Disinfection Fundamentals

Disinfection is a critical treatment step used to reduce disease-causing microorganisms to an acceptable level. Operators should understand what disinfection can and cannot do, how different disinfectants work, and why dose, residual, contact time, water quality, temperature, pH, and mixing all affect performance.

Disinfection is not the same as removing particles, treating chemical contaminants, or sterilizing water. It is one part of a treatment system and works best when upstream treatment has already reduced solids and other substances that can interfere with microbial inactivation.

What Is Disinfection?

Disinfection is the treatment process used to inactivate or destroy many disease-causing microorganisms.

The goal is to reduce the risk from pathogens such as:

• bacteria;
• viruses;
• protozoa;
• other microorganisms of health concern.

Disinfection Versus Sterilization

Sterilization means destruction or removal of all viable microorganisms under defined conditions.

Routine drinking water and wastewater treatment generally relies on disinfection, not sterilization.

Disinfection Is Not Physical Removal

Disinfection inactivates microorganisms but does not necessarily remove them from the water.

Physical treatment processes such as:

• sedimentation;
• filtration;
• membrane treatment

can remove microorganisms and particles from the water.

Multiple Barriers

Water treatment commonly uses multiple barriers rather than relying on one disinfection step.

Examples include:

• source-water protection;
• coagulation and clarification;
• filtration;
• disinfection;
• distribution-system control.

Why Upstream Treatment Matters

Particles and organic material can reduce disinfection effectiveness by:

• shielding microorganisms;
• consuming disinfectant;
• increasing chemical demand;
• reducing UV transmission.

Pathogens

Pathogens are microorganisms capable of causing disease.

Different pathogen groups can respond differently to disinfection methods.

Bacteria

Many bacteria are relatively susceptible to common drinking-water and wastewater disinfectants when appropriate operating conditions are maintained.

Viruses

Viruses can require different levels of disinfectant exposure than bacteria.

Disinfection performance therefore should not be judged only by how one microorganism responds.

Protozoa

Some protozoan organisms and their resistant life stages can be much more resistant to certain chemical disinfectants than common bacteria.

This is one reason filtration and other treatment barriers are important.

Common Disinfection Methods

Common methods used in water and wastewater treatment include:

• chlorine-based disinfection;
• ultraviolet light;
• ozone.

Other disinfectants and treatment approaches may also be used depending on facility design.

Characteristics of an Effective Disinfectant

A useful disinfectant should provide sufficient microbial inactivation under the intended treatment conditions.

Operational considerations include:

• effectiveness;
• contact requirements;
• water chemistry;
• safety;
• cost;
• residual behavior;
• byproduct formation.

Disinfectant Dose

Dose is the amount of disinfectant applied to the water or wastewater.

Dose is commonly expressed as a concentration such as:

mg/L

Disinfectant Demand

Demand is the amount of disinfectant consumed by reactions with substances in the water before the remaining disinfectant is measured as residual.

Substances that can contribute to demand include:

• organic matter;
• ammonia;
• iron;
• manganese;
• reduced sulfur compounds;
• other reactive materials.

Disinfectant Residual

Residual is the amount of disinfectant remaining after demand and reactions have occurred.

Dose, Demand, and Residual

A useful simplified relationship is:

Demand = Dose - Residual

Demand Example

If chlorine dose is 4.0 mg/L and measured residual is 1.5 mg/L:

Demand = 4.0 - 1.5 = 2.5 mg/L

The 2.5 mg/L difference represents chlorine consumed by reactions under those conditions.

Residual Is Not the Same as Dose

Operators should not assume that the chemical feed setting or calculated applied dose equals the disinfectant concentration actually available downstream.

Residual must be measured where required.

Contact Time

Contact time is the amount of time the disinfectant remains in contact with the water or wastewater before the point being evaluated.

In general, greater contact time can increase inactivation when other conditions remain suitable.

The CT Concept

For some disinfection applications, performance is evaluated using the product of disinfectant concentration and effective contact time.

A simplified relationship is:

CT = C × T

where:

• C = disinfectant concentration;
• T = effective contact time.

CT Example

If disinfectant concentration is 1.5 mg/L and effective contact time is 30 minutes:

CT = 1.5 × 30 = 45 mg-min/L

Whether this CT is sufficient depends on the disinfectant, microorganism, temperature, pH, and applicable treatment requirements.

Effective Contact Time

Actual effective contact time can be less than theoretical tank detention time because of:

• short-circuiting;
• poor baffling;
• dead zones;
• uneven flow distribution.

Theoretical Detention Time

A simplified theoretical detention-time relationship is:

Detention Time = Volume ÷ Flow

Actual disinfection performance may require consideration of hydraulic efficiency rather than theoretical detention time alone.

Short-Circuiting

Short-circuiting occurs when part of the flow travels through a basin faster than intended.

This reduces effective contact time.

Baffling

Baffles can improve contact-basin hydraulics by:

• reducing direct short paths;
• improving flow distribution;
• increasing effective contact time.

Flow Affects Contact Time

If basin volume stays constant and flow increases, detention time decreases.

This means high-flow conditions can reduce available disinfection contact time.

Temperature

Temperature affects many disinfection reactions.

For chemical disinfectants, lower temperature often slows inactivation reactions.

Operators should therefore consider seasonal temperature changes when evaluating performance.

pH

pH can strongly influence the chemistry and effectiveness of some disinfectants.

For chlorine, pH affects the balance between hypochlorous acid and hypochlorite ion.

Hypochlorous Acid and Hypochlorite

Free chlorine in water commonly exists mainly as:

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

Lower pH within the normal treatment range favors a larger fraction of hypochlorous acid.

Why Chlorine Species Matter

Hypochlorous acid is generally a more effective disinfecting species than hypochlorite ion.

This is why pH can affect chlorine disinfection performance even when total free chlorine concentration is unchanged.

Free Chlorine

Free chlorine generally refers to chlorine present as hypochlorous acid and hypochlorite ion.

Combined Chlorine

Combined chlorine includes chloramine compounds formed when chlorine reacts with ammonia or other nitrogen compounds.

Total Chlorine

A simplified relationship is:

Total Chlorine = Free Chlorine + Combined Chlorine

Free Versus Combined Chlorine

Free and combined chlorine have different:

• disinfection characteristics;
• reaction rates;
• residual persistence.

Operators must know which residual is being measured and controlled.

Breakpoint Chlorination Concept

When chlorine is added to water containing ammonia and other reactive compounds, chlorine can pass through several stages of demand and combined-chlorine formation before a stronger free-chlorine residual develops.

This overall behavior is commonly described as breakpoint chlorination.

Chlorine Demand Can Change

Demand can increase because of changes in:

• organic loading;
• ammonia;
• iron;
• manganese;
• reduced compounds;
• source-water quality.

Do Not Control Chlorine Feed from Dose Alone

Operators should evaluate both:

• applied dose;
• measured residual.

A constant dose can produce a changing residual if demand changes.

Mixing

Good mixing is important immediately after chemical disinfectant addition.

Poor mixing can create:

• local high concentrations;
• areas receiving insufficient disinfectant;
• unrepresentative residual measurements.

Injection Point

A chemical injection point should support:

• rapid distribution;
• appropriate mixing;
• adequate downstream contact.

Turbidity and Particles

High turbidity can interfere with disinfection.

Particles may shield microorganisms from:

• chemical disinfectants;
• UV radiation.

Organic Matter

Organic matter can:

• consume disinfectant;
• increase chemical demand;
• contribute to disinfection byproduct formation with some disinfectants.

Disinfection Byproducts

Certain disinfectants can react with substances in water and form unwanted byproducts.

Disinfection therefore requires balancing:

• microbial inactivation;
• residual needs;
• chemical dose;
• byproduct control.

Chlorine-Based Disinfection

Chlorine-based systems are widely used because they can provide:

• strong microbial inactivation;
• measurable residual;
• continuing protection after the initial contact period.

Chlorine Forms Used in Treatment

Depending on facility design, chlorine may be supplied through systems using:

• chlorine gas;
• sodium hypochlorite;
• calcium hypochlorite;
• other chlorine-based processes.

Chlorine Safety

Chlorine chemicals are hazardous and require appropriate:

• PPE;
• ventilation;
• storage;
• leak response;
• chemical compatibility controls.

Ultraviolet Disinfection

Ultraviolet, or UV, disinfection uses light energy at germicidal wavelengths to damage microbial genetic material and prevent reproduction.

UV Does Not Create a Chemical Residual

Unlike chlorine-based disinfection, UV normally does not provide a persistent disinfectant residual downstream.

UV Dose Concept

UV performance depends on factors including:

• UV intensity;
• exposure time;
• UV transmittance;
• lamp condition;
• sleeve cleanliness;
• hydraulics.

UV Transmittance

UV transmittance describes how much UV light can pass through the water.

Low UV transmittance reduces the amount of useful UV energy reaching microorganisms.

Factors Reducing UV Performance

UV performance can be reduced by:

• high suspended solids;
• high turbidity;
• poor UV transmittance;
• dirty quartz sleeves;
• aging lamps;
• hydraulic short-circuiting.

UV Sleeve Fouling

Deposits on quartz sleeves can reduce transmitted UV intensity.

Operators should monitor:

• cleaning systems;
• lamp intensity;
• alarm status;
• sleeve condition.

UV Lamp Aging

UV lamp output decreases with use.

A lamp can still be illuminated while producing less effective germicidal output than when new.

Ozone

Ozone is a strong oxidizing disinfectant.

It can provide effective microbial inactivation and oxidation of certain water constituents.

Ozone Characteristics

Ozone:

• is generated onsite;
• is highly reactive;
• does not provide a long-lasting distribution residual;
• requires specialized generation and contacting equipment.

Disinfectant Selection

The appropriate disinfection method depends on:

• treatment objective;
• water quality;
• target microorganisms;
• required residual;
• facility design;
• safety;
• cost.

Primary Disinfection

Primary disinfection refers to microbial inactivation during treatment before water enters the distribution system or before treated wastewater is discharged.

Secondary Disinfection

In drinking-water systems, secondary disinfection commonly refers to maintaining a disinfectant residual within the distribution system.

Why Distribution Residual Matters

A distribution disinfectant residual can help:

• limit microbial regrowth;
• provide an indicator of water-quality changes;
• provide some continuing protection.

Residual Decay

Disinfectant residual can decrease as water moves through a system.

Residual decay can increase with:

• water age;
• temperature;
• organic matter;
• pipe deposits;
• microbial activity.

Water Age

Long water age can contribute to:

• lower disinfectant residual;
• greater disinfectant reaction time;
• microbial regrowth risk;
• water-quality deterioration.

Contact Basin Sampling

Disinfectant residual should be measured at locations that correspond to the treatment objective.

An upstream sample may not represent the residual after the required contact period.

Sampling Time Matters

Disinfectant residual can change quickly after sample collection.

Residual measurements should therefore be performed promptly according to the applicable method.

Chlorine Residual Measurement

Common chlorine testing methods use colorimetric chemistry.

Potential errors include:

• delayed testing;
• dirty sample cells;
• wrong reagent;
• incorrect timing;
• sample interference;
• instrument calibration problems.

High Residual

An unexpectedly high residual may indicate:

• excess chemical feed;
• reduced demand;
• incorrect flow pacing;
• instrument error.

Low Residual

An unexpectedly low residual may indicate:

• insufficient chemical feed;
• increased demand;
• poor mixing;
• long contact or water age;
• feed-system problem;
• measurement error.

Flow-Paced Disinfection Feed

Chemical feed may be paced according to flow.

If flow measurement is incorrect, disinfectant dose can also become incorrect.

Feed Rate and Dose

A chemical-feed system should provide the required chemical mass for the actual process flow.

Operators should understand the relationship among:

• flow;
• chemical feed rate;
• solution strength;
• applied dose;
• measured residual.

Changes in Demand

If dose stays constant while residual decreases, demand has increased.

If dose stays constant while residual increases, demand may have decreased.

Disinfection and Solids

Poor upstream solids removal can reduce disinfection performance.

In wastewater, elevated effluent suspended solids can shield microorganisms from:

• chlorine;
• UV.

Wastewater Disinfection

Wastewater disinfection occurs after biological treatment and solids separation in many treatment systems.

Performance depends strongly on final effluent quality.

Wastewater Chlorine Demand

Wastewater can have significant chlorine demand because of:

• ammonia;
• organic matter;
• reduced compounds;
• suspended solids.

Dechlorination

Where chlorine-based wastewater disinfection is used, a dechlorination step may be used before discharge.

Operators then need to balance:

• adequate upstream disinfection;
• appropriate downstream removal of residual chlorine.

Overfeeding Disinfectant

Applying more disinfectant than necessary can:

• increase chemical cost;
• increase byproduct formation;
• create excessive residual;
• increase dechlorination demand in wastewater.

Underfeeding Disinfectant

Insufficient disinfectant can reduce pathogen inactivation and produce inadequate residual where residual is required.

Disinfection Control Requires Balance

Good operation aims for:

• adequate dose;
• adequate contact;
• appropriate residual;
• stable process conditions;
• minimal unnecessary chemical use.

Monitor Related Process Data

Useful related measurements can include:

• flow;
• pH;
• temperature;
• turbidity;
• TSS;
• ammonia;
• UV transmittance;
• disinfectant residual.

Example: Low Chlorine Residual

Suppose chlorine dose remains constant but residual decreases.

Review:

• flow;
• ammonia;
• organic loading;
• chemical strength;
• feed-pump output;
• sample method.

Example: Adequate Residual but Poor Microbial Results

Possible causes include:

• insufficient effective contact time;
• short-circuiting;
• poor mixing;
• high solids;
• sampling contamination;
• analytical problems.

Example: UV Alarm During High Flow

Possible contributing factors include:

• reduced exposure time;
• lower UV transmittance;
• high suspended solids;
• lamp or sleeve condition;
• instrument problems.

Verify Measurements Before Major Changes

If a disinfection measurement changes unexpectedly, verify:

• sample location;
• instrument calibration;
• reagent condition;
• flow;
• chemical-feed operation;
• related water-quality parameters.

Common Disinfection Mistakes

• Confusing disinfection with sterilization.
• Assuming dose is the same as residual.
• Ignoring disinfectant demand.
• Ignoring contact time.
• Using theoretical tank detention time without considering short-circuiting.
• Ignoring pH and temperature effects.
• Assuming more disinfectant is always better.
• Ignoring turbidity or suspended solids.
• Delaying chlorine-residual testing.
• Assuming UV provides a persistent chemical residual.
• Ignoring UV sleeve fouling or lamp aging.
• Changing feed based on one questionable measurement.

A Practical Chemical-Disinfection Review

1. Confirm process flow.
2. Confirm chemical strength.
3. Confirm feed-pump or feeder output.
4. Calculate or review applied dose.
5. Measure residual at the correct location.
6. Evaluate demand.
7. Review pH and temperature.
8. Review effective contact conditions.
9. Review turbidity or solids.
10. Verify unusual measurements before major adjustments.

A Practical UV Review

1. Review process flow.
2. Review UV intensity.
3. Review UV transmittance where measured.
4. Inspect lamp status.
5. Review quartz-sleeve condition.
6. Review cleaning-system operation.
7. Review turbidity and suspended solids.
8. Check alarms and instrumentation.
9. Verify that the system is operating within its validated operating conditions.

What to Remember for the Exam

• Disinfection reduces disease-causing microorganisms but is not the same as sterilization.
• Disinfection inactivates microorganisms but does not necessarily physically remove them.
• Upstream particle removal improves disinfection performance.
• Different microorganisms have different resistance to different disinfectants.
• Dose is the amount of disinfectant applied.
• Demand is the amount consumed by reactions.
• Residual is the disinfectant remaining after demand has been satisfied.
• A simplified relationship is Demand = Dose - Residual.
• Disinfection performance depends on both disinfectant concentration and contact time.
• A simplified CT relationship is CT = Concentration × Time.
• Actual effective contact time can be less than theoretical detention time because of short-circuiting and poor hydraulics.
• Increasing flow through a fixed-volume contact basin decreases detention time.
• Temperature and pH can strongly affect chemical disinfection performance.
• Free chlorine includes hypochlorous acid and hypochlorite ion.
• Lower pH within normal treatment conditions favors a greater fraction of hypochlorous acid.
• Total chlorine equals free chlorine plus combined chlorine.
• Organic matter, ammonia, metals, and reduced compounds can increase disinfectant demand.
• UV performance depends on intensity, exposure, transmittance, lamp condition, sleeve cleanliness, solids, and hydraulics.
• UV normally does not provide a persistent downstream disinfectant residual.
• Good disinfection control requires adequate dose, effective contact, appropriate residual, reliable measurements, and good upstream treatment.