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