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ROTATING BIOLOGICAL CONTACTOR (RBC)(4th

DC 930)

History

RBCs were first installed in West Germany in 1960 and later introduced in the United States.

A submerged RBC design was introduced in the early 1980s.

Rotating Biological Contactor,American Falls, Idaho (2003)

b) Conventional RBC in enclosed reactor

a) Conventional RBC with mechanical

drive and optional air input

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Description

An RBC consists of a series of closely spaced circular disks of polystyrene or polyvinyl

chloridethat are submerged in wastewater and rotated through it.

The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unitsizes of approximately 3.5 m (12 ft) in diameter and 7.5 m (25 ft) in length.

Photos/ Diagrams

Fig 9-11 (4th

ME 931): Typical RBC units.

Fig. 9-12 (4th

ME 934): Typical RBC staging arrangements.Fig. 12.38 (VH, p.491), Fig 12.39 (VH, p.492), Fig 12.40 (VH, p.493)

Standard unit:

Surface area of the disks: 9300 m2(100,000 ft2)

Revolution: 1 1.6 revolutions per min. Submergence of RBC: 40%; Submerged RBC: 70-90%

Major Components

- consists of a shaft of circular plastic disks (media) revolving partly submerged in a contour-

bottomed tank.

a) Shafts

b) Media

c) Drive Systems

d) Tankagee) Enclosures

f) Settling Tanks

a) SBC equipped with air capture cups; air isused both to rotate and to aerate the biodisks

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Shafts

1) to support and rotates the plastic media- max shaft length is limited to 27 ft (8.23 m) with 25 ft (7.62 m) occupied by media.

- shorted shaft length: 5-25 ft (1.52-7.62 m)

Media1) larger-diameter flat disks fabricated from expanded polystyrene beads, polyethylene media.

2) made of high-density polyethylene (HDPE)

3) different configurations and corrugation patterns

corrugation increase the available surface area and enhance structural stability

types of media are classified based on the area of media:a) low-(or standard) density: surface area 100,000 ft

2(9290 m

2) per 27 ft (8.23 m) shaft

b) medium-density & high-density: surface area 120,000 - 180,000 ft2(11,149-16,723 m2)

per 27 ft (8.23 m) shaft

Drive Systems1) Mechanical driver to rotate the units

2) Air-driven unit

o use buoyant forceo an array of cups is fixed to the periphery of the disks and diffused aeration is used

to direct air to the cup to cause rotation

A submerged RBC: air-drive units are used to provide oxygen and rotation.

Tankage

1) optimized at 0.12 gal/ft2(0.0049 m3/m2) of media- a stage volume of 12,000 gal (45.42 m

3) for a 100,000 ft2 (9290 m

2) shaft.

- a detention time of 1.44 hrs for a hydraulic loading of 2 gal/ft2.d (0.08 m

3/m

2.d)

2) a typical side-water depth: 5 ft (1.52 m) to accommodate a 40 % submergence of the

media.

Enclosures1) RBCs are enclosed to

a) protect the plastic media from deterioration due to UV lightb) protect the process from low temperatures

c) protect the media and equipment from damage

d) control the building of algae in the process

- Segmented fiberglass-reinforced plastic covers are usually provided over each shaft.

- the RBC units are covered to prevent algae growth, protect the plastic disks from the effects of UV exposure, and to prevent excessive heatloss in cold weather

2) In some cases, units have been housed in a building (see Fig. 10-36b) for:

a) protection against cold weather,

b) improved access, orc) aethetic reasons.

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Settling Tanks

1) similar to trickling-filter settling tanks

Operation

- During submergence, the wastewater can enter between the surface.

- Air enters the spaces while the liquid trickles out over films of biological growth attached tothe media.

Operational Problems (most problems have been fixed)1) shaft failure

a) most serious equipment problem

- the loss of a process unit from service- possible damage to the media

b) Causes are attributed to:

- inadequate structural design- metal failure

- excessive biomass accumulation on the media

2) media breakagea) caused by:

- exposure to heat, organic solvents, UV light

- inadequate design of the media support system3) bearing failure

a) attributed to inadequate lubrication

4) odor problemsa) caused by excessive organic loadings

Advantages:1) Reduced loadings on the shaft and bearings2) Improved biomass control by air agitation3) Ability to use large bundles of disks4) Ease of retrofit into existing aeration tanks

Other advantages:

1) Low power consumption

2) Good process stability3) Superior in performance as compared to other fixed film system due to:

a) lower organic loading per mass of biological solids

b) longer SRT

c) better control of short circuiting

Disadvantages:

a. possible oxygen limitation of biological activity because of the low levels of DO in the liquid

Other disadvantages

1) Fatigue failures - meadia failure, broken shafts2) High replacement cost of damaged media

- Replacement of damaged media is difficult and costly.

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3) Sensitive to cold temperature (in-door, plastic covers, in building)

C. Design Considerations

1) Staging of the RBC units

2) Loading criteria

3) Effluent characteristics

4) Settling tank requirement

1. Staging of RBC Units

1) Staging is the compartmentalization of the RBC media to form a series of independent cells.

2) Staging can be accomplished by :

a) using baffles in a single tankb) use of separate tanks in series

3) Staging promotes a variety conditions where different organisms can flourish

in varying degrees.- As the wastewater flows through the system, each subsequent stage receive an influent

with a lower organic concentration than the previous stage.

Cure for overloading problem:

1) by removing baffles between first and second stages

a) to reduce surface loading, andb) to increase oxygen-transfer capacity

2) by installing supplemental air systems

3) step feed4) recycle from the last stage.

RBC system can be designed to provide:a) Secondary level treatment

- effluent BOD5characteristics are comparable to well-operated activated sludge processes.

b) Advanced level treatment

- RBCs can be used to provide:

a) combined treatment for BOD and ammonia, orb) separate nitrification of secondary effluent

c) denitrification - the media shaft is totally submerged

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Typical schematic for RBC in the secondary treatment.

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Typical arrangement of RBCs

a) Flow parallel to shaft

b) Flow perpendicular to shaft

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c) Step feed

d) Tapered feed

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Design Parameters (4th

ME 933)

Rotating Biological Contactors- RBC(4th

ME 930)

- An RBC consists of a series of polystyrene or polyvinyl chloride that are submerged in

wastewater rotated through it.

Standard unit size:

Diameter 3.5 m (12 ft)Length 7.5 m (25 ft)

Surface area 9300 m2(100 000 ft

2)

Specific sBOD loading 12 20 g sBOD/m2. d (2.5 - 4.1 lb sBOD/103ft2. d)

Table 9-8.

Typical design information for rotating biological contactors (*4th ME)

Treatment level

Unit BOD removal

BOD removal

and nitrification

Separate

nitrification

Hydraulic loading m3/m

2.d 0.08 - 0.16 0.03 - 0.08 0.04 - 0.10

Organic loading g sBOD/m2.d 4 - 10 2.5 - 8 0.5 - 1.0

g BOD/m2.d 8 - 20 5 - 16 1 - 2

Maximum 1st-stage

organic loading g sBOD/m2.d 12 - 15 12 - 15

g sBOD/m2.d 24 - 30 24 - 30

NH3 loading g N/m2.d 0.75 - 1.5

Hydraulic retention

time hr 0.7 - 1.5 1.5 - 4 1.2 - 3

Effluent BOD mg/L 15 - 30 7 - 15 7 - 15

Effluent NH4-N mg/L 1 - 2

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BOD Removal (4th

ME 937)

SAs Q S

As Q

nn

=

+ +

1 1 4 0 00974

2 0 00974

1( )( . )( / )

( )( . )( / )

(9-27; 4th

ME 937)

where Sn= sBOD concentration in stage n, mg/L

As = disk surface area on stage n, m2

Q = flow rate, m3/d

Loading rate criteria:

The first-stage RBC soluble unit organic loading rate should be equal to or less than12 -15 g sBOD/m

2. d to determine the first-stage disk area.

Example9-7 Staged RBC Design for BOD Removal (4th

MD 938)

Given the following design conditions, develop a process design for a staged RBC system.

--------------------------------------------------------------------------Parameter Unit Primary effluent Target effluent

Flow rate m3/d 4000

BOD g/m3 140 20

sBOD g/m

3

90 10TSS g/m3 70 20

--------------------------------------------------------------------------

SOLUTION

1. Determine number of RBC shafts for the first stage.

a.

Use 1

st

-stage sBOD loading rate criteria = 15 g /m

2

. d

b. sBOD loading = QSo = (4000 m3/d)(90 g sBOD /m3) = 360,000 g sBOD /d

sBOD loading (360,000 g/d)c. Total disk area required = ------------------------ = ----------------- = 24,000 m2

Loading rate criteria (15 g /m2. d)

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Use standard unit size of 9300 m2/shaft (p. 931)

(24,000 m2)

d. Number of shafts = ---------------------- = 2.6

(9300 m2/shaft)

Use 3 shafts for the first stage at 9300 m2/shaft.

2. Select number of trains and number of stages.

a. Assume 3 trains with 3 stages/train.

(4,000 m3/d)

Flow rate /train = ------------------ = 1333.3 m3

/d3 trains

3. Calculate sBOD concentration in each stage using the shaft area and flow to each train.

BOD Removal is calculate using

SAs Q S

As Qn

n=

+ +

1 1 4 0 00974

2 0 00974

1( )( . )( / )

( )( . )( / )

a. Stage 1

SAs Q S

As Q

o1

1 1 4 0 00974

2 0 00974=

+ + ( )( . )( / )

( )( . )( / )

where So = 90 g/m3 (given)

As/Q = (9300 m2) / (1333.3 m

3/d) = 6.97 d/m

Influent To secondary

clarifier

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S g m131 1 4 0 00974 6 97 90

2 0 0 0974 6 9 7

1 1 24 44

0135829 8=

+ +

= + +

=

( )( . )( . )( )

( )( . )( . )

.

.. /

Repeat calculation similar to (a) above.

S g m231 1 4 0 00974 6 97 29 8

2 0 0 0974 6 97

1 1 8 09

0135814 8=

+ +

= + +

=

( )( . )( . )( . )

( )( . )( . )

.

.. /

S g m331 1 4 0 00974 6 97 14 8

2 000974 697

1 1 4 02

013589 1=

+ +

= + +

=

( )( . )( . )( . )

( )( . )( . )

.

.. /

Because the goal was 10 g/m3for S3, the proposed design is satisfactory.

4. Determine the organic and hydraulic loadings.a. 1ststage organic loading

( )( )( )( )

3 3

2

2

4000 / 90 /

3 9300

12.9 / .

org

m d g sBOD mQ SoL

Total Surface Aera trains m

g sBOD m d

= =

=

Typical design value (Table 9-8) = 12-15 g sBOD/m2. d

b. Overall organic loading

( )( )( ) ( )

3 3

2

2

4000 / 90 /

3 (3 ) 9300

4.3 / .

org

m d g sBOD mQ SoL

Total Surface Aera trains stages m

g sBOD m d

= =

=

Typical design value (Table 9-8) = 4-10 g sBOD/m2. d

c. Hydraulic loading

( )( ) ( )

L Q

Total Surface Aera

m d

trains stages m

m m d

org = =

=

4000

3 3 9300

0 05

3

2

3 2

/

( )

. / .

Typical design value (Table 9-8) = 0.08 - 0.16 m3/m

2. d