SRT Control

Solids retention time control for activated sludge process optimization

Overview

Solids Retention Time (SRT), also known as Mean Cell Residence Time (MCRT) or sludge age, is the fundamental design and operating parameter for activated sludge systems. Proper SRT control ensures stable biological treatment, adequate nitrification, and efficient sludge production.

Control Objectives

Primary Goals

Objective	Target	Rationale
Maintain SRT setpoint	Process-specific	Treatment objectives
Ensure nitrification	SRT > SRT_min	Complete ammonia removal
Control MLSS	2,000-5,000 mg/L	Settling, oxygen transfer
Stable F/M ratio	0.1-0.5 kg BOD/kg MLSS/d	Process stability

SRT Requirements by Process

Process	Minimum SRT	Typical SRT
BOD removal only	3 days	3-5 days
Nitrification (15°C)	5 days	8-12 days
Nitrification (10°C)	8 days	12-20 days
Denitrification	6 days	10-15 days
Bio-P removal	3-5 days	5-10 days
Full BNR	10 days	15-25 days

SRT Calculation

Basic Formula

$$SRT = \frac{V \times X_{MLSS}}{Q_{WAS} \times X_{WAS} + Q_e \times X_e}$$

Where:

• V = Aeration volume (m³)
• X_MLSS = Mixed liquor suspended solids (mg/L)
• Q_WAS = Waste activated sludge flow (m³/d)
• X_WAS = WAS concentration (mg/L)
• Q_e = Effluent flow (m³/d)
• X_e = Effluent TSS (mg/L)

Simplified Formula

For well-designed clarifiers (X_e << X_WAS):

$$SRT \approx \frac{V \times X_{MLSS}}{Q_{WAS} \times X_{WAS}}$$

Dynamic SRT

$$\frac{d(V \times X)}{dt} = Y \times r_s - b_d \times V \times X - Q_{WAS} \times X_{WAS}$$

Where:

• Y = Yield coefficient (kg VSS/kg BOD)
• r_s = Substrate removal rate (kg/d)
• b_d = Decay rate (d⁻¹)

Control Theory

Manipulated Variable

WAS flow rate is the primary manipulated variable:

$$Q_{WAS} = \frac{V \times X_{MLSS}}{SRT_{target} \times X_{WAS}}$$

Process Dynamics

Parameter	Response Time
MLSS to WAS change	1-5 days
SRT to WAS change	1-2 × SRT
F/M to load change	Hours

Minimum SRT for Nitrification

Temperature-dependent minimum SRT:

$$SRT_{min} = \frac{1}{\mu_{max,A} \times e^{(T-20) \times \ln(1.103)} - b_A}$$

Safety factor of 1.5-2.5 typically applied:

$$SRT_{design} = SF \times SRT_{min}$$

Temperature	SRT_min (d)	SRT_design (d)
10°C	8.5	15-20
15°C	4.8	8-12
20°C	2.8	5-8
25°C	1.7	3-5

Control Strategies

1. Manual SRT Control

Traditional approach with operator adjustment:

┌──────────────┐
│  Calculate   │
│    SRT       │──► Operator ──► Manual WAS Rate
│  Weekly      │
└──────────────┘

Limitations:

• Labor intensive
• Slow response
• Inconsistent performance

2. MLSS-Based Control

Control MLSS through WAS flow:

         ┌─────────────┐
SP(MLSS) ►│             │
         │     PID     ├──► WAS Flow
MLSS ────►│  Controller │
         └─────────────┘

$$Q_{WAS} = K_p \times (MLSS - MLSS_{SP}) + \int K_i \times e , dt$$

Advantages:

• Simple implementation
• Fast response
• Continuous measurement available

Limitations:

• Indirect SRT control
• Affected by influent variations
• May not optimize for nitrification

3. Direct SRT Control

Calculate and control SRT directly:

         ┌──────────────┐
         │ SRT          │
         │ Calculator   │──► Actual SRT
MLSS ────►│              │
WAS ─────►│              │
         └──────┬───────┘
                │
                ▼
         ┌─────────────┐
SP(SRT) ─►│             │
         │     PID     ├──► WAS Flow
Actual ──►│  Controller │
SRT       └─────────────┘

Equation:

$$Q_{WAS}(t) = Q_{WAS}(t-1) + K_p \times \left(\frac{1}{SRT_{actual}} - \frac{1}{SRT_{SP}}\right)$$

4. Adaptive SRT Control

Adjust SRT based on load and temperature:

$$SRT_{SP} = f(T, BOD_{load}, NH_4)$$

Temperature Adaptation:

$$SRT_{SP}(T) = SRT_{base} \times 1.103^{(20-T)}$$

Load Adaptation:

• High load: Reduce SRT (increase WAS)
• Low load: Increase SRT (reduce WAS)

5. Ammonia-Feedback SRT

Adjust SRT based on effluent ammonia:

         ┌─────────────┐
NH4_eff ─►│   Ammonia   │
         │   Control   ├──► SRT Setpoint ──► SRT Controller
NH4_SP ──►│             │
         └─────────────┘

If NH4 > target: Increase SRT If NH4 < target and stable: Decrease SRT (save energy)

Process Calculations

Sludge Production

$$P_X = Y_{obs} \times Q \times (S_0 - S) + Q \times X_{I,in}$$

Where:

• Y_obs = Observed yield = Y / (1 + b_d × SRT)
• S_0, S = Influent/effluent substrate (mg/L)
• X_I,in = Influent inert solids (mg/L)

F/M Ratio Relationship

$$F/M = \frac{Q \times BOD_{in}}{V \times MLSS}$$

Relationship to SRT:

$$SRT = \frac{1}{Y \times F/M - b_d}$$

Oxygen Requirement

$$OD = \frac{Q \times (S_0 - S)}{f} \times (1 - 1.42 \times Y_{obs}) + 4.57 \times Q \times TKN_{nitrified}$$

Where:

• OD = Oxygen demand (kg O2/d)
• f = Conversion factor (typically 0.68 for BOD to COD)

Parameters

Control Parameters

Parameter	Symbol	Default	Range
SRT setpoint	SRT_SP	12 d	3-30 d
Kp (for 1/SRT)	Kp	100	50-200
Ti	Ti	2 d	1-5 d
WAS flow min	Q_WAS,min	50 m³/d	Site-specific
WAS flow max	Q_WAS,max	500 m³/d	Site-specific

Process Parameters

Parameter	Default	Range
MLSS setpoint	3500 mg/L	2000-6000 mg/L
WAS concentration	8000 mg/L	5000-15000 mg/L
Safety factor	2.0	1.5-2.5
Y (heterotrophic)	0.5 kg/kg	0.4-0.6
b_d (decay)	0.06 d⁻¹	0.04-0.10

Configuration

Example Configuration

{
  "controller": {
    "type": "SRT",
    "name": "SRT_Control",
    "inputs": {
      "mlss": "MLSS_Tank1",
      "wasFlow": "WAS_Flow",
      "wasConcentration": "WAS_TSS",
      "volume": 8000
    },
    "setpoint": 12,
    "manipulatedVariable": "WAS_Pump_SP",
    "parameters": {
      "Kp": 100,
      "Ti": 172800,
      "outputMin": 50,
      "outputMax": 500,
      "safetyFactor": 2.0,
      "temperatureCompensation": true
    },
    "constraints": {
      "mlssMin": 2000,
      "mlssMax": 5000,
      "srtMin": 5,
      "srtMax": 30
    }
  }
}

MLSS-Based Configuration

{
  "controller": {
    "type": "PID",
    "name": "MLSS_Control",
    "processVariable": "MLSS_Tank1",
    "setpoint": 3500,
    "manipulatedVariable": "WAS_Flow",
    "parameters": {
      "Kp": 0.5,
      "Ti": 86400,
      "Td": 0,
      "outputMin": 50,
      "outputMax": 500,
      "action": "reverse"
    }
  }
}

Monitoring

Key Performance Indicators

KPI	Target	Alarm
SRT	SP ± 20%	SP ± 50%
MLSS	SP ± 10%	SP ± 25%
F/M	0.15-0.3	<0.1 or >0.5
SVI	<150 mL/g	>200 mL/g
NH4 effluent	<1 mg/L	>3 mg/L

Sludge Quality Indicators

Parameter	Good	Concern
SVI	80-150	>200 bulking
DSVI	60-100	>150
ISV30	<400 mL/g	>500
Microscopy	Floc-formers	Filaments

Operational Modes

Normal Operation

• Maintain target SRT
• Daily WAS adjustment
• Weekly trend analysis

Load Variation Response

Condition	Action
High load	Increase SRT temporarily
Low load	Decrease SRT (optimize)
Storm event	Protect biomass (reduce WAS)

Seasonal Adjustment

Season	Temperature	SRT Action
Winter	10-15°C	Increase SRT
Summer	20-25°C	Decrease SRT
Transition	Variable	Gradual adjustment

Troubleshooting

Issue	Cause	Solution
Rising MLSS	Low WAS rate	Increase WAS
Falling MLSS	High WAS rate	Decrease WAS
Incomplete nitrification	SRT too low	Increase SRT setpoint
High sludge production	SRT too low	Increase SRT
Poor settling	Filaments, SRT issue	Adjust SRT, chlorinate
High NH4 (winter)	Temperature drop	Increase SRT preemptively

Integration with Other Controls

RAS Control

Coordinate SRT with RAS rate:

• High RAS = thicker WAS = less WAS volume needed
• Low RAS = dilute WAS = more WAS volume needed

DO Control

SRT affects oxygen demand:

• Higher SRT = more nitrification = more O2
• Lower SRT = less endogenous respiration = less O2

Chemical Dosing

SRT influences:

• P removal (chemical vs biological)
• Alkalinity consumption (nitrification)

Related Topics

• RAS/WAS Control
• DO Control
• Aeration Control
• CSTR
• ASM1

References

1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
2. WEF (2010). Design of Municipal Wastewater Treatment Plants. MOP 8.
3. Grady, C.P.L. (1999). Biological Wastewater Treatment, 2nd Ed.
4. Olsson, G. (2012). ICA and Bioprocesses. IWA Publishing.