DO Control

Dissolved oxygen control strategies for activated sludge aeration optimization

Overview

Dissolved oxygen (DO) control is the most critical process control loop in activated sludge systems. Maintaining optimal DO levels ensures efficient biological treatment while minimizing energy consumption - typically 50-60% of total plant energy.

Control Objectives

Primary Goals

Objective	Target	Rationale
Maintain DO setpoint	1.5-2.5 mg/L	Biological activity
Minimize energy	kWh/kg O2	Cost reduction
Prevent over-aeration	<4.0 mg/L	Energy waste, foaming
Prevent under-aeration	>1.0 mg/L	Treatment failure

Secondary Goals

• Stable effluent quality
• N2O emission minimization
• Equipment protection
• Operator workload reduction

Control Theory

Process Dynamics

DO concentration follows:

$$\frac{dDO}{dt} = k_La \cdot (DO_{sat} - DO) - OUR$$

Where:

• k_La = Oxygen transfer coefficient (h⁻¹)
• DO_sat = Saturation concentration (~9 mg/L at 20°C)
• OUR = Oxygen uptake rate (mg/L/h)

Time Constants

Parameter	Typical Value
DO response time	2-10 minutes
Blower response	5-30 seconds
Valve response	10-60 seconds
Diffuser lag	30-120 seconds

Transfer Function

$$G(s) = \frac{DO(s)}{Air(s)} = \frac{K_p}{(1 + \tau_1 s)(1 + \tau_2 s)}$$

Where:

• K_p = Process gain (mg/L per % air)
• tau_1 = Primary time constant (blower)
• tau_2 = Secondary time constant (diffuser/basin)

Control Strategies

1. Simple Feedback (PID)

         ┌─────────────┐
SP(DO) ──►│             │
         │    PID      ├──► Air Flow
DO ──────►│  Controller │
         └─────────────┘

Equation:

$$MV(t) = K_p \left[ e(t) + \frac{1}{T_i} \int_0^t e(\tau)d\tau + T_d \frac{de(t)}{dt} \right]$$

Typical Tuning:

Parameter	Value	Notes
Kp	20-100	Process dependent
Ti	120-300 s	Slow integration
Td	0-30 s	Optional derivative

2. Cascade Control

         ┌─────────────┐      ┌─────────────┐
SP(DO) ──►│   Master    │      │   Slave     │
         │     PID     ├─SP──►│    PID      ├──► Valve
DO ──────►│             │      │             │
         └─────────────┘      └─────────────┘
                                    ▲
                              Air Flow FB

Inner loop controls air flow, outer loop controls DO.

Benefits:

• Faster disturbance rejection
• Handles supply pressure variations
• Improved stability

3. Feedforward + Feedback

                    Feedforward
          ┌────────────────────────┐
Load ─────►│ FF = f(BOD, NH4, Flow)│─────┐
          └────────────────────────┘     │
                                         ▼  ┌──────┐
         ┌─────────────┐                 Σ──►│Blower│
SP(DO) ──►│  Feedback  │─────────────────►   └──────┘
DO ──────►│    PID     │
         └─────────────┘

Feedforward Model:

$$FF = \frac{Q \cdot (BOD_{in} - BOD_{target})}{SOTE \cdot \eta}$$

4. ABAC (Ammonia-Based Aeration Control)

DO setpoint varies with ammonia concentration:

$$DO_{SP} = DO_{min} + K_{ABAC} \cdot (NH_4 - NH_{4,target})$$

NH4 (mg/L)	DO Setpoint
<0.5	1.0 mg/L
1.0	1.5 mg/L
2.0	2.0 mg/L
3.0	2.5 mg/L
>5.0	3.0 mg/L

Benefits:

• 15-30% energy savings
• Automatic load following
• Reduced N2O emissions

5. AVN Control (Ammonia vs NOx)

Balance nitrification capacity with denitrification demand:

$$\frac{NH_4}{NO_x} \approx 1.0$$

Manipulates DO to achieve target ratio.

Aeration Equipment Control

Blower Control

Type	Control Method	Efficiency
Positive displacement	VFD speed	Good
Single-stage centrifugal	IGV + VFD	Better
Multi-stage centrifugal	VFD	Best
Turbo blower	VFD + IGV	Excellent

Turndown Ratio:

Blower Type	Turndown
Positive displacement	30-100%
Centrifugal	50-100%
Turbo	40-100%

Diffuser Control

Drop leg valves:

• Modulate air to individual zones
• Enable tapered aeration
• Allow zone isolation

Control valve sizing:

$$C_v = \frac{Q_{air}}{1360} \times \sqrt{\frac{SG \cdot T}{P_1 \cdot \Delta P}}$$

Zone Configuration

Multi-Zone Control

┌─────────────────────────────────────────────────────────┐
│                     AERATION BASIN                      │
│                                                         │
│  ┌────────┐   ┌────────┐   ┌────────┐   ┌────────┐    │
│  │ Zone 1 │   │ Zone 2 │   │ Zone 3 │   │ Zone 4 │    │
│  │ DO=2.5 │   │ DO=2.0 │   │ DO=1.5 │   │ DO=1.0 │    │
│  │ High   │   │ Medium │   │ Low    │   │ Tail   │    │
│  └───┬────┘   └───┬────┘   └───┬────┘   └───┬────┘    │
│      │            │            │            │          │
│      ▼            ▼            ▼            ▼          │
│  ┌────────────────────────────────────────────────┐   │
│  │              AIR HEADER                         │   │
│  └─────────────────────┬──────────────────────────┘   │
│                        │                               │
│                   ┌────▼────┐                         │
│                   │ BLOWER  │                         │
│                   └─────────┘                         │
└─────────────────────────────────────────────────────────┘

Zone Setpoint Strategy

Strategy	Description	Application
Uniform	Same setpoint all zones	Simple systems
Tapered	Decreasing setpoint	BOD-limited
Variable	Demand-based	Large plants
SND	Low DO for N removal	BNR systems

Parameters

Tuning Parameters

Parameter	Symbol	Default	Range
Proportional gain	Kp	50	20-150
Integral time	Ti	180 s	60-600 s
Derivative time	Td	0	0-60 s
Deadband	DB	0.1 mg/L	0-0.5 mg/L
Output minimum	MV_min	30%	20-50%
Output maximum	MV_max	100%	80-100%

Process Parameters

Parameter	Typical	Range
DO setpoint	2.0 mg/L	0.5-4.0 mg/L
Response time	5 min	2-15 min
k_La	5-15 h⁻¹	2-30 h⁻¹
OUR	20-60 mg/L/h	10-100 mg/L/h

Configuration

Example Configuration

{
  "controller": {
    "type": "PID",
    "name": "DO_Control_Zone1",
    "processVariable": "DO_Zone1",
    "setpoint": 2.0,
    "manipulatedVariable": "AirValve_Zone1",
    "parameters": {
      "Kp": 50,
      "Ti": 180,
      "Td": 0,
      "deadband": 0.1,
      "outputMin": 30,
      "outputMax": 100,
      "action": "reverse",
      "antiWindup": "backCalculation"
    },
    "alarms": {
      "lowLow": 0.5,
      "low": 1.0,
      "high": 3.5,
      "highHigh": 4.0
    }
  }
}

ABAC Configuration

{
  "controller": {
    "type": "ABAC",
    "name": "Ammonia_Based_DO",
    "ammoniaInput": "NH4_Effluent",
    "doSetpointOutput": "DO_Setpoint",
    "parameters": {
      "NH4target": 1.0,
      "DOmin": 1.0,
      "DOmax": 3.0,
      "gainKABAC": 0.5,
      "filterTime": 300
    }
  }
}

Performance Metrics

Control Performance

Metric	Target	Formula
IAE	<0.5 mg/L·min	Σ
ISE	<0.2 (mg/L)²·min	Σe²·dt
Overshoot	<20%	(DO_max - SP)/SP
Rise time	<10 min	Time to 90% SP
Settling time	<20 min	Time to ±5% SP

Energy Performance

Metric	Target
SOTE (Standard Oxygen Transfer Efficiency)	25-35%
SAE (Specific Aeration Efficiency)	>3 kg O2/kWh
kWh/kg BOD removed	<1.0

Tuning Procedures

Ziegler-Nichols Method

1. Set Ti = ∞, Td = 0
2. Increase Kp until sustained oscillation
3. Record Ku (ultimate gain) and Pu (period)
4. Apply formulas:

Controller	Kp	Ti	Td
P	0.5 Ku	-	-
PI	0.45 Ku	Pu/1.2	-
PID	0.6 Ku	Pu/2	Pu/8

Lambda Tuning

More conservative, less oscillatory:

$$K_p = \frac{\tau}{\lambda \cdot K} \quad T_i = \tau$$

Where:

• tau = Process time constant
• K = Process gain
• lambda = Closed-loop time constant (typically 1-3 × tau)

Auto-Tuning

ErgoPlanner includes automatic tuning:

1. Step test to identify process model
2. Calculate recommended parameters
3. Simulate response before applying
4. Fine-tune in operation

Troubleshooting

Issue	Cause	Solution
Oscillation	Kp too high	Reduce Kp by 20%
Slow response	Kp too low	Increase Kp
Offset	Ti too long	Reduce Ti
Noise amplification	Td too high	Reduce Td, add filter
Valve hunting	Deadband too small	Increase deadband
Low DO peaks	Blower undersized	Add capacity
High energy	Over-aeration	Reduce setpoint

Related Topics

• Aeration Control - Advanced strategies
• Cascade Controller - Nested loops
• Feedforward Controller - Disturbance rejection
• SRT Control - Solids management
• Aerobic Zone

References

1. Olsson, G. & Newell, B. (1999). Wastewater Treatment Systems. IWA Publishing.
2. WEF (2008). Automation of Wastewater Treatment Facilities. MOP 21.
3. Åström, K.J. & Hägglund, T. (2006). Advanced PID Control. ISA.
4. Rieger, L. et al. (2012). "Aeration Control" Water Science & Technology.