Trickling Filters and Rotary Trickling Filters and Rotary Biological ContactorsBiological Contactors

CE - 370CE - 370

Trickling FiltersTrickling Filters Trickling filters are composed of:Trickling filters are composed of:

Influent pipeInfluent pipe Rotary distributionRotary distribution Filter bedFilter bed Underdrain systemUnderdrain system Effluent pipeEffluent pipe

At downstream, a sedimentation tank is used to At downstream, a sedimentation tank is used to remove microbial growth that sloughs from the remove microbial growth that sloughs from the mediummedium

Medium Medium Crushed stoneCrushed stone Large gravelLarge gravel SlagSlag Plastic Plastic redwood redwood

Figs 17.1-17.3Figs 17.1-17.3

Classifications of Trickling FiltersClassifications of Trickling Filters Trickling filters are classified into:Trickling filters are classified into:

Low-rate or standard-rateLow-rate or standard-rateIntermediate-rateIntermediate-rateHigh-rateHigh-rateSuper-rateSuper-rate

Classification is according to:Classification is according to:Organic loadingOrganic loadingUnit liquid loadingUnit liquid loadingRecycle employedRecycle employed

Low-rate or Standard-rate FiltersLow-rate or Standard-rate Filters Organic loading = 0.08 to 0.32 kg BODOrganic loading = 0.08 to 0.32 kg BOD55/m/m33-day-day Unit liquid loading = 1 to 4 mUnit liquid loading = 1 to 4 m33/m/m22-day-day Bed depth = 1.5 to 3 mBed depth = 1.5 to 3 m Recycle is employed at times when flow is not enough to Recycle is employed at times when flow is not enough to

turn the rotary distributor (night time)turn the rotary distributor (night time) Usually single-stageUsually single-stage BODBOD55 removal = 90 to 95 % removal = 90 to 95 % Effluent BODEffluent BOD55 = 12 to 25 mg/l = 12 to 25 mg/l Can achieve better nitrification than high-rate filtersCan achieve better nitrification than high-rate filters Media volume is much greater than high-rate filtersMedia volume is much greater than high-rate filters

Intermediate-rate FiltersIntermediate-rate Filters Organic loading = 0.24 to 0.48 kg BODOrganic loading = 0.24 to 0.48 kg BOD55/m/m33-day-day Unit liquid loading = 4 to 10 mUnit liquid loading = 4 to 10 m33/m/m22-day-day Bed depth = Bed depth = Recycle may or may not be employed on Recycle may or may not be employed on

continuous basis, it is employed at times when continuous basis, it is employed at times when flow is not enough to turn the rotary distributor flow is not enough to turn the rotary distributor (night time)(night time)

May be single-stage or two-stageMay be single-stage or two-stage BODBOD55 removal = 85 to 90 % removal = 85 to 90 % Effluent BODEffluent BOD55 = 20 to 30 mg/l = 20 to 30 mg/l

High-rate FiltersHigh-rate Filters Organic loading = 0.32 to 1.0 kg BODOrganic loading = 0.32 to 1.0 kg BOD55/m/m33-day-day Unit liquid loading = 10 to 40 mUnit liquid loading = 10 to 40 m33/m/m22-day-day Bed depth = 1 to 2 mBed depth = 1 to 2 m Recycle is employed continuouslyRecycle is employed continuously May be single-stage or two-stageMay be single-stage or two-stage BODBOD55 removal removal

Single-stage = 75 to 80 %Single-stage = 75 to 80 % Two-stage = 85 to 90 %Two-stage = 85 to 90 %

Effluent BODEffluent BOD55 Single-stage = 40 to 50 mg/lSingle-stage = 40 to 50 mg/l Two-stage = 20 to 30 mg/lTwo-stage = 20 to 30 mg/l

Can not achieve a highly nitrified effluentCan not achieve a highly nitrified effluent

Super-rate FiltersSuper-rate Filters Organic loading = 0.80 to 6.0 kg BODOrganic loading = 0.80 to 6.0 kg BOD55/m/m33-day-day Unit liquid loading = 40 to 200 mUnit liquid loading = 40 to 200 m33/m/m22-day-day Bed depth = 4.5 to 12 mBed depth = 4.5 to 12 m Recycle is continuousRecycle is continuous Medium is always synthetic plastic (most common) or Medium is always synthetic plastic (most common) or

redwoodredwood Plastic media has a specific surface area (surface area per Plastic media has a specific surface area (surface area per

unit volume) that is from 2 to 5 time that for stone mediaunit volume) that is from 2 to 5 time that for stone media Microbial growth is proportional to surface areaMicrobial growth is proportional to surface area Can achieve a nitrified effluent at low loadingsCan achieve a nitrified effluent at low loadings

Trickling FiltersTrickling Filters

Energy cost per mass of BODEnergy cost per mass of BOD55 removed is less removed is less than that of activated sludgethan that of activated sludge

Super-rate filters can be used ahead of Super-rate filters can be used ahead of activated sludge (for high-strength wastewater)activated sludge (for high-strength wastewater)

Single-stage and two-stage trickling filters are Single-stage and two-stage trickling filters are upgraded by adding activated sludge process upgraded by adding activated sludge process or a shallow polishing pond downstreamor a shallow polishing pond downstream

Mechanisms in Biological FiltrationMechanisms in Biological Filtration

When wastewater passes over a microbial When wastewater passes over a microbial growth, organic matter and Ogrowth, organic matter and O22 are sorbed by are sorbed by the growththe growth

Aerobic bio-oxidation occursAerobic bio-oxidation occurs End-products are releasedEnd-products are released As time passes, the microbial film becomes As time passes, the microbial film becomes

thicker due to growth of new cellsthicker due to growth of new cells The film slough off the mediumThe film slough off the medium

Mechanisms in Biological FiltrationMechanisms in Biological Filtration Most of the biological growth is aerobicMost of the biological growth is aerobic Growth immediate to the medium surface is usually Growth immediate to the medium surface is usually

anaerobicanaerobic The amount of organic removed per meter of bed depth is The amount of organic removed per meter of bed depth is

greatest at top of the filter and smallest at bottomgreatest at top of the filter and smallest at bottom Nitrifying growth is taking place at the lower depths of Nitrifying growth is taking place at the lower depths of

the filterthe filter Recycling the effluent increases the removal efficiencyRecycling the effluent increases the removal efficiency Microbial growth sloughed will be removed in final Microbial growth sloughed will be removed in final

clarifierclarifier The sludge is sent to the head of the plant and removed in The sludge is sent to the head of the plant and removed in

the primary sedimentation tankthe primary sedimentation tank

Filter PerformanceFilter Performance Kinetic EquationsKinetic Equations

The rate of organic removal per interval of depth is The rate of organic removal per interval of depth is proportional to the remaining concentration of removable proportional to the remaining concentration of removable organic matter:organic matter:

- dL/dD = kL- dL/dD = kL dL is an increment of remaining organic concentrationdL is an increment of remaining organic concentration dD is an increment of depthdD 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, thenSince dD is proportional to an increment of time dt, then (L(LDD / L) = 10 / L) = 10-kD-kD

LLDD = removable ultimate first-stage BOD concentration at depth D = removable ultimate first-stage BOD concentration at depth D L = removable ultimate fist-stage BOD concentration applied to the L = removable ultimate fist-stage BOD concentration applied to the

bedbed k = rate constantk = rate constant D = depth of bed, ft (m)D = depth of bed, ft (m)

Kinetic EquationsKinetic Equations For low-rate filters and flow of 1.88 to 5.61 mFor low-rate filters and flow of 1.88 to 5.61 m33/m/m22--

dayday k = 0.574k = 0.574 Removable fraction = 90%Removable fraction = 90%

For high-rate filters operated at flow 18.8 mFor high-rate filters operated at flow 18.8 m33/m/m22-day-day k = 0.494k = 0.494 Removable fraction = 78.4%Removable fraction = 78.4%

The following equation was developedThe following equation was developed kk22 = k = k2020 . 1.047 . 1.047 (T(T

22- 20)- 20)

kk22 is rate at T is rate at T22 ; k20 is the rate at 20 ; k20 is the rate at 20 C C

Kinetic EquationsKinetic Equations Another performance equation was developed Another performance equation was developed

based on the specific rate of substrate removalbased 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, (1/X)(dS/dt) = specific rate of substrate utilization, mass/(mass microbes)(time)mass/(mass microbes)(time)

(dS/dt) = rate of substrate utilization, mass/(volume)(dS/dt) = rate of substrate utilization, mass/(volume)(time)(time)

k = rate constant, volume/(mass microbes)(time)k = rate constant, volume/(mass microbes)(time)S = substrate concentration, mass/volumeS = substrate concentration, mass/volume

Kinetic EquationsKinetic Equations Re-arrange and integrateRe-arrange and integrate

(S(Stt/S/S00) = e) = e-k-kXtXt

k = rate constantk = rate constant X = average cell mass concentration, mass/volumeX = average cell mass concentration, mass/volume SStt = substrate concentration after the contact time t, mass/volume = substrate concentration after the contact time t, mass/volume SS00 = substrate concentration applied to the filter, mass/volume = substrate concentration applied to the filter, mass/volume

X is proportional to the specific surface area of the X is proportional to the specific surface area of the packing, Apacking, Ass (m (m22 of the surface per bulk m of the surface per bulk m33 of the volume) of the volume)

X ~ AX ~ Ass

Kinetic EquationsKinetic 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 timet = mean contact time D = depth of filterD = depth of filter QQLL = unit liquid loading or surface loading = unit liquid loading or surface loading C, n = experimental constantsC, n = experimental constants

nLQ

CDt

Kinetic EquationsKinetic Equations From the above equationsFrom the above equations

SStt = substrate concentration in the filter effluent, mass/volume = substrate concentration in the filter effluent, mass/volume SS00 = substrate concentration applied to the filter, mass/volume = substrate concentration applied to the filter, mass/volume K = rate constantK = rate constant AAss = specific surface area of the packing, area/volume = specific surface area of the packing, area/volume D = filter depthD = filter depth QQLL = unit liquid loading or surface loading = unit liquid loading or surface loading m, n = experimental constants m, n = experimental constants

nL

ms QDKAt e

SS /

0

Kinetic EquationsKinetic Equations Value of nValue of n

Depends of the flow characteristics through the packingDepends of the flow characteristics through the packing Usually between 0.5 to 0.67Usually between 0.5 to 0.67

The equation can be further simplified by combing The equation can be further simplified by combing KAKAss

mm

K is the new rate constantK is the new rate constant K is between 0.01 and 0.1K is between 0.01 and 0.1 K can be obtained from pilot-scale plantK can be obtained from pilot-scale plant

nLQKDt e

SS /

0

Kinetic EquationsKinetic Equations To correct K for temperatureTo correct K for temperature

KKTT = rate constant at temperature T, = rate constant at temperature T, C C KK2020 = rte constant at 20 = rte constant at 20 C C T = temperature, T = temperature, C C

)20(20 035.1 T

T KK

Kinetic EquationsKinetic Equations A more common kinetic equation for filter with stone media A more common kinetic equation for filter with stone media

is:is:

SStt = BOD = BOD55 in the filter effluent, mg/l in the filter effluent, mg/l SS00 = BOD = BOD55 in the wastewater discharged on the filter bed, mg/l in the wastewater discharged on the filter bed, mg/l C = 2.5 for USCS units and 5.358 for SI unitsC = 2.5 for USCS units and 5.358 for SI units D = filter depth, ft (m)D = filter depth, ft (m) QQLL = unit liquid loading, MG/acre-day (m = unit liquid loading, MG/acre-day (m33/m/m22-day) -day)

50.0

67.00 1

1

L

t

QDC

SS

Examples 17.1 and 17.2Examples 17.1 and 17.2

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

Filter DetailsFilter Details

Operational ProblemsOperational Problems Less problems than activated sludge processesLess problems than activated sludge processes ProblemsProblems

Floating sludge may be encountered if anaerobic Floating sludge may be encountered if anaerobic conditions occur within the settled sludgeconditions occur within the settled sludge

Presence of fly that breed in the filterPresence of fly that breed in the filterCan be controlled by allowing the medium to stay Can be controlled by allowing the medium to stay

submergedsubmergedOdor when low-rate filters are employed and Odor when low-rate filters are employed and

wastewater is stale when it reaches the plant wastewater is stale when it reaches the plant

Rotary Biological ContactorsRotary Biological Contactors

Main CharacteristicsMain Characteristics Composed of multiple discs mounted on a horizontal shaft that Composed of multiple discs mounted on a horizontal shaft that

passes through the center of the discspasses through the center of the discs Wastewater flow is perpendicular to the shaftWastewater flow is perpendicular to the shaft About 40% of the total disc area is submergedAbout 40% of the total disc area is submerged Biological film grows on the discBiological film grows on the disc As the shaft rotates, the biological growth (film) sorbs organic As the shaft rotates, the biological growth (film) sorbs organic

matter from wastewatermatter from wastewater Oxygen is adsorbed from air to keep aerobic conditionOxygen is adsorbed from air to keep aerobic condition Multiple stages of RBC is used to achieve greater BOD5 Multiple stages of RBC is used to achieve greater BOD5

removalremoval Sloughed biological growths are removed in final clarifiersSloughed biological growths are removed in final clarifiers No recycle is employedNo recycle is employed Biological activities are reduced during cold weatherBiological activities are reduced during cold weather In cold climates, RBCs are covered to avoid heat loss and In cold climates, RBCs are covered to avoid heat loss and

protect against freezing protect against freezing

DesignDesign The main design parameter is the wastewater flowrate The main design parameter is the wastewater flowrate

per surface area of the discsper 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 ratioIndirectly represents the F/M ratio

Wastewater flowrate is related to mass of substrateWastewater flowrate is related to mass of substrate Disc surface area is related to mass of microbesDisc surface area is related to mass of microbes

For municipal wastewater, four (4) stages are used, For municipal wastewater, four (4) stages are used, but if nitrification is required, five (5) stages are but if nitrification is required, five (5) stages are employedemployed

AdvantagesAdvantages Low energy requirement compared to activated sludgeLow energy requirement compared to activated sludge Ability to handle shock loadingsAbility to handle shock loadings Ability of multistage to achieve high degree of nitrificationAbility of multistage to achieve high degree of nitrification

KineticsKinetics Kinetic equation of RBC is based on Kinetic equation of RBC is based on

substrate removalsubstrate removal (1/X)(dS/dt) = specific rate of substrate (1/X)(dS/dt) = specific rate of substrate

utilizationutilization (dS/dt) = rate of substrate utilization(dS/dt) = rate of substrate utilization k = rate constantk = rate constant S = substrate concentrationS = substrate concentration Q = flowrateQ = flowrate S0 = influent substrate concentrationS0 = influent substrate concentration Se = effluent substrate concentrationSe = effluent substrate concentration X = cell massX = cell mass A = disc areaA = disc area

ee

ee

e

kSSSAQ

AX

kSSSQX

SSQdtdS

kSdtdS

X

)(

)(1

)(

1

0

0

0

KineticsKinetics

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

kSratereactionrSSAQIf

)( 0

KineticsKinetics

n

n

AQkS

S

orA

QkS

S

AQkS

Sarrangere

AQk

SSS

SandAQby

kSSSAQDivide

1

1

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2

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Example 17.5Example 17.5