SBR-Flexible - Flexible Cycle Sequencing Batch Reactor

Advanced SBR configuration with programmable cycle phases, adaptive timing, and automated control for optimal performance under varying load conditions.

Overview

The SBR-Flexible extends the standard SBR concept with programmable cycle phases, real-time optimization, and adaptive control strategies. This enables the reactor to respond dynamically to varying influent loads, achieve specific treatment targets, and handle storm events efficiently.

When to Use

• Variable or unpredictable flow patterns
• Seasonal load variations (tourism, industry)
• Storm water handling requirements
• Stringent nutrient limits requiring optimization
• Plants requiring maximum operational flexibility

Key Characteristics

• Fully programmable phase durations
• Real-time cycle optimization
• Multiple stored cycle profiles
• Adaptive aeration control
• Automated storm mode switching

Flexible Cycle Architecture

Standard Cycle Components

Fill Phase        React Phase       Settle   Decant   Idle
[Static/Mixed/   [Aerobic/Anoxic/  [No      [Effluent [Optional
 Aerated]         Alternating]      Mix]     Withdraw] Standby]

Programmable Sub-Phases

Phase	Sub-Phases	Variables
Fill	Static, Mixed, Aerated	Duration, mixing intensity
React	Aerobic, Anoxic, Anaerobic	Duration, DO target, sequences
Settle	Single phase	Duration, minimum depth
Decant	Variable speed	Rate, end level, TSS trigger
Idle	Active/Passive	WAS timing, next cycle prep

Phase Transition Triggers

Trigger Type	Parameter	Application
Time-based	Fixed duration	Standard operation
Level-based	Water depth	Fill, Decant
Quality-based	NH4, NO3, PO4	React optimization
Event-based	Storm signal	Emergency mode

Control Strategies

Feed-Forward Control

Adjust cycle based on predicted load: $$Cycle_{adjusted} = Cycle_{base} \times \frac{Load_{predicted}}{Load_{design}}$$

Input signals:

• Flow rate
• Time of day
• Day of week
• Upstream conductivity

Feedback Control

Real-time optimization based on measured parameters:

Ammonia-Based Aeration Control (ABAC): $$t_{aerobic} = t_{aerobic,base} + K_p(NH_{4,target} - NH_{4,measured})$$

ORP-Based Phase Control:

• Anoxic phase: ORP < -50 mV
• Aerobic phase: ORP > +50 mV
• Nitrate knee detection

Multi-Parameter Optimization

Optimize cycle for multiple objectives: $$J = w_1(NH_4 - NH_{4,target})^2 + w_2(NO_3 - NO_{3,target})^2 + w_3(Energy)$$

Advanced Phase Programming

BNR Cycle Example

Phase	Duration	Mode	DO	Purpose
Fill 1	20 min	Anaerobic	0	P release
Fill 2	40 min	Anoxic	<0.3	Denitrification
Fill 3	30 min	Aerobic	2.0	Start nitrification
React 1	60 min	Aerobic	2.0	BOD, nitrification
React 2	45 min	Anoxic	<0.3	Post-denit
React 3	30 min	Aerobic	2.0	P uptake polish
Settle	60 min	-	-	Clarification
Decant	30 min	-	-	Effluent withdraw
Idle	15 min	-	-	WAS, standby

Total: 5.5 hours

Storm Mode Cycle

Phase	Duration	Mode	Changes
Fill	Variable	Aerated	Extended, high rate
React	60 min	Aerobic	Shortened
Settle	45 min	-	Shortened
Decant	45 min	-	Extended, faster

Total: ~3 hours (increased throughput)

Weekend/Low-Load Cycle

Phase	Duration	Mode	Changes
Fill	120 min	Mixed	Extended collection
React	240 min	Variable	Enhanced treatment
Settle	90 min	-	Extended settling
Decant	45 min	-	Slow withdrawal

Total: 8+ hours (2-3 cycles/day)

Biokinetics Considerations

Batch Kinetics

Unlike continuous systems, SBR operates in batch mode:

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

Integrated form for substrate depletion: $$t = \frac{Y}{\mu_{max} X}\left[(S_0 - S) + K_s \ln\frac{S_0}{S}\right]$$

Variable MLSS Through Cycle

MLSS varies with fill volume: $$MLSS_{fill} = MLSS_{react} \times \frac{V_{react}}{V_{fill}} = MLSS_{react} \times \frac{1}{1-VER}$$

Where VER = Volume Exchange Ratio

Oxygen Demand Profile

OD varies significantly through react phase:

React Time	OD (relative)	Dominant Process
0-30 min	100%	BOD oxidation
30-60 min	70%	BOD + nitrification
60-90 min	50%	Nitrification dominant
90-120 min	30%	Polishing

Design Equations

Flexible Volume Design

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

Where:

• V_min = Minimum operating volume (sludge blanket)
• V_fill = Maximum fill volume per cycle
• V_freeboard = Storm surge capacity

Variable Cycle Capacity

$$Q_{daily} = V_{fill} \times n_{cycles} \times n_{basins}$$

With flexible cycles: $$Q_{max} = V_{fill} \times n_{max} \times n_{basins}$$

Decant Rate Calculation

$$Q_{decant} = \frac{V_{fill}}{t_{decant}}$$

Maximum rate limited by:

• Weir overflow rate
• Sludge blanket disturbance
• TSS carryover

Parameters

Cycle Parameters

Parameter	Unit	Default	Range	Description
Base cycle time	hours	6	4-12	Normal operation
Min cycle time	hours	3	2-4	Storm mode
Max cycle time	hours	12	8-24	Low-load mode
Fill phases	-	3	1-5	Sub-phase count
React phases	-	3	1-6	Sub-phase count
VER	%	25	15-40	Volume exchange

Control Parameters

Parameter	Unit	Default	Range	Description
NH4 target	mg/L	2	0.5-5	Aerobic endpoint
NO3 target	mg/L	5	2-10	Anoxic endpoint
DO setpoint	mg/L	2.0	1.5-4.0	Aerobic phase
ORP anoxic	mV	-100	-200 to 0	Anoxic indicator
Storm trigger	-	2xQ	1.5-3x	Storm mode entry

Decanter Parameters

Parameter	Unit	Default	Range	Description
Decanter type	-	floating	fixed/floating	Withdrawal device
Max rate	m3/h	500	100-1500	Withdrawal rate
Submergence	m	0.3	0.2-0.5	Below surface
Min depth	m	2.5	2-4	Sludge protection
TSS limit	mg/L	30	10-50	Quality cutoff

Configuration

Example Configuration

{
  "type": "sbrFlexible",
  "id": "sbr-basin-1",
  "parameters": {
    "volume": 3000,
    "minVolume": 1800,
    "depth": 5.5,
    "numberOfBasins": 3,
    "cycleProfiles": {
      "normal": {
        "totalTime": 6,
        "phases": [
          {"name": "fill-anaerobic", "duration": 0.5, "mode": "anaerobic", "fill": true},
          {"name": "fill-anoxic", "duration": 0.75, "mode": "anoxic", "fill": true},
          {"name": "fill-aerobic", "duration": 0.5, "mode": "aerobic", "fill": true, "do": 2.0},
          {"name": "react-aerobic", "duration": 1.5, "mode": "aerobic", "do": 2.0},
          {"name": "react-anoxic", "duration": 0.75, "mode": "anoxic"},
          {"name": "react-aerobic-polish", "duration": 0.5, "mode": "aerobic", "do": 1.5},
          {"name": "settle", "duration": 1.0},
          {"name": "decant", "duration": 0.5}
        ]
      },
      "storm": {
        "totalTime": 3,
        "phases": [
          {"name": "fill-aerated", "duration": 1.0, "mode": "aerobic", "fill": true, "do": 3.0},
          {"name": "react-aerobic", "duration": 0.75, "mode": "aerobic", "do": 3.0},
          {"name": "settle", "duration": 0.5},
          {"name": "decant", "duration": 0.75, "rate": "high"}
        ]
      },
      "lowLoad": {
        "totalTime": 8,
        "phases": [
          {"name": "fill-extended", "duration": 2.0, "mode": "mixed", "fill": true},
          {"name": "react-bnr", "duration": 4.0, "mode": "alternating", "aerobic": 0.6, "anoxic": 0.4},
          {"name": "settle", "duration": 1.5},
          {"name": "decant", "duration": 0.5}
        ]
      }
    },
    "control": {
      "cycleSelection": "automatic",
      "stormTrigger": 2.0,
      "lowLoadTrigger": 0.5,
      "abacEnabled": true,
      "nh4Target": 2.0,
      "no3Target": 5.0
    },
    "decanter": {
      "type": "floating",
      "maxRate": 400,
      "submergence": 0.3,
      "tssLimit": 25
    }
  }
}

Worked Example

Given

• Design flow: 8,000 m3/d
• Peak flow (storm): 20,000 m3/d
• BOD: 180 mg/L, TN: 45 mg/L, TP: 8 mg/L
• Targets: BOD <15, TN <8, TP <1 mg/L
• 3 basins

Design

Step 1: Normal Operation

• Cycles per day: 4 (6-hour cycles)
• Fill volume per basin: 8,000 / (4 x 3) = 667 m3

Step 2: Basin Sizing (VER = 25%) $$V_{basin} = 667 / 0.25 = 2,668 \text{ m}^3$$

Round to 2,750 m3 per basin.

Step 3: Storm Mode Verification

• Storm cycles: 8 (3-hour cycles)
• Fill volume: 2,750 x 0.35 = 963 m3 (increased VER)
• Capacity: 963 x 8 x 3 = 23,100 m3/d > 20,000

Step 4: Cycle Design

Normal BNR Cycle (6 hours):

Phase	Duration	Mode	Fill	DO
Fill-Anaerobic	30 min	Ana	Yes	0
Fill-Anoxic	45 min	Anox	Yes	<0.3
Fill-Aerobic	30 min	Aer	Yes	2.0
React-Aerobic	90 min	Aer	No	2.0
React-Anoxic	45 min	Anox	No	<0.3
React-Aerobic	30 min	Aer	No	1.5
Settle	60 min	-	No	-
Decant	30 min	-	No	-

Step 5: MLSS and SRT At MLSS = 4,500 mg/L: $$SRT = \frac{2,750 \times 4.5}{Q_w} \approx 18 \text{ days}$$

Step 6: Oxygen Demand Peak OD during aerobic phases:

• BOD oxidation: 8,000 x 0.18 / 3 = 480 kg/d per basin
• Nitrification: 8,000 x 0.045 x 4.6 / 3 = 552 kg/d per basin
• Total: ~1,030 kg O2/d per basin

Step 7: Aeration System Peak rate during react-aerobic: $$SOTR = 1,030 / (0.6 \times 4/24) = 10,300 \text{ kg O}_2\text{/d equivalent}$$

Design for 1.5x safety: 15,500 kg/d capacity

Automation Features

Cycle Selection Logic

IF flow > storm_trigger THEN
    SELECT storm_cycle
ELSE IF flow < low_load_trigger THEN
    SELECT low_load_cycle
ELSE
    SELECT normal_cycle
END IF

Phase Termination Logic

AEROBIC PHASE:
    IF NH4 < target AND time > min_duration THEN
        GOTO next_phase
    ELSE IF time > max_duration THEN
        ALARM and GOTO next_phase
    END IF

ANOXIC PHASE:
    IF NO3 < target OR ORP < threshold THEN
        GOTO next_phase
    END IF

Troubleshooting

Issue	Cause	Solution
Incomplete nitrification	React too short	Extend aerobic phase
	Low DO	Increase aeration
Poor denitrification	No anoxic time	Add anoxic phase
	Insufficient carbon	Earlier anoxic or supplement
High effluent TSS	Short settle	Extend settle time
	Fast decant	Reduce decant rate
Storm overflow	Cycle too long	Enable storm mode
Floating sludge	Denit in settle	More pre-denit
Erratic performance	Control hunting	Tune PID parameters

Related Topics

• SBR - Standard Sequencing Batch Reactor
• CSTR - Continuously Stirred Tank Reactor
• Granular-SBR - Granular Sludge SBR
• BNR-Fundamentals
• Process-Control

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.
4. Olsson, G. & Newell, B. (1999). Wastewater Treatment Systems: Modelling, Diagnosis and Control. IWA Publishing.