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RAS/WAS Control ​

Return activated sludge and waste activated sludge flow control for clarifier performance and SRT management

Overview ​

Return Activated Sludge (RAS) and Waste Activated Sludge (WAS) control are critical for maintaining secondary clarifier performance, controlling solids inventory, and achieving target SRT. These two flows must be coordinated to optimize the entire activated sludge system.

Control Objectives ​

RAS Control Objectives ​

ObjectiveTargetRationale
Maintain sludge blanket0.3-1.0 mClarifier performance
Prevent blanket rise<1.5 mEffluent quality
Minimize RAS flowEnergy savingsCost optimization
Thicken RAS>8,000 mg/LReduce WAS volume

WAS Control Objectives ​

ObjectiveTargetRationale
Control SRTProcess-specificTreatment objectives
Maintain MLSS2,000-5,000 mg/LOxygen transfer
Stable sludge productionkg/dDownstream capacity
Consistent WAS concentration>8,000 mg/LThickener performance

RAS Control Theory ​

Mass Balance ​

At steady state:

$$Q_{in} \times X_{in} + Q_{RAS} \times X_{RAS} = (Q_{in} + Q_{RAS}) \times X_{MLSS}$$

Solving for RAS ratio:

$$R = \frac{Q_{RAS}}{Q_{in}} = \frac{X_{MLSS}}{X_{RAS} - X_{MLSS}}$$

RAS Ratio Determination ​

MLSS (mg/L)RAS Conc (mg/L)Required RAS Ratio
3,0008,0000.60
3,5008,0000.78
4,0008,0001.00
3,50010,0000.54
3,50012,0000.41

Clarifier State Point Analysis ​

Limiting flux determines RAS requirement:

$$G_L = X_{MLSS} \times \left(\frac{Q_{in}}{A} + \frac{Q_{RAS}}{A}\right)$$

Critical settling parameters:

  • Overflow rate (OR) = Q/A
  • Solids loading rate (SLR) = (Q_in + Q_RAS) × X_MLSS / A
  • Underflow rate = Q_RAS / A

Sludge Blanket Dynamics ​

$$\frac{dH_{blanket}}{dt} = \frac{(Q_{in} + Q_{RAS}) \times X_{MLSS} - Q_{RAS} \times X_{RAS}}{A \times X_{blanket}}$$

Response time: 15-60 minutes for RAS changes

WAS Control Theory ​

SRT Control ​

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

Sludge Mass Balance ​

$$\frac{d(V \times X)}{dt} = Y_{obs} \times r_s - Q_{WAS} \times X_{WAS} - Q_e \times X_e$$

At steady state:

$$Q_{WAS} = \frac{Y_{obs} \times r_s - Q_e \times X_e}{X_{WAS}}$$

Control Strategies ​

1. Constant RAS Ratio ​

Simplest approach:

$$Q_{RAS} = R \times Q_{in}$$

Typical ratios:

ProcessRAS Ratio
Conventional AS25-50%
Extended aeration50-100%
BNR50-100%
High-rate100-200%

Limitations:

  • Doesn't respond to settling changes
  • Energy inefficiency during low flow
  • Blanket may vary

2. Flow-Paced RAS ​

RAS proportional to influent:

         ┌─────────────┐
Q_in ────►│   Ratio    │
         │  Control   ├──► Q_RAS = R × Q_in
Ratio SP ►│            │
         └─────────────┘

Benefits:

  • Simple implementation
  • Follows diurnal pattern
  • Stable MLSS in aeration

3. Sludge Blanket Control ​

Feedback from blanket level:

         ┌─────────────┐
SP ──────►│             │
         │     PID     ├──► RAS Flow
Blanket ─►│  Controller │
Level     └─────────────┘

Control equation:

$$Q_{RAS} = Q_{RAS,base} + K_p \times (H_{blanket} - H_{SP})$$

Setpoints:

ConditionBlanket Target
Normal0.5-0.8 m
Storm0.3-0.5 m
Low flow0.8-1.2 m

4. MLSS-Based RAS Control ​

Maintain MLSS through RAS adjustment:

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

Note: This is complementary to WAS control (slower loop).

5. Clarifier Stress Control ​

Reduce RAS during high load:

$$Q_{RAS} = f(SLR, SVI, Q_{in})$$

When SLR approaches limit:

  • Increase RAS to draw down blanket
  • Reduce loading per clarifier

6. Integrated RAS/WAS Control ​

Coordinate both flows:

                    ┌─────────────┐
         ┌─────────►│    RAS      │──► RAS Flow
         │         │  Control    │
         │         └─────────────┘
MLSS ────┤
SRT  ────┤         ┌─────────────┐
         │         │    WAS      │
         └─────────►│  Control    │──► WAS Flow
                   └─────────────┘

Coordination logic:

  • WAS controls SRT/MLSS (slow loop, daily)
  • RAS controls blanket/thickening (fast loop, minutes)

WAS Control Strategies ​

1. Constant WAS Flow ​

Fixed rate adjusted periodically:

$$Q_{WAS} = constant$$

Application: Small plants, stable loads

2. MLSS-Based WAS ​

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

Tuning (slow response):

  • Kp: 0.1-0.5 m³/d per mg/L
  • Ti: 1-5 days
  • Td: 0

3. SRT-Based WAS ​

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

4. Sludge Mass Flow Control ​

Control mass rather than volume:

$$M_{WAS} = Q_{WAS} \times X_{WAS}$$

Benefits:

  • Independent of WAS concentration variations
  • More stable SRT

Equipment Configuration ​

RAS Pumping ​

TypeTypical RangeControl
Submersible20-1000 L/sVFD
AirliftVariableAirflow
Progressive cavity5-200 L/sVFD
Recessed impeller50-500 L/sVFD

WAS Removal ​

MethodApplication
From RAS lineMost common
From clarifierDirect withdrawal
From aerationCo-settling systems

Flow Measurement ​

TechnologyAccuracyApplication
Magnetic flow±0.5%Clean RAS
Ultrasonic±1-2%Non-intrusive
Weir±3-5%Simple systems
Pump curves±5-10%Backup only

Parameters ​

RAS Control Parameters ​

ParameterDefaultRange
RAS ratio0.750.25-2.0
Blanket setpoint0.6 m0.3-1.5 m
RAS flow min25% Q_designSite-specific
RAS flow max150% Q_designPump capacity
Kp (blanket)5020-100
Ti (blanket)600 s300-1800 s

WAS Control Parameters ​

ParameterDefaultRange
SRT setpoint12 d3-30 d
MLSS setpoint3500 mg/L2000-6000 mg/L
WAS flow min5% designSite-specific
WAS flow max10% designSite-specific
Kp (MLSS)0.20.1-0.5
Ti (MLSS)86400 s43200-259200 s

Configuration ​

RAS Control Configuration ​

json
{
  "controller": {
    "type": "RAS",
    "name": "RAS_Control_Clarifier1",
    "mode": "blanket_control",
    "inputs": {
      "influentFlow": "Influent_Flow",
      "blanketLevel": "Blanket_Sensor_C1",
      "rasConcentration": "RAS_TSS_C1"
    },
    "setpoint": {
      "blanketLevel": 0.6,
      "minRatio": 0.25,
      "maxRatio": 1.5
    },
    "manipulatedVariable": "RAS_Pump_C1_SP",
    "parameters": {
      "Kp": 50,
      "Ti": 600,
      "outputMin": 100,
      "outputMax": 500,
      "rampRate": 10
    }
  }
}

WAS Control Configuration ​

json
{
  "controller": {
    "type": "WAS",
    "name": "WAS_Control",
    "mode": "SRT_control",
    "inputs": {
      "mlss": "MLSS_Aeration",
      "wasConcentration": "WAS_TSS",
      "volume": 10000
    },
    "setpoint": {
      "srt": 12,
      "mlss": 3500
    },
    "manipulatedVariable": "WAS_Pump_SP",
    "parameters": {
      "Kp": 0.2,
      "Ti": 86400,
      "outputMin": 20,
      "outputMax": 200
    },
    "constraints": {
      "srtMin": 5,
      "srtMax": 25,
      "mlssMin": 2000,
      "mlssMax": 5000
    }
  }
}

Integrated Control ​

json
{
  "integratedControl": {
    "name": "RAS_WAS_Coordinator",
    "ras": {
      "primary": "blanket_control",
      "secondary": "ratio_limit"
    },
    "was": {
      "primary": "SRT_control",
      "secondary": "mlss_override"
    },
    "coordination": {
      "mlssTarget": 3500,
      "srtTarget": 12,
      "thickeningTarget": 10000,
      "priorityMode": "SRT_first"
    }
  }
}

Operational Considerations ​

Diurnal Variations ​

Time PeriodTypical RAS Strategy
Morning peakIncrease RAS
AfternoonMaintain/reduce
Night lowMinimum RAS
Storm eventMaximum RAS

Seasonal Adjustments ​

SeasonRASWASReason
WinterSimilarDecreaseSlower growth
SummerSimilarIncreaseFaster growth
StormsIncreaseDecreaseProtect blanket

Process Upsets ​

EventRAS ActionWAS Action
High flowIncreaseDecrease
Toxic slugNormalStop
BulkingIncreaseEvaluate
StartupLowStop

Performance Metrics ​

MetricTargetAlarm
Blanket level0.3-1.0 m>1.5 m
RAS concentration>8,000 mg/L<5,000 mg/L
Effluent TSS<20 mg/L>30 mg/L
SRTTarget ±20%Target ±50%
MLSSTarget ±10%Target ±25%

Troubleshooting ​

IssueCauseSolution
Rising blanketRAS too lowIncrease RAS flow
Thin RASBlanket too lowReduce RAS
High effluent TSSBlanket too highIncrease RAS
Variable MLSSWAS rate unstableReview WAS control
Septic RASLong retentionIncrease RAS
Bulking sludgeProcess issueSVI diagnosis, RAS adjust
Clarifier failureOverloadDistribute load, increase RAS

References ​

  1. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
  2. WEF (2005). Clarifier Design. MOP FD-8.
  3. Ekama, G.A. et al. (1997). Secondary Settling Tanks. IAWQ.
  4. Olsson, G. (2015). Water and Energy. IWA Publishing.