Sequencing Batch Reactor (SBR)

Time-sequenced activated sludge variant in single tank

Overview

SBR performs fill, react, settle, decant, and idle phases in a single tank, eliminating separate clarifiers. Ideal for small-medium WWTPs and flow equalization. Achieves biological nutrient removal (BNR) through controlled aeration cycles. Flexible operation but requires sophisticated controls.

Theory - Basic Mode

Same as activated sludge with time-based operation:

$$\text{Cycle time} = t_{fill} + t_{react} + t_{settle} + t_{decant} + t_{idle}$$

Typical: 4-6 hour cycles (4-6 cycles/day)

Theory - Advanced Mode

SRT Calculation:

$$\theta_c = \frac{V \times X}{Q_w \times X_w}$$

Same as AS but based on waste volume per cycle

Volume Fraction:

$$f = \frac{V_{react}}{V_{total}}$$

Typical: f = 0.25-0.33 (fill ratio)

Parameters

Parameter	Typical Range	Units
Cycle Time	4-6	hours
Fill Time	25-50%	of cycle
React Time	35-50%	of cycle
Settle Time	15-25%	of cycle
Decant Time	5-15%	of cycle
SRT	5-25	days
MLSS	2,500-4,000	mg/L

Worked Example

Given: SBR for 5,000 m³/d, 6h cycles Calculate: Tank volume

1. Cycles per day: 24 / 6 = 4 cycles
2. Volume per cycle: 5,000 / 4 = 1,250 m³
3. With fill fraction 0.33: Total volume = 1,250 / 0.33 = 3,788 m³
4. Use 2 tanks × 2,000 m³ = 4,000 m³

References

1. Metcalf & Eddy (2014)
2. EPA (1999). SBR Design Manual