Moving Bed Biofilm Reactor (MBBR)

Attached-growth process using free-floating plastic carriers in aerated tank

Overview

MBBR combines suspended growth (AS) and attached growth (biofilm) in a single tank. Plastic carriers (10-25mm, 40-60% fill) provide >500 m²/m³ specific surface area for biofilm growth. No backwashing required, low headloss, and compact design. BOD removal: 70-90%, nitrification capable.

Theory - Basic Mode

$$\text{BOD}{out} = \text{BOD} \times (1 - \eta)$$

Typical removal: 75-90% (depends on fill fraction and HRT)

Theory - Advanced Mode

Area-Based Design:

$$\text{Loading} = \frac{Q \times S_{in}}{A_{total}}$$

$$A_{total} = V \times \text{Fill fraction} \times a$$

Where:

• a = specific surface area (m²/m³)
• Fill fraction = 0.40-0.60
• Loading = 5-20 g BOD/m²·d (carbonaceous)

Volume-Based Design:

$$V = \frac{Q \times (S_{in} - S_{out})}{k \times X \times f}$$

Where k = removal rate, X = biofilm concentration, f = fill fraction

Parameters

Parameter	Carbonaceous	Nitrification	Units
HRT	1.5-4.0	3-8	hours
Fill Fraction	40-60	50-70	%
Surface Loading	5-20	0.5-2.0	g/m²·d
DO	2-3	3-4	mg/L
Temperature	>10	>12	°C

Worked Example

Given: MBBR for 5,000 m³/d, BOD=200 mg/L Calculate: Required volume

1. Target removal: 90% (BOD_out=20 mg/L)
2. BOD removed: 180 mg/L
3. Assume loading: 10 g/m²·d
4. Carrier: 500 m²/m³, fill 50%
5. Effective area: 500 × 0.5 = 250 m²/m³
6. Volume = (5,000 × 0.18) / (10 × 250) = 360 m³
7. HRT = 360 / (5,000/24) = 1.7 hours

References

1. Metcalf & Eddy (2014)
2. Ødegaard, H. (2006). "Innovations in wastewater treatment: the moving bed biofilm process"