrate may be estimated by setting the draft equal to the sum of the head losses that Fig. 8-6 Typical under-drain system for Tower filter