Time On/Off Controller

Programmable time-based cycling control

Overview

The Time On/Off Controller implements programmable cycling with multiple phases. Each phase has a defined duration and output value, enabling complex time-based control sequences like intermittent aeration or batch operations.

When to Use

• Intermittent aeration: Nitrification/denitrification cycling
• Batch processes: SBR phase sequencing
• Energy optimization: Off-peak operation
• Scheduled operations: Periodic chemical dosing

Common Applications

Application	On Time	Off Time	Purpose
Intermittent DO	2-4 hours	1-2 hours	N removal
SBR aeration	2-3 hours	0.5-1 hour	Cycle phases
Sludge wasting	1 hour	7 hours	SRT control
Chemical feed	30 min	2 hours	Phosphorus removal

Control Theory

Concept

graph LR
    subgraph Cycle
        P1[Phase 1: ON] --> P2[Phase 2: OFF]
        P2 --> P3[Phase 3: RAMP]
        P3 --> P1
    end

Time-Based Sequencing

Output
100% ─────┐     ┌─────┐     ┌─────
          │     │     │     │
 50% ─────│─────│─────│─────│─────
          │     │     │     │
  0% ─────└─────┘     └─────┘

Time   0   1   2   3   4   5   6
      |--On--|--Off--|--On--|--Off--|

Cycle Calculation

$$t_{cycle} = \sum_{i=1}^{n} t_{phase_i}$$

$$phase_{current} = \left\lfloor \frac{t_{elapsed} \mod t_{cycle}}{\sum_{j=1}^{i} t_{phase_j}} \right\rfloor$$

Parameters

Parameter	Unit	Range	Default	Description
Phases	-	1-10	2	Array of phase definitions
Phase.name	-	-	"On"/"Off"	Phase identifier
Phase.duration	days	0.001-1	0.25	Phase length
Phase.output	%	0-100	varies	Output during phase
Phase.rampTime	days	0-duration	0	Ramp to output
Repeat	-	true/false	true	Repeat cycle
Cycle Count	-	0-∞	0 (infinite)	Number of cycles
Start Offset	days	0-1	0	Delay before start

Phase Definition

{
  "phases": [
    {
      "name": "Aeration",
      "duration": 0.125,
      "output": 100,
      "rampTime": 0.01
    },
    {
      "name": "Anoxic",
      "duration": 0.0625,
      "output": 10,
      "rampTime": 0.005
    }
  ]
}

Time Conversion:

• 0.125 days = 3 hours
• 0.0625 days = 1.5 hours
• 0.01 days = ~15 minutes

Tuning Guidelines

Cycle Timing

1. Consider process dynamics: Cycle longer than response time
2. Match biological kinetics: Nitrifier growth rates
3. Account for lag: Allow time for DO to respond

Intermittent Aeration Timing

Process	On Time	Off Time	Total Cycle
Nitrification only	4 h	2 h	6 h
SND process	3 h	1.5 h	4.5 h
Enhanced N removal	2 h	2 h	4 h
Energy saving	3 h	3 h	6 h

Configuration

Example: Intermittent Aeration

{
  "id": "intermittent-aeration-1",
  "type": "TimeOnOff",
  "enabled": true,
  "targetNodeId": "aerobic-reactor-1",
  "manipulatedVariable": "aerationMode",
  "phases": [
    {
      "name": "Aerobic",
      "duration": 0.125,
      "output": 100,
      "rampTime": 0.01
    },
    {
      "name": "Anoxic",
      "duration": 0.0625,
      "output": 10,
      "rampTime": 0.005
    }
  ],
  "cyclePeriod": 0.1875,
  "repeat": true,
  "cycleCount": 0,
  "startOffset": 0,
  "mode": "auto"
}

Example: SBR Cycle

{
  "id": "sbr-cycle-1",
  "type": "TimeOnOff",
  "enabled": true,
  "targetNodeId": "sbr-reactor-1",
  "manipulatedVariable": "phaseOutput",
  "phases": [
    { "name": "Fill", "duration": 0.042, "output": 0 },
    { "name": "React-Aero", "duration": 0.125, "output": 100 },
    { "name": "React-Anox", "duration": 0.042, "output": 10 },
    { "name": "Settle", "duration": 0.042, "output": 0 },
    { "name": "Decant", "duration": 0.042, "output": 0 }
  ],
  "cyclePeriod": 0.293,
  "repeat": true,
  "cycleCount": 0,
  "startOffset": 0,
  "mode": "auto"
}

Integration Example

Simultaneous Nitrification-Denitrification (SND)

Scenario: Cycle between aerobic and anoxic conditions

Aeration  ON ─────┐     ┌─────┐     ┌─────
                  │     │     │     │
         OFF ─────└─────┘     └─────┘

DO (mg/L) 2.0 ─────┐
                   └─→ drops during anoxic
          0.5 ──────────┐
                        └─→ rises during aerobic

NH4      High ─────┐     ┌────
                   └─────┘    (oxidizes during aerobic)

NO3            ┌────┐
         Low ──┘    └──── (reduces during anoxic)

Benefits:

• Single reactor handles N removal
• Energy savings from reduced aeration
• Flexible timing adjustment

Best Practices

1. Start with proven timing

   • Literature values for similar processes
   • Conservative cycles initially

2. Monitor during transitions

   • DO lag after aeration stops
   • Ammonia/nitrate response

3. Consider ramp times

   • Gradual equipment changes
   • Process settling time

4. Allow process adaptation

   • Biomass needs time to adjust
   • Start with longer cycles

5. Coordinate with other equipment

   • Mixers may need to run during anoxic
   • Chemical feed timing

Troubleshooting

Issue	Cause	Solution
Incomplete nitrification	On time too short	Increase aerobic phase
Poor denitrification	Off time too short	Increase anoxic phase
Rising ammonia	Cycle too long	Shorten total cycle
Equipment stress	No ramp time	Add ramp transitions
Out of sync	Start offset wrong	Adjust start offset

Advanced Features

Ramp Transitions

Smooth transitions between phases:

Output
100% ────────────┐        ╭────────
                 │      ╭─╯
                 │    ╭─╯
                 │  ╭─╯
 10% ────────────└──╯

     |--Aero--|Ramp|--Anox--|Ramp|

Multiple Cycles

Run specific number of cycles then stop:

{
  "repeat": true,
  "cycleCount": 4
}

After 4 cycles, controller stops at last phase output.

Related Controllers

• Deadband-Controller - PV-triggered on/off
• Switch-Controller - Smooth PV-based transitions
• Cascade-Controller - DO control during aerobic phase

References

1. Tchobanoglous, G. et al. "Wastewater Engineering" (5th Edition)
2. Grady, C.P.L. "Biological Wastewater Treatment" (3rd Edition)
3. WEF "Nutrient Removal" (MOP 34)