Fixed Film Systems

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Fixed Film Systems Attached Growth


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Remember Activated Sludge?


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Attached Growth(fixed film)


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Fixed Film Systems Attached Growth

OBJECTIVE: Introduce some concepts and process analysis of trickling

filters. Give some introduction to rotating biological contactors (RBCs).

Trickling filters are a fairly old wastewater treatment system. The first was. . .,

the current popularity of activated sludge systems. Many municipalitiesstill operate trickling filters as their major biological treatment system.Rotating biological contactors which have appeared in the last two decades

are now beginning to gain acceptance in public treatment systems.

Aerobic Process little or no odor


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Trickling Filters


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Typical Trickling Filter


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Trickling Filters

Basically a tank or container with some sort of media upon which

microorganisms grow. The wastewater is percolated through the filter

from the top.

Media typically consists of:river rock

redwood slatsplastic various proprietary design

Filter shape is typically circular with a depth of 4 10 ft (6 typical). Plasticmedial may be up to 20 - 40 ft deep due to light weight of media.


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Trickling Filters

Disadvantages

Advantages

neven sur acewetting

May promote solids

retention & be hardto clean

oo a r exc ange Simple to construct

Almost any material

will work


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Mechanisms of Removal

Little or no actual filtration occurs. Treatment is primarily biological andoccurs in the layer of growth on the filter media. Microorganisms in themicrobial film use organics from the waste and O2 from the air.

Microbial

film(slime layer)

Waste

water

Organics Air

Treatedwater

BedMedia

a

cultative/Anaerob

ic

Aerobic

AirCO2

Otheroxidizedproducts

(~100) (~1mm)


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Three Indefinite Zones in the Microbial Film Layer

1. Surface high F/M, good O2 availability exponential growth rapid organic removal

2. Meso deeper into the film F/M decreases O2 availability decreases eventuall leads to endo enous res .

Microbialfilm

Wastewater

3. Bottom right next to media face may or may not be anaerobic

depending upon film thickness

BedMedia

Facultative/Anaerobic

Ae

robic

s me ayer

Organics

AirCO2

Otheroxidizedproducts

(~100) (~1mm)

Air


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Sloughing - the falling off of film which represents

actual waste removal from the system.

Bed

Facultative/Ana

Aero

Flowingwat

ro

bic

ic

r


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Sloughing - the falling off of film which represents actual waste removalfrom the system. Four mechanisms have been suggested.

1. CO2, CH4 bubbles build up inside due to anaerobic activity mechanicalweakening

2. acid byproducts of anaerobic activity build up faster than methane

formers can use them. This lowers pH and kills organisms weakeningin the deep layers

3. as the film thickness increases, the space for wastewater to flowdecreases. Hydraulic shear thus increases until the film is torn loose.

4. lack of food deep in the film causes organisms to undergo endogenousrespiration (including consumption of the slime capsule) therebyweakening the film

Bed

Media

Faculta

tive/Anaerobic

Aerobic

Flowingwater


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Recirculation


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Recirculation see various configurations in the text, Fig. 9.2p. 895, 4th Ed. Some schemes recirculate sludge

as well as wastewater

Recirculation is practical because:

1. reduces influent concentration

2. provides some load equalization

.

4. ensures that entire filter is kept wet

5. reduces temperature fluctuations

6. prevents filter fly growth psychoda (gnat-sized)

7. provides adequate hydraulic shear


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Trickling Filter Applications


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Roughing Filters

specially designed high-rate trickling filters

high hydraulic loading rates

reduces load to conventional trickling filters

High-rate filters that treat organic loads in excess of 1.6 kg/m3/d.

Usually used to pretreat wastewater prior to secondary treatment.


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Process Analysis

National Research Council 1946

Empirical formulation based on operation of U.S. Army facilities:

VF

WE

4432.01

100

+

=

Where:

E = removal efficiencyW = BOD5 loading, kg/d

V = volume of media, m3

F = recirculation factor

note: the data which let to this formulationhave a great deal of scatter


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Rotating Biological Contactors, RBCs, Biodisks


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RBC facts, continued

normally no recycling scheme is employed. Therefore, to get adequatetreatment the systems are usually plug flow (more efficient than completelymixed) for practical construction purposes.

4-stage treatment is typical.

clarification is usually accomplished by a mechanical clarifier with the sludgebeing either directly collected or returned to the primary clarifier. Sludgeproduction ranges around 0.5 lb solids/lb BOD applied.

Rotating Biological Contactor North Scotland

design is based on a combination of hydraulic loading and BOD removaldata from pilot studies. Temperature effects should be considered. Mostmanufacturers have pilot units available.


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RBC, cross-section


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Package treatment - Rotating biological contactors


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Rotating Biological Contactor - Failure

Normal Loading

Overloaded

Manistique, MI


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Failure


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De Biocombi is speciaal ontworpen voor

communale zuiveringen.


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)/)(00974.0)(2(

)/)(00974.0)(4(111

QA

QA

Ss

ns

n

++

=

Process Analysis

(U.S EPA, 1985)

Where Sn = sBOD concentration in stage n, mg/LAs = disk surface area on stage n, m

2

Q = flowrate, m3/d

Metcalf & Eddy, 4th Edition


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Combined Fixed Film Systems


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RBC - facts

rotating circular plates submerged, typically about 40 % in acontoured tank.

when submerged, wastewater contacts the disks and waste organics start to

diffuse into the slime layer. When rotated out of the wastewater, the mediacontacts air and the wastestream moves over the slime in sheet flow. The

sequence is not unlike what is seen by the media in a trickling filter dosedwith a revolving distributor.

lower power consumption and sludge production than activated sludge.

better removal than a trickling filter.

disks typically 10-12 in diameter, mounted on a shaft on 1-2 centers.

Fixed Film Systems

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