trickling filters

Trickling Filters*Sanitary EngineeringCWS-presentation

Submitted ByPrakhar Deroliya, 2013UCE1492Purkha Ram, 2014UCE1247K. Anirudh, 2014UCE1376Rishabh Srivastava, 2014UCE1300

SUBMITTED TO Prof A. B. Gupta

INDEX 1. Introduction2. Classification and Design criteria3. Basic flow diagrams 4. Empirical equations 5. Operational Problems 6. Advantages and disadvantages 7. References

INTRODUCTION • Whereas Activated Sludge

Process (ASP) is a “suspended growth” process, Trickling Filters* (TFs) and its variants i.e. Rotatory Biological Contractors (RBCs) are “attached growth” processes. • In a typical TF, wastewater trickles

over medium. • Bacteria grow on medium, thus

creating Biofilm.

INTRODUCTION (CONTD.)• Trickling filters are conventional aerobic

biological wastewater treatment units. • A typical TF consists of a cylindrical tank and is

filled with a high specific surface area material, such as rocks, gravel, shredded PVC bottles, or special pre-formed plastic filter media.

• A high specific surface provides a large area for biofilm formation.

• Organisms that grow in the biofilm over the surface of the media oxidize the organic load in the wastewater to CO2 and water, which generating new biomass.

• The incoming pre-treated wastewater is trickled over the filter with the use of rotatory sprinkler.

INTRODUCTION (CONTD.)• In this way, the filter media goes through cycles of being dosed and exposed to air. • However, oxygen is depleted within the biomass and the inner layers may be

anoxic or anaerobic. • The properties of an ideal material for trickling filter are:

• Low cost• Durable • High surface to volume ratio• Light• Allows air to circulate

• Generally used filter media are gravels, corrugated plastic sheets or hollow plastic cylinders.

INTRODUCTION (CONTD.)• Adequate air flow is important to

ensure sufficient treatment, performance and prevent odors. • Rotary sprinkler/distributor is most

often used which consists of a hollow vertical centre column carrying two or more radial pipes or arms. • Each arm(radial pipe) contains a

number of nozzles or orifices for discharging the wastewater onto the bed.

INTRODUCTION (CONTD.)• The underdrains should provide a passageway for air at the maximum

filling rate. • A perforated slab supports the bottom of the filter, allowing the

effluent and excess sludge to be collected. • TFs are usually designed with a recirculation pattern for the effluent to

improve wetting and flushing of the filter material. • Due to anaerobic condition, sloughing1 takes place. Microbes fall off

the medium and are carried with the effluent. The under-drain system allows transporting these solid to a clarifier, where the solids settle and separate from the treated effluent.

BASIC DEFINITIONS • Biofilm layer: The layer of

microorganisms developed on the filter medium due to aerobic utilization of substrate, typically 10 micron to 10 mm thick. • Diffusion layer or stagnant boundary

layer: The layer separating biofilm and moving liquid. • Anaerobic/endogenous zone: The

zone in which oxygen cannot penetrate and hence anaerobic conditions prevail.

SUBSTRATE CONCENTRATION (CONTD.)

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SUBSTRATE CONCENTRATION (CONTD.)

• Rate of substrate flux across boundary layer: ) where:rsf = rate of substrate surface fluxDw=molecular diffusion co-efficient for substrate in water Sb=substrate concentration in bulk liquid Ss=substrate concentration at biofilm surface dS/dX=substrate concentration gradient

• Within biofilm, rate of movement

where: rbf= rate of substrate flux De=effective molecular diffusion coefficient in biofilm (De< Dw)

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SUBSTRATE CONCENTRATION (CONTD.)

• Substrate utilization rate:

• Mass balance for biofilm under steady-state: -=0

or De= ……………………………………………………………………..(1)

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SUBSTRATE CONCENTRATION (CONTD.)

• In equation (1), if we assume S<<ks (i.e. low concentration)• Then the equation gives the following expression • S=Ss )………………………………………………………………………(2)where = sqrt(DeksY/µmaxX)

• Based on and Lf we can classify biofilms in two categories• When Lf> biofilm-substrate does not penetrate far • When Lf< , biofilm is a fully penetrated biofilm

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DEEP VERSUS SHALLOW BIOFILM

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BIOMASS KINETICS • Mass balance within the biofilm

maxSf/(Sf+ks)Xfdz - klossXfdz……………………………………………..(3)where dz=unit thickness of biofilm Sf= substrate concentration within biofilm Xf= biomass concentration within biofilm kloss= overall loss rate for biofilm

• Biofilm lose mass constantly by erosion of small pieces and sloughing of large sections

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BIOMASS KINETICS • Equation (3) can be integrated

over full thickness of biofilm and set to steady-state conditions to get, with substitutes we get the following expression.

XfLf=JY/kloss

or Lf= JY/Xfkloss

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BIOMASS KINETICS • Sb(minimum)= • When Sb < Sb(minimum), there is no biofilm. • When Sb >> Sb(minimum), biofilm is deep

CLASSIFICATION OF TFS• Based on organic loadings• Low rate or standard rate trickling

filters: 0.08 to 0.30 kg BOD(5)/cum-day• High rate trickling filters: 0.5 to 1.0

kg BOD(5)/cum-day• Super rate trickling filters: 1-2 kg

BOD(5)/cum-day

• Based on the number of units used in series• Single-stage trickling filter • Two-stage trickling filter

DESIGN CRITERIA Design Criteria Standard Rate or low

rate TFHigh rate TF (stone media)

Super rate TF (plastic media)

Hydraulic loading (m3/m2-d)

1-4 10-40a 40-200

Organic loading (BOD5 kg/d-m3)

0.08-0.35 0.35-2.4b 1-6

Depth of filter (m) 1.5-3.0 1.0-2.0 2-12Recirculation ratio, R 0.0 1-4 1-14BOD5 removal efficiency (%)

80-85 65-80 65-85

a including recirculation b excluding recirculation Adopted from Wastewater treatment: concept and design approach by Kari and Christian

TYPICAL FLOW DIAGRAMS (LOW RATE, SINGLE-STAGE TRICKLING FILTERS WITHOUT RECIRCULATION)

Primary Settling Tank Trickling Filter Secondary Settling Tank

Influent Effluent

HIGH RATE SINGLE STAGE TFS WITH EFFLUENT RECIRCULATION

PST

PST

PST

TF

TF

TF

SST

SST

SST

IN

IN

IN OUT

OUT

OUT

BASIC DESIGN EQUATIONS • National Research Council Equation (NRC-Equation)• For a low rate or single-stage TF• For a high rate or two-stage TF• Modified National Research Council Equation

• Eckenfelder Equation • Without circulation • With circulation

• Velz Equation • Without circulation • With circulation

NRC EQUATION • For a low rate or single stage TF

E1=

• For a high rate or two-stage TF

E2=

• Recirculation factor can be calculated using the following formula

F=

Here E1=BOD removal efficiency of the filter of a single-stage TF system or first filter of two-stage TF system in %E2=BOD removal efficiency of the second filter of two-stage TF system in %W1=BOD loading of settled influent to the filter of a single-stage TF system or first filter of a two-stage TF system, kg BOD(5) per dayW2=BOD loading to second filter of a two-stage TF system, kg BOD(5) per dayV1=volume of second filter of a two-stage TF system, cum F1= recirculation factor for first stage filterF2= recirculation factor for second stage filter f=treatability factor which can be assumed as 0.9 for domestic wastewater

ECKENFELDER EQUATION • Without recirculation

• With recirculation

Sa is given by

Sa=

here Se=effluent substrate concentration, BOD-5, mg/LS0=influent substrate concentration, BOD-5 mg/LSa=substrate concentration of influent and recycled flow, BOD-5, mg/Lk= treatability or waste removal constant (depends on type of wastewater and filter media and usually varies from 0.01 to 1 per minute n= empirical or filter media constant (usually assumed 0.5)Q= hydraulic loading rate cum/sqm-minR= recirculation ratio

VELZ EQUATION • Without circulation

=10-kD

• With circulation =10-kD

where Ca is given by Ca=

HereCD=concentration removal of BOD remaining at depth D, mg/LC=concentration removal of applied BOD of influent, mg/LCa=concentration of total applied BOD of influent and recycled flow, mg/LCi=concentration of influent BOD excluding BOD excluding recirculation, mg/LCe=effluent BOD, mg/Lk=Bod removable rate constant, per day D=depth of filter media, meters

OPERATIONAL PROBLEMS• Disagreeable Odours from Filter • Ponding on Filter Media• Filter Files (Psychoda) • Icing • Rotating Distributor Slows Down or Stops

DISAGREEABLE ODOURS FROM FILTER

• Potential Cause: Excessive organic load causing anaerobic decomposition in filter. • Remedy: Reduce loading; increase BOD removal in primary settling tanks;

enhance aerobic conditions in treatment units by adding chemical oxidants, pre-aerating, recycling plant effluent, or increasing air to aerated grit chambers; use plastic media instead of rock • Potential Cause: Inadequate ventilation • Remedy: Increase hydraulic loading to wash out excess biological growth;

remove debris from filter effluent channels, underdrains, and the top of filter media; unclog vent pipes; reduce hydraulic loading rates if underdrains are flooded; install fans to induce draft through filters

PONDING ON FILTER MEDIA• Potential Cause: Excessive biological growth or foreign matter in or on

the filter • Remedy: Reduce organic loading; increase hydraulic loading to

increase sloughing; use high-pressure stream of water to flush filter surface; maintain 1-2 mg/L residual chlorine on the filter for several hours; flood filter for 24 hours; shut down filter to dray out media; replace media if necessary; remove debris.

FILTER FLIES (PSYCHODA)• Potential Cause: Inadequate filter media moisture • Remedy: Increase hydraulic loading; unplug spray orifices or nozzles;

use orifice opening at end of rotating distributor arms to spray filter walls; flood filter for several hours each week during fly season; maintain 1-2 mg/L residual chlorine on the filter for several hours • Potential Cause: Poor housekeeping • Remedy: Mow area surrounding filter and remove weeds and shrubs.

ICING • Potential Cause: Low temperature of wastewater • Remedy: Decrease recirculation; use high-pressure stream of water to

remove ice from orifices, nozzles, and distributor arms; reduce number of filters in service as long as effluent limits can still be met; reduce retention time in pre-treatment and primary treatment units; construct windbreak or covers

ROTATING DISTRIBUTOR SLOWS DOWN OR STOPS

• Potential Cause: Insufficient flow to turn distributor. • Remedy: Increase hydraulic loading • Potential Cause: Clogged arms or orifices • Remedy: Flush out arms by opening end plates; remove solids from influent

wastewater; flush out orifices • Potential Cause: Clogged distributor arm vent pipe• Remedy: Remove material from vent pipe by rodding or flushing; remove

solids from influent wastewater • Potential Cause: Distributor arms not level and Distributing rods hitting media • Remedy: Level Media; remove some media

ADVANTAGES• Less energy needed (Natural aeration)• Simpler operation • No bulking sludge problem • Better sludge thickening • Can withstand varied loading • Suitable in areas where ASP cannot be installed. • Less skilled person and technical expertise required. • Less wear and tear, thus low operation and maintenance cost

DISADVANTAGES • Poorer effluent quality, hence additional treatment may be required• Sensitive to low temperature • Produces odors • Sloughing events can create lots of sludge in short time • Filter files • Nitrogen removal is difficult • Limited flexibility and control as comparison with ASP.

LIST OF REFERENCES • www.sswm.com/trickling-filters• Trickling Filter Fly Control, M. Livingston Vol.23, No.2 • NPTEL • Trickling Filter Achieving Nitrification, ETI • www.Wikipedia.com • Rittman and McCarly for biofilm modelling.

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