The short answer
Chemical Feed
Chemical feed in a water plant is controlled by pacing each feeder to the flow it treats, at a dose set by the operator or trimmed by a downstream analyzer, with the feeder interlocked so that it cannot run without proven water flow. Metering pumps are controlled by speed and stroke, dry feeders by motor speed, and gas feeders by an automatic valve, each with a feedback that proves chemical is actually moving. Day tanks with level measurement, calibration columns for drawdown tests, and loss-of-feed alarms verify delivery, and overfeed protections such as maximum feed limits, day tank sizing, and high residual alarms guard the chemicals, fluoride above all, that can hurt people when overdosed.
Key points
- Dose times flow is the feed rate; pace every feeder to the flow it treats and trim from the residual where one is measured.
- No feed without proven flow, and no flow without feed for disinfection: two interlocks that are not negotiable.
- Prove delivery: pump stroke feedback, a flow switch or pulse flowmeter, day tank level rate, and a drawdown test on a schedule.
- Day tanks sized so a full tank cannot overdose the plant at maximum feed, for fluoride especially.
- Fallback dose when the pacing flow signal is bad; never pace to zero or to a spike.
- The chemical, the concentration, and the units are on the loop sheet and the HMI; the math is where feed errors hide.
The chemicals
| Chemical | Purpose | Usual feed point | Control basis |
|---|---|---|---|
| Coagulant: alum, ferric, polyaluminum chloride | Particle removal | Rapid mix | Flow paced; dose from jar tests, streaming current, or raw turbidity |
| Polymer | Flocculation and filter aid | Flocculation or filter influent | Flow paced |
| Pre-oxidant: permanganate, chlorine dioxide, ozone | Iron, manganese, taste and odor, algae | Raw water or head of plant | Flow paced; residual or demand |
| pH adjustment: lime, caustic, soda ash, carbon dioxide | Coagulation pH and corrosion control | Rapid mix and finished water | Flow paced with pH trim |
| Disinfectant: hypochlorite, chlorine gas, chloramine | Disinfection and residual | Clearwell inlet, finished water | Flow paced with residual trim |
| Fluoride | Dental health | Finished water | Flow paced; strict overfeed protection |
| Corrosion inhibitor: orthophosphate | Lead and copper control | Finished water | Flow paced |
| Powdered activated carbon | Taste, odor, organics | Raw water or rapid mix | Flow paced, often seasonal |
Feeders and how they are controlled
- Diaphragm metering pump
- Output is stroke length times stroke frequency. Stroke length is set manually or by a positioner; frequency is controlled by a 4-20 mA speed signal or a pulse input. Accurate at moderate turndown; loses accuracy at very short strokes.
- Peristaltic pump
- Output is proportional to speed, controlled by 4-20 mA. Tolerant of gas in the chemical, gentle on polymers, tube life is the maintenance item.
- Gas chlorinator
- A vacuum-operated feeder with an automatic valve positioned by a signal proportional to flow, and a residual analyzer trimming. Leak detection and ventilation are part of the installation.
- Dry feeder
- Volumetric feeders meter by screw speed; gravimetric feeders weigh the discharge and correct. Motor speed follows the flow-paced signal; a dissolver and a carrier water flow are interlocked.
- Lime slaker
- Dry lime fed to a slaker producing slurry, with slaker temperature and grit removal; slurry feed by pump or gravity.
The arithmetic
Feed, lb/day = Dose, mg/L × Flow, MGD × 8.34
- Dose = the concentration of the chemical to be added, as the pure chemical
- Flow = the water flow being treated, in million gallons per day
- 8.34 = pounds per gallon of water; the conversion that makes the units work
For a liquid chemical the pounds per day are divided by the pounds of active chemical per gallon of the solution as supplied, which comes from its density and its strength, to get gallons per day, and then converted to the pump units. Hypochlorite is the usual trap: it is bought as a percentage available chlorine by weight or by volume, it loses strength in storage, and a dose computed from the nominal strength drifts as the solution ages. Put the concentration on the HMI as an operator entry with a date, and confirm the delivered dose with a drawdown test and the residual.
Control strategies
- Flow pacing: the feed rate follows the treated flow at a dose setpoint. The basis of every feed. The flow signal must be validated, and a fallback dose rate held when it is bad.
- Residual trim: a downstream analyzer adjusts the dose setpoint slowly, within limits, to hold a residual. Chlorine and pH are the usual cases. The trim is a slow outer loop; the pacing does the fast work.
- Manual dose: the operator sets the dose from a jar test or experience, and the pacing follows flow. Coagulant is usually run this way, sometimes with a streaming current monitor as a guide.
- Batch: polymer makeup and lime slaking are sequences that make a batch and then feed from it.
Proving that chemical is moving
A metering pump that has lost prime, a plugged injection quill, an empty day tank, or a closed valve all leave the pump running and the plant untreated. Proof of feed comes from more than one source: a stroke or flow pulse from the pump, a flow switch or a small flowmeter on the discharge, the day tank level falling at the expected rate, and the residual analyzer downstream. A loss-of-feed alarm compares the expected and the proven, and for disinfection it is an interlock: no proven chlorine feed for a set time, and the plant is alarmed and, at the level the plant decides, the finished water pumps stop. A calibration column, a graduated cylinder on the pump suction, allows a drawdown test that measures actual pump output against the expected value; do it on a schedule and after every pump repair.
Interlocks and overfeed protection
| Protection | How | Applies to |
|---|---|---|
| No feed without flow | Feeder enable requires the treated water flow above a minimum for a delay; feed stops when flow stops | Every chemical |
| No flow without feed | Loss of disinfectant feed alarms and, after a delay, stops finished water delivery | Disinfection |
| Maximum feed limit | The controller clamps the feed rate at a value that cannot exceed the maximum dose | Fluoride, chlorine, and any chemical with a health limit |
| Day tank sizing | A day tank holds no more than one day of feed at maximum rate, so an overfeed is bounded | Fluoride especially; good practice for all |
| High residual alarm and shutoff | Analyzer high alarm stops the feed | Fluoride, chlorine, pH chemicals |
| Carrier water interlock | Feed only when carrier water flow is proven | Dry feeders, dissolvers, injection quills with carrier water |
| Leak detection | Gas detection with alarm, ventilation, and feed shutoff | Chlorine gas, ammonia, chlorine dioxide |
What the operator sees
- Per chemical: dose setpoint, computed feed rate, feeder output, proven feed status, day tank level and days remaining, residual where measured, and the concentration entered with its date.
- Alarms: loss of feed, day tank low, residual high and low, feeder fault, leak detection, carrier water lost.
- Trends: dose, residual, and flow together, so a residual excursion can be read against what the feed was doing.
- Totals: chemical used per day, which is also the inventory and the regulatory report.
Frequently asked questions
- Why pace from flow rather than control from the residual alone?
- Because the residual is measured downstream after a delay, and a controller that waits for the residual to move reacts late and overshoots. Flow pacing changes the feed the instant the flow changes; the residual loop then corrects slowly for demand changes. Residual-only control oscillates in most plants.
- How do I know the pump is delivering what the controller thinks?
- Drawdown test with a calibration column: isolate the day tank, let the pump draw from the column for a timed interval, and compute the output. Compare with the expected rate at that speed and stroke. Do it weekly for critical chemicals and after any repair, and record the results; the trend catches wear before the residual does.
- What should the feed do if the flowmeter fails?
- Hold a fallback: the last good feed rate, or a rate computed from a fixed assumed flow, with an alarm. Pacing from a failed signal reading zero stops disinfection; pacing from a failed signal reading full scale overdoses. Signal validation on the pacing flow is part of every feed loop.
- Stroke or speed control on a diaphragm pump?
- Speed for the automatic control, because it is linear and repeatable over a wide range. Set the stroke length so that the pump runs in the middle of its speed range at the normal dose, and leave it. Pumps controlled by stroke with a positioner exist, but speed control is simpler and more accurate.
Related topics
- Disinfection ControlControlling chlorine and chloramine dosing at a water plant: the CT concept and how contact time is credited, flow-paced and residual-trimmed feed, gas, hypochlorite, and on-site generation systems, the point-of-entry residual as the compliance measurement, and the interlocks that stop a feed system from over- or under-dosing.
- 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.
- Control StrategiesChoosing how a loop is controlled before choosing how to program it: on-off with deadband, staging, PID, ratio and feedforward, cascade, split range, and override, with the water and wastewater applications each fits and the questions that pick between them.
- Raw WaterControlling the front of a surface water plant: intake and screens, low-lift pumping, the raw water flow that paces the whole plant, the online quality instruments that warn of a changing source, and the alarms and ramp limits that keep the train stable.
- 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.
Direct contact
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