the similar low organic loadings rates (less than 1.0 kg bOd/m.d), the performance of rock filters compared to filters with plastic packing is similar. At higher organic loading rates, however, the rformance of filters with plastic packing is superior. The higher porosity, which provides for better air circulation and biofilm sloughing, is a likely explanation for the improved performance Dosing Rate. The dosing rate on a trickling filter is the depth of liquid discharged on top of the packing for each pass of the distributor. For higher distributor rotational speeds, the dosing rate is lower. In the past, typical rotational speeds for distributors were about 0.5 to 2 min per revolution. With two to four arms, the trickling filter is dosed every 10 to 60 s. Results from various investigators have indicated that reducing the distributor speed results in better filter performance. Hawkes(1963)showed that rock trickling filters dosed every 30 to 55 min/rev outperformed a more conventional operation of I to 5 min/rev. Besides improved BOD removal, there were dramatic reductions in the Psychoda and Anisopus fly population, biofilm thickness and odors. Albertson and Davies (1984)showed similar advantages from an investigation of reduced distributor speed. At a higher dosing rate, the larger water volume applied per revolution()provides greater wetting efficiency, (2)results in greater agitation, which causes more solids to flush out of the packing, (3) results in a thinner biofilm, and (4)helps to wash away fly eggs. The thinner biofilm creates more surface area and results in a more aerobic biofilm If the high dosing rate is sustained to control the biofilm thickness, the treatment efficiency may be decreased because the liquid contact time in the filter is less. a daily intermittent high dose, referred to as a flushing dose, is used to control the biofilm thickness and solids inventory. A combination of a once-per-day high flushing rate and a lower daily sustained dosing rate is recommended as a function of the BOd loading as shown in Table 8-3. The data in Table 8-3 are guidelines to establish a dosing range Optimization of the dosing rate and flushing rate and frequency is best determined from field operation Flexibility in the distributor design is Tab. 8-3 loading ng dose, Flushit needed to provide a range of dosing dding filter dosing rates to optimize the trickling filter performance. d BOD loading ≥200 40-120 Distribution Systems. a distributor 60-180 consists of two or more arms that are 80240 ≥800 mounted on a pivot in the center of the filter and revolve in a horizontal plane(see Fig. 8-4) Figure 9-4 8-4 The arms are hollow andTypical distributors used to apply wastewater to contain nozzles through which tickling Hler pocking he wastewater is discharged over the filter bed. The rock filter with twoorm distributor assembly may be [b]view of early (circa driven either by the dynamic 1920) rock filter with a reaction of the wastewater fixed distribution system discharging from the nozzles and led view of op or by an electric motor. The tower trickling filter flow-driven rotary distributor with four-arm rotary for trickling filtration has been I distributor ed traditionally process because it is reliable and easy to maintain. Motor drives are used in more recent designs. Clearance of 150 to 225 mm should be allowed between the bottom of the distributor arm and the top of the bed. The clearance permits the wastewater streams from the nozzles to spread out and cover the bed uniformly, and it prevents ice accumulations from interfering with the distributor motion during freezing weather Distributors are manufactured for trickling filters with diameters up to 60 m. Distributor arms may be of constant cross section for small units, or they may be tapered to maintain minimum transport velocity Nozzles are spaced unevenly so that greater flow per unit of length is achieved near the periphery of the filter than at the center. For uniform distribution over the area of the filter, the flowrate per unit of length 78-7 the similar low organic loadings rates (less than 1.0 kg BOD/m3·d), the performance of rock filters compared to filters with plastic packing is similar. At higher organic loading rates, however, the performance of filters with plastic packing is superior. The higher porosity, which provides for better air circulation and biofilm sloughing, is a likely explanation for the improved performance. Dosing Rate. The dosing rate on a trickling filter is the depth of liquid discharged on top of the packing for each pass of the distributor. For higher distributor rotational speeds, the dosing rate is lower. In the past, typical rotational speeds for distributors were about 0.5 to 2 min per revolution. With two to four arms, the trickling filter is dosed every 10 to 60 s. Results from various investigators have indicated that reducing the distributor speed results in better filter performance. Hawkes (1963) showed that rock trickling filters dosed every 30 to 55 min/rev outperformed a more conventional operation of 1 to 5 min/rev. Besides improved BOD removal, there were dramatic reductions in the Psychoda and Anisopus fly population, biofilm thickness ,and odors. Albertson and Davies (1984) showed similar advantages from an investigation of reduced distributor speed. At a higher dosing rate, the larger water volume applied per revolution (1) provides greater wetting efficiency, (2) results in greater agitation, which causes more solids to flush out of the packing, (3) results in a thinner biofilm, and (4) helps to wash away fly eggs. The thinner biofilm creates more surface area and results in a more aerobic biofilm. If the high dosing rate is sustained to control the biofilm thickness, the treatment efficiency may be decreased because the liquid contact time in the filter is less. A daily intermittent high dose, referred to as a flushing dose, is used to control the biofilm thickness and solids inventory. A combination of a once-per-day high flushing rate and a lower daily sustained dosing rate is recommended as a function of the BOD loading as shown in Table 8-3. The data in Table 8-3 are guidelines to establish a dosing range. Optimization of the dosing rate and flushing rate and frequency is best determined from field operation. Flexibility in the distributor design is needed to provide a range of dosing rates to optimize the trickling filter performance. Distribution Systems. A distributor consists of two or more arms that are mounted on a pivot in the center of the filter and revolve in a horizontal plane (see Fig. 8-4). The arms are hollow and contain nozzles through which the wastewater is discharged over the filter bed. The distributor assembly may be driven either by the dynamic reaction of the wastewater discharging from the nozzles or by an electric motor. The flow-driven rotary distributor for trickling filtration has been used traditionally for the process because it is reliable and easy to maintain. Motor drives are used in more recent designs. Clearance of 150 to 225 mm should be allowed between the bottom of the distributor arm and the top of the bed. The clearance permits the wastewater streams from the nozzles to spread out and cover the bed uniformly, and it prevents ice accumulations from interfering with the distributor motion during freezing weather. Distributors are manufactured for trickling filters with diameters up to 60 m. Distributor arms may be of constant cross section for small units, or they may be tapered to maintain minimum transport velocity. Nozzles are spaced unevenly so that greater flow per unit of length is achieved near the periphery of the filter than at the center. For uniform distribution over the area of the filter, the flowrate per unit of length Fig. 8-4 Tab. 8-3