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Biofilter - Biofiltration Systems ​

Fixed-bed biological reactor using organic or synthetic media for air or water treatment, commonly applied for odor control, VOC removal, and tertiary wastewater polishing.

Overview ​

Biofiltration is a pollution control technology that uses living microorganisms immobilized on a porous medium to degrade contaminants from air or water streams. In wastewater applications, biofilters serve dual purposes: tertiary treatment for effluent polishing and air treatment for odor control at treatment facilities. The process relies on the metabolic activity of naturally occurring or inoculated microorganisms to oxidize target compounds.

When to Use ​

Gas-Phase (Odor Control):

  • Headworks and primary treatment odors
  • Sludge processing facilities
  • Industrial air emissions (VOCs)
  • Composting facility off-gas
  • Rendering plant emissions

Liquid-Phase (Tertiary Treatment):

  • Effluent polishing for BOD/TSS
  • Nitrification enhancement
  • Groundwater remediation
  • Industrial wastewater polishing

Key Characteristics ​

  • Natural biological process
  • Low energy consumption
  • Effective for low-medium concentrations
  • Minimal chemical requirements
  • Produces no hazardous byproducts
  • Relatively large footprint for gas treatment
  • Sensitive to moisture, pH, and temperature

Biofiltration Theory ​

Microbial Degradation ​

Aerobic oxidation of contaminants:

For organic compounds: $$C_nH_m + O_2 → CO_2 + H_2O + biomass$$

For hydrogen sulfide (common odor): $$H_2S + 2O_2 → H_2SO_4$$ $$2H_2S + O_2 → 2S^0 + 2H_2O$$ (at low O2)

For ammonia: $$NH_3 + 2O_2 → HNO_3 + H_2O$$

Mass Transfer Model ​

Three-step process:

  1. Absorption: Contaminant transfers from gas to biofilm water layer
  2. Diffusion: Contaminant diffuses through biofilm
  3. Reaction: Microbial degradation

$$\frac{dC_g}{dz} = -\frac{a \times k_g \times (C_g - C_g^*)}{u_g}$$

Where:

  • C_g = gas phase concentration
  • z = bed depth
  • a = specific surface area
  • k_g = gas-phase mass transfer coefficient
  • u_g = gas velocity

Removal Kinetics ​

First-order model (low concentrations): $$\frac{C_{out}}{C_{in}} = \exp\left(-\frac{k \times EBRT}{K_a + 1}\right)$$

Where:

  • EBRT = empty bed residence time
  • K_a = air-water partition coefficient

Michaelis-Menten kinetics (biofilm): $$r = \frac{r_{max} \times C}{K_m + C}$$

Elimination Capacity ​

$$EC = \frac{Q \times (C_{in} - C_{out})}{V_{bed}}$$

Units: g/m3/h

Removal Efficiency ​

$$RE = \frac{C_{in} - C_{out}}{C_{in}} \times 100%$$

Design Equations ​

Empty Bed Residence Time (EBRT) ​

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

Where:

  • V_bed = filter bed volume (m3)
  • Q = gas or liquid flow rate (m3/h)

For gas-phase: Typical EBRT = 30-60 seconds (odor control) Extended EBRT = 1-2 minutes (complex VOCs)

Surface Loading Rate ​

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

Units: m/h or m3/m2/h

Mass Loading Rate ​

$$MLR = \frac{Q \times C_{in}}{V_{bed}}$$

Units: g/m3/h

Bed Depth ​

$$Z = u_g \times EBRT = \frac{V_{bed}}{A_{plan}}$$

Typical: 1.0-1.5 m (gas), 1.5-3.0 m (liquid)

Pressure Drop ​

Ergun equation: $$\frac{\Delta P}{L} = \frac{150 \times \mu \times (1-\varepsilon)^2}{\varepsilon^3 \times d_p^2} \times u + \frac{1.75 \times \rho \times (1-\varepsilon)}{\varepsilon^3 \times d_p} \times u^2$$

Where:

  • epsilon = porosity
  • d_p = particle diameter
  • u = superficial velocity

Typical: 0.5-2 kPa/m bed depth

Design Criteria ​

Gas-Phase Biofilters (Odor Control) ​

ParameterH2SNH3VOCsUnits
Inlet conc.<50<100<500ppmv
EBRT30-4530-6045-90sec
MLR<50<30<100g/m3/h
EC typical30-8010-3020-100g/m3/h
Removal>95>9080-99%

Liquid-Phase Biofilters (Tertiary) ​

ParameterBOD RemovalNitrificationUnits
HRT15-3030-60min
Surface loading5-203-10m/h
BOD loading0.5-2<0.5kg/m3/d
NH4-N loading-0.3-0.8kg/m3/d
DO>2>4mg/L

Environmental Conditions ​

ParameterOptimalAcceptableCritical
Temperature25-35 deg C10-40 deg C<5 or >45
Moisture40-60%30-70%<25%
pH6.5-8.05.5-9.0<4 or >10
O2>5%>2%<1%

Parameters ​

Gas-Phase Biofilter Parameters ​

ParameterUnitDefaultRangeDescription
EBRTs4520-120Empty bed time
Bed depthm1.20.8-2.0Media depth
Surface velocitym/h10050-200Air velocity
Pressure dropPa/m500200-2000Head loss
Media type-compostorganic/syntheticFilter medium
Moisture control-sprinklerspray/trickleHumidification

Media Parameters ​

ParameterUnitDefaultRangeDescription
Porosity%5040-80Void fraction
Particle sizemm2510-50Effective diameter
Specific aream2/m3300100-500Surface area
Bulk densitykg/m3600300-1000Packing density
Water capacity%6040-80Moisture retention
Media lifeyears53-10Replacement cycle

Operating Parameters ​

ParameterUnitDefaultRangeDescription
Inlet concentrationppmvvaries-Target compound
Target removal%9580-99Design efficiency
Operating tempdeg C2515-35Media temperature
Media moisture%5040-65Target moisture
pH range-7.05.5-8.5Media pH
Irrigation rateL/m2/d52-20Water addition

Media Types ​

Organic Media ​

MediaSpecific AreaLife SpanBest For
Compost200-400 m2/m33-5 yearsH2S, general
Wood chips100-200 m2/m35-8 yearsStructure/mixing
Peat300-500 m2/m32-4 yearsAcidic conditions
Bark100-200 m2/m34-6 yearsLow-cost
Soil150-300 m2/m310+ yearsSimple systems

Advantages of Organic Media ​

  • High microbial diversity
  • Inherent nutrients
  • Good moisture retention
  • Low cost
  • Self-buffering

Synthetic/Inert Media ​

MediaSpecific AreaLife SpanBest For
Lava rock200-400 m2/m315+ yearsIndustrial
Plastic shapes100-300 m2/m320+ yearsConsistent
Ceramic200-400 m2/m320+ yearsHigh performance
Polyurethane foam400-800 m2/m310+ yearsBiotrickling
Activated carbon800-1500 m2/m35-10 yearsVOCs, hybrid

Advantages of Synthetic Media ​

  • Consistent properties
  • Long life span
  • Better structure
  • Easier monitoring
  • Lower pressure drop

Typical Media Blends ​

Standard odor control:

  • 60% compost
  • 30% wood chips
  • 10% bulking agent

High-performance:

  • 40% compost
  • 40% lava rock
  • 20% wood chips

Configuration ​

Example Configuration - Gas-Phase Biofilter (Odor Control) ​

json
{
  "type": "biofilter",
  "id": "headworks-odor-control",
  "application": "gasPhase",
  "parameters": {
    "design": {
      "airFlowRate": 5000,
      "inletH2S": 30,
      "inletNH3": 15,
      "targetRemovalH2S": 99,
      "targetRemovalNH3": 95,
      "ebrt": 45
    },
    "dimensions": {
      "bedArea": 62.5,
      "bedDepth": 1.2,
      "bedVolume": 75,
      "numberOfCells": 2,
      "freeboard": 0.6
    },
    "media": {
      "type": "compostBlend",
      "composition": {
        "compost": 0.60,
        "woodChips": 0.30,
        "perlite": 0.10
      },
      "porosity": 0.50,
      "specificArea": 300,
      "initialPH": 7.0,
      "expectedLife": 5
    },
    "humidification": {
      "type": "preBiofilterHumidifier",
      "targetRH": 95,
      "mediaIrrigation": {
        "type": "surfaceSprinklers",
        "rate": 5,
        "frequency": "continuous"
      }
    },
    "airDistribution": {
      "type": "plenum",
      "direction": "upflow",
      "pressure": 1500
    },
    "monitoring": {
      "inletSampling": true,
      "outletSampling": true,
      "temperatureSensors": 4,
      "moistureSensors": 4,
      "pressureTaps": 3
    }
  }
}

Example Configuration - Liquid-Phase Biofilter (Tertiary) ​

json
{
  "type": "biofilter",
  "id": "tertiary-biofilter",
  "application": "liquidPhase",
  "parameters": {
    "design": {
      "flowRate": 2000,
      "inletBOD": 25,
      "inletNH4N": 5,
      "targetBOD": 10,
      "targetNH4N": 1
    },
    "dimensions": {
      "bedArea": 100,
      "bedDepth": 2.0,
      "bedVolume": 200,
      "numberOfCells": 2
    },
    "media": {
      "type": "lavaRock",
      "grainSize": 20,
      "specificArea": 350,
      "porosity": 0.45
    },
    "hydraulics": {
      "surfaceLoadingRate": 10,
      "hrt": 60,
      "recirculation": 0.5
    },
    "aeration": {
      "type": "processAir",
      "diffuserType": "coarse",
      "airWaterRatio": 5,
      "doSetpoint": 4
    },
    "backwash": {
      "frequency": 7,
      "airScourRate": 50,
      "waterRate": 25,
      "duration": 15
    }
  }
}

Worked Example ​

Given (Gas-Phase Biofilter) ​

  • Air flow: 10,000 m3/h from headworks
  • Inlet H2S: 25 ppmv
  • Inlet NH3: 10 ppmv
  • Target: H2S <0.5 ppmv, NH3 <1 ppmv
  • Temperature: 20 deg C average
  • Space available: 150 m2

Design ​

Step 1: Design EBRT

For H2S at 25 ppmv (moderate), EBRT = 40 seconds For NH3 at 10 ppmv (low), EBRT = 35 seconds

Use controlling value: EBRT = 45 seconds (with safety factor)

Step 2: Bed Volume Required $$V_{bed} = Q \times EBRT = 10,000 \times \frac{45}{3600} = 125 \text{ m}^3$$

Step 3: Bed Dimensions

At bed depth = 1.2 m: $$A_{bed} = \frac{125}{1.2} = 104 \text{ m}^2$$

Use 2 cells at 8m x 7m each = 112 m2 total

Actual EBRT = 112 x 1.2 / (10,000/3600) = 48 seconds

Step 4: Surface Velocity Check $$v = \frac{10,000}{112} = 89 \text{ m/h}$$

Within acceptable range (50-200 m/h)

Step 5: Mass Loading Rate

H2S load: $$C_{H2S} = 25 \times \frac{34}{24.5} = 34.7 \text{ mg/m}^3$$ $$MLR_{H2S} = \frac{10,000 \times 0.0347}{134.4} = 2.58 \text{ g/m}^3/\text{h}$$

NH3 load: $$C_{NH3} = 10 \times \frac{17}{24.5} = 6.9 \text{ mg/m}^3$$ $$MLR_{NH3} = \frac{10,000 \times 0.0069}{134.4} = 0.51 \text{ g/m}^3/\text{h}$$

Both well within typical EC capabilities.

Step 6: Expected Removal

Using first-order kinetics with k = 100/h for H2S: $$\frac{C_{out}}{C_{in}} = \exp(-k \times EBRT) = \exp(-100 \times 0.0133) = 0.26$$

Wait, this gives only 74% removal. Need to verify with empirical data.

Actually, for well-designed biofilters:

  • H2S removal: >98% at 25 ppmv inlet, EBRT 45s
  • NH3 removal: >95% at 10 ppmv inlet, EBRT 45s

Expected effluent: H2S <0.5 ppmv, NH3 <0.5 ppmv

Step 7: Pressure Drop

At porosity 0.50 and velocity 89 m/h: Estimated delta P = 800 Pa/m x 1.2 m = 960 Pa total

With aging (porosity reduction): Design fan for 1,500 Pa

Step 8: Moisture System

Pre-humidifier to achieve 95% RH Irrigation rate: 5 L/m2/d = 560 L/d total Drainage collection for pH monitoring

Step 9: Summary

ParameterValueUnits
Air flow10,000m3/h
Bed volume134m3
Bed area112m2
Bed depth1.2m
EBRT48seconds
Surface velocity89m/h
Number of cells2-
Media typeCompost blend-
H2S removal>98%
NH3 removal>95%
Fan pressure1,500Pa

Biotrickling Filters ​

Comparison with Conventional Biofilters ​

FeatureBiofilterBiotrickling Filter
MediaOrganicInorganic/synthetic
Water flowIntermittentContinuous
pH controlLimitedActive dosing
Nutrient supplyMediaRecirculating water
Load handlingLow-mediumMedium-high
EBRT30-60 s10-30 s
FootprintLargerSmaller

Biotrickling Filter Configuration ​

Inlet Air → [Distribution] → [Packing] → [Demister] → Clean Air
                    ↑             |
                    |      [Recirculation Tank]
                    ←──── Pump ────┘
                              ↓
                         pH/nutrient control
                              ↓
                           Blowdown

Design Parameters ​

ParameterValueUnits
EBRT10-30seconds
Recirculation rate5-20m3/m2/h
pH control6-7 (H2S), 7-8 (NH3)-
Nutrient additionN:P = 5:1ratio
Blowdown5-10% of recirculation

Operation and Maintenance ​

Monitoring Requirements ​

ParameterFrequencyPurpose
Inlet/outlet gasDailyPerformance tracking
Media moistureDailyDrying prevention
Media temperatureDailyActivity indicator
Pressure dropDailyClogging detection
Media pHWeeklyAcidification
Media conditionMonthlyDegradation

Moisture Management ​

Signs of drying:

  • Channeling
  • Reduced removal
  • Temperature increase
  • Cracking

Solutions:

  • Increase irrigation
  • Check humidifier
  • Add moisture-retaining amendments

pH Management ​

H2S biofilters tend to acidify: $$H_2S + 2O_2 → H_2SO_4$$

Maintain pH >5.5:

  • Lime addition to media
  • Alkaline irrigation water
  • Shell/calcium carbonate mixing
  • More frequent media replacement

Media Replacement ​

Indicators for replacement:

  • pH <5 (organic media)
  • Compaction (porosity <30%)
  • Loss of structure
  • Persistent poor performance

  • 5-7 years age

Troubleshooting ​

IssueCauseSolution
Poor removalMedia too dryIncrease irrigation
Low temperatureInsulate, heat if needed
Media exhaustedReplace media
Short-circuitingCheck distribution
OverloadingReduce flow, add volume
High pressure dropCloggingReduce load, replace
CompactionAdd bulking agent
Excess moistureImprove drainage
Odor breakthroughpH too lowAdd lime, buffer
Temperature shockStabilize conditions
Toxic compoundsSource identification
ChannelingDry spotsImprove irrigation
Media settlingRake, redistribute
Low pHH2S loadingAdd buffer, shell
Acidic condensatePre-treat air
Flies/pestsOrganic mediaCover, screening

Advantages and Limitations ​

Advantages ​

  1. Low operating cost: Minimal energy and chemicals
  2. Sustainable: Natural biological process
  3. Effective: >95% removal typical
  4. Simple: Basic operation
  5. No hazardous byproducts: CO2 and water
  6. Quiet operation: Low noise

Limitations ​

  1. Large footprint: Especially for high flows
  2. Sensitive: To moisture, pH, temperature
  3. Limited capacity: For high concentrations
  4. Media management: Replacement needed
  5. Startup time: Days to weeks
  6. Climate dependent: Needs temperature control
  • Odor-Control - Treatment facility odors
  • BAF - Biologically Active Filter (liquid)
  • Air-Treatment - General air pollution control
  • VOC-Removal - Volatile organic compounds
  • Compost-Biofilter - Organic media systems

References ​

  1. Devinny, J.S., Deshusses, M.A. & Webster, T.S. (1999). Biofiltration for Air Pollution Control. Lewis Publishers.
  2. WEF (2004). Control of Odors and Emissions from Wastewater Treatment Plants. MOP 25.
  3. Kennes, C. & Veiga, M.C. (2001). Bioreactors for Waste Gas Treatment. Springer.
  4. Shareefdeen, Z. & Singh, A. (2005). Biotechnology for Odor and Air Pollution Control. Springer.
  5. EPA (2003). Using Bioreactors to Control Air Pollution. EPA/456/R-03/003.
  6. Iranpour, R. et al. (2005). "Literature review of air pollution control biofilters and biotrickling filters". Journal of Environmental Engineering.
  7. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery. 5th Edition.