SBR - Sequencing Batch Reactor

Time-based treatment system combining biological reaction and settling in a single tank through sequential operational phases.

Overview

The Sequencing Batch Reactor (SBR) performs all treatment steps in a single vessel through timed phases: Fill, React, Settle, Decant, and Idle. This eliminates the need for a separate secondary clarifier and provides operational flexibility.

When to Use

• Small to medium plants (<50,000 PE)
• Variable flow applications
• Limited space sites
• Nutrient removal requirements
• Phased construction projects

Key Characteristics

• Single vessel for reaction and settling
• Time-based rather than space-based treatment
• Excellent nutrient removal capability
• Ideal settling conditions (quiescent)
• Flexible operation for varying loads

Operating Cycle

Standard 5-Phase Cycle

graph LR
    A[Fill] --> B[React]
    B --> C[Settle]
    C --> D[Decant]
    D --> E[Idle]
    E --> A

Phase Details

Phase	Duration	Activity	Typical %
Fill	1-3 h	Influent addition	15-25%
React	2-6 h	Mixing/aeration	35-50%
Settle	0.5-1.5 h	Quiescent settling	10-20%
Decant	0.5-1 h	Effluent withdrawal	10-15%
Idle	0-1 h	Standby/sludge waste	0-10%

Total cycle: 4-12 hours (typically 6-8 hours)

Nutrient Removal Cycles

BNR Configuration:

1. Fill (anoxic) - denitrification with RAS nitrate
2. Fill (anaerobic) - P release, VFA uptake
3. React (aerobic) - BOD oxidation, nitrification, P uptake
4. React (anoxic) - post-denitrification
5. Settle - clarification
6. Decant - effluent withdrawal

Biokinetics

Fill Phase Kinetics

Static Fill (no mixing):

• Stratification occurs
• Minimal reaction
• Anaerobic conditions develop

Mixed Fill (anoxic): $$r_{denit} = \mu_{max} \times \eta_g \times \frac{S}{K_s + S} \times \frac{NO_3}{K_{NO} + NO_3} \times X$$

Aerated Fill:

• Begins BOD oxidation
• Dilutes high substrate concentrations

React Phase Kinetics

Standard ASM kinetics apply - see CSTR for equations.

Key difference: Batch kinetics rather than continuous:

$$\frac{dS}{dt} = -\mu_{max} \times \frac{S}{K_s + S} \times X / Y$$

Integrated for batch: $$t = \frac{Y}{μ_{max} \times X} \times \left[S_0 - S + K_s \times \ln\frac{S_0}{S}\right]$$

Design Equations

Volume Calculation

$$V_{total} = V_{fill} + V_{sludge} + V_{freeboard}$$

Where: $$V_{fill} = Q_{avg} \times t_{cycle} \times n_{cycles/day}$$

Decant Volume

$$V_{decant} = V_{fill} = \frac{Q_{avg}}{n_{cycles}}$$

Number of Basins

For continuous flow treatment:

• Minimum 2 basins for redundancy
• 3+ basins for larger flows
• Allows continuous receipt of influent

Exchange Ratio

$$VER = \frac{V_{fill}}{V_{total}} = 15-35%$$

Lower VER → higher treatment intensity per cycle.

F:M Ratio

$$F:M = \frac{Q \times S_0}{V \times X \times VER}$$

Adjusted for batch operation.

Parameters

Operational Parameters

Parameter	Unit	Default	Range	Description
Cycle time	hours	6	4-12	Total cycle duration
Fill time	hours	1.5	0.5-3	Influent addition
React time	hours	3	2-6	Biological treatment
Settle time	hours	1	0.5-2	Clarification
Decant time	hours	0.5	0.3-1	Effluent withdrawal
VER	%	25	15-35	Volume exchange ratio
Cycles/day	-	4	2-6	Number of cycles

Tank Parameters

Parameter	Unit	Default	Range	Description
Volume	m³	calculated	-	Per basin
Depth	m	5	4-7	Operating depth
Freeboard	m	0.5	0.3-1	Above max level
Number of basins	-	2	2-6	For continuity
Decanter type	-	floating	fixed/floating	Withdrawal

Biological Parameters

Parameter	Unit	Default	Range	Description
MLSS	mg/L	4000	3000-6000	Mixed liquor
SRT	days	15	10-25	Solids retention
DO setpoint	mg/L	2.0	1.5-4.0	During aeration
SVI	mL/g	100	50-150	Sludge volume index

Nutrient Removal

Nitrogen Removal

Pre-anoxic strategy:

• Anoxic fill uses influent COD
• RAS provides nitrate
• 60-80% N removal typical

Post-anoxic strategy:

• Additional anoxic phase after nitrification
• May require supplemental carbon

Phosphorus Removal

Anaerobic/aerobic cycling:

1. Anaerobic fill - VFA uptake, P release
2. Aerobic react - luxury P uptake
3. Settle and waste P-rich sludge

Achieve <1 mg/L effluent P.

Combined BNR

Phase	DO	Function
Fill 1	0 (anaerobic)	P release
Fill 2	0 (anoxic)	Denitrification
React 1	>2 (aerobic)	BOD, nitrification, P uptake
React 2	0 (anoxic)	Post-denit
Settle	0	Clarification

Configuration

Example Configuration

{
  "type": "sbr",
  "id": "sbr-basin-1",
  "parameters": {
    "volume": 2500,
    "depth": 5,
    "numberOfBasins": 2,
    "cycleTime": 6,
    "phases": {
      "fill": {
        "duration": 1.5,
        "mode": "anoxic-then-aerobic",
        "anoxicFraction": 0.6
      },
      "react": {
        "duration": 3.0,
        "aerobicFraction": 0.7,
        "anoxicFraction": 0.3
      },
      "settle": {
        "duration": 1.0
      },
      "decant": {
        "duration": 0.5,
        "decanterType": "floating"
      }
    },
    "mlss": 4000,
    "srt": 15,
    "doSetpoint": 2.0
  }
}

Worked Example

Given

• Flow: 5,000 m³/d
• BOD: 200 mg/L, TN: 40 mg/L, TP: 8 mg/L
• Targets: BOD <20, TN <10, TP <1 mg/L
• 6-hour cycle time

Design

Step 1: Number of Cycles $$n = 24/6 = 4 \text{ cycles/day}$$

Step 2: Fill Volume Per Cycle $$V_{fill} = 5,000/4 = 1,250 \text{ m}^3/\text{cycle}$$

Step 3: Total Volume (VER = 25%) $$V_{total} = 1,250/0.25 = 5,000 \text{ m}^3$$

Step 4: With 2 Basins $$V_{basin} = 5,000/2 = 2,500 \text{ m}^3$$

Step 5: Dimensions At depth = 5m: $$A = 2,500/5 = 500 \text{ m}^2$$

Square: 22.4m × 22.4m

Step 6: MLSS Check $$F:M = \frac{5,000 \times 200 \times 10^{-3}}{5,000 \times 4 \times 0.25} = 0.2 \text{ kg/kg/d}$$ ✓

Troubleshooting

Issue	Cause	Solution
Poor settling	Short settle time	Extend settle phase
	High SVI	Reduce SRT, add selector
Floating sludge	Denitrification in settle	Extend anoxic phase
Poor N removal	Insufficient anoxic time	Adjust phase times
Poor P removal	No anaerobic phase	Add anaerobic fill
Effluent TSS	Decanter issues	Adjust withdrawal rate

Advantages vs Conventional

Aspect	SBR	Conventional
Clarifier	Not needed	Required
RAS pumping	Not needed	Required
Footprint	30-50% smaller	Baseline
Flexibility	High	Low
Equalization	Built-in	Separate
Complexity	Control system	Simple

Related Topics

• CSTR - Continuously Stirred Tank Reactor
• Granular-SBR - Granular Sludge SBR
• Nutrient-Removal
• SBR-Control-Strategies

References

1. Wilderer, P.A. et al. (2001). Sequencing Batch Reactor Technology. IWA Publishing.
2. EPA (1999). Sequencing Batch Reactors. EPA/625/R-99/010.
3. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.