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Trickling Filters and Rotary Biological Contactors CE - 370.

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Presentation on theme: "Trickling Filters and Rotary Biological Contactors CE - 370."— Presentation transcript:

1 Trickling Filters and Rotary Biological Contactors CE - 370

2 Trickling Filters Trickling filters are composed of: Trickling filters are composed of: Influent pipe Influent pipe Rotary distribution Rotary distribution Filter bed Filter bed Underdrain system Underdrain system Effluent pipe Effluent pipe At downstream, a sedimentation tank is used to remove microbial growth that sloughs from the medium At downstream, a sedimentation tank is used to remove microbial growth that sloughs from the medium Medium Medium Crushed stone Crushed stone Large gravel Large gravel Slag Slag Plastic Plastic redwood redwood

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4 Classifications of Trickling Filters Trickling filters are classified into: Trickling filters are classified into: Low-rate or standard-rate Low-rate or standard-rate Intermediate-rate Intermediate-rate High-rate High-rate Super-rate Super-rate Classification is according to: Classification is according to: Organic loading Organic loading Unit liquid loading Unit liquid loading Recycle employed Recycle employed

5 Low-rate or Standard-rate Filters Organic loading = 0.08 to 0.32 kg BOD 5 /m 3 -day Organic loading = 0.08 to 0.32 kg BOD 5 /m 3 -day Unit liquid loading = 1 to 4 m 3 /m 2 -day Unit liquid loading = 1 to 4 m 3 /m 2 -day Bed depth = 1.5 to 3 m Bed depth = 1.5 to 3 m Recycle is employed at times when flow is not enough to turn the rotary distributor (night time) Recycle is employed at times when flow is not enough to turn the rotary distributor (night time) Usually single-stage Usually single-stage BOD 5 removal = 90 to 95 % BOD 5 removal = 90 to 95 % Effluent BOD 5 = 12 to 25 mg/l Effluent BOD 5 = 12 to 25 mg/l Can achieve better nitrification than high-rate filters Can achieve better nitrification than high-rate filters Media volume is much greater than high-rate filters Media volume is much greater than high-rate filters

6 Intermediate-rate Filters Organic loading = 0.24 to 0.48 kg BOD 5 /m 3 -day Organic loading = 0.24 to 0.48 kg BOD 5 /m 3 -day Unit liquid loading = 4 to 10 m 3 /m 2 -day Unit liquid loading = 4 to 10 m 3 /m 2 -day Bed depth = Bed depth = Recycle may or may not be employed on continuous basis, it is employed at times when flow is not enough to turn the rotary distributor (night time) Recycle may or may not be employed on continuous basis, it is employed at times when flow is not enough to turn the rotary distributor (night time) May be single-stage or two-stage May be single-stage or two-stage BOD 5 removal = 85 to 90 % BOD 5 removal = 85 to 90 % Effluent BOD 5 = 20 to 30 mg/l Effluent BOD 5 = 20 to 30 mg/l

7 High-rate Filters Organic loading = 0.32 to 1.0 kg BOD 5 /m 3 -day Organic loading = 0.32 to 1.0 kg BOD 5 /m 3 -day Unit liquid loading = 10 to 40 m 3 /m 2 -day Unit liquid loading = 10 to 40 m 3 /m 2 -day Bed depth = 1 to 2 m Bed depth = 1 to 2 m Recycle is employed continuously Recycle is employed continuously May be single-stage or two-stage May be single-stage or two-stage BOD 5 removal BOD 5 removal Single-stage = 75 to 80 % Single-stage = 75 to 80 % Two-stage = 85 to 90 % Two-stage = 85 to 90 % Effluent BOD 5 Effluent BOD 5 Single-stage = 40 to 50 mg/l Single-stage = 40 to 50 mg/l Two-stage = 20 to 30 mg/l Two-stage = 20 to 30 mg/l Can not achieve a highly nitrified effluent Can not achieve a highly nitrified effluent

8 Super-rate Filters Organic loading = 0.80 to 6.0 kg BOD 5 /m 3 -day Organic loading = 0.80 to 6.0 kg BOD 5 /m 3 -day Unit liquid loading = 40 to 200 m 3 /m 2 -day Unit liquid loading = 40 to 200 m 3 /m 2 -day Bed depth = 4.5 to 12 m Bed depth = 4.5 to 12 m Recycle is continuous Recycle is continuous Medium is always synthetic plastic (most common) or redwood Medium is always synthetic plastic (most common) or redwood Plastic media has a specific surface area (surface area per unit volume) that is from 2 to 5 time that for stone media Plastic media has a specific surface area (surface area per unit volume) that is from 2 to 5 time that for stone media Microbial growth is proportional to surface area Microbial growth is proportional to surface area Can achieve a nitrified effluent at low loadings Can achieve a nitrified effluent at low loadings

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11 Trickling Filters Energy cost per mass of BOD 5 removed is less than that of activated sludge Energy cost per mass of BOD 5 removed is less than that of activated sludge Super-rate filters can be used ahead of activated sludge (for high-strength wastewater) Super-rate filters can be used ahead of activated sludge (for high-strength wastewater) Single-stage and two-stage trickling filters are upgraded by adding activated sludge process or a shallow polishing pond downstream Single-stage and two-stage trickling filters are upgraded by adding activated sludge process or a shallow polishing pond downstream

12 Mechanisms in Biological Filtration When wastewater passes over a microbial growth, organic matter and O 2 are sorbed by the growth When wastewater passes over a microbial growth, organic matter and O 2 are sorbed by the growth Aerobic bio-oxidation occurs Aerobic bio-oxidation occurs End-products are released End-products are released As time passes, the microbial film becomes thicker due to growth of new cells As time passes, the microbial film becomes thicker due to growth of new cells The film slough off the medium The film slough off the medium

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14 Mechanisms in Biological Filtration Most of the biological growth is aerobic Most of the biological growth is aerobic Growth immediate to the medium surface is usually anaerobic Growth immediate to the medium surface is usually anaerobic The amount of organic removed per meter of bed depth is greatest at top of the filter and smallest at bottom The amount of organic removed per meter of bed depth is greatest at top of the filter and smallest at bottom Nitrifying growth is taking place at the lower depths of the filter Nitrifying growth is taking place at the lower depths of the filter Recycling the effluent increases the removal efficiency Recycling the effluent increases the removal efficiency Microbial growth sloughed will be removed in final clarifier Microbial growth sloughed will be removed in final clarifier The sludge is sent to the head of the plant and removed in the primary sedimentation tank The sludge is sent to the head of the plant and removed in the primary sedimentation tank

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16 Filter Performance Kinetic Equations Kinetic Equations The rate of organic removal per interval of depth is proportional to the remaining concentration of removable organic matter: The rate of organic removal per interval of depth is proportional to the remaining concentration of removable organic matter: - dL/dD = kL - dL/dD = kL dL is an increment of remaining organic concentration dL is an increment of remaining organic concentration dD is an increment of depth dD is an increment of depth L is the remaining removable organic concentration (BOD) L is the remaining removable organic concentration (BOD) Since dD is proportional to an increment of time dt, then Since dD is proportional to an increment of time dt, then (L D / L) = 10 -kD (L D / L) = 10 -kD L D = removable ultimate first-stage BOD concentration at depth D L D = removable ultimate first-stage BOD concentration at depth D L = removable ultimate fist-stage BOD concentration applied to the bed L = removable ultimate fist-stage BOD concentration applied to the bed k = rate constant k = rate constant D = depth of bed, ft (m) D = depth of bed, ft (m)

17 Kinetic Equations For low-rate filters and flow of 1.88 to 5.61 m 3 /m 2 - day For low-rate filters and flow of 1.88 to 5.61 m 3 /m 2 - day k = k = Removable fraction = 90% Removable fraction = 90% For high-rate filters operated at flow 18.8 m 3 /m 2 -day For high-rate filters operated at flow 18.8 m 3 /m 2 -day k = k = Removable fraction = 78.4% Removable fraction = 78.4% The following equation was developed The following equation was developed k 2 = k (T ) k 2 = k (T ) k 2 is rate at T 2 ; k20 is the rate at 20  C k 2 is rate at T 2 ; k20 is the rate at 20  C

18 Kinetic Equations Another performance equation was developed based on the specific rate of substrate removal Another performance equation was developed based on the specific rate of substrate removal (-1/X)(dS/dt) = kS (-1/X)(dS/dt) = kS (1/X)(dS/dt) = specific rate of substrate utilization, mass/(mass microbes)(time) (1/X)(dS/dt) = specific rate of substrate utilization, mass/(mass microbes)(time) (dS/dt) = rate of substrate utilization, mass/(volume)(time) (dS/dt) = rate of substrate utilization, mass/(volume)(time) k = rate constant, volume/(mass microbes)(time) k = rate constant, volume/(mass microbes)(time) S = substrate concentration, mass/volume S = substrate concentration, mass/volume

19 Kinetic Equations Re-arrange and integrate Re-arrange and integrate (S t /S 0 ) = e -k  Xt (S t /S 0 ) = e -k  Xt k = rate constant k = rate constant  X = average cell mass concentration, mass/volume  X = average cell mass concentration, mass/volume S t = substrate concentration after the contact time t, mass/volume S t = substrate concentration after the contact time t, mass/volume S 0 = substrate concentration applied to the filter, mass/volume S 0 = substrate concentration applied to the filter, mass/volume  X is proportional to the specific surface area of the packing, A s (m 2 of the surface per bulk m 3 of the volume)  X is proportional to the specific surface area of the packing, A s (m 2 of the surface per bulk m 3 of the volume)  X ~ A s  X ~ A s

20 Kinetic Equations The mean contact time, t, for a filter is given by: The mean contact time, t, for a filter is given by: t = mean contact time t = mean contact time D = depth of filter D = depth of filter Q L = unit liquid loading or surface loading Q L = unit liquid loading or surface loading C, n = experimental constants C, n = experimental constants

21 Kinetic Equations From the above equations From the above equations S t = substrate concentration in the filter effluent, mass/volume S t = substrate concentration in the filter effluent, mass/volume S 0 = substrate concentration applied to the filter, mass/volume S 0 = substrate concentration applied to the filter, mass/volume K = rate constant K = rate constant A s = specific surface area of the packing, area/volume A s = specific surface area of the packing, area/volume D = filter depth D = filter depth Q L = unit liquid loading or surface loading Q L = unit liquid loading or surface loading m, n = experimental constants m, n = experimental constants

22 Kinetic Equations Value of n Value of n Depends of the flow characteristics through the packing Depends of the flow characteristics through the packing Usually between 0.5 to 0.67 Usually between 0.5 to 0.67 The equation can be further simplified by combing KA s m The equation can be further simplified by combing KA s m K is the new rate constant K is the new rate constant K is between 0.01 and 0.1 K is between 0.01 and 0.1 K can be obtained from pilot-scale plant K can be obtained from pilot-scale plant

23 Kinetic Equations To correct K for temperature To correct K for temperature K T = rate constant at temperature T,  C K T = rate constant at temperature T,  C K 20 = rte constant at 20  C K 20 = rte constant at 20  C T = temperature,  C T = temperature,  C

24 Kinetic Equations A more common kinetic equation for filter with stone media is: A more common kinetic equation for filter with stone media is: S t = BOD 5 in the filter effluent, mg/l S t = BOD 5 in the filter effluent, mg/l S 0 = BOD 5 in the wastewater discharged on the filter bed, mg/l S 0 = BOD 5 in the wastewater discharged on the filter bed, mg/l C = 2.5 for USCS units and for SI units C = 2.5 for USCS units and for SI units D = filter depth, ft (m) D = filter depth, ft (m) Q L = unit liquid loading, MG/acre-day (m 3 /m 2 -day) Q L = unit liquid loading, MG/acre-day (m 3 /m 2 -day)

25 Examples 17.1 and 17.2

26 Flowsheets for Intermediate- and High-Rate Trickling Filter Plants

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29 Filter Details

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31 Operational Problems Less problems than activated sludge processes Less problems than activated sludge processes Problems Problems Floating sludge may be encountered if anaerobic conditions occur within the settled sludge Floating sludge may be encountered if anaerobic conditions occur within the settled sludge Presence of fly that breed in the filter Presence of fly that breed in the filter Can be controlled by allowing the medium to stay submerged Can be controlled by allowing the medium to stay submerged Odor when low-rate filters are employed and wastewater is stale when it reaches the plant Odor when low-rate filters are employed and wastewater is stale when it reaches the plant

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35 Rotary Biological Contactors

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40 Main Characteristics Composed of multiple discs mounted on a horizontal shaft that passes through the center of the discs Composed of multiple discs mounted on a horizontal shaft that passes through the center of the discs Wastewater flow is perpendicular to the shaft Wastewater flow is perpendicular to the shaft About 40% of the total disc area is submerged About 40% of the total disc area is submerged Biological film grows on the disc Biological film grows on the disc As the shaft rotates, the biological growth (film) sorbs organic matter from wastewater As the shaft rotates, the biological growth (film) sorbs organic matter from wastewater Oxygen is adsorbed from air to keep aerobic condition Oxygen is adsorbed from air to keep aerobic condition Multiple stages of RBC is used to achieve greater BOD5 removal Multiple stages of RBC is used to achieve greater BOD5 removal Sloughed biological growths are removed in final clarifiers Sloughed biological growths are removed in final clarifiers No recycle is employed No recycle is employed Biological activities are reduced during cold weather Biological activities are reduced during cold weather In cold climates, RBCs are covered to avoid heat loss and protect against freezing In cold climates, RBCs are covered to avoid heat loss and protect against freezing

41 Design The main design parameter is the wastewater flowrate per surface area of the discs The main design parameter is the wastewater flowrate per surface area of the discs Is called the hydraulic loading (m3/day-m2) Is called the hydraulic loading (m3/day-m2) Indirectly represents the F/M ratio Indirectly represents the F/M ratio Wastewater flowrate is related to mass of substrate Wastewater flowrate is related to mass of substrate Disc surface area is related to mass of microbes Disc surface area is related to mass of microbes For municipal wastewater, four (4) stages are used, but if nitrification is required, five (5) stages are employed For municipal wastewater, four (4) stages are used, but if nitrification is required, five (5) stages are employed Advantages Advantages Low energy requirement compared to activated sludge Low energy requirement compared to activated sludge Ability to handle shock loadings Ability to handle shock loadings Ability of multistage to achieve high degree of nitrification Ability of multistage to achieve high degree of nitrification

42 Kinetics Kinetic equation of RBC is based on substrate removal Kinetic equation of RBC is based on substrate removal (1/X)(dS/dt) = specific rate of substrate utilization (1/X)(dS/dt) = specific rate of substrate utilization (dS/dt) = rate of substrate utilization (dS/dt) = rate of substrate utilization k = rate constant k = rate constant S = substrate concentration S = substrate concentration Q = flowrate Q = flowrate S0 = influent substrate concentration S0 = influent substrate concentration Se = effluent substrate concentration Se = effluent substrate concentration X = cell mass X = cell mass A = disc area A = disc area

43 Kinetics This equation is in the form of ( y = mx ) This equation is in the form of ( y = mx )

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45 Kinetics

46 Example 17.5


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