8 Attached Growth Biological Treatment Processes 8-1 Background Evolution of Attached growth Processes Attached growth processes can be grouped into three general classes: (1)nonsubmerged attached growth processes, (2)suspended growth processes with fixed-film packing, and (3)submerged attached growth aerobic processes Nonsubmerged Attached Growth Processes. Trickling filters with rock packing have been a common simple, and low-energy process used for secondary treatment since the early 1900s. A trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continously. Treatment occurs as the liquid flows over the attached biofilm. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Originally they were watertight basins filled with broken stones and were operated in a cyclic mode. The bed was filled with wastewater from the top, and the wastewater was allowed to contact the packing for a short time. The bed was then drained and allowed to rest before the cycle was repeated a typical cycle required 12 h(6 h for operation and 6h of resting). The limitations of the contact filter included a relatively high incidence of clogging, the long rest period required, headloss, and the relatively low loading that could be used Because of the clogging problems, larger packing was used until a rock size of 50 to 100 mm was reached In the 1950s, plastic packing began to replace rock in the United States. The use of plastic packing allowed the use of higher loading rates and taller filters(also known as biotowers) with less land area, improved process efficiency, and reduced clogging In the 1960s, practical designs were developed for otating biological contactors(RBCs), which provided an alternative attached growth process where the packing is rotated in the wastewater treatment tank, versus pumping and applying the wastewater over a static packing. Both trickling filters and RBCs have been used as aerobic attached growth processes for BUD removal only, combined BOD removal and nitrification, and for tertiary nitrification after secondary treatment by suspended growth or attached growth processes. The principal advantages claimed for these aerobic attached growth processes over the activated-sludge process are as follows ed Simpler operation with no issues of mixed liquor inventory control and sludge wasting No problems of bulking sludge in secondary clarifiers Better sludge thickening properties Less equipment maintenance needs Better recovery from shock toxic loads In comparison to the activated-sludge process, disadvantages encountered for trickling filters are a poorer effluent quality in terms of BOD and TSS concentrations, greater sensitivity to lower temperatures, odor production, and uncontrolled solids sloughing events. In general, the actual limitations of the processes(1 make it difficult to accomplish biological nitrogen and phosphorus removal compared to single-sludge biological nutrient removal suspended growth designs, and(2)result in an effluent with a higher turbidity than activated-sludge treatment. Trickling filters and RBCs have also been used in combined processes with activated sludge to utilize the benefits of both processes, in terms of energy savings and effluent alit Suspended Growth Processes with Fixed-Film Packing. The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith proce Present-day designs use more engineered packings and include the use of packing materials that are spended in the aeration tank with the mixed liquor fixed packing material placed in portions of the aeration tank, as well as submerged RBCs. The advantages claimed for these activated-sludge process enhancements are as follows Increased treatment capacit Greater process stability Reduced sludge production Enhanced sludge settleability Reduced solids loadings on the secondary clarifier No increase in operation and maintenance costs Submerged Attached Growth Processes. Beginning in the 1970s and extending into the 1980s, a nev class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are upflow and downflow packed-bed reactors and fluidized-bed reactors that do not use
8-1 8 Attached Growth Biological Treatment Processes 8-1 Background Evolution of Attached Growth Processes Attached growth processes can be grouped into three general classes: (1) nonsubmerged attached growth processes, (2) suspended growth processes with fixed-film packing, and (3) submerged attached growth aerobic processes. Nonsubmerged Attached Growth Processes. Trickling filters with rock packing have been a common, simple, and low-energy process used for secondary treatment since the early 1900s. A trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continously. Treatment occurs as the liquid flows over the attached biofilm. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Originally they were watertight basins filled with broken stones and were operated in a cyclic mode. The bed was filled with wastewater from the top, and the wastewater was allowed to contact the packing for a short time. The bed was then drained and allowed to rest before the cycle was repeated. A typical cycle required 12 h (6 h for operation and 6h of resting). The limitations of the contact filter included a relatively high incidence of clogging, the long rest period required, headloss, and the relatively low loading that could be used. Because of the clogging problems, larger packing was used until a rock size of 50 to 100 mm was reached. In the 1950s, plastic packing began to replace rock in the United States. The use of plastic packing allowed the use of higher loading rates and taller filters (also known as biotowers) with less land area, improved process efficiency, and reduced clogging. In the 1960s, practical designs were developed for rotating biological contactors (RBCs), which provided an alternative attached growth process where the packing is rotated in the wastewater treatment tank, versus pumping and applying the wastewater over a static packing. Both trickling filters and RBCs have been used as aerobic attached growth processes for BUD removal only, combined BOD removal and nitrification, and for tertiary nitrification after secondary treatment by suspended growth or attached growth processes. The principal advantages claimed for these aerobic attached growth processes over the activated-sludge process are as follows: . Less energy required . Simpler operation with no issues of mixed liquor inventory control and sludge wasting . No problems of bulking sludge in secondary clarifiers . Better sludge thickening properties . Less equipment maintenance needs . Better recovery from shock toxic loads In comparison to the activated-sludge process, disadvantages encountered for trickling filters are a poorer effluent quality in terms of BOD and TSS concentrations, greater sensitivity to lower temperatures, odor production, and uncontrolled solids sloughing events. In general, the actual limitations of the processes (1) make it difficult to accomplish biological nitrogen and phosphorus removal compared to single-sludge biological nutrient removal suspended growth designs, and (2) result in an effluent with a higher turbidity than activated-sludge treatment. Trickling filters and RBCs have also been used in combined processes with activated sludge to utilize the benefits of both processes, in terms of energy savings and effluent quality. Suspended Growth Processes with Fixed-Film Packing. The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith processes. Present-day designs use more engineered packings and include the use of packing materials that are suspended in the aeration tank with the mixed liquor, fixed packing material placed in portions of the aeration tank, as well as submerged RBCs. The advantages claimed for these activated-sludge process enhancements are as follows: . Increased treatment capacity . Greater process stability . Reduced sludge production . Enhanced sludge settleability . Reduced solids loadings on the secondary clarifier . No increase in operation and maintenance costs Submerged Attached Growth Processes. Beginning in the 1970s and extending into the 1980s, a new class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are upflow and downflow packed-bed reactors and fluidized-bed reactors that do not use
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 8-2
8-2 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
moist Influent wastewater is normally applied at the top of the packing through distributor arms that extend across the trickling filter inner diameter and have variable openings to provide a uniform application rate per unit area. The distributor arms are rotated by the force of the water exiting through their opening or by the use of electric drives. The electric drive designs provide more control flexibility and a wider range of distributor rotational speeds than possible by the simple hydraulic designs. In some cases, especially for square or rectangular filters, fixed flat-spray nozzles have been used Primary clarification is necessary before rock trickling filters, and generally used also before trickling filters with plastic packing, though fine screens(smaller than 3mm openings) have been used successfully with plastic packing. With increases in plastic and rubber floatable materials in wastewater, screening of these materials is important to reduce fouling of the packing. In some installations a wire-mesh screen is olaced over the top of plastic packing to collect debris that can be vacuumed off periodically a slime layer develops on the rock or plastic packing in the trickling filters and contains the microorganisms for biodegradation of the substrates to be removed from the liquid flowing over the packing. The biological community in the filter includes aerobic and facultative bacteria, fungi, algae, and protozoans. Higher animals, such as worms, insect larvae, and snails, are also present Facultative bacteria are the predominating organisms in trickling filters, and decompose the organic material in the wastewater along with aerobic and anaerobic bacteria Achromobacter Flavobacterium Pseudomonas, and Alcaligenes are among the bacterial species commonly associated with the trickling filter. Within the slime layer, where adverse conditions prevail with respect to growth, the filamentous forms Sphaerotilus natans and Beggiatoa will be found. In the lower reaches of the filter, the nitrifying bacteria will be present. The fungi present are also responsible for waste stabilization, but their role is usually important only under low-pH conditions or with certain industrial wastes. At times, fungi growth can be so rapid that the filter clogs and ventilation becomes restricted. Among the fungi species that have been identified are Fusarium, Mucor: Penicillium, Geotrichum, Sporatichum, and various yeasts Algae can grow only in the upper reaches of the filter where sunlight is available Phormidium, chlorella and Ulothrix are among the algae species commonly found in trickling filters. Generally, algae do not wastewater.From an operational standpoint, the algae may be troublesome because they can cau o take a direct part in waste degradation, but during the daylight hours they add oxygen to the percolating logging of the filter surface. The protozoa in the filter are predominantly of the ciliate group, including Vorticella, Opercularia, and Epistylis. Their function is to feed on the biological films and, as a result, effluent turbidity decreases and the biofilm is maintained in a higher growth state. The higher animals, such as worms, snails, and insects, feed on the biological film. Snails are especially troublesome in trickling filters used mainly for nitrification, where they have been known to consume enough of the nitrifying bacteria to significantly reduce treatment efficiency The slime layer thickness can reach depths as much as 10 mm. Organic material from the liquid is sorbed onto the biological film or slime layer In the outer portions of the biological slime layer(0. 1 to 0.2 mm), the organic material is degraded by aerobic microorganisms. As the microorganisms grow and the slime layer thickness increases, oxygen is consumed before it can penetrate the full depth, and an anaerobic environment is established near the surface of the packing. As the slime layer increases in Bacteria in the slime layer enter an endogenous respiration state and lose their ability to cling to the packing surface. The liquid then washes the slime off the packing, and a new slime layer starts to grow. The phenomenon of losing the slime layer is called sloughing and is primarily a function of the organic and hydraulic loading on the filter. The hydraulic loading accounts for shear velocities, and the organic loading accounts for the rate of metabolism in the slime layer. Hydraulic loading and trickling filter sloughing can be controlled by using a wastewater distributor with an electric motor drive to vary The mechanisms of biological film loss in plastic and rock packing are different. Continuous, small-scale sloughing of the film occurs in high-rate plastic filters due to hydraulic shear, while large-scale, spring-time sloughing occurs in rock filters located in temperate zones. Sloughing is due to the activity of insect larvae, which become active in the warmer spring temperatures and consume and mechanically dislodge thick biofilms that accumulate over the winter. when a rock filter sloughs. the effluent before settling will contain higher amounts of BOD and TSS than the applied wastewater Trickling Filter Classification and Applications Trickling filter applications and loadings, based on historical terminology developed originally for rock filter designs. are summarized in Table 8-1
8-3 moist. Influent wastewater is normally applied at the top of the packing through distributor arms that extend across the trickling filter inner diameter and have variable openings to provide a uniform application rate per unit area. The distributor arms are rotated by the force of the water exiting through their opening or by the use of electric drives. The electric drive designs provide more control flexibility and a wider range of distributor rotational speeds than possible by the simple hydraulic designs. In some cases, especially for square or rectangular filters, fixed flat-spray nozzles have been used. Primary clarification is necessary before rock trickling filters, and generally used also before trickling filters with plastic packing, though fine screens (smaller than 3mm openings) have been used successfully with plastic packing. With increases in plastic and rubber floatable materials in wastewater, screening of these materials is important to reduce fouling of the packing. In some installations a wire-mesh screen is placed over the top of plastic packing to collect debris that can be vacuumed off periodically. A slime layer develops on the rock or plastic packing in the trickling filters and contains the microorganisms for biodegradation of the substrates to be removed from the liquid flowing over the packing. The biological community in the filter includes aerobic and facultative bacteria, fungi, algae, and protozoans. Higher animals, such as worms, insect larvae, and snails, are also present. Facultative bacteria are the predominating organisms in trickling filters, and decompose the organic material in the wastewater along with aerobic and anaerobic bacteria Achromobacter, Flavobacterium, Pseudomonas, and Alcaligenes are among the bacterial species commonly associated with the trickling filter. Within the slime layer, where adverse conditions prevail with respect to growth, the filamentous forms Sphaerotilus natans and Beggiatoa will be found. In the lower reaches of the filter, the nitrifying bacteria will be present. The fungi present are also responsible for waste stabilization, but their role is usually important only under low-pH conditions or with certain industrial wastes. At times, fungi growth can be so rapid that the filter clogs and ventilation becomes restricted. Among the fungi species that have been identified are Fusarium, Mucor, Penicillium, Geotrichum, Sporatichum, and various yeasts. Algae can grow only in the upper reaches of the filter where sunlight is available Phormidiun, Chlorella and Ulothrix are among the algae species commonly found in trickling filters . Generally, algae do not take a direct part in waste degradation, but during the daylight hours they add oxygen to the percolating wastewater. From an operational standpoint, the algae may be troublesome because they can cause clogging of the filter surface. The protozoa in the filter are predominantly of the ciliate group, including Vorticella, Opercularia, and Epistylis. Their function is to feed on the biological films and, as a result, effluent turbidity decreases and the biofilm is maintained in a higher growth state. The higher animals, such as worms, snails, and insects, feed on the biological film. Snails are especially troublesome in trickling filters used mainly for nitrification, where they have been known to consume enough of the nitrifying bacteria to significantly reduce treatment efficiency. The slime layer thickness can reach depths as much as 10 mm. Organic material from the liquid is adsorbed onto the biological film or slime layer. In the outer portions of the biological slime layer (0.1 to 0.2 mm), the organic material is degraded by aerobic microorganisms. As the microorganisms grow and the slime layer thickness increases, oxygen is consumed before it can penetrate the full depth, and an anaerobic environment is established near the surface of the packing. As the slime layer increases in thickness, the substrate in the wastewater is used before it can penetrate the inner depths of the biofilm. Bacteria in the slime layer enter an endogenous respiration state and lose their ability to cling to the packing surface. The liquid then washes the slime off the packing, and a new slime layer starts to grow. The phenomenon of losing the slime layer is called sloughing and is primarily a function of the organic and hydraulic loading on the filter. The hydraulic loading accounts for shear velocities, and the organic loading accounts for the rate of metabolism in the slime layer. Hydraulic loading and trickling filter sloughing can be controlled by using a wastewater distributor with an electric motor drive to vary rotational speed. The mechanisms of biological film loss in plastic and rock packing are different. Continuous, small-scale sloughing of the film occurs in high-rate plastic filters due to hydraulic shear, while large-scale, spring-time sloughing occurs in rock filters located in temperate zones. Sloughing is due to the activity of insect larvae, which become active in the warmer spring temperatures and consume and mechanically dislodge thick biofilms that accumulate over the winter. When a rock filter sloughs, the effluent before settling will contain higher amounts of BOD and TSS than the applied wastewater. Trickling Filter Classification and Applications Trickling filter applications and loadings, based on historical terminology developed originally for rock filter designs, are summarized in Table 8-1
Tab. 8-I Trickling filter fsbrical classification of trickling filters applications designs are classified ow. or by hydraulic or organic loading rates ipe of pocking Rock Rock Rock looding 40-200 have been classified anic looding 0.07-0.22 0.24-0.48 0.4-2.4 standard-rate tortulofion ratio intermediate-rate Continuous Continuous and high-rate. Plastic 0.9-6 mo removal efficiency, packing is used -90 40-70 品 ent quality No nitrification No nitrification 10-20 however packing has also been used at lower organic loadings, near the high end of those used for intermediate-rate rock filters. Much higher organic loadings have been used for rock or plastic packing designs roughing"applications where only partial BOD removal occurs Low-Rate Filters. A low-rate filter is a relatively simple, highly dependable device that produces an effluent of consistent quality with an influent of varying strength The filters may be circular rectangular in shape. Generally, feed flow from a dosing tank is maintained by suction level controlled pumps or a dosing siphon. Dosing tanks are small, usually with only a 2-min detention time based or twice the average design flow, so that intermittent dosing is minimized. Even so, at small plants, low nighttime flows may result in intermittent dosing and recirculation may be necessary to keep the packing moist. If the interval between dosing is longer than 1 or 2 h, the efficiency of the process deteriorates because the character of the biological slime is altered by a lack of moisture In most low-rate filters, only the top 0.6 to 1. 2 m of the filter packing will have appreciable biological slime. As a result, the lower portions of the filter may be populated by autotrophic nitrifying bacteria, hich oxidize ammonia nitrogen to nitrite and nitrate forms. If the nitrifying population is sufficiently well established, and if climatic conditions and wastewater characteristics are favorable, a well-operated low rate filter can provide good BOD removal and a highly nitrified effluent With a favorable hydraulic gradient, the ability to use gravity flow is a distinct advantage. If the site is too flat to permit gravity flow, pumping will be required. Odors are a common problem, especially if the wastewater is stale or septic, or if the weather is warm. Filters should not be located where the odors would create a nuisance. Filter flies(Psychoda) may breed in the filters unless effective control measures are used
8-4 Trickling filter designs are classified by hydraulic or organic loading rates. Rock filter designs have been classified as low- or standard-rate, intermediate-rate, and high-rate. Plastic packing is used typically for high-rate designs; however, plastic packing has also been used at lower organic loadings, near the high end of those used for intermediate-rate rock filters. Much higher organic loadings have been used for rock or plastic packing designs in "roughing" applications where only partial BOD removal occurs. Low-Rate Filters. A low-rate filter is a relatively simple, highly dependable device that produces an effluent of consistent quality with an influent of varying strength. The filters may be circular or rectangular in shape. Generally, feed flow from a dosing tank is maintained by suction level controlled pumps or a dosing siphon. Dosing tanks are small, usually with only a 2-min detention time based on twice the average design flow, so that intermittent dosing is minimized. Even so, at small plants, low nighttime flows may result in intermittent dosing and recirculation may be necessary to keep the packing moist. If the interval between dosing is longer than 1 or 2 h, the efficiency of the process deteriorates because the character of the biological slime is altered by a lack of moisture. In most low-rate filters, only the top 0.6 to 1.2 m of the filter packing will have appreciable biological slime. As a result, the lower portions of the filter may be populated by autotrophic nitrifying bacteria, which oxidize ammonia nitrogen to nitrite and nitrate forms. If the nitrifying population is sufficiently well established, and if climatic conditions and wastewater characteristics are favorable, a well-operated low rate filter can provide good BOD removal and a highly nitrified effluent. With a favorable hydraulic gradient, the ability to use gravity flow is a distinct advantage. If the site is too flat to permit gravity flow, pumping will be required. Odors are a common problem, especially if the wastewater is stale or septic, or if the weather is warm. Filters should not be located where the odors would create a nuisance. Filter flies (Psychoda) may breed in the filters unless effective control measures are used. Tab. 8-1
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 treatment
8-5 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
Design of Physical Facilities Factors that must be considered in the design of trickling filters include (1) type and physical characteristics of filter packing to be used;(2) dosing rate;(3) type and dosing characteristics of the distribution system;(4)configuration of the underdrain system; (5) provision for adequate airflow (ie ventilation), either natural or forced air; and(6)sealing tank design Filter Packing. The ideal Tab.8-2 physical properties of trickling filter packing materials filter packing is a material that has a high surface area per unit of volume. is low in cost. has a high durability, and has a high Packing material enough porosity so that 800-1000 50 CNN clogging is minimized and good tich 6595 CN air circulation can occur Typical trickling filter packing slie random pocking 3060 materials are shown on Fig 8-3 Plastic random pocking hig 50-B0 N The physical characteristics of commonly used filter packings, including those shown on Fig. 8-3, are reported in Table 8-2. Until the mid-1960s, the material used was either high-quality granite or blast-furnace slag. Since the 1960s, plastic packing material, either cross-flow or vertical-flow, has become the packing of choice in the United States of low cost. the most suitable material is rounded fiver rock or a uniform size so that 95 e ithin th ange of 75 to 100 mm. uniformity is a way of ensuring adequate pore flow and air circulation. Other Important characteristics of filter typical packing material for tricking filters: (al rock, ( b) and (c) plastic verticol-Row, Id) plastic cross Rlow. (e) redwood stre horinontal, and ( f) random pock. / Figs, fe) and (d) fom American Surfpoc Corp, (e) from Neptune Microfloc, ond ff) from Products, Ine. Note: the random pock material is often used in air stripping towers Durability may be determined by odium sulfate test, which is used to test the soundness of concrete aggregates. Because of the weight of the packing, the depth of rock filters is usually on the order of 2 m. The low void volume of rock limits the space available for airflow and increases the potential for plugging and flow short circuiting Because of plugging, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/m.d Various forms of plastic packings are shown on Fig 8-3. Molded plastic packing materials have the appearance of a honeycomb. Flat and corrugated sheets of polyvinyl chloride are bonded together in rectangular modules. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Each layer of modules is turned at right angles to the previous layer to further improve wastewater distribution. The two basic types of corrugated plastic sheet packing are vertical and cross flow(see Fig. 8-3b, c, and d). Both types of packing are reported to be effective in BOD and TSS removal over a wide range of loadings. Biotowers as deep as 12 m have been constructed using plastic packing with depths in the range of 6 m being more common In biotowers with vertical plastic packing, cross-flow packing can be used for the uppermost layers to enhance the distribution across the top of the filter. The high hydraulic capacity, high void ratio, and resistance to plugging offered by these types of packing can best be used in a high-rate-type filter. Redwood or other wood packings have been used in the past, but with the limited availability of redwood, wood packing is seldom used currently. Plastic packing has the advantage of requiring less land area for the filter structure than rock due to the ability to use higher loading rates and taller trickling filters. Grady et al. (1999)noted that when loaded
8-6 Design of Physical Facilities Factors that must be considered in the design of trickling filters include (1) type and physical characteristics of filter packing to be used; (2) dosing rate; (3) type and dosing characteristics of the distribution system; (4) configuration of the underdrain system; (5) provision for adequate airflow (i.e., ventilation), either natural or forced air; and (6) sealing tank design. Filter Packing. The ideal filter packing is a material that has a high surface area per unit of volume, is low in cost, has a high durability, and has a high enough porosity so that clogging is minimized and good air circulation can occur. Typical trickling filter packing materials are shown on Fig. 8-3. The physical characteristics of commonly used filter packings, including those shown on Fig. 8-3, are reported in Table 8-2. Until the mid-1960s, the material used was either high-quality granite or blast-furnace slag. Since the 1960s, plastic packing material, either cross-flow or vertical- flow, has become the packing of choice in the United States. Where locally available, rock has the advantage of low cost. The most suitable material is rounded fiver rock or crashed stone, graded to a uniform size so that 95 percent is within the range of 75 to 100 mm. The specification of size uniformity is a way of ensuring adequate pore space for wastewater flow and air circulation. Other important characteristics of filter packing materials are strength and durability. Durability may be determined by the sodium sulfate test, which is used to test the soundness of concrete aggregates. Because of the weight of the packing, the depth of rock filters is usually on the order of 2 m. The low void volume of rock limits the space available for airflow and increases the potential for plugging and flow short circuiting. Because of plugging, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/m3·d. Various forms of plastic packings are shown on Fig. 8-3. Molded plastic packing materials have the appearance of a honeycomb. Flat and corrugated sheets of polyvinyl chloride are bonded together in rectangular modules. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Each layer of modules is turned at right angles to the previous layer to further improve wastewater distribution. The two basic types of corrugated plastic sheet packing are vertical and cross flow (see Fig. 8-3b, c, and d). Both types of packing are reported to be effective in BOD and TSS removal over a wide range of loadings. Biotowers as deep as 12 m have been constructed using plastic packing, with depths in the range of 6 m being more common. In biotowers with vertical plastic packing, cross-flow packing can be used for the uppermost layers to enhance the distribution across the top of the filter. The high hydraulic capacity, high void ratio, and resistance to plugging offered by these types of packing can best be used in a high-rate-type filter. Redwood or other wood packings have been used in the past, but with the limited availability of redwood, wood packing is seldom used currently. Plastic packing has the advantage of requiring less land area for the filter structure than rock due to the ability to use higher loading rates and taller trickling filters. Grady et al. (1999) noted that when loaded at Fig. 8-3 Tab. 8-2
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 7
8-7 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
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 that
8-8 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
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 89
8-9 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
distributed in the trickling filter, the biofilm thickness can vary, the biofilm solids concentration may range from 40 to 100 g/L, and the liquid does not uniformly flow over the entire packing surface area, which is referred to as the wetting efficiency. With the inability to quantify the biological and hydrodynan volumetr organic een used as design and operating parameters to relat sign T he esearch Council (NRC)in ld data for BOD removal ere was a at the organic loading rate 0 d BOD removal ication removal ng rate in ed for rock eating on treatment y be used as an ues at the ts. The BOD dicts treatment ow Chemical is 0.5 L/m2.s e minimum wetting o have little benefi ion may improve u a Eic n rates, recirculation tewater ount of the n for trickling filters is at what condition occurs. s may be produced due to anaerobic a in the literature, for influent BOD transfer may become limiting. Hinton and Stensel substrate removal rates at soluble to accomplish biological nitrification in trickling e secondary treatment process may be a oaches based on pilot-plant and full-scale the difficulty in predicting the actual dissolved oxygen luent wastewater bacteria an investigations ha ation Studies by Harrem es(1982) showed that nit maximum rate at soluble BOD(SBOD) concentrations belo( as OD concentration above 5 mg/ and(3)was insignificant, in proportion to the sBOD concentration of30mg/ or more. In study with 8-10
8-10 distributed in the trickling filter, the biofilm thickness can vary, the biofilm solids concentration may range from 40 to 100 g/L, and the liquid does not uniformly flow over the entire packing surface area, which is referred to as the wetting efficiency. With the inability to quantify the biological and hydrodynamic properties of field trickling filter systems, broader parameters such as volumetric organic loading, unit area loadings, and hydraulic application rates have been used as design and operating parameters to relate to treatment efficiency. For BOD removal, the volumetric BOD loading has been correlated well with treatment performance for both BOD removal and nitrification in combined BOD and nitrification trickling filter designs. The original design model for rock trickling filters was developed by the National Research Council (NRC) in the early 1940s at military installations. The NRC formulations were based on field data for BOD removal efficiency and the organic loading rate. The NRC design model was used even though there was a significant amount of data scatter. Bruce and Merkens (1970 and 1973) found that the organic loading rate controlled trickling filter performance and not the hydraulic application rate. For combined BOD removal and nitrification systems, nitrification efficiency has been related to the volumetric BOD loading. For tertiary nitrification applications, very little BOD is applied to the trickling filter and a thin biofilm develops on the packing that consists of a high proportion of nitrifying bacteria. The nitrification removal efficiency is related to the packing surface area and correlated with the specific nitrogen loading rate in terms of g NH4-N removed/m2 packing surface area·d. BOD Removal Design. The first empirical design equations for BOD removal were developed for rock trickling filters from an analysis of trickling filter performance at 34 plants at military installations treating domestic wastewater. The effect of volumetric BOD loading and recirculation ratio on treatment performance was accounted for in the equations. The equations given below should only be used as an estimate of performance as they are based on a limited data base and the influent BOD values at the installations sampled were relatively high compared to most municipal primary effluents. The BOD removal includes the effect of the secondary clarifier, so that if the equation overpredicts treatment performance, improved and deeper secondary clarifier designs used today may help in meeting expected treatment performance. Recirculatlon. The minimum hydraulic application rate recommended by Dow Chemical is 0.5 L/m2 .s to provide maximum efficiency. Shallow tower designs require recirculation to provide minimum wetting rates. When above the minimum hydraulic application rate, recirculation was reported to have little benefit. For filters with low hydraulic application rates and higher organic loadings, recirculation may improve efficiency. For design systems such as rock filters with low hydraulic application rates, recirculation provides a higher flow to improve wetting and flushing of the filter packing. Solids Production. Solids production from trickling filter processes will depend on the wastewater characteristics and the trickling filter loading. At lower organic loading rates, a greater amount of the particulate BOD is degraded, the biomass has a longer SRT, and, as a result, less biomass is produced. Mass Transfer Limitations. One of the concerns in the process design for trickling filters is at what organic loading the filter performance becomes limited by oxygen transfer. When this condition occurs, treatment efficiency at the higher organic load is limited and odors may be produced due to anaerobic activity in the biofilm. Based on an evaluation of the data in the literature, for influent BOD concentrations in the range of 400 to 500 mg/L, oxygen transfer may become limiting. Hinton and Stensel (1994) reported that oxygen availability controlled organic substrate removal rates at soluble biodegradable COD loadings above 3.3 kg/m3 .d. Nitrification Design Two types of process design approaches have been used to accomplish biological nitrification in trickling filters, either in a combined system along with BOD removal or in a tertiary application following secondary treatment and clarification for BOD removal. The secondary treatment process may be a suspended growth or fixed-film process. Empirical design approaches based on pilot-plant and full-scale plant results are again used to guide nitrification designs in view of the difficulty in predicting the actual biofilm coverage area, wetting efficiency, and biofilm thickness and density. Major impacts on nitrification performance are the influent BOD concentration and dissolved oxygen concentration within the trickling filter bulk liquid. As the BOD to TKN ratio of the influent wastewater increases, a greater proportion of the trickling filter packing area is covered by heterotrophic bacteria and the apparent nitrification rate (kg/m3 .d) based on the total trickling filter volume is decreased. A number of investigations have shown that BOD, if at high enough concentration, inhibits nitrification. Studies by Harrem es (1982) showed that nitrification (1) could occur at a maximum rate at soluble BOD (sBOD) concentrations below 5 mg/L, (2) was inhibited in proportion to the sBOD concentration above 5 mg/L, and (3) was insignificant, in proportion to the sBOD concentration of 30 mg/L or more. In a study with a