Granular SBR - Aerobic Granular Sludge Sequencing Batch Reactor

Advanced biological treatment system utilizing aerobic granular sludge in a sequencing batch reactor configuration for simultaneous COD, nitrogen, and phosphorus removal in a single compact reactor, exemplified by the NEREDA technology.

Overview

The Granular SBR (also known as Aerobic Granular Sludge or AGS technology) represents a paradigm shift in activated sludge treatment. Instead of conventional flocculent sludge, biomass forms dense, fast-settling granules (0.5-3 mm diameter) with a layered microbial structure. This enables simultaneous carbon oxidation, nitrification, denitrification, and biological phosphorus removal within a single granule, dramatically reducing footprint and energy consumption.

When to Use

• New plants requiring compact footprint (50-75% reduction)
• Biological nutrient removal (N and P) without chemicals
• Retrofitting existing SBR or activated sludge plants
• Industrial wastewater with high-strength loads
• Water reuse applications
• Plants requiring excellent settling performance

Key Characteristics

• Dense granular biomass (settling velocity 20-80 m/h)
• Simultaneous COD/N/P removal in single reactor
• No separate anoxic or anaerobic zones required
• 50-75% smaller footprint than conventional AS
• 20-30% energy reduction
• No secondary clarifier required (SBR configuration)
• Excellent effluent quality

Aerobic Granulation Technology

Granule Formation Mechanism

Granules form through selective pressure:

1. Feast-Famine Regime: Short anaerobic feed creates substrate gradient
2. Settling Selection: Fast settling granules retained, slow flocs wasted
3. Hydraulic Selection: Short settling time selects for dense biomass
4. Shear: Aeration provides shear for compact structure

Formation Timeline:

• Week 1-2: Flocs begin aggregating
• Week 3-6: Proto-granules form (0.2-0.5 mm)
• Week 6-12: Mature granules develop (0.5-2 mm)
• Week 12+: Stable granular sludge bed

Granule Structure

Cross-section of Mature Granule:

        Bulk Liquid (O2, NO3, NH4, COD)
              |
    +-----------------------------+
    |   Aerobic Layer (0-200 um)  | <- Nitrifiers, heterotrophs
    |   O2: 2-6 mg/L              |   COD oxidation, nitrification
    +-----------------------------+
    |   Anoxic Layer (200-500 um) | <- Denitrifiers
    |   O2: <0.5 mg/L, NO3 present|   Denitrification
    +-----------------------------+
    |   Anaerobic Core (>500 um)  | <- PAOs (Bio-P)
    |   O2: 0, NO3: 0             |   P release/uptake
    +-----------------------------+
        Granule Center (1-3 mm total diameter)

Granule Properties

Property	Flocculent Sludge	Granular Sludge	Units
Particle size	0.05-0.2	0.5-3.0	mm
Settling velocity	1-5	20-80	m/h
SVI	100-200	30-60	mL/g
Density	1.01-1.02	1.04-1.08	g/mL
Biomass concentration	3-5	8-15	g/L
Specific surface area	High	Low	m2/m3
Shear resistance	Low	High	-

Selection Pressure Criteria

Critical Factors for Granulation:

Parameter	Requirement	Effect
Settling time	<10 min (initially <5 min)	Washout of slow settlers
Fill ratio	40-60%	Feast-famine gradient
Anaerobic feed	Required	Substrate gradient
Minimum shear	>1 W/m3	Compact structure
OLR	>1 kg COD/m3/d	Selection pressure
SRT	15-30 days	Granule maturity

Granule Size and Settling Velocity

Size Distribution

Typical granule size distribution:

Size Fraction	% by Mass	Settling Velocity
<0.2 mm	5-15%	5-15 m/h
0.2-0.5 mm	10-20%	15-25 m/h
0.5-1.0 mm	25-35%	25-40 m/h
1.0-2.0 mm	30-40%	40-60 m/h
>2.0 mm	10-20%	60-80 m/h

Mean Diameter: $$d_{50} = 0.8 - 1.5 \text{ mm}$$ (typical mature granules)

Settling Velocity Equations

Stokes Law (small particles): $$v_s = \frac{g \times (\rho_p - \rho_f) \times d^2}{18 \times \mu}$$

Allen/Intermediate Regime (granules): $$v_s = \sqrt{\frac{4 \times g \times (\rho_p - \rho_f) \times d}{3 \times C_D \times \rho_f}}$$

Empirical Correlation for Granules: $$v_s = 10 \times d^{0.6}$$ (v_s in m/h, d in mm)

SVI Correlation

Diluted SVI (DSVI): $$DSVI = \frac{V_{30}}{X}$$

For granular sludge: DSVI = 30-60 mL/g (vs 100-200 for flocs)

SVI5 (5-minute settling): More representative for granules due to rapid settling.

NEREDA-Type Process

Process Description

NEREDA (developed by DHV/Royal HaskoningDHV) is the leading commercial AGS technology:

1. Simultaneous Feeding and Discharge: Influent enters bottom, effluent exits top
2. Anaerobic Feeding: Creates feast condition for PAOs
3. Aeration: Provides oxygen for aerobic processes
4. Settling: Short settling phase (3-10 min)
5. Continuous Operation: Multiple tanks operate in sequence

Typical Cycle

Phase 1: Feed/Decant (30-60 min)
- Bottom feed (anaerobic)
- Top decant (effluent)
- Creates plug flow profile
- PAO substrate uptake

Phase 2: Aeration (60-120 min)
- Nitrification
- COD oxidation
- P uptake by PAOs
- Denitrification in granule core

Phase 3: Settle (3-10 min)
- Rapid granule settling
- Clear supernatant
- No clarifier needed

Cycle repeats 4-8 times/day

Cycle Time Distribution

Phase	Duration	% of Cycle	Function
Feed/Decant	45 min	30-40%	Anaerobic substrate uptake, effluent discharge
Aeration	90 min	50-60%	BOD, N, P removal
Settle	5 min	5-10%	Solids separation
Total	140 min	100%	~4-5 cycles/day

Simultaneous COD/N/P Removal

Layered Removal Mechanism

In Granule Structure:

Layer	Zone	Process	Key Organisms
Outer	Aerobic	Nitrification	AOB, NOB
Outer	Aerobic	COD oxidation	Heterotrophs
Middle	Anoxic	Denitrification	Denitrifiers
Core	Anaerobic	P release	PAOs
All	Aerobic phase	P uptake	PAOs

Nitrogen Removal

Simultaneous Nitrification-Denitrification (SND):

During aeration:

• Outer layer (DO > 2 mg/L): NH4 -> NO2 -> NO3 (nitrification)
• Inner layer (DO ~ 0): NO3 -> N2 (denitrification)

$$\eta_{TN} = 70-90%$$ typical

N Mass Balance: $$N_{in} = N_{eff} + N_{assimilated} + N_{denitrified}$$

Phosphorus Removal

Enhanced Biological P Removal (EBPR):

1. Anaerobic feed: PAOs uptake VFAs, release P
2. Aerobic phase: PAOs uptake P (luxury uptake), store as polyphosphate

$$P_{released}/VFA_{uptake} = 0.3-0.5 \text{ mol P/mol C}$$ $$P_{uptake}/P_{release} = 1.5-2.5$$ (net P removal)

P Removal Efficiency: $$\eta_{TP} = 80-95%$$ (without chemicals)

Carbon Requirements

C/N Ratio for Denitrification: Minimum C/N = 4-6 kg COD/kg N

C/P Ratio for Bio-P: Minimum VFA/P = 15-25 mg COD/mg P

Overall C Balance: $$COD_{available} > COD_{denitrification} + COD_{EBPR} + COD_{heterotrophs}$$

Design Equations

Volume Calculation

Based on OLR: $$V = \frac{Q \times S_0}{OLR}$$

Where OLR = 1.5-3.0 kg COD/m3/d typical

Based on HRT: $$V = Q \times HRT$$

Where HRT = 4-8 hours typical

Exchange Ratio

$$VER = \frac{V_{feed}}{V_{total}} = 40-60%$$

Higher VER creates stronger feast-famine.

Number of Reactors

For continuous treatment: $$n_{reactors} \geq 3$$ (minimum for 24/7 operation)

Sizing: Each reactor handles 1/n of daily flow.

Oxygen Demand

$$OD_{total} = OD_C + OD_N - OD_{credit,denit}$$

$$OD_C = \frac{Q \times (COD_0 - COD_e)}{1.42}$$ (excluding N component)

$$OD_N = 4.57 \times Q \times NH_4_{nitrified}$$

$$OD_{credit} = 2.86 \times Q \times NO_3_{denitrified}$$

Aeration Efficiency

Granular sludge alpha factor: $$\alpha_{granular} = 0.6-0.8$$ (higher than flocculent due to lower SMP)

Parameters

Reactor Parameters

Parameter	Symbol	Unit	Default	Range	Description
Volume	V	m3	calculated	-	Per reactor
Depth	H	m	7	5-10	Operating depth
VER	-	%	50	40-60	Volume exchange
Cycle time	t_c	min	180	120-360	Total cycle
Feed time	t_f	min	60	30-90	Anaerobic feed
Aeration time	t_a	min	110	60-180	Aerobic phase
Settle time	t_s	min	5	3-10	Clarification

Granule Parameters

Parameter	Symbol	Unit	Default	Range	Description
Mean diameter	d_50	mm	1.0	0.5-2.0	Size
Settling velocity	v_s	m/h	40	20-80	At d_50
DSVI	-	mL/g	45	30-60	Diluted SVI
SVI5	-	mL/g	40	25-50	5-min settling
Granule density	rho	g/mL	1.05	1.03-1.08	Wet density
Granule fraction	-	%	80	60-95	Of total biomass

Biological Parameters

Parameter	Symbol	Unit	Default	Range	Description
MLSS	X	g/L	10	8-15	High concentration
SRT	-	days	25	15-40	Long retention
HRT	-	hours	6	4-10	Short retention
OLR	-	kg COD/m3/d	2.0	1.5-4.0	High loading
F/M	-	kg/kg/d	0.15	0.1-0.3	Per cycle average
DO (aeration)	-	mg/L	3.0	2-5	During aeration

Nutrient Removal Parameters

Parameter	Unit	Default	Range	Description
TN removal	%	80	70-90	Via SND
TP removal	%	90	80-95	Bio-P
NH4-N effluent	mg/L	1	<0.5-3	Nitrification
NO3-N effluent	mg/L	5	3-10	Post-denit limited
PO4-P effluent	mg/L	0.5	<0.2-1	Bio-P

Configuration

Example Configuration

{
  "type": "granularSBR",
  "id": "nereda-reactor-1",
  "parameters": {
    "reactor": {
      "volume": 3500,
      "diameter": 25,
      "depth": 7.5,
      "freeboard": 0.5,
      "numberOfReactors": 3
    },
    "cycle": {
      "totalTime": 180,
      "feedDecant": {
        "duration": 60,
        "mode": "simultaneous",
        "feedLocation": "bottom",
        "decantLocation": "top"
      },
      "aeration": {
        "duration": 115,
        "doSetpoint": 3.0,
        "doControl": "cascade"
      },
      "settle": {
        "duration": 5
      }
    },
    "granuleParameters": {
      "targetMeanDiameter": 1.0,
      "targetDSVI": 45,
      "settlingVelocity": 40,
      "granuleFraction": 0.8
    },
    "biological": {
      "mlss": 10000,
      "srt": 25,
      "biokinetics": {
        "model": "ASM2d",
        "muH": 6.0,
        "muA": 0.8,
        "muPAO": 1.0,
        "qPHA": 3.0,
        "qPP": 1.5
      }
    },
    "nutrientRemoval": {
      "nitrogenRemoval": true,
      "phosphorusRemoval": true,
      "targetTN": 10,
      "targetTP": 1.0,
      "sndEnabled": true
    },
    "aeration": {
      "type": "fineBubble",
      "floor_coverage": 0.15,
      "sote": 0.30,
      "alpha": 0.7
    },
    "wasteSludge": {
      "selectionMethod": "overflow",
      "targetSRT": 25,
      "granuleRetention": 0.95
    }
  }
}

Worked Example

Given

• Design flow: 20,000 m3/d
• Influent: COD 600 mg/L, BOD 280 mg/L, TSS 300 mg/L
• Influent: TKN 55 mg/L, NH4-N 45 mg/L, TP 10 mg/L
• Effluent targets: BOD <10 mg/L, TSS <10 mg/L, TN <10 mg/L, TP <1 mg/L
• Temperature: 12-22C
• VFA in influent: 80 mg/L as COD

Design Calculations

Step 1: Verify C/N and C/P Ratios

C/N ratio: $$C/N = \frac{COD}{TKN} = \frac{600}{55} = 10.9$$ - adequate (>6 required)

VFA/P ratio: $$VFA/P = \frac{80}{10} = 8$$ - low (15-25 preferred)

Consider primary fermentation or VFA supplementation for reliable Bio-P.

Step 2: Select OLR and Calculate Volume

Select OLR = 2.0 kg COD/m3/d (moderate for nutrient removal):

$$V_{total} = \frac{Q \times COD_0}{OLR} = \frac{20,000 \times 0.6}{2.0} = 6,000 \text{ m}^3$$

Step 3: Number of Reactors

Select 3 reactors for continuous operation:

$$V_{reactor} = \frac{6,000}{3} = 2,000 \text{ m}^3$$

For robust operation, increase to 2,500 m3 each (7,500 m3 total).

Step 4: Reactor Dimensions

At depth H = 7 m: $$A = \frac{2,500}{7} = 357 \text{ m}^2$$ $$D = \sqrt{4 \times 357/\pi} = 21.3 \text{ m}$$

Select D = 22 m (380 m2, V = 2,660 m3 each)

Step 5: Cycle Design

Total daily volume per reactor: $$Q_{reactor} = \frac{20,000}{3} = 6,667 \text{ m}^3\text{/d}$$

With VER = 50%: $$V_{exchange} = 0.5 \times 2,660 = 1,330 \text{ m}^3$$

Cycles per day: $$n_{cycles} = \frac{6,667}{1,330} = 5 \text{ cycles/day}$$

Cycle time: $$t_{cycle} = \frac{24 \times 60}{5} = 288 \text{ min} \approx 4.8 \text{ hours}$$

Step 6: Phase Distribution

Phase	Duration	% of Cycle
Feed/Decant	90 min	31%
Aeration	190 min	66%
Settle	8 min	3%
Total	288 min	100%

Step 7: Oxygen Demand

Carbonaceous: $$OD_C = Q \times (COD_0 - COD_e) \times (1 - Y_{obs})/1.42$$ $$OD_C = 20,000 \times (0.6 - 0.04) \times 0.75 / 1.42 = 5,915 \text{ kg O}_2\text{/d}$$

Nitrogenous: $$OD_N = 4.57 \times Q \times (NH_4 - NH_{4,eff})$$ $$OD_N = 4.57 \times 20,000 \times (0.045 - 0.001) = 4,015 \text{ kg O}_2\text{/d}$$

Denitrification credit (assume 80% TN removal): $$NO_3_{denit} = 0.8 \times 55 - 3 = 41 \text{ mg/L}$$ $$OD_{credit} = 2.86 \times 20,000 \times 0.041 = 2,345 \text{ kg O}_2\text{/d}$$

Net oxygen demand: $$OD_{net} = 5,915 + 4,015 - 2,345 = 7,585 \text{ kg O}_2\text{/d}$$

Step 8: Design Summary

Parameter	Value	Unit
Total reactor volume	7,500	m3
Number of reactors	3	-
Reactor diameter	22	m
Reactor depth	7	m
MLSS	10,000	mg/L
SRT	25	days
HRT	9	hours
OLR	1.6	kg COD/m3/d
VER	50	%
Cycle time	288	min
Aeration time	190	min
Settle time	10	min
Expected TN removal	80	%
Expected TP removal	85	%
Footprint	1,140	m2

Troubleshooting

Issue	Possible Cause	Diagnostic	Solution
Granule disintegration	High shear	Check aeration intensity	Reduce air flow
	Toxic shock	Analyze influent	Source control
	Low substrate	Check OLR	Increase loading
Poor settling	Filamentous outgrowth	Microscopy	Shorter settling selection
	Young granules	Time from startup	Wait for maturation
	High SVI	Measure SVI5	Increase selection pressure
Poor nitrification	Low DO in bulk	DO measurement	Increase aeration
	Short aeration time	Review cycle	Extend aeration
	Granule too large	Size distribution	Controlled shearing
Poor denitrification	Insufficient anoxic core	Granule structure	Increase granule size
	Low C/N ratio	Analyze influent	Add carbon source
Poor Bio-P	Low VFA	VFA analysis	Fermentation, supplement
	NO3 in feed phase	Check cycle	Ensure anaerobic feed
	PAO washout	Monitor ortho-P	Protect PAO population
Floating granules	Gas bubble entrapment	Visual observation	Improve settling, degassing
	Low density granules	Density measurement	Selection adjustment
Effluent TSS	Floc carryover	Microscopy	Longer settling, better selection
	Granule breakup	Inspect granules	Reduce shear

Advantages vs Conventional Activated Sludge

Aspect	Granular SBR	Conventional AS
Footprint	50-75% smaller	Baseline
Energy	20-30% lower	Baseline
Chemicals	None for N/P	Often required
Settling velocity	20-80 m/h	1-5 m/h
Effluent quality	Excellent	Good
Startup time	2-6 months	2-4 weeks
Operator skill	Higher	Standard
CAPEX	Similar	Baseline
OPEX	15-25% lower	Baseline

Related Topics

• SBR - Sequencing Batch Reactor
• EBPR - Enhanced Biological Phosphorus Removal
• Nitrification-Fundamentals
• Denitrification
• NEREDA-Technology

References

1. de Kreuk, M.K., Heijnen, J.J., and van Loosdrecht, M.C.M. (2005). "Simultaneous COD, nitrogen, and phosphate removal by aerobic granular sludge." Biotechnology and Bioengineering, 90(6), 761-769.
2. Pronk, M., et al. (2015). "Full scale performance of the aerobic granular sludge process for sewage treatment." Water Research, 84, 207-217.
3. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Ed. McGraw-Hill.
4. Royal HaskoningDHV (2023). NEREDA Technology Handbook.
5. Liu, Y., and Tay, J.H. (2004). "State of the art of biogranulation technology for wastewater treatment." Biotechnology Advances, 22(7), 533-563.
6. Bassin, J.P., et al. (2012). "Effect of different salt adaptation strategies on the microbial diversity, activity, and settling of nitrifying sludge in sequencing batch reactors." Applied Microbiology and Biotechnology, 93(3), 1281-1294.