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BAF - Biologically Active Filter ​

Submerged fixed-film reactor combining biological treatment with filtration, providing high-rate secondary or tertiary treatment in a compact footprint.

Overview ​

The Biologically Active Filter (BAF) is a fixed-bed biofilm reactor where wastewater flows through a submerged bed of media that supports attached microbial growth. Unlike conventional biofilters, BAFs are fully submerged and typically aerated, functioning as both a biological reactor and a physical filter. The process requires periodic backwashing to remove accumulated solids and excess biomass.

When to Use ​

  • Space-constrained treatment facilities
  • Tertiary nitrification or denitrification
  • Industrial pretreatment
  • Combined secondary treatment and filtration
  • Upgrade of existing facilities
  • Cold climate applications (enclosed systems)

Key Characteristics ​

  • Compact footprint (high volumetric loading)
  • Combined biological treatment + filtration
  • No secondary clarifier required
  • Periodic backwash needed
  • Enclosed system option for odor control
  • Modular, expandable design

Process Configurations ​

Upflow vs Downflow ​

ConfigurationFlow DirectionCommon ApplicationCharacteristics
Upflow aeratedWastewater up, air upBOD removal, nitrificationCounter-current, efficient O2
Downflow aeratedWastewater down, air upNitrification, tertiaryCo-current water/gravity
Upflow anoxicWastewater up, no airDenitrificationMethanol addition
Downflow anoxicWastewater down, no airDenitrificationPre-settled feed

Upflow Configuration ​

Effluent
    ^
    |
[========] ← Effluent collection
    |
[  Media  ] ← Biofilm growth zone
[  Bed    ]
[        ]
    ^   ^
    |   Air
    |
[========] ← Support floor / nozzles
    |
Influent

Downflow Configuration ​

Influent
    |
    v
[========] ← Distribution system
    |
[  Media  ] ← Biofilm growth zone
[  Bed    ]
[        ]
    ^
    Air (counter-current)
    |
[========] ← Underdrain / nozzles
    |
    v
Effluent

Biofilm Theory ​

Substrate Removal in Fixed Bed ​

For biofilm systems with both diffusion and reaction:

$$\eta = \tanh(\phi) / \phi$$

Where:

  • eta = effectiveness factor
  • phi = Thiele modulus

$$\phi = L_f \sqrt{\frac{k_1 \times X_f}{D_f}}$$

Removal Rate ​

$$r_v = \eta \times k_1 \times X_f \times a \times \frac{S}{K_s + S}$$

Where:

  • r_v = volumetric removal rate (kg/m3/d)
  • a = specific surface area of media (m2/m3)
  • X_f = biofilm density (kg/m3)
  • L_f = biofilm thickness (m)

Modified Eckenfelder Equation ​

For practical BAF design:

$$\frac{S_e}{S_0} = \exp\left(-\frac{k \times a \times D}{v^n}\right)$$

Where:

  • S_e = effluent concentration
  • S_0 = influent concentration
  • k = rate constant (m/d)
  • D = bed depth (m)
  • v = hydraulic loading rate (m/h)
  • n = empirical exponent (0.5-0.7)

Design Equations ​

Volumetric Loading Rate ​

$$BV = \frac{Q \times S_0}{V_{media}}$$

Units: kg BOD/m3/d or kg COD/m3/d

Surface Area Loading Rate ​

$$BS = \frac{Q \times S_0}{A_{media}}$$

Where A_media = V_media x a (specific area) Units: g/m2/d

Hydraulic Loading Rate ​

$$HLR = \frac{Q}{A_{plan}}$$

Units: m/h or m3/m2/h

Empty Bed Contact Time (EBCT) ​

$$EBCT = \frac{V_{bed}}{Q}$$

$$EBCT = \frac{D}{HLR}$$

Air:Water Ratio ​

For aerobic BAF: $$\frac{Q_{air}}{Q_{water}} = \frac{OUR \times V}{Q_{air} \times \rho_{O2} \times E}$$

Typical: 4-15:1 (v/v)

Backwash Frequency ​

$$t_{BW} = \frac{V_{void} \times \Delta p_{max}}{Q \times SS_{captured}}$$

Typical: 12-48 hours between backwash cycles

Design Criteria ​

BOD Removal ​

Loading (kg BOD/m3/d)Removal (%)Application
1-3>90Tertiary polishing
3-680-90Secondary treatment
6-1060-80High-rate roughing
>10<60Industrial pretreatment

Nitrification ​

Loading (kg NH4-N/m3/d)Removal (%)Notes
0.3-0.5>95Complete nitrification
0.5-1.080-95Good performance
1.0-1.560-80Partial nitrification
>1.5<60Overloaded

Temperature correction: $$k_T = k_{20} \times 1.047^{(T-20)}$$

Denitrification ​

Loading (kg NO3-N/m3/d)Removal (%)Carbon Source
1-2>90Methanol
2-470-90Methanol
1-380-95Ethanol
0.5-1.570-85Endogenous

Hydraulic Design ​

ParameterTertiarySecondaryUnits
HLR (normal)5-103-6m/h
HLR (peak)15-208-12m/h
EBCT15-3030-60min
Bed depth2-43-5m

Parameters ​

Media Parameters ​

ParameterUnitDefaultRangeDescription
Media type-expandedClayclay/plastic/lavaMaterial
Grain sizemm42-8Effective diameter
Specific aream2/m3400200-800Surface area
Porosity%4535-55Void fraction
Densitykg/m31400900-2000Bulk density
Bed depthm32-5Media depth

Process Parameters ​

ParameterUnitDefaultRangeDescription
Configuration-upflowupflow/downflowFlow direction
HLRm/h63-15Hydraulic loading
EBCTmin3015-60Contact time
BOD loadingkg/m3/d41-10Organic loading
NH4 loadingkg/m3/d0.80.3-1.5Nitrogen loading
Temperaturedeg C158-25Operating temp

Aeration Parameters ​

ParameterUnitDefaultRangeDescription
Air:water ratiov/v84-15Volumetric ratio
DO setpointmg/L42-6Target DO
Diffuser type-coarsecoarse/processAeration system
PressurekPa5030-80Air supply

Backwash Parameters ​

ParameterUnitDefaultRangeDescription
Frequencyh2412-72Between cycles
Durationmin2010-30Per cycle
Air ratem/h6040-100Scour velocity
Water ratem/h3020-50Rinse velocity
Expansion%2515-40Bed expansion

Media Types ​

Expanded Clay (Biolite, Filtralite) ​

PropertyValue
Specific gravity1.3-1.6
Surface area300-500 m2/m3
Grain size2-6 mm
Porosity35-45%
AdvantagesDurable, good biofilm attachment
DisadvantagesHeavy, higher backwash energy

Plastic Media ​

PropertyValue
Specific gravity0.95-1.1
Surface area200-400 m2/m3
Grain size3-10 mm
Porosity85-95%
AdvantagesLight, low head loss
DisadvantagesCan float, requires retention

Polystyrene Beads ​

PropertyValue
Specific gravity0.03-0.05
Surface area1000-2000 m2/m3
Diameter2-4 mm
Porosity35-40%
AdvantagesVery high surface area, buoyant
DisadvantagesRequires top retention, floating bed

Granular Activated Carbon (GAC) ​

PropertyValue
Specific gravity1.3-1.5
Surface area800-1200 m2/g (internal)
Grain size0.8-2.4 mm
Porosity40-50%
AdvantagesAdsorption + biodegradation
DisadvantagesExpensive, periodic replacement

Configuration ​

Example Configuration - Nitrifying BAF ​

json
{
  "type": "baf",
  "id": "tertiary-nitrification-baf",
  "parameters": {
    "configuration": "upflow",
    "numberOfCells": 4,
    "dimensions": {
      "length": 8,
      "width": 8,
      "mediaDepth": 3.5,
      "totalDepth": 5
    },
    "media": {
      "type": "expandedClay",
      "grainSize": 4,
      "specificArea": 400,
      "porosity": 0.45
    },
    "operatingConditions": {
      "hlr": 8,
      "ebct": 26,
      "nh4Loading": 0.8,
      "doSetpoint": 4,
      "temperature": 15
    },
    "aeration": {
      "type": "processAir",
      "airWaterRatio": 10,
      "diffuserType": "coarseBubble",
      "coverage": "full"
    },
    "backwash": {
      "frequency": 24,
      "airScourRate": 60,
      "waterRinseRate": 35,
      "duration": 20,
      "expansion": 25
    },
    "underdrain": {
      "type": "nozzle",
      "spacing": 0.3,
      "material": "stainless316"
    }
  }
}

Example Configuration - Denitrifying BAF ​

json
{
  "type": "baf",
  "id": "denitrification-baf",
  "parameters": {
    "configuration": "upflow",
    "numberOfCells": 2,
    "dimensions": {
      "length": 6,
      "width": 6,
      "mediaDepth": 3,
      "totalDepth": 4.5
    },
    "media": {
      "type": "expandedClay",
      "grainSize": 5,
      "specificArea": 350,
      "porosity": 0.50
    },
    "operatingConditions": {
      "hlr": 6,
      "ebct": 30,
      "no3Loading": 2.0,
      "doSetpoint": 0,
      "temperature": 15
    },
    "carbonSource": {
      "type": "methanol",
      "dosing": "inline",
      "ratio": 3.0
    },
    "backwash": {
      "frequency": 48,
      "waterRinseRate": 30,
      "duration": 15
    }
  }
}

Worked Example ​

Given ​

  • Flow: 20,000 m3/d
  • Secondary effluent: BOD 15 mg/L, NH4-N 25 mg/L
  • Target: BOD <5 mg/L, NH4-N <1 mg/L
  • Temperature: 12 deg C
  • Available area: 400 m2

Design ​

Step 1: Nitrification Loading NH4-N load = 20,000 x 25/1000 = 500 kg NH4-N/d

Temperature-corrected rate at 12 deg C: $$k_{12} = k_{20} \times 1.047^{(12-20)} = 0.69 \times k_{20}$$

Design loading = 0.7 x 0.69 = 0.48 kg NH4-N/m3/d

Step 2: Media Volume Required $$V_{media} = \frac{500}{0.48} = 1,042 \text{ m}^3$$

Step 3: Bed Dimensions At 3.5m media depth: $$A_{plan} = 1042/3.5 = 298 \text{ m}^2$$

Use 4 cells at 8m x 9.5m each = 304 m2 total

Step 4: HLR Check $$HLR = \frac{20,000/24}{304} = 2.7 \text{ m/h}$$

Within typical range (3-10 m/h) - acceptable for nitrification

Step 5: EBCT Check $$EBCT = \frac{3.5}{2.7} = 1.3 \text{ h} = 78 \text{ min}$$

Adequate for complete nitrification at 12 deg C

Step 6: Aeration Requirement O2 for nitrification = 500 kg NH4-N/d x 4.57 = 2,285 kg O2/d

At 20% transfer efficiency: $$Q_{air} = \frac{2,285}{0.20 \times 0.28 \times 1.2} = 34,000 \text{ m}^3/\text{d} = 1,417 \text{ m}^3/\text{h}$$

Air:water ratio = 1,417 / 833 = 1.7:1

This is below typical (4-15:1), so process air also provides mixing. Increase to 5:1 for proper operation: Q_air = 4,165 m3/h

Step 7: Backwash System Backwash water rate = 35 m/h x 76 m2/cell = 2,660 m3/h per cell Duration = 20 min Volume per backwash = 2,660 x 20/60 = 887 m3/cell

Total backwash volume (4 cells, staggered) = 3,548 m3/d Backwash as % of flow = 3,548/20,000 = 17.7%

Step 8: Summary

ParameterValueUnit
Media volume1,064m3
Plan area304m2
Number of cells4-
Media depth3.5m
HLR2.7m/h
EBCT78min
Air flow4,165m3/h
Backwash volume17.7% of flow

Backwash Optimization ​

Backwash Sequence ​

  1. Drain down (if downflow): Lower water level
  2. Air scour: 5-10 min at 40-80 m/h
  3. Combined air+water: 5 min (optional)
  4. Water rinse: 10-15 min at 20-50 m/h
  5. Settle: 2-5 min
  6. Return to service: Gradually increase flow

Backwash Triggers ​

TriggerThresholdPriority
Head loss>1.5-2.0 mPrimary
Time24-48 hSecondary
Effluent qualityDeteriorationEmergency

Backwash Water Management ​

  • Return to head of works (small plants)
  • Dedicated equalization tank (large plants)
  • Backwash water typically 10-25% of forward flow
  • High TSS (200-1000 mg/L) and BOD (50-200 mg/L)

Troubleshooting ​

IssueCauseSolution
Rapid head lossExcess solidsImprove pretreatment
Biomass overgrowthMore frequent backwash
Poor effluent qualityUnderloadingReduce backwash frequency
Temperature dropReduce loading
Inadequate DOIncrease air flow
Media lossBackwash too intenseReduce rates
Screen damageRepair/replace
ChannelingPoor distributionInspect/clean nozzles
Media compactionBackwash more vigorously
Short filter runsHigh influent TSSAdd pretreatment
Media degradationReplace media
Poor nitrificationLow DOIncrease air:water
Cold temperatureReduce loading
Inhibitory compoundsIdentify source

Advantages and Limitations ​

Advantages ​

  1. Compact: 3-5x smaller than AS for same load
  2. Combined process: Biology + filtration
  3. No clarifier: Inherent solids separation
  4. Modular: Easy expansion
  5. Enclosed option: Odor control
  6. Stable performance: Attached growth resilient

Limitations ​

  1. Backwash: Complexity and water use
  2. Head loss: Pumping costs
  3. Pretreatment: Required for high TSS
  4. Media costs: Higher capital
  5. Operator skill: More complex than AS

Commercial Technologies ​

TechnologyManufacturerMedia Type
BiostyrVeoliaPolystyrene beads
BioforVeoliaExpanded clay
BiocarboneOTVGAC
DeepBedLeopoldSand/anthracite
BiobeadUK industry practicePolystyrene

References ​

  1. Mendoza-Espinosa, L. & Stephenson, T. (1999). "A review of biological aerated filters (BAFs) for wastewater treatment". Environmental Engineering Science.
  2. Canler, J.P. et al. (2003). "Biological Aerated Filtration: Assessment of the Process". Water Science & Technology.
  3. WEF (2011). Biofilm Reactors. MOP 35.
  4. EPA (2000). Wastewater Technology Fact Sheet: Trickling Filter Nitrification.
  5. Rogalla, F. et al. (1990). "Biological aerated filters for nitrogen removal". WPCF.
  6. Stensel, H.D. & Ødegaard, H. (2011). "Moving bed biofilm reactors". In: Treatise on Water Science.