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