IFAS - Integrated Fixed-Film Activated Sludge

Hybrid biological treatment system combining suspended growth activated sludge with attached growth biofilm media to enhance treatment capacity, particularly for nitrification, within existing reactor volumes.

Overview

The Integrated Fixed-Film Activated Sludge (IFAS) process adds free-floating or fixed biofilm carrier media to conventional activated sludge reactors. This hybrid configuration leverages the benefits of both suspended and attached growth systems: the robustness and flexibility of activated sludge combined with the high biomass concentration and cold-temperature performance of biofilm processes.

When to Use

• Plant capacity upgrades without new tankage
• Nitrification enhancement in existing basins
• Cold climate applications requiring stable nitrification
• Industrial pretreatment upgrades
• Retrofit of overloaded activated sludge plants
• BNR systems requiring additional nitrifier biomass

Key Characteristics

• Hybrid suspended/attached biomass system
• Increases effective SRT for nitrifiers without raising MLSS
• Uses existing activated sludge infrastructure
• 50-100% capacity increase in same volume
• Excellent cold weather nitrification performance
• Maintains existing secondary clarifier operation

Process Configuration

IFAS vs Related Processes

Process	Suspended Growth	Attached Growth	Clarifier
Activated Sludge	Yes	No	Required
MBBR	No (minor)	Yes	Required
IFAS	Yes	Yes	Required
MBR	Yes	No	Membrane

Media Locations

Influent → Anaerobic → Anoxic → Aerobic (IFAS) → Clarifier → Effluent
              ↓            ↓           ↓              |
           No media     Optional    Primary           ↓
                        (sponge)    location         RAS

Preferred Zones for Media:

1. Aerobic zone (nitrification) - most common
2. Anoxic zone (denitrification) - specialized applications
3. Aerobic + anoxic (maximum capacity)

Biofilm Kinetics + ASM Model Integration

Dual Biomass System

Total biomass in IFAS:

$$X_{total} = X_{suspended} + X_{biofilm}$$

Effective Biomass Concentration: $$X_{eff} = MLSS + \frac{A_m \times L_f \times X_f}{V}$$

Where:

• A_m = media surface area in reactor (m2)
• L_f = biofilm thickness (m)
• X_f = biofilm density (g VSS/L biofilm)
• V = reactor volume (m3)

Suspended Growth Kinetics (ASM1)

Heterotrophic Growth: $$r_{X_H,susp} = \mu_{H,max} \times \frac{S_S}{K_S + S_S} \times \frac{S_O}{K_O + S_O} \times X_{H,susp}$$

Autotrophic Growth: $$r_{X_A,susp} = \mu_{A,max} \times \frac{S_{NH}}{K_{NH} + S_{NH}} \times \frac{S_O}{K_{O,A} + S_O} \times X_{A,susp}$$

Biofilm Kinetics

Half-Order Kinetics for Diffusion-Limited Biofilm: $$r_{A,biofilm} = k_{0.5,A} \times \sqrt{S_{NH}} \times A_{media}$$

Where:

• k_0.5,A = half-order nitrification rate constant (g^0.5 N/m2/d)
• Typical: 0.8-2.0 g^0.5 N/m2/d at 15C

Full Monod Kinetics for Thin Biofilm: $$r_{A,biofilm} = \mu_{A,max,f} \times \frac{S_{NH}}{K_{NH,f} + S_{NH}} \times \frac{S_O}{K_{O,f} + S_O} \times X_{A,f} \times L_f \times A_m$$

Biofilm-Bulk Liquid Mass Transfer

Substrate Flux: $$J = k_L \times (S_{bulk} - S_{surface})$$

Where:

• k_L = mass transfer coefficient (m/d)
• Function of turbulence and diffusion

Effective Half-Saturation (accounting for mass transfer): $$K_{eff} = K_s + \frac{\mu_{max} \times X_f \times L_f}{k_L}$$

Temperature Effects

Biofilm nitrifiers are more stable at low temperatures:

$$k_{0.5}(T) = k_{0.5,15} \times 1.09^{(T-15)}$$

Temperature	Suspended mu_A	Biofilm k_0.5	Notes
20C	0.8 d-1	1.5 g^0.5/m2/d	Baseline
15C	0.47 d-1	1.0 g^0.5/m2/d	Reference
10C	0.28 d-1	0.65 g^0.5/m2/d	Cold climate
5C	0.16 d-1	0.42 g^0.5/m2/d	Very cold

Advantage: Biofilm provides stable nitrifier population protected from washout during low SRT periods.

Media Types and Fill Fraction

Carrier Media Options

Media Type	Shape	Specific Area	Fill Fraction	Application
K1 (AnoxKaldnes)	Wheel	500 m2/m3	40-70%	Standard
K3 (AnoxKaldnes)	Cylinder	500 m2/m3	40-60%	High strength
K5 (AnoxKaldnes)	Chip	800 m2/m3	50-70%	High rate
BioPortz	Cylinder	600 m2/m3	40-60%	General
ActiveCell	Sponge	300-400 m2/m3	20-40%	Retrofit
Linpor	Sponge cube	200-300 m2/m3	15-30%	Anoxic

Fill Fraction Guidelines

$$\phi = \frac{V_{media}}{V_{reactor}}$$

Application	Fill Fraction	Rationale
Nitrification upgrade	30-50%	Moderate enhancement
Major capacity increase	50-70%	Maximum biofilm area
Anoxic denitrification	20-40%	Lower due to mixing
New construction	40-60%	Balanced design

Maximum Fill Fraction Limits:

• Hollow plastic media: 70% (mixing constraints)
• Sponge media: 40% (compression, transport)
• Fixed media: 50% (flow distribution)

Surface Area Calculations

Total Protected Surface Area: $$A_{total} = \phi \times V_{reactor} \times a$$

Where a = specific surface area of media (m2/m3 media)

Available Biofilm Area: $$A_{biofilm} = A_{total} \times f_{coverage}$$

Where f_coverage = 0.7-0.9 (not all surface colonized)

Design Equations

Nitrification Enhancement

Required Media Area for Target NH4: $$A_m = \frac{Q \times (NH_{4,in} - NH_{4,target})}{SALR_{NH4}}$$

Where SALR_NH4 = Surface Area Loading Rate for ammonia (g N/m2/d)

Design SALR Values:

Temperature	SALR (g NH4-N/m2/d)	Expected Removal
20C	1.0-1.5	>90%
15C	0.6-1.0	>90%
10C	0.4-0.7	>85%
5C	0.2-0.4	>80%

Combined System Nitrification Rate

$$r_{nitr,total} = r_{nitr,susp} + r_{nitr,biofilm}$$

$$r_{nitr,total} = \mu_A \times X_{A,susp} \times V + k_{0.5} \times \sqrt{S_{NH}} \times A_m$$

DO Requirements

Higher DO needed due to diffusion limitations in biofilm:

System	Minimum DO	Recommended	Notes
Suspended AS	1.5 mg/L	2.0 mg/L	Standard
IFAS	3.0 mg/L	4-6 mg/L	Biofilm limited
MBBR	3.0 mg/L	4-6 mg/L	Pure biofilm

DO Penetration Depth: $$\delta_{O2} = \sqrt{\frac{2 \times D_{O2} \times S_{O2}}{q_{O2} \times X_f}}$$

At DO = 4 mg/L: delta_O2 ~ 100-200 um

Aeration for Media Mixing

Minimum Air Rate for Media Suspension:

• Coarse bubble: 8-15 Nm3/m2/h (floor area)
• Fine bubble: Not recommended for media mixing

Combined Aeration:

• Process oxygen: Fine bubble diffusers
• Media mixing: Coarse bubble or mechanical mixers
• Energy optimization: Staged aeration

Parameters

Media Parameters

Parameter	Symbol	Unit	Default	Range	Description
Media type	-	-	K5	K1/K3/K5/sponge	Carrier type
Fill fraction	phi	%	50	30-70	Volume percentage
Specific area	a	m2/m3	800	300-1200	Protected surface
Media density	rho	kg/m3	960	920-1000	Carrier density
Coverage factor	f_cov	-	0.8	0.7-0.9	Active biofilm

Biological Parameters

Parameter	Symbol	Unit	Default	Range	Description
MLSS	X	mg/L	3500	2500-5000	Suspended solids
SRT	-	days	8	5-15	For suspended growth
Biofilm SRT	-	days	20-50	-	Self-regulating
DO setpoint	-	mg/L	4.0	3-6	Higher than AS
Temperature	T	C	15	5-25	Operating temp

Kinetic Parameters (Biofilm)

Parameter	Symbol	Unit	Default	Range	Description
k_0.5 BOD	-	g^0.5/m2/d	12	8-18	BOD removal
k_0.5 NH4	-	g^0.5/m2/d	1.2	0.8-2.0	Nitrification
Biofilm thickness	L_f	um	200	50-400	Active layer
Biofilm density	X_f	g VSS/L	40	20-60	Volatile solids
Theta	-	-	1.09	1.07-1.12	Temp coefficient

Loading Parameters

Parameter	Symbol	Unit	Default	Range	Description
SALR BOD	-	g/m2/d	10	5-20	BOD loading
SALR NH4	-	g/m2/d	0.8	0.3-1.5	Ammonia loading
Volumetric load	-	kg BOD/m3/d	1.5	0.5-3.0	Total reactor
F/M (suspended)	-	kg/kg/d	0.2	0.1-0.4	For MLSS only

Configuration

Example Configuration

{
  "type": "ifas",
  "id": "ifas-aerobic-1",
  "parameters": {
    "reactor": {
      "volume": 3000,
      "depth": 5.5,
      "length": 25,
      "width": 22,
      "numberOfCells": 2
    },
    "suspendedGrowth": {
      "mlss": 3500,
      "srt": 8,
      "biokinetics": {
        "model": "ASM1",
        "muHMax": 6.0,
        "muAMax": 0.8,
        "yH": 0.67,
        "yA": 0.24,
        "bH": 0.4,
        "bA": 0.15
      }
    },
    "media": {
      "type": "K5",
      "fillFraction": 0.5,
      "specificArea": 800,
      "coverageFactor": 0.8,
      "density": 960
    },
    "biofilm": {
      "thickness": 200,
      "density": 40,
      "k05_BOD": 12,
      "k05_NH4": 1.2,
      "theta": 1.09
    },
    "aeration": {
      "doSetpoint": 4.5,
      "process": {
        "type": "fineBubble",
        "sote": 0.25,
        "alpha": 0.55
      },
      "mediaMixing": {
        "type": "coarseBubble",
        "airRate": 10,
        "coverage": "supplemental"
      }
    },
    "retentionScreens": {
      "type": "cylindrical",
      "meshSize": 8,
      "numberOfScreens": 4
    }
  }
}

Worked Example

Given

• Existing activated sludge plant: 15,000 m3/d, 4,000 m3 aerobic volume
• Current performance: NH4-N effluent = 8 mg/L (failing permit of 3 mg/L)
• Influent: NH4-N = 35 mg/L, BOD = 180 mg/L
• Current MLSS = 3,200 mg/L, SRT = 7 days
• Temperature minimum: 10C
• Goal: Achieve ❤️ mg/L NH4-N effluent year-round

Design Calculations

Step 1: Assess Current Nitrification Capacity

At 10C: $$\mu_{A,max} = 0.8 \times 1.10^{(10-20)} = 0.31 \text{ d}^{-1}$$

$$SRT_{min} = \frac{1}{0.31 - 0.09} = 4.5 \text{ days}$$

Current SRT = 7 days provides only 1.5x safety factor - insufficient

Step 2: Calculate Required Nitrification Rate

$$r_{nitr} = Q \times (NH_{4,in} - NH_{4,out}) = 15,000 \times (35 - 3) \times 0.001 = 480 \text{ kg N/d}$$

Step 3: Estimate Current Suspended Growth Contribution

Assuming 5% of MLSS is nitrifiers: $$X_A = 0.05 \times 3200 = 160 \text{ mg/L}$$

At 10C, with substrate limitation: $$r_{A,susp} = \mu_A \times X_A \times V = 0.31 \times 0.16 \times 4000 = 198 \text{ kg N/d}$$

Current capacity is only ~200 kg N/d (actual ~250 at 8 mg/L effluent)

Step 4: Required Biofilm Contribution

$$r_{A,biofilm} = 480 - 198 = 282 \text{ kg N/d}$$

Add safety factor of 1.5: $$r_{A,biofilm,design} = 282 \times 1.5 = 423 \text{ kg N/d}$$

Step 5: Calculate Required Media Area

At 10C, design SALR = 0.5 g NH4-N/m2/d (conservative):

$$A_m = \frac{423 \times 1000}{0.5} = 846,000 \text{ m}^2$$

Step 6: Select Media and Fill Fraction

Using K5 media (800 m2/m3):

$$V_{media} = \frac{846,000}{800} = 1,058 \text{ m}^3$$

$$\phi = \frac{1,058}{4,000} = 26%$$

This is low - increase to 40% for robustness:

$$V_{media} = 0.40 \times 4,000 = 1,600 \text{ m}^3$$

$$A_m = 1,600 \times 800 \times 0.8 = 1,024,000 \text{ m}^2$$

Actual SALR = 423,000 / 1,024,000 = 0.41 g/m2/d - acceptable margin

Step 7: Verify DO Requirements

For biofilm with 0.41 g N/m2/d loading:

• Minimum DO = 3.0 mg/L
• Design DO = 4.5 mg/L

Check oxygen demand increase:

• Additional nitrification: 423 kg N/d x 4.57 = 1,933 kg O2/d
• Current aeration may need 30-50% upgrade

Step 8: Check Clarifier Impact

IFAS does not significantly increase MLSS (biofilm stays on media)

• Current MLSS: 3,200 mg/L
• With biofilm contribution to clarifier: Negligible (biofilm sloughing is low)
• Clarifier should remain adequate

Step 9: Screen Sizing

Media retention screens:

• Flow: 15,000 m3/d = 625 m3/h peak (assuming 2.5 PF)
• Screen velocity: <1.0 m/s to prevent media damage
• Screen area: 625 / (1.0 x 3600) = 0.17 m2 minimum per screen
• Use 4 cylindrical screens, 0.5 m diameter x 1.5 m long each

Step 10: Design Summary

Parameter	Current	With IFAS
Aerobic volume	4,000 m3	4,000 m3 (same)
Media fill	0%	40%
Media area	0	1,024,000 m2
MLSS	3,200 mg/L	3,500 mg/L
SRT (suspended)	7 days	8 days
DO setpoint	2.0 mg/L	4.5 mg/L
NH4-N effluent	8 mg/L	❤️ mg/L
Aeration capacity	Baseline	+50%

Troubleshooting

Issue	Possible Cause	Diagnostic	Solution
Poor nitrification	Low DO in bulk liquid	Measure DO profile	Increase aeration
	Insufficient media area	Calculate SALR	Add more media
	Low temperature	Check temp correction	Reduce loading
	Biofilm washout	Inspect media	Reduce mixing intensity
Media accumulation	Dead zones	Visual inspection	Improve mixing pattern
	Screen clogging	Check headloss	Clean screens
Screen clogging	Excessive biomass	TSS measurement	Increase WAS
	Debris accumulation	Visual check	Install prescreening
Poor settling	High filaments	Microscopy	Add selector, adjust F/M
	Biofilm sloughing	Check MLSS trend	Maintain consistent DO
Uneven biofilm	Non-uniform aeration	DO mapping	Balance air distribution
	Media stratification	Observe mixing	Increase air rate
High effluent TSS	Clarifier overload	SLR calculation	Reduce MLSS or add clarifier
	Pin floc	Floc characteristics	Polymer addition

IFAS Advantages

Compared to AS Upgrade (New Tankage)

Aspect	IFAS Retrofit	New AS Tank
Construction time	2-4 months	12-24 months
Capital cost	40-60% of new	Baseline
Footprint	None additional	Significant
Downtime	Minimal	Moderate
Permit risk	Low	Moderate

Compared to Pure MBBR

Aspect	IFAS	MBBR
Robustness	High (dual biomass)	Moderate
Cold weather	Excellent	Good
Flexibility	High	Moderate
MLSS management	Existing RAS/WAS	Simpler
Sludge handling	Existing infrastructure	May need upgrade

Related Topics

• MBBR - Moving Bed Biofilm Reactor
• CSTR - Continuously Stirred Tank Reactor
• MBR - Membrane Bioreactor
• Nitrification-Fundamentals
• Biofilm-Theory

References

1. Sen, D. et al. (2006). "Integrated Fixed-Film Activated Sludge (IFAS) Process - Description and Preliminary Design Criteria." WEF/IWA Biofilm Reactor Technology Conference.
2. Ødegaard, H. (2006). "Innovations in wastewater treatment: the moving bed biofilm process." Water Science and Technology, 53(9), 17-33.
3. WEF (2011). Biofilm Reactors, WEF MOP 35.
4. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed.
5. Randall, C.W., and Sen, D. (1996). "Full-Scale Evaluation of an Integrated Fixed-Film Activated Sludge (IFAS) Process for Enhanced Nitrogen Removal." Water Science and Technology, 33(12), 155-162.
6. Sriwiriyarat, T., and Randall, C.W. (2005). "Performance of IFAS wastewater treatment processes for biological phosphorus removal." Water Research, 39(16), 3873-3884.