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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water and Wastewater Treatment Processes › Lesson 6
AGR-WATE71-03 · Lesson 6 · 14.5 clock hours · lab

Disinfection: Chlorination, UV, and Alternative Methods

Objective. Students will calculate chlorine dose, demand, and residual for a water sample, explain CT concept for pathogen inactivation, and demonstrate safe operation of a chlorine feed system including cylinder handling and leak detection.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Disinfection: Chlorination, UV, and Alternative Methods

Disinfection is the treatment step that inactivates or removes pathogens—bacteria, viruses, and protozoa—from drinking water and treated wastewater. It is one of the most critical responsibilities of a licensed operator. Without effective disinfection, waterborne illnesses can spread rapidly through a community. State and federal regulations require operators to meet specific inactivation targets and to maintain a measurable disinfectant residual throughout the distribution system.

Why Disinfection Matters

The goal of disinfection is not to sterilize water (remove all microorganisms) but to reduce pathogens to levels that pose no significant health risk. The Safe Drinking Water Act and state primacy rules require drinking water systems to achieve specified log-inactivation credits for organisms such as Giardia lamblia, Cryptosporidium, and viruses. A 1-log reduction means 90% of organisms are inactivated; 2-log means 99%; 3-log means 99.9%.

Chlorination

Chlorine is the most widely used disinfectant in water and wastewater treatment. It can be applied as:

When chlorine is added to water, it reacts with naturally occurring organic matter, ammonia, and other substances before any disinfection occurs. The amount of chlorine consumed by these reactions is called the chlorine demand. The chlorine remaining after demand is satisfied is the chlorine residual—the portion that actually protects the water.

Applied dose = Chlorine demand + Chlorine residual

Operators must measure demand, adjust the applied dose, and confirm that an adequate residual is maintained at the point of application and throughout the distribution system.

The CT Concept

Effective disinfection depends not just on how much disinfectant is present, but on how long water is in contact with it. The CT value expresses this relationship:

CT = C × T

where C is the disinfectant residual in mg/L at the outlet of the contact chamber, and T is the contact time in minutes (specifically T₁₀, the time it takes 10% of the water to travel through the contact zone). Published CT tables—available in state study guides and EPA guidance documents—list the CT values required to achieve specific log-inactivation credits for target pathogens at given pH and temperature conditions. Lower water temperature and higher pH generally require a higher CT value to achieve the same inactivation.

Operators compare the CT they are delivering to the CT required in the applicable table. If the delivered CT meets or exceeds the required CT, the inactivation credit is achieved.

UV Disinfection

Ultraviolet (UV) light damages pathogen DNA so organisms cannot reproduce. UV is particularly effective against Cryptosporidium and Giardia, which are resistant to chlorine at normal doses. UV systems do not add a chemical residual to the water, so they are often used in combination with a chlorine residual for distribution system protection. UV dose is expressed in mJ/cm² and must be validated to receive regulatory credit.

Alternative Methods

Ozone (O₃) is a powerful oxidant and disinfectant used at some larger facilities. It provides excellent virus and protozoa inactivation but leaves no residual and must be generated on-site. Chloramines (formed by combining chlorine and ammonia) are used as a secondary disinfectant to maintain a longer-lasting residual in distribution systems and to reduce disinfection byproduct formation. Chlorine dioxide is another option for systems dealing with taste, odor, or specific pathogens.

Safe Chlorine Cylinder Handling

Chlorine gas is a severe health hazard. Operators must follow all applicable safety regulations, including OSHA requirements, when working with chlorine cylinders.

PPE required: Chemical splash goggles, face shield, chemical-resistant gloves, chemical-resistant apron, and respiratory protection appropriate to the task. Self-contained breathing apparatus (SCBA) must be available and operators must be trained in its use before working with gas cylinders.

Cylinder change-out procedure:

1. Don all required PPE before approaching the cylinder.

2. Inspect the new cylinder for damage, proper labeling, and valve cap.

3. Confirm the correct pressure-reducing regulator is used for that cylinder type.

4. Connect the regulator per manufacturer instructions; do not use pipe dope or tape on chlorine connections—use only the correct gaskets.

5. Slowly open the cylinder valve and apply an approved leak-detection solution (such as ammonia vapor or commercial leak-detection solution) to all connections. Bubbling indicates a leak—close the valve immediately and correct the problem before proceeding.

6. Record the tare weight and gross weight of the cylinder on the chemical feed log to track usage rate.

7. Apply lockout/tagout (LOTO) procedures whenever performing maintenance on chlorine feed equipment that is not part of normal operational adjustments.

Emergency: If a leak is detected and cannot be quickly stopped, evacuate the area, activate your facility's emergency response plan, and contact the appropriate emergency response team. Never attempt to plug a large chlorine leak without proper SCBA and training.

Measuring Chlorine Residual

The DPD (N,N-diethyl-p-phenylenediamine) colorimetric method is the standard field method for measuring free and total chlorine residual. A DPD reagent tablet or powder is added to a water sample; the resulting pink color is compared to a color wheel or read by a spectrophotometer. Free chlorine is measured first; total chlorine is measured with a second reagent. The difference between total and free chlorine is the combined (chloramine) residual.

Documentation

Accurate record-keeping is a regulatory requirement. Operators must log disinfectant doses, residuals, CT values, and cylinder weights on approved forms. These records are reviewed during state inspections and are essential for demonstrating compliance.

Key terms9

Chlorine demandThe amount of chlorine consumed by reactions with organic matter, ammonia, and other substances in a water sample before a residual is established; equal to the applied dose minus the measured residual.
Chlorine residualThe concentration of chlorine remaining in water after demand has been satisfied; the portion available to inactivate pathogens and protect the distribution system.
CT valueThe product of disinfectant residual concentration (C, in mg/L) and contact time (T, in minutes); used to quantify disinfection effectiveness and compare delivered disinfection to regulatory inactivation requirements.
Log inactivationA measure of pathogen reduction expressed as powers of ten; 1-log = 90% reduction, 2-log = 99% reduction, 3-log = 99.9% reduction.
DPD methodA colorimetric analytical method using N,N-diethyl-p-phenylenediamine reagent to measure free and total chlorine residual in a water sample; the standard field and laboratory test for chlorine.
T₁₀The time it takes 10% of the water to pass through a contact chamber under actual flow conditions; used as the contact time (T) in CT calculations to account for hydraulic short-circuiting.
Applied doseThe total amount of disinfectant added to the water; equal to chlorine demand plus the desired residual.
Lockout/tagout (LOTO)An OSHA-required safety procedure that isolates energy sources (electrical, chemical, mechanical) and physically locks equipment in a safe state before maintenance, preventing accidental startup or exposure.
UV disinfectionA physical disinfection process that uses ultraviolet light to damage pathogen DNA, preventing reproduction; effective against chlorine-resistant protozoa but does not leave a chemical residual.

Worked examples4

1. A water sample requires a chlorine dose of 3.2 mg/L to be applied. After 30 minutes of contact, the measured chlorine residual is 0.8 mg/L. What is the chlorine demand of this sample?
  1. Write the relationship: Applied dose = Chlorine demand + Chlorine residual
  2. Rearrange: Chlorine demand = Applied dose − Chlorine residual
  3. Substitute values: Chlorine demand = 3.2 mg/L − 0.8 mg/L
  4. Calculate: Chlorine demand = 2.4 mg/L
Answer: Chlorine demand = 2.4 mg/L
2. An operator wants to maintain a chlorine residual of 1.5 mg/L after treatment. The chlorine demand of the water is 4.6 mg/L. What applied dose (mg/L) must the operator feed?
  1. Write the relationship: Applied dose = Chlorine demand + Chlorine residual
  2. Substitute values: Applied dose = 4.6 mg/L + 1.5 mg/L
  3. Calculate: Applied dose = 6.1 mg/L
Answer: Applied dose = 6.1 mg/L
3. A contact chamber has a T₁₀ of 25 minutes. The chlorine residual measured at the outlet of the chamber is 1.2 mg/L. The CT table requires a CT of 27 mg·min/L for 3-log Giardia inactivation at the current temperature and pH. Does this scenario achieve the required inactivation credit?
  1. Calculate delivered CT: CT = C × T = 1.2 mg/L × 25 min
  2. CT delivered = 30 mg·min/L
  3. Compare to required CT: 30 mg·min/L (delivered) vs. 27 mg·min/L (required)
  4. Since 30 ≥ 27, the delivered CT meets the requirement.
Answer: Yes. The delivered CT of 30 mg·min/L exceeds the required CT of 27 mg·min/L, so 3-log Giardia inactivation credit is achieved.
4. A second scenario has a T₁₀ of 18 minutes and a measured chlorine residual at the contact chamber outlet of 1.4 mg/L. The required CT for the target inactivation credit at current conditions is 27 mg·min/L. Does this scenario achieve the required credit?
  1. Calculate delivered CT: CT = C × T = 1.4 mg/L × 18 min
  2. CT delivered = 25.2 mg·min/L
  3. Compare to required CT: 25.2 mg·min/L (delivered) vs. 27 mg·min/L (required)
  4. Since 25.2 < 27, the delivered CT does not meet the requirement.
Answer: No. The delivered CT of 25.2 mg·min/L is less than the required CT of 27 mg·min/L. The operator must increase residual, increase contact time, or both.

The math in this lesson2

Learners compute CT as residual concentration times contact time for disinfection credit.
A-SSE.1A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Perform a chlorine demand test on two water samples using a colorimetric method (DPD or equivalent); calculate chlorine demand, applied dose, and resulting residual for each sample and record results on a lab data sheet. · Use the CT concept (concentration × time = CT value) and a published CT table to determine whether a given disinfection scenario achieves the required log inactivation credit for Giardia; practice three calculation problems with varied flow, contact time, and residual. · Participate in a chlorine cylinder change-out demonstration: inspect the cylinder, confirm the correct pressure-reducing regulator, apply the leak-detection solution to connections, and document the cylinder weight change on a chemical feed log—all steps performed with appropriate PPE and under instructor supervision.
Materials: DPD colorimetric test kit or spectrophotometer, water samples, sodium hypochlorite solution, CT value reference table (from state study guide or EPA guidance), chemical feed log form, chlorine cylinder (water-filled training prop) or demonstration video, leak-detection solution, PPE (goggles, gloves, apron).
Check: Chlorine demand calculations correct to within ±0.1 mg/L on at least two of three problems; CT calculation correct on at least two of three problems; cylinder change-out demonstration completed with zero safety errors on the instructor checklist.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Treatment – Disinfection
1. A water sample has an applied chlorine dose of 5.0 mg/L. After contact, the measured residual is 1.2 mg/L. What is the chlorine demand?
Water Treatment – Disinfection / Regulatory Compliance
2. The CT concept in disinfection calculations combines which two variables?
Water Treatment – Disinfection
3. An operator measures a chlorine residual of 2.0 mg/L at the outlet of a contact chamber with a T₁₀ of 15 minutes. What is the delivered CT value?
Water Treatment – Disinfection Methods
4. Which of the following best describes why UV disinfection is often combined with chlorination in drinking water systems?
Safety – Hazardous Chemical Handling
5. Before beginning a chlorine gas cylinder change-out, what is the correct first action?
Water Treatment – Microbiology / Regulatory Requirements
6. A 3-log inactivation credit means what percentage of a target pathogen population has been inactivated?
Water Treatment – Laboratory Procedures
7. When using the DPD colorimetric method, what does the color change in the sample indicate?
Water Treatment – Disinfection / Regulatory Compliance
8. A delivered CT of 22 mg·min/L is calculated for a contact chamber. The applicable CT table requires 27 mg·min/L for the required inactivation credit at current temperature and pH. What should the operator do?
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