Chemical Feed Fundamentals

Chemical feed systems convert an operator's treatment target into a controlled amount of chemical delivered to the process. Reliable operation requires understanding the relationship among flow, dose, chemical strength, feeder output, mixing, and the actual process response.

A feeder setting alone does not prove that the correct chemical dose is reaching the water or wastewater. Operators must verify the feed system, calculate the expected dose, calibrate equipment, and compare chemical feed with process measurements.

What Is Chemical Feed?

Chemical feed is the controlled addition of a treatment chemical to water, wastewater, sludge, or another process stream.

Chemicals may be added for purposes such as:

• disinfection;
• coagulation;
• pH adjustment;
• alkalinity addition;
• corrosion control;
• phosphorus removal;
• odor control;
• dechlorination.

Dose

Dose is the amount of chemical applied per unit volume of process flow.

Dose is commonly expressed as:

mg/L

Feed Rate

Feed rate is the amount of chemical delivered per unit time.

Feed rate may be expressed as:

• lb/day;
• lb/hr;
• gal/day;
• mL/min;
• other feeder-specific units.

Dose and Feed Rate Are Different

Dose describes chemical concentration applied to the process.

Feed rate describes how much chemical the equipment delivers over time.

The required feed rate changes when process flow changes.

Basic Mass Feed Formula

A common operator relationship is:

Chemical Required, lb/day = Flow, MGD × Dose, mg/L × 8.34

Mass Feed Example

A plant treats 2.0 MGD and requires a dose of 5.0 mg/L.

Feed = 2.0 × 5.0 × 8.34

Feed = 83.4 lb/day

This is the required chemical mass based on the dose calculation.

Finding Dose from Feed Rate

The formula can be rearranged:

Dose, mg/L = Feed, lb/day ÷ (Flow, MGD × 8.34)

Dose Example

A feeder delivers 50 lb/day to a process flowing at 1.5 MGD.

Dose = 50 ÷ (1.5 × 8.34)

Dose = 50 ÷ 12.51

Dose ≈ 4.0 mg/L

Concentration Versus Mass

A chemical solution contains both:

• active chemical;
• carrier liquid or other material.

Commercial product volume is therefore not the same as active chemical mass.

Chemical Strength

Chemical strength describes how much active chemical is present in a product or solution.

Strength may be expressed as:

• percent by weight;
• percent by volume;
• lb/gal;
• mg/L;
• another product-specific basis.

Always Know the Strength Basis

A value such as 12 percent is incomplete unless the operator knows whether it represents:

• weight percent;
• volume percent;
• another defined concentration basis.

Commercial Product Versus Active Chemical

If a process requires a certain active chemical mass, the required commercial product amount depends on product strength.

A weaker product requires more total product to deliver the same active chemical mass.

Simple Strength Example

If 20 lb/day of active chemical is required and a dry product is 50 percent active by weight:

Commercial Product Required = 20 ÷ 0.50

Commercial Product Required = 40 lb/day

Liquid Chemical Feed

Liquid chemicals may be fed using:

• metering pumps;
• diaphragm pumps;
• peristaltic pumps;
• other positive-displacement equipment.

Dry Chemical Feed

Dry chemicals may be delivered using:

• screw feeders;
• volumetric feeders;
• gravimetric feeders;
• solution preparation systems.

Dry Feed Requires Consistent Material Flow

Dry chemical feed can be affected by:

• bridging;
• rat-holing;
• moisture;
• clumping;
• changing bulk density;
• feeder wear.

Positive-Displacement Metering Pumps

Many liquid chemical feeders are positive-displacement pumps.

They deliver a measured volume during each stroke or pumping cycle.

Output may be controlled by changing:

• stroke length;
• stroke frequency;
• motor speed;
• electronic control signal.

Feeder Setting Is Not Guaranteed Output

A pump set to 50 percent does not automatically mean that it delivers exactly 50 percent of nameplate capacity.

Actual output can be affected by:

• backpressure;
• chemical viscosity;
• suction conditions;
• check-valve condition;
• air or gas in the pump head;
• mechanical wear.

Calibration

Calibration determines actual feeder output under real operating conditions.

Calibration connects:

• feeder setting;
• actual chemical volume or mass delivered.

Why Calibration Matters

If the actual feeder output differs from the assumed output, calculated dose will also be wrong.

Regular calibration is especially important when:

• chemical strength changes;
• feed equipment is serviced;
• tubing or check valves are replaced;
• process results no longer match expected dose.

Calibration Column

A calibration column allows operators to measure the volume of liquid chemical removed over a known period.

A simple relationship is:

Feed Rate = Volume Used ÷ Time

Calibration Example

A pump removes 250 mL from a calibration column in 5 minutes.

Feed Rate = 250 ÷ 5

Feed Rate = 50 mL/min

Convert Calibration Results Carefully

To convert a measured liquid volume to chemical mass, operators may need:

• solution density;
• chemical strength;
• appropriate unit conversions.

Flow-Paced Feed

Flow pacing changes chemical-feed rate in response to process flow.

If the desired dose stays constant:

higher process flow requires higher chemical feed.

lower process flow requires lower chemical feed.

Flow-Pacing Example

A system is designed to maintain a constant 3 mg/L dose.

If flow doubles from 1 MGD to 2 MGD, required active chemical mass also doubles.

Flow Signal Accuracy

Flow-paced chemical feed depends on an accurate flow measurement.

If the flow meter reads low, the feed system may underfeed.

If the flow meter reads high, the feed system may overfeed.

Feedforward Control

Flow pacing is a form of feedforward control.

The system responds to an expected process requirement, such as flow, before the downstream result is measured.

Feedback Control

Feedback control adjusts chemical feed using a measured downstream result.

Examples include:

• chlorine residual;
• pH;
• orthophosphate residual;
• another process variable.

Feedforward Plus Feedback

Some systems combine:

• flow pacing for base chemical dose;
• downstream measurement for trim adjustment.

This can improve control when both flow and chemical demand change.

Process Delay

Feedback control must account for delay between:

• chemical injection;
• mixing;
• reaction;
• downstream measurement.

Do Not Adjust Too Quickly

If an operator changes feed repeatedly before the previous adjustment reaches the measurement point, the system can:

• overshoot;
• undershoot;
• cycle;
• become unstable.

Injection Point

The injection point should allow the chemical to enter the process safely and mix effectively.

Poor injection can cause:

• local chemical concentration;
• incomplete mixing;
• precipitation;
• corrosion;
• poor treatment performance.

Mixing

Mixing brings chemical into contact with the process stream.

Mixing may be provided by:

• rapid mixers;
• pipeline turbulence;
• static mixers;
• mechanical mixers;
• hydraulic structures.

Fast-Reaction Chemicals Need Rapid Mixing

When a chemical reacts quickly, poor initial mixing can create uneven treatment.

Examples may include:

• coagulants;
• chlorine;
• pH-adjustment chemicals.

Contact Time After Feed

Some chemical processes require reaction time after mixing.

Examples include:

• disinfection;
• oxidation;
• coagulation reactions;
• pH stabilization.

Dilution

A concentrated chemical may be diluted before feed to:

• improve mixing;
• improve feeder control;
• reduce localized concentration;
• match equipment operating range.

Dilution Does Not Change Chemical Mass

Adding water lowers solution concentration but does not change the mass of active chemical already present.

Dilution Formula

A common relationship is:

C1V1 = C2V2

where:

• C1 = initial concentration;
• V1 = initial solution volume;
• C2 = final concentration;
• V2 = final total volume.

Dilution Example

Prepare 100 gallons of 2 percent solution from a 10 percent stock solution.

10 × V1 = 2 × 100

V1 = 20 gallons

Therefore:

• 20 gallons of 10 percent stock solution;
• enough dilution water to produce 100 gallons total.

Do Not Confuse Final Volume with Water Added

In the example, final volume is 100 gallons.

The required dilution water is approximately:

100 - 20 = 80 gallons

Solution Preparation

When preparing a chemical solution, follow the approved procedure for:

• order of addition;
• mixing;
• temperature;
• chemical compatibility;
• PPE.

Never Assume Chemicals Can Be Mixed

Incompatible chemicals can cause:

• toxic gas;
• heat;
• fire;
• violent reaction;
• precipitation;
• equipment damage.

Chemical Storage

Storage systems should protect chemical quality and provide safe containment.

Important considerations include:

• compatible tank material;
• secondary containment;
• venting;
• temperature;
• sunlight;
• chemical segregation.

Chemical Age

Some chemicals lose strength during storage.

This can cause actual dose to decrease even when feed equipment output remains constant.

Day Tanks

A day tank may provide a smaller controlled volume between bulk storage and the feeder.

Advantages can include:

• easier inventory tracking;
• controlled dilution;
• separation from bulk storage;
• improved feed-system management.

Level Monitoring

Chemical tanks may use:

• level transmitters;
• switches;
• sight tubes;
• load cells;
• manual measurements.

Inventory as a Feed Check

Chemical inventory can provide an independent check on expected feed.

If calculated consumption is 100 gallons per day but actual tank level drops only 40 gallons per day, investigate the difference.

Possible Inventory Differences

Differences may result from:

• incorrect feeder calibration;
• wrong tank-level measurement;
• leaks;
• incorrect flow data;
• incorrect chemical-strength assumption.

Suction Side of a Chemical Feed Pump

The suction side should provide reliable chemical supply to the pump.

Problems can include:

• empty tank;
• closed valve;
• plugged strainer;
• air leak;
• crystallized chemical;
• excessive suction lift.

Air Binding and Gas Lock

Gas in a metering-pump head can reduce or stop chemical delivery.

Possible causes include:

• air entering suction piping;
• chemical off-gassing;
• empty suction line.

Check Valves

Metering pumps commonly use suction and discharge check valves.

Dirty or worn check valves can cause:

• loss of prime;
• reduced output;
• backflow;
• inconsistent feed.

Backpressure

Some metering systems require adequate and stable discharge backpressure for reliable feed.

Changes in process pressure can affect actual output depending on feeder design.

Anti-Siphon Protection

Where chemical tank elevation or downstream vacuum can cause uncontrolled flow, anti-siphon protection may be required by system design.

Chemical should not continue flowing simply because the pump stops.

Injection Check Valve

An injection check valve can help prevent process water from flowing backward into the chemical line.

Plugged Injection Point

Injection points can plug because of:

• scale;
• chemical crystallization;
• precipitation;
• debris.

A running feed pump does not prove chemical is entering the process.

Loss of Chemical Feed

If the process result suggests no chemical addition, check:

• chemical supply;
• suction valves;
• pump operation;
• check valves;
• tubing;
• injection point;
• calibration.

Overfeed

Chemical overfeed can result from:

• incorrect setpoint;
• incorrect chemical strength;
• flow-meter error;
• control-loop problem;
• feeder calibration error.

Underfeed

Chemical underfeed can result from:

• weak chemical;
• low pump output;
• plugged line;
• gas lock;
• incorrect flow pacing;
• empty tank.

Verify Process Response

The final proof of chemical-feed performance is not feeder movement alone.

Operators should verify appropriate downstream indicators such as:

• chlorine residual;
• pH;
• alkalinity;
• orthophosphate;
• turbidity;
• another relevant process result.

Example: pH Chemical Feed

If caustic feed increases but downstream pH does not change, possible causes include:

• no actual chemical delivery;
• wrong chemical strength;
• high process buffering;
• poor mixing;
• faulty pH measurement.

Example: Chlorine Feed

If chlorine feeder output increases but residual remains low, investigate:

• increasing chlorine demand;
• weak chemical;
• feed-system failure;
• sample or analyzer error.

Example: Coagulant Feed

If coagulant dose increases but settled-water turbidity worsens, more chemical may not be the solution.

Review:

• raw-water quality;
• pH;
• alkalinity;
• mixing;
• flocculation;
• feeder calibration.

Feeder Redundancy

Critical chemical systems may include:

• duty feeder;
• standby feeder;
• automatic transfer;
• manual backup procedures.

Standby Equipment Must Be Ready

A standby pump provides little protection if it:

• has not been exercised;
• is not calibrated;
• has plugged tubing;
• has closed isolation valves.

Alarm Conditions

Useful alarms may include:

• low chemical level;
• feed-pump failure;
• loss of flow;
• high or low process residual;
• leak detection.

Do Not Rely on One Alarm

Good chemical-feed monitoring combines:

• equipment status;
• chemical inventory;
• process measurement;
• operator inspection.

Calibration Records

Useful records include:

• feeder ID;
• date;
• chemical;
• feeder setting;
• measured output;
• chemical strength;
• person performing calibration.

Trend Chemical Use

Chemical consumption trends can reveal:

• seasonal demand changes;
• feeder problems;
• chemical-strength changes;
• process inefficiency.

Normalize Chemical Use

When plant flow varies, total chemical use alone can be misleading.

Useful normalized measures include:

• lb per million gallons;
• gal of solution per million gallons;
• mg/L dose.

Chemical Feed Safety

Chemical-feed systems can involve corrosive, oxidizing, toxic, or reactive materials.

Operators should use appropriate:

• PPE;
• ventilation;
• secondary containment;
• emergency procedures;
• eyewash and shower access where required;
• chemical compatibility controls.

Never Mix Incompatible Chemicals

Particular care is required when handling:

• hypochlorite and acids;
• oxidizers and reducing agents;
• chemicals that form hazardous gases;
• chemicals that react strongly with water or each other.

Lockout and Isolation

Maintenance on chemical-feed equipment may require:

• electrical lockout;
• chemical isolation;
• pressure relief;
• draining or flushing;
• appropriate PPE.

Common Chemical Feed Mistakes

• Confusing dose with feed rate.
• Ignoring commercial chemical strength.
• Assuming feeder percent setting equals actual output.
• Failing to calibrate the feeder.
• Ignoring changing process flow.
• Ignoring flow-meter errors in flow-paced systems.
• Making repeated adjustments before process response reaches the measurement point.
• Ignoring poor mixing or a plugged injection point.
• Assuming a running pump proves chemical is reaching the process.
• Ignoring chemical degradation during storage.
• Failing to compare chemical inventory with calculated use.
• Mixing incompatible chemicals.

A Practical Chemical Feed Calculation

1. Determine process flow.
2. Determine required dose.
3. Calculate active chemical mass required.
4. Determine actual commercial chemical strength.
5. Convert active chemical requirement to product quantity.
6. Set the feeder within its normal operating range.
7. Calibrate actual output.
8. Verify downstream process response.

A Practical Low-Feed Troubleshooting Sequence

1. Confirm chemical is available in the tank.
2. Confirm suction and discharge valves are correctly positioned.
3. Inspect suction tubing and strainers.
4. Check for air or gas lock.
5. Inspect pump check valves.
6. Inspect discharge tubing and injection point.
7. Calibrate actual pump output.
8. Confirm chemical strength.
9. Verify downstream process response.

A Practical Feed-Control Review

1. Confirm process flow measurement.
2. Confirm chemical-feed setpoint.
3. Confirm feeder calibration.
4. Confirm chemical strength.
5. Review mixing and injection conditions.
6. Review downstream measurement.
7. Account for process response delay.
8. Make controlled adjustments.
9. Trend chemical use and process response.

What to Remember for the Exam

• Dose is the amount of chemical applied per unit process volume, commonly expressed as mg/L.
• Feed rate is the amount of chemical delivered per unit time.
• A common mass-feed formula is lb/day = MGD × mg/L × 8.34.
• Commercial product strength must be considered when converting active chemical demand to actual product feed.
• A weaker chemical product requires more product to deliver the same active chemical mass.
• Liquid and dry chemical feeders require calibration to determine actual output.
• A feeder percent setting does not guarantee a matching percent of actual capacity.
• Flow-paced feed changes chemical-feed rate with process flow.
• An inaccurate flow signal can cause chemical overfeed or underfeed.
• Feedback control uses a downstream process measurement to adjust feed.
• Process delay must be considered before repeated feed adjustments are made.
• Good mixing and proper injection-point location are essential for effective chemical treatment.
• Dilution changes concentration but does not change the active chemical mass already present.
• The dilution relationship C1V1 = C2V2 is commonly used for solution preparation.
• A running metering pump does not prove chemical is entering the process.
• Plugged injection points, dirty check valves, gas lock, and suction problems can reduce actual feed.
• Chemical inventory provides an independent check on expected feed-system performance.
• Actual chemical strength can change during storage.
• Process measurements should be used to verify that the expected chemical effect is occurring.
• Chemical compatibility, PPE, containment, isolation, and emergency procedures are essential parts of chemical-feed operation.