Trickling filters and rotating biological contactors: attached growth processes

Diederik RousseauTineke Hooijmans This presentation is based on lecture notes of dr.

Peter van der Steen, UNESCO-IHE

• Introduction to attached growth• What is a trickling filter?• Trickling filter bed media

• Trickling filter types

• Trickling filter configurations

• Rotating biological contactors

Contents

Biofilm• a biological slime layer

consisting of bacteria, inert material etc.

• biofilm will develop on almost anything

Attached growth: biofilm

Suspended growth systems such as activated sludge

Wastewater

BOD

O2

NH4

CO2

NO3

Aerobic biofilm processes

1. Wastewater components and oxygen diffuse into the biofilm

2. Bacteria convert compounds3. End products diffuse out of

biofim into bulk water

Support media

Biofilm

O2

BODNH4

Aerobic Anaerobic/Anoxic

Water phase

Air

Combined aerobic and anaerobic biofilm processes

Oxygen diffusion is limited to upper biofilm layer; lower layers are anoxic or even anaerobic

• A packed or fixed bed of media covered with slime or film over which wastewater is passed.

• Attached growth wastewater treatment process.

What is a trickling filter?

!! Ventilation !! Provision of oxygen to bacteria Venting of waste gases (CO2 etc.)

Schematics of a typical trickling filter

• Simplicity of operation• Resistance to shock loads• Low biosolids yield• Low power requirements

• Relatively Low BOD Removal (85%)• High Suspended Solids in the

Effluent (20 to 30 mg/l) (“sloughing” of biofilm)

• Very little operational control

Advantages Disadvantages

Why trickling filters?

•Media type and depth

•Hydraulic and organic loading

•Ventilation

•Filter staging

•Recirculation rate

•Flow distribution

Factors affecting performance

• Rock Media – Filter depth 1 to 2.5 m• Rocks 3 to 10 cm in diameter• Heavy so only suitable for small filter depths

• Plastic Media – Filter depth 10 to 13 m• Greater surface area than rocks so more

attachment opportunity for bacteria• Much Lighter so suitable for larger filter depths• Larger filter depths means smaller surface areas

Trickling filter media types

• Standard or low rate trickling filter• single stage rock media units• loading rates of 1-4 m3 wastewater/m2 filter cross-

sectional area-day• large area required

• High rate trickling filter• single stage or two-stage rock media units• loading rates of 10-40 m3 wastewater/m2 filter

cross-sectional area-day• re-circulation ratio 1-3

Trickling filter types using rocks

• synthetic plastic media units• modules or random packed• specific surface areas 2-5 times greater than rock• much lighter than rocks• can be stacked higher than rocks

• loading rates of 40-200 m3 wastewater/m2 filter cross-sectional area-day

• plastic media depths of 5-10 m

Super rate trickling filter

PCSecondary

TF

Influ

ent

Primary

TFSCPC

PreTreatment

Disinfection

Biosolids TreatmentEffluent

Recirculation

Disposal

Typical trickling filter plant scheme

• Reduce strength of the filter influent and/or dilute toxic wastes

• Maintain a constant wetting rate• Force sloughing to occur due to increased shear

forces.

Why Recirculation?

Organic Surface Loading Rate (gBOD/m2/day) =

)()/(*)/(

2

33

mAmgBODdaymQOSLR

afiltermedi

=

)/(*)()/(*)/(

323

33

mmAmVmgBODdaymQOSLR

specificfilter

=

Organic surface loading rate: definition

Low loaded High loaded

OSLR 1-7 gBOD/m2/day 4-38 gBOD/m2/day

Hydr. Load 0.1-0.3 m3/m2/h 0.4-2 m3/m2/h

Effl. BOD < 25 mg/l > 30 mg/l

BODremoval 80-90% 50-80%

Nitrification 60-80% 0-50%

Design criteria for trickling filters

Design criteria for trickling filtersTable 10.5

Typical Design Criteria for Trickling Filters

Item Low-rate filter High-rate filter Super-rate filter

Hydraulic loading (m3/m2-d) 1 - 4 10 - 40 40 - 200

Organic loading (kg BOD5/m3-d) 0.08 - 0.32 0.32 - 1.0 0.8 - 6.0

Depth (m) 1.5 - 3.0 1.0 - 2.0 4.5 - 12.0

Recirculation ratio 0 1 - 3 1 - 4

Filter media Rock, slag, etc. Rock, slag,synthetics

Filter flies Many Few, larvae arewashed away

Few or none

Sloughing Intermittent Continuous Continuous

Dosing intervals < 5 min < 15 s Continuous

Effluent Usually fullynitrified

Nitrified at lowloadings

Nitrified at lowloadings

• Consist of 2-4 m diameter disks, closely spaced on a rotating horizontal shaft

• Disks are covered with biofilm that rotates in and out of the wastewater to repeatedly wet and aerate the biofilm

• Shaft rotates at 1-2 rpm

Rotating biological contactors

WastewaterWastewater

Film of MicroorganismsFilm of MicroorganismsRotationRotation

Package RBC system

• Shafts• max. length limited to 9m with 8m occupied by media

• Disks (Media)• polyethylene provided in different configurations or corrugation

patterns.• Drive systems

• rotated by direct mechanical drive units, air-drive• Enclosures

• segmented fiberglass-reinforced plastic covers or housed in a building; for protection of plastic media from UV attack, for low temperature control, for protection of equipment, and for control of the buildup of algae in the process

• Settling tanks• similar to trickling filter settling tanks

• Operating problems• shaft failures, media breakage, bearing failure, and odor problems

Rotating biological contactors

Flow diagrams for RBCs

•Number of stages

•Organic loading

•Hydraulic loading

•Recirculation rate

•Submergence

•Rotational speed

•Oxygen levels

Factors affecting RBC performance

Treatment levelParameters Combined Separate

Secondary nitrification nitrificationHydraulic loading, m3/m2·day 0.08~0.16 0.03~0.08 0.04~0.1Organic loadinggSBOD5/m2 ·day 3.7~9.8 2.4~7.3 0.5~1.5gTBOD5/m2 ·day 9.8~17.2 7.3~14.6 1.0~2.9

Maximum loading on first stagegSBOD5/m2 ·day 19~29 19~29gTBOD5/m2 ·day 39~59 39~59

NH3 loading, gN/m2 ·day 0.7~1.5 1.0~2.0Hydraulic retention time, hr 0.7~1.5 1.5~4 1.2~2.9Effluent BOD5, mg/L 15~30 7~15 7~15Effluent NH3, mg-N/L < 2 < 2

RBC design and operational parameters

Unit Process Energy Usage (kWh/yr)Activated Sludge 1,000,000

RBC 120,000

Waste Stabilization Pond 0

Notes:Flow = 3.785 m3/dayInfluent BOD5 = 350 mg/LExcludes Pumping and Pretreatment Costs

Energy requirements of RBCs