MBBR - Moving Bed Biofilm Reactor

Biofilm process using free-floating carrier media to provide high surface area for attached microbial growth while maintaining mixed-liquor flexibility.

Overview

The Moving Bed Biofilm Reactor (MBBR) uses plastic carrier media that move freely within the reactor. Biofilm grows on the protected inner surfaces of the media, combining the benefits of fixed-film and suspended-growth processes.

When to Use

• Capacity upgrades to existing plants
• Nitrification enhancement
• Space-constrained sites
• Cold climate applications
• Industrial pretreatment

Key Characteristics

• No sludge recirculation required
• Compact footprint (high loading)
• Excellent nitrification capability
• Self-regulating biofilm thickness
• Temperature tolerant

Biofilm Theory

Half-Order Kinetics

For diffusion-limited biofilm reactions:

$$r = k_{0.5} \times \sqrt{S}$$

Where:

• r = substrate removal rate (g/m²/d)
• k₀.₅ = half-order rate constant
• S = bulk substrate concentration (mg/L)

Biofilm Diffusion

Substrate flux into biofilm:

$$J = D_f \times \frac{dS}{dz}$$

Where:

• J = flux (g/m²/d)
• D_f = diffusion coefficient in biofilm
• z = depth into biofilm

Effective Biofilm Depth

$$\delta_{eff} = \sqrt{\frac{2 \times D_f \times S}{k_1 \times X_f}}$$

Typical δ_eff = 50-300 μm

Design Equations

Surface Area Loading Rate (SALR)

$$SALR = \frac{Q \times (S_{in} - S_{out})}{A_{media}}$$

Units: g/m²/d

Media Fill Fraction

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

Typical: 40-70%

Specific Surface Area

$$a = \frac{A_{media}}{V_{media}}$$

Typical media: 500-1200 m²/m³

Total Protected Surface Area

$$A_{total} = \phi \times V_{reactor} \times a$$

Removal Rate

$$r = k \times S^n \times T_{factor}$$

Where n = 0.5 for biofilm processes

Design Criteria

BOD Removal

SALR (g BOD/m²/d)	Expected Removal	Application
<5	>90%	Tertiary
5-10	80-90%	Secondary
10-20	60-80%	High-rate
>20	<60%	Roughing

Nitrification

SALR (g NH₄-N/m²/d)	Expected Removal	Notes
<0.5	>95%	Complete
0.5-1.0	80-95%	Good
1.0-1.5	60-80%	Partial
>1.5	<60%	Limited

Temperature effects: $$k(T) = k_{15} \times 1.09^{(T-15)}$$

At 10°C: reduce design SALR by 40%

DO Requirements

Process	Minimum DO	Recommended
BOD removal	2 mg/L	3-4 mg/L
Nitrification	3 mg/L	4-6 mg/L
Combined	4 mg/L	5-6 mg/L

Parameters

Media Parameters

Parameter	Unit	Default	Range	Description
Media type	-	K5	K1/K3/K5/chips	Carrier type
Fill fraction	%	50	40-70	Volume % media
Specific area	m²/m³	800	500-1200	Protected area
Media density	kg/m³	960	920-1000	Slightly <water
Media diameter	mm	25	10-50	Carrier size

Process Parameters

Parameter	Unit	Default	Range	Description
HRT	hours	2	1-6	Hydraulic retention
SALR (BOD)	g/m²/d	10	5-25	BOD loading
SALR (NH₄)	g/m²/d	0.8	0.3-1.5	Ammonia loading
DO setpoint	mg/L	4	2-6	Dissolved oxygen
Temperature	°C	15	10-25	Operating temp

Biofilm Parameters

Parameter	Unit	Default	Range	Description
Biofilm thickness	μm	200	50-500	Active biofilm
Biofilm density	g VS/L	40	20-60	Volatile solids
k₀.₅ BOD	g^0.5/m²/d	12	8-18	Rate constant
k₀.₅ NH₄	g^0.5/m²/d	1.5	1.0-2.5	Nitrification

Media Types

Type	Specific Area	Applications
AnoxKaldnes K1	500 m²/m³	Standard
AnoxKaldnes K3	500 m²/m³	High strength
AnoxKaldnes K5	800 m²/m³	Nitrification
BioPortz	600 m²/m³	General
ActiveCell	650 m²/m³	General
Chips	1200 m²/m³	High area

Mixing and Aeration

Aerobic MBBR

• Coarse bubble diffusers for mixing + oxygen
• Air flow: 3-6 m³/m²/h (floor area)
• Maintains media suspension
• Provides oxygen for biofilm

Anoxic MBBR

• Mechanical mixers required
• Power: 10-30 W/m³
• No aeration (DO < 0.5 mg/L)
• Media must remain suspended

Retention Screens

• Cylindrical or flat screens
• Mesh size: 5-10 mm
• Prevents media loss
• Self-cleaning designs preferred

Configuration

Example Configuration

{
  "type": "mbbr",
  "id": "nitrification-mbbr",
  "parameters": {
    "volume": 1000,
    "mediaType": "K5",
    "fillFraction": 0.5,
    "specificArea": 800,
    "operatingMode": "aerobic",
    "doSetpoint": 4.5,
    "temperature": 12,
    "aeration": {
      "type": "coarseBubble",
      "airFlowRate": 4,
      "coverage": "full"
    },
    "screen": {
      "type": "cylindrical",
      "meshSize": 8
    }
  }
}

Worked Example

Given

• Flow: 10,000 m³/d
• Influent NH₄-N: 35 mg/L
• Target: <2 mg/L
• Temperature: 12°C

Design

Step 1: Required Removal $$\Delta NH_4 = 35 - 2 = 33 \text{ mg/L}$$

Step 2: Temperature-Corrected SALR

At 12°C: $$k = k_{15} \times 1.09^{(12-15)} = k_{15} \times 0.77$$

Design SALR = 0.8 × 0.77 = 0.62 g NH₄-N/m²/d

Step 3: Required Surface Area $$A = \frac{10,000 \times 33 \times 10^{-3}}{0.62} = 532,000 \text{ m}^2$$

Step 4: Media Volume (K5, 800 m²/m³) $$V_{media} = 532,000 / 800 = 665 \text{ m}^3$$

Step 5: Reactor Volume (50% fill) $$V_{reactor} = 665 / 0.5 = 1,330 \text{ m}^3$$

Step 6: HRT Check $$HRT = 1,330 / (10,000/24) = 3.2 \text{ hours}$$ ✓

Step 7: Dimensions At depth 5m: $$A_{floor} = 1,330/5 = 266 \text{ m}^2$$

Tank: 16m × 17m × 5m

MBBR Applications

Standalone

• Complete secondary treatment
• Small to medium flows
• New construction

Hybrid (IFAS)

• MBBR + activated sludge
• Nitrification upgrade
• Retrofit applications
• See IFAS

Multi-Stage

Stage 1 (BOD) → Stage 2 (Nitrification) → Stage 3 (Denit)
   SALR: 15        SALR: 0.8            Anoxic

Troubleshooting

Issue	Cause	Solution
Poor nitrification	Low DO	Increase air
	Low temperature	Reduce loading
Media accumulation	Dead zones	Improve mixing
Excess biofilm	Overloading	Reduce SALR
Media loss	Screen damage	Repair screens
Foaming	SRT issues	Adjust wasting

Advantages

1. Compact: 2-4× smaller than AS
2. Robust: Self-regulating biofilm
3. Cold tolerant: Better than AS at low T
4. No recirculation: Simple operation
5. Modular: Easy expansion
6. Retrofit friendly: Fits existing tanks

Related Topics

• IFAS - Integrated Fixed-film AS
• MBR - Membrane Bioreactor
• Trickling-Filter
• Nitrification-Fundamentals

References

1. Ødegaard, H. (2006). "Innovations in MBBR Technology". Water Science & Technology.
2. WEF (2011). Biofilm Reactors. WEF MOP 35.
3. Rusten, B. et al. (2006). "Design and Operations of MBBR Plants". Water Science & Technology.