The short answer
Disinfection Control
Disinfection control feeds chlorine, or chlorine and ammonia for chloramination, at a dose that achieves the required CT, the product of residual concentration and contact time, and holds a target residual at the point of entry to distribution. The feed is paced to flow with a feedforward, trimmed by a residual analyzer feedback, and interlocked so that loss of flow, loss of sample, or an analyzer fault cannot drive the dose to an extreme. The residual analyzer at the point of entry is the compliance instrument, and the record it produces is what the regulator reads.
Key points
- The regulatory requirement is CT: residual times contact time, credited by the contact tank geometry and the flow.
- Flow-paced feedforward does the work; residual feedback trims it slowly.
- Loss of sample flow to the analyzer is a hardwired interlock to the feed, not an alarm.
- Chloramination adds an ammonia feed and a ratio to hold. The order of addition and the ratio decide the chemistry.
- The point-of-entry residual and the CT calculation are compliance records. Log them at the required interval with validated data.
What is being controlled
Disinfection kills or inactivates pathogens by exposing them to a disinfectant residual for a period of time. The surface water treatment rules express the requirement as CT: the residual concentration in mg/L multiplied by the contact time in minutes, compared with a required CT that depends on the disinfectant, the temperature, the pH, and the log inactivation the plant must achieve. The control system feeds enough disinfectant to hold a residual at the end of the contact tank that, at the current flow through the tank, gives the required CT with margin, and then feeds whatever more is needed to hold a residual at the point of entry to distribution, so that the water carries a residual to the customers.
Two things about that are unusual for a control loop. The controlled variable is measured a long way downstream of the actuator, through a contact tank with a hydraulic residence time of tens of minutes, so the loop has an enormous dead time. And the compliance is a calculation, CT, that the controller can perform continuously from the residual, the flow, and the tank geometry, which makes the calculation itself a control system deliverable.
CT in the controller
CT = C × T₁₀ = C × (V / Q) × (T₁₀ / T)
- C = disinfectant residual at the contact tank outlet, in mg/L
- V = contact tank volume, in gallons
- Q = flow through the tank, in gallons per minute
- T₁₀ / T = the baffling factor of the tank, from a tracer study or the regulatory table, typically 0.1 for an unbaffled tank to 0.7 or better for a serpentine one
- CT is compared with the required CT for the temperature and pH from the regulatory tables
The controller computes CT continuously, compares it with the required CT for the current temperature and pH, and alarms when the margin falls below a limit. At low temperature the required CT rises; at high flow the contact time falls; a plant that runs near its CT limit in winter at peak flow is a plant that needs the calculation on the screen. The calculation also supports a residual setpoint that floats with flow and temperature, raising the residual when contact time is short and lowering it when it is long, which saves chemical and reduces disinfection byproducts.
Feed control
- 1
Flow pacing
The feedforward. The dose in mg/L times the flow gives the feed rate; the controller sets the feeder output from the flow and the dose setpoint. This alone holds the residual reasonably well when demand and water quality are steady.
- 2
Residual trim
A slow PID loop on the residual analyzer, adjusting the dose setpoint up or down within limits. Because of the contact tank dead time, the loop is tuned very slowly, with a long integral time and a small gain, and often with a sample-and-hold or a dead time compensator.
- 3
Demand compensation
Where the raw water chlorine demand varies, from turbidity, organics, or ammonia, a second analyzer at the injection point or a demand calculation from the raw water quality adjusts the feedforward before the residual loop sees the change.
- 4
Limits
A minimum and maximum dose, a maximum feeder output, and a rate-of-change limit, so that no combination of feedforward and trim can drive the feed to an extreme.
- 5
Manual and fallback
Manual dose entry for the operator, and a fallback to flow pacing at the last good dose when the residual analyzer is invalid.
Feed systems
| System | How the dose is delivered | Control interface | Notes |
|---|---|---|---|
| Chlorine gas | A vacuum regulator at the cylinder or container, an automatic gas feeder with a modulating valve, an ejector that draws the gas into carrier water | A 4-20 mA signal to the feeder valve; feed rate feedback from the rotameter transmitter | Leak detection, room ventilation interlocks, and cylinder changeover are part of the control scope; the ejector water supply is an interlock |
| Sodium hypochlorite | A metering pump, diaphragm or peristaltic, from a bulk tank or day tank | Speed and stroke by 4-20 mA or a pulse signal; pump running and loss-of-prime feedback | Hypochlorite degrades with heat and time and off-gasses; the dose calculation uses the current strength, which is measured, and the pump loses prime on gas bubbles |
| On-site generation | A brine electrolysis system producing dilute hypochlorite into a storage tank, fed to the process by metering pumps | The generator runs on tank level; the feed pumps as for hypochlorite | The generator is its own packaged control system; the feed control is the same as bulk hypochlorite at a known strength |
| Chloramination | Chlorine as above plus an ammonia feed: aqueous ammonia or ammonium sulfate by metering pump | A ratio controller holds the chlorine-to-ammonia ratio, typically near 4 to 1 by weight, from the two feed rates | Order of addition and mixing decide whether monochloramine forms; free ammonia and monochloramine analyzers close the loop |
Interlocks
- Loss of plant flow stops the feed. A feeder that runs with no flow delivers concentrated chemical into a stagnant pipe.
- Loss of sample flow to the residual analyzer drops the residual loop to manual at the last good dose and alarms. The analyzer reads low with no sample, and the loop would raise the dose without limit.
- Analyzer bad quality, out of range, or frozen does the same.
- Feeder fault, loss of prime, or loss of ejector water alarms and, where a standby feeder exists, transfers to it.
- A high residual at the point of entry above a limit alarms and, at a set higher limit, stops the feed.
- Day tank low level stops the feed and alarms before the pump runs dry.
- For chloramination, loss of either feed stops the other, because free chlorine without ammonia, or ammonia without chlorine, sent to a chloraminated system is a water quality event.
The compliance record
The residual at the point of entry, the CT calculation, the temperature, and the pH are recorded at the interval the rule requires, commonly every 15 minutes for the residual with a daily minimum reported. The historian collects them from validated tags: a residual with bad quality or a lost sample is recorded as invalid, not as zero, and the report shows the gap and the reason. The residual analyzer at the point of entry is calibrated and verified against a grab sample on the schedule the chlorine analyzer page describes, and the calibration record sits beside the residual record.
Frequently asked questions
- Why is the residual loop so slow to correct?
- The contact tank. A dose change at the injection point reaches the outlet analyzer after the hydraulic residence time, which may be 30 minutes or more, and the loop cannot respond to what it has not seen. The feedforward on flow does the fast work; the residual loop adjusts the dose over hours. Tuning it faster produces a slow oscillation of the residual through the day.
- What residual should we target at the point of entry?
- Enough to satisfy the CT requirement at the contact tank outlet under the worst expected temperature and flow, plus enough to carry a detectable residual to the far end of distribution, and below the level that forms excessive disinfection byproducts or draws taste complaints. The utility sets it with its regulator; the control system holds it and records it.
- Should the dose be controlled on the point-of-entry residual or the contact tank outlet residual?
- The contact tank outlet residual is the CT measurement and the primary control point. The point-of-entry residual is the compliance measurement for distribution and may be trimmed by a second, downstream feed point where the plant has one. Controlling the single feed on the point-of-entry residual alone adds the clearwell residence time to the loop dead time and makes it worse.
- How do we handle hypochlorite strength decay in the dose calculation?
- Measure the strength when a delivery arrives and periodically after, enter it as a parameter, and let the controller compute the feed rate from the dose and the current strength. A controller that assumes 12.5 percent from a tank that has decayed to 9 percent under-doses by a quarter, and the residual loop chases it until it runs out of range.
Related topics
- Chlorine Residual AnalyzersAmperometric and colorimetric residual analyzers: what each measures, free versus total chlorine, pH and flow dependence, sample line design, calibration against a DPD grab sample, and using the signal for feed control.
- pH MeasurementGlass electrodes, reference junctions, and why pH is the measurement that drifts: how the sensor works, temperature compensation, two-point buffer calibration, mounting rules, the failure modes of the reference, and using the signal for chemical feed control.
- PID Control for Pumps and ValvesWhat each term actually does, how to tune a loop on a real pump station, and the handful of mistakes that cause most of the oscillation people blame on tuning.
- Signal ValidationCatching a failed analog input in logic before it runs a pump on a dead transmitter: range checks, module status bits, frozen-value detection, rate checks, and what the program does with a bad value.
- Magnetic FlowmetersFaraday's law applied to a pipe: how a mag meter works, the conductivity and full-pipe requirements, and why grounding decides whether it reads.
- How to Create a PID Loop in a PLCSet up a PID instruction from scratch: scale the PV and CV, pick the action, set the execution rate, configure limits and anti-windup, tune conservatively, and test the manual and auto transitions before it controls anything.
Direct contact
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