Chlorine Demand, Dose & Residual

Chlorine disinfection depends on more than simply adding chlorine to water or wastewater. Part of the applied chlorine reacts with substances in the water, and only the remaining portion appears as measurable residual. Operators must understand the relationship among dose, demand, residual, ammonia, pH, contact time, and water quality.

The most important basic relationship is simple:

Chlorine Demand = Chlorine Dose - Chlorine Residual

However, interpreting chlorine behavior requires understanding which chlorine species are present and how quickly they react.

What Is Chlorine Dose?

Chlorine dose is the amount of chlorine applied to the water or wastewater.

Dose is commonly expressed as:

mg/L

The applied dose may come from:

• chlorine gas;
• sodium hypochlorite;
• calcium hypochlorite;
• another approved chlorine source.

What Is Chlorine Demand?

Chlorine demand is the amount of chlorine consumed by chemical and biological reactions before a measurable residual remains.

Demand may be created by:

• organic matter;
• ammonia;
• nitrite;
• iron;
• manganese;
• hydrogen sulfide;
• other reduced compounds;
• microorganisms.

What Is Chlorine Residual?

Chlorine residual is the measurable chlorine remaining after demand has been satisfied to the point of measurement.

Residual provides information about how much chlorine remains available after reactions have occurred.

Dose, Demand, and Residual Relationship

A simplified relationship is:

Demand = Dose - Residual

This can also be rearranged:

Residual = Dose - Demand

or:

Dose = Demand + Residual

Basic Chlorine Demand Example

If chlorine dose is 4.0 mg/L and measured residual is 1.2 mg/L:

Demand = 4.0 - 1.2

Demand = 2.8 mg/L

Second Example

If chlorine demand is estimated at 2.5 mg/L and the desired residual is 1.0 mg/L:

Dose = 2.5 + 1.0

Dose = 3.5 mg/L

Dose Is Not the Same as Feed Rate

Chlorine dose is a concentration applied to the process.

Feed rate describes the amount of chlorine delivered per unit time.

Feed rate may be expressed as:

• lb/day;
• lb/hr;
• gal/day of solution;
• other equipment-specific units.

Chlorine Mass Feed Requirement

A common water and wastewater relationship is:

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

Chlorine Feed Example

A plant treats 2.5 MGD and applies a chlorine dose of 3.0 mg/L.

Chlorine Required = 2.5 × 3.0 × 8.34

Chlorine Required = 62.55 lb/day

This is the chlorine mass required, not necessarily the mass or volume of commercial chemical solution.

Commercial Solution Strength

When sodium hypochlorite or another chlorine solution is used, operators must account for product strength.

A weaker solution requires a larger volume to deliver the same chlorine mass.

Chemical Strength Can Change

Sodium hypochlorite can lose strength during storage.

Degradation can be accelerated by:

• heat;
• sunlight;
• long storage time;
• contamination.

An aging product can therefore produce a lower actual dose even when the feed-pump setting has not changed.

Free Chlorine

Free chlorine generally includes:

• hypochlorous acid, HOCl;
• hypochlorite ion, OCl-.

The relative proportion of these species depends strongly on pH.

Hypochlorous Acid

Hypochlorous acid is generally the more effective disinfecting form of free chlorine.

Within typical treatment conditions, lower pH favors a greater proportion of HOCl.

Hypochlorite Ion

As pH increases, a greater portion of free chlorine exists as hypochlorite ion.

This changes disinfection effectiveness even when measured free chlorine concentration remains the same.

Combined Chlorine

Combined chlorine includes chlorine compounds formed by reaction with ammonia or nitrogen-containing compounds.

These compounds are commonly called chloramines.

Total Chlorine

A simplified relationship is:

Total Chlorine = Free Chlorine + Combined Chlorine

Example of Total Chlorine

If:

• free chlorine = 0.8 mg/L;
• combined chlorine = 1.4 mg/L;

then:

Total Chlorine = 0.8 + 1.4 = 2.2 mg/L

Know Which Residual Is Being Measured

Operators must distinguish among:

• free chlorine residual;
• combined chlorine residual;
• total chlorine residual.

These measurements are not interchangeable.

Chlorine and Ammonia

Chlorine reacts readily with ammonia.

This reaction can form chloramines and create significant chlorine demand.

Ammonia can therefore strongly influence:

• free residual;
• combined residual;
• chlorine dose requirement;
• breakpoint behavior.

Breakpoint Chlorination

Breakpoint chlorination describes the changing chlorine residual that occurs as increasing chlorine dose reacts with ammonia and other reactive substances.

A simplified sequence includes:

1. initial chlorine is consumed by readily reactive substances;
2. chloramines form and combined residual increases;
3. additional chlorine oxidizes chloramines and other nitrogen compounds;
4. combined residual decreases;
5. after the breakpoint, additional chlorine produces a stronger free-chlorine residual.

Before the Breakpoint

Before breakpoint, added chlorine may be consumed rapidly or may appear mainly as combined chlorine.

A large increase in chlorine feed may therefore produce only a small increase in free residual.

Near the Breakpoint

Near breakpoint, chlorine reactions can be complex and residual may change rapidly with dose.

Operators should make controlled adjustments and allow adequate mixing and reaction time.

After the Breakpoint

After breakpoint, additional chlorine generally produces a more predictable increase in free residual, assuming demand remains reasonably stable.

Breakpoint Is a Concept, Not One Universal Dose

The chlorine dose required to reach breakpoint depends on:

• ammonia concentration;
• other reducing compounds;
• organic matter;
• pH;
• temperature;
• reaction time.

Chloramine Disinfection

Some drinking-water systems intentionally maintain combined chlorine, commonly chloramine, as a secondary disinfectant.

Combined chlorine generally:

• reacts more slowly than free chlorine;
• can persist longer in distribution systems;
• has different disinfection characteristics.

Free and Combined Chlorine Serve Different Purposes

Operators should follow the treatment objective and system design rather than assuming one form is always preferable.

Chlorine Decay

Chlorine decay is the reduction of chlorine residual over time as chlorine continues reacting.

Decay can occur in:

• contact basins;
• clearwells;
• storage tanks;
• distribution systems;
• wastewater contact tanks.

Factors Increasing Chlorine Decay

Residual decay can increase with:

• higher temperature;
• greater organic matter;
• ammonia;
• pipe deposits;
• biofilm;
• long water age.

Water Age

Long water age gives chlorine more time to react.

This can result in lower residual at distant or low-use locations.

Temperature

Higher temperatures generally increase many chlorine reaction rates.

Operators may therefore observe faster residual decay during warm conditions.

Organic Matter

Natural organic matter and wastewater organics can consume chlorine.

Higher organic loading can increase demand and reduce residual for a constant dose.

Iron and Manganese

Reduced iron and manganese can react with chlorine.

Part of the applied chlorine may therefore be consumed by oxidation before a disinfectant residual develops.

Hydrogen Sulfide

Hydrogen sulfide creates strong oxidant demand.

Where present, chlorine can be consumed by sulfur oxidation reactions.

Nitrite

Nitrite can exert significant chlorine demand.

This can be important in some wastewater and biological-treatment conditions.

Demand Can Change Quickly

Changes in:

• source water;
• influent wastewater;
• ammonia;
• organic loading;
• temperature

can change chlorine demand even when process flow remains stable.

Constant Dose Does Not Guarantee Constant Residual

If dose remains constant but demand rises, residual falls.

If demand decreases while dose remains constant, residual rises.

Example: Increasing Demand

Suppose dose remains 4.0 mg/L.

Earlier:

Residual = 1.5 mg/L

Later:

Residual = 0.6 mg/L

Estimated demand changed from:

4.0 - 1.5 = 2.5 mg/L

to:

4.0 - 0.6 = 3.4 mg/L

This suggests an increase in chlorine demand if dose and measurements are correct.

Flow-Paced Chlorine Feed

Many systems adjust chlorine feed according to process flow.

This can maintain approximately constant dose when flow changes.

Flow Signal Errors

If the flow measurement is wrong, flow-paced chlorine feed can also become wrong.

For example:

• flow reading too low can underfeed chlorine;
• flow reading too high can overfeed chlorine.

Residual-Controlled Feed

Some systems use residual measurements to trim chlorine feed automatically.

This requires reliable:

• analyzer calibration;
• sample flow;
• response time;
• control logic.

Feedforward and Feedback

A disinfection control system may combine:

• flow pacing as feedforward control;
• residual measurement as feedback control.

This can help respond to both flow changes and demand changes.

Mixing Matters

Chlorine must contact the process stream effectively.

Poor mixing can cause:

• uneven residual;
• local overdosing;
• local underdosing;
• unrepresentative sampling.

Reaction Time Matters

Immediately after chlorine injection, reactions may still be occurring.

A residual measured too close to the injection point may not represent the residual after demand and mixing have stabilized.

Sample Location Matters

Chlorine samples should be collected at locations appropriate for the question being evaluated.

Examples include:

• immediately after feed for process troubleshooting;
• end of contact basin for disinfection performance;
• distribution system for residual maintenance.

Chlorine Samples Should Be Tested Promptly

Chlorine continues reacting after a sample is collected.

Delayed testing can produce a lower residual than existed at the sampling point.

Free and Total Chlorine Testing

Colorimetric methods are commonly used to measure chlorine residual.

Potential errors include:

• wrong reagent;
• incorrect timing;
• dirty sample cell;
• sample color;
• sample turbidity;
• instrument calibration error.

Confirm the Correct Test

A free chlorine test should not be assumed to represent total chlorine, and a total chlorine test should not be assumed to represent free chlorine.

High Residual Troubleshooting

Possible causes include:

• excess chlorine feed;
• decreased demand;
• lower process flow;
• incorrect flow pacing;
• analyzer error;
• stronger-than-expected chemical solution.

Low Residual Troubleshooting

Possible causes include:

• insufficient feed;
• increased chlorine demand;
• weak chemical solution;
• feed-pump failure;
• plugged injection line;
• poor mixing;
• incorrect flow signal;
• measurement error.

No Residual

If no residual is detected, do not automatically assume that no chlorine is being fed.

Possible explanations include:

• very high demand;
• chemical feed failure;
• sampling error;
• test error.

Residual Spike

A sudden residual increase can indicate:

• reduced demand;
• flow decrease;
• feed-control problem;
• process loading change;
• instrument error.

Check Process Data Together

Useful related data include:

• flow;
• chlorine feed rate;
• chemical strength;
• free residual;
• total residual;
• pH;
• temperature;
• ammonia;
• turbidity or TSS.

Chlorine Dose Calculation from Flow

A common dose relationship can be rearranged from the mass-loading formula:

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

Dose Calculation Example

A facility feeds 50 lb/day chlorine at 2.0 MGD.

Dose = 50 ÷ (2.0 × 8.34)

Dose = 50 ÷ 16.68

Dose ≈ 3.0 mg/L

Finding Required Feed Rate

If required dose is known:

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

Required Feed Example

Flow is 4 MGD and desired chlorine dose is 2.5 mg/L.

Feed = 4 × 2.5 × 8.34

Feed = 83.4 lb/day

Changing Flow

If flow doubles and the desired dose stays the same, required chlorine mass feed also doubles.

Changing Demand

If demand increases but flow stays constant, chlorine dose may need to increase to maintain the same residual.

Do Not Chase Residual Too Aggressively

Large rapid feed adjustments can cause:

• residual overshoot;
• control instability;
• excess chemical use.

Allow appropriate mixing, contact, and analyzer response time before making repeated large adjustments.

Chlorine Analyzer Lag

Online residual analyzers may have delay caused by:

• sample-line travel time;
• instrument response time;
• process detention time.

Operators should understand this delay when evaluating feed changes.

Sampling-Line Problems

Online chlorine analyzers can be affected by:

• plugged sample line;
• low sample flow;
• air bubbles;
• stagnant sample;
• dirty analyzer components.

Wastewater Chlorination

Wastewater can have substantial chlorine demand because of:

• ammonia;
• organic matter;
• TSS;
• reduced compounds.

Wastewater Ammonia Effect

Ammonia can produce combined chlorine and significantly affect chlorine dose requirements.

Operators should therefore review ammonia when chlorine demand changes unexpectedly.

Wastewater Solids

High suspended solids can:

• increase chlorine demand;
• shield microorganisms;
• reduce disinfection effectiveness.

Dechlorination

Where wastewater chlorination is followed by dechlorination, operators must maintain enough upstream chlorine for disinfection while avoiding excessive downstream residual.

Excess Chlorine Increases Dechlorination Demand

Overfeeding chlorine can increase:

• dechlorination chemical use;
• operating cost;
• control difficulty.

Distribution-System Residual

In drinking-water systems, residual may decline through the distribution system because of:

• water age;
• temperature;
• pipe-wall reactions;
• biofilm;
• organic matter.

Low Residual at One Distribution Location

Possible causes include:

• long water age;
• dead-end conditions;
• local demand;
• poor tank turnover;
• sample or test problem.

Chlorine Safety

Chlorine gas and concentrated hypochlorite solutions can create serious hazards.

Operators should follow facility procedures for:

• PPE;
• ventilation;
• chemical transfer;
• storage;
• spill or leak response;
• chemical compatibility.

Hypochlorite and Acids

Hypochlorite should not be mixed with acids.

Dangerous chlorine gas can be released.

Hypochlorite and Ammonia Chemicals

Uncontrolled mixing of chlorine chemicals with ammonia-containing chemicals can create hazardous reactions.

Chemical compatibility and facility procedures must be followed.

Common Chlorine Mistakes

• Confusing chlorine dose with chlorine residual.
• Ignoring chlorine demand.
• Confusing free chlorine with total chlorine.
• Assuming constant dose will always produce constant residual.
• Ignoring ammonia when residual changes.
• Ignoring chemical-strength degradation.
• Sampling too close to the injection point.
• Delaying residual testing.
• Changing feed before allowing process and analyzer response.
• Ignoring flow-meter errors in flow-paced systems.
• Assuming a low analyzer value automatically means chemical feed failure.
• Overfeeding chlorine without considering downstream effects.

A Practical Low-Residual Troubleshooting Sequence

1. Verify the residual measurement.
2. Confirm whether free or total chlorine is being measured.
3. Check sample location and sample flow.
4. Review chlorine feed rate.
5. Review process flow.
6. Confirm chemical strength.
7. Inspect feed equipment and injection point.
8. Review ammonia and organic loading.
9. Review pH and temperature.
10. Make controlled feed adjustments if justified.
11. Allow adequate mixing and response time.

A Practical High-Residual Troubleshooting Sequence

1. Verify the residual measurement.
2. Review process flow.
3. Review chemical feed rate.
4. Check flow pacing and automatic control.
5. Review recent demand changes.
6. Check chemical strength.
7. Reduce feed carefully if appropriate.
8. Continue monitoring downstream residual.

A Practical Chlorine Dose Review

1. Confirm flow in MGD.
2. Confirm required chlorine dose in mg/L.
3. Calculate chlorine mass using MGD × mg/L × 8.34.
4. Convert required chlorine mass to commercial chemical feed using actual product strength.
5. Confirm feeder calibration.
6. Measure downstream residual.
7. Calculate or estimate demand.
8. Adjust according to actual process response.

What to Remember for the Exam

• Chlorine dose is the amount of chlorine applied to the process.
• Chlorine demand is the amount consumed by chemical and biological reactions.
• Chlorine residual is the amount remaining after demand has been satisfied to the point of measurement.
• A basic relationship is Demand = Dose - Residual.
• Total chlorine equals free chlorine plus combined chlorine.
• Free chlorine mainly includes hypochlorous acid and hypochlorite ion.
• pH affects the relative amounts of hypochlorous acid and hypochlorite ion.
• Combined chlorine commonly includes chloramines formed by reactions between chlorine and ammonia.
• Breakpoint chlorination describes the changing residual pattern as chlorine reacts with ammonia and other demand.
• Ammonia, organic matter, nitrite, iron, manganese, and hydrogen sulfide can increase chlorine demand.
• A constant chlorine dose does not guarantee a constant residual.
• Chlorine residual can decay with time, temperature, water age, and continued reactions.
• A common feed calculation is lb/day = MGD × mg/L × 8.34.
• If flow doubles and desired dose remains constant, required chlorine mass feed doubles.
• Flow-paced feed depends on an accurate flow signal.
• Residual-control systems depend on accurate analyzers and appropriate response time.
• Chlorine samples should be tested promptly because residual continues changing after collection.
• Low residual can result from insufficient feed, increased demand, weak chemical, feed-system failure, or measurement error.
• High residual can result from excess feed, reduced demand, lower flow, or measurement error.
• Good chlorine control requires review of dose, demand, residual, flow, chemical strength, pH, temperature, ammonia, and process conditions together.