Chick's Law and Its Application in Disinfection
Initial and Remaining Organisms:
o N0: Initial microorganisms count.
o N: Microorganisms remaining after time t.
Disinfection Rate Constant:
k: Dieoff constant, depends on disinfectant concentration and other factor
Factors Affecting Disinfection:
Concentration of Disinfectant: Higher concentration improves disinfection.
Contact Time: Longer time increases effectiveness.
Practical Applications:
1. Microorganism Removal Efficiency: Disinfection efficiency is a function of disinfectant concentration and
contact time.
2. Disinfection Strategies: Helps design effective disinfection protocols.
3. Regulatory Compliance: Ensures effluent meets health standards for safe discharge.
Chlorine Disinfection Process in Wastewater Treatment
Purpose: Kills or inactivates harmful microorganisms (bacteria, viruses, protozoa) in wastewater.
Common Forms of Chlorine:
Gaseous Chlorine (Cl₂): Powerful but toxic, requires careful handling.
Sodium Hypochlorite (NaOCl): Stable liquid form.
Calcium Hypochlorite (Ca(OCl)₂): Solid form used for disinfection.
Chlorine Dioxide (ClO₂): Effective alternative with fewer harmful byproducts.
Process:
1. Introduction of Chlorine: Added to the wastewater in contact basins, after secondary treatment.
2. Chemical Reactions: Chlorine forms hypochlorous acid (HOCl) and chloramines by reacting with water and
organic matter.
3. Contact Time: Time spent in contact basins to ensure chlorine reacts with pathogens.
4. Breakpoint Chlorination: The point where enough chlorine is added to meet demand and provide
residual chlorine.
5. Dechlorination: Chemicals like sodium bisulfite are used to neutralize residual chlorine before discharge.
Advantages:
1. Highly effective (9899% pathogen reduction).
2. Economical and widely available.
3. Controls odors, reduces phenols, stabilizes sludge, and mitigates foaming.
Considerations:
Disinfection ByProducts (DBPs): Formation of harmful compounds like trihalomethanes (THMs) with high
chlorine concentrations.
Influencing Factors: pH, temperature, TSS, BOD, and ammonia levels affect disinfection efficiency.
De-chlorination Process in Water Treatment
Dechlorination refers to the removal or neutralization of chlorine from water, crucial for ensuring water is safe
for discharge into the environment or for specific applications like aquatic habitats.
Importance of Dechlorination
1. Environmental Protection: Prevents harm to aquatic life by removing chlorine before discharge.
2. Water Quality Improvement: Reduces chlorine-induced taste and odor issues.
3. Infrastructure Protection: Prevents damage to downstream treatment processes and equipment.
Common Methods of Dechlorination
Chemical De-chlorination:
Uses chemicals (e.g., Sodium Bisulfite, Sodium Metabisulfite, Sulfur Dioxide) to neutralize chlorine.
Advantages: Quick, effective, and controllable.
Activated Carbon Filtration:
Water passes through activated carbon filters, removing chlorine and organic compounds.
Advantages: Improves overall water quality.
Ultraviolet (UV) Dechlorination:
UV light breaks down chlorine molecules.
Advantages: No chemical residues, effective against both chlorine and chloramines.
Reverse Osmosis (RO):
Water is filtered through a semi-permeable membrane to remove chlorine and other contaminants.
Advantages: High-quality water.
Aeration:
Chlorine dissipates naturally when exposed to air.
Advantages: Simple and cost-effective for small volumes.
Boiling:
Heating water causes chlorine to evaporate.
Advantages: Practical for small-scale use.
Natural Dissipation:
Chlorine naturally evaporates when water sits exposed to air for 24-48 hours.
Considerations
1) Regulatory Compliance: Must adhere to standards for allowable residual chlorine.
2) Monitoring: Continuous monitoring ensures effective dechlorination.
3) Chemical Safety: Proper handling of chemicals is required for safety.
Ozone Disinfection Process
Mechanism:
1. Oxidation Process: Ozone penetrates microbial cells, oxidizing proteins, enzymes, DNA, and RNA, leading
to cell breakdown and pathogen death.
2. Rapid Reaction: Ozone acts quickly, often requiring less than 2 minutes of contact time for effective
disinfection.
3. CT Values: Measured as concentration × time; e.g., a CT value of 2 mg·min/L can kill 99% of bacteria and
viruses.
Advantages of Ozone Disinfection:
1. Higher Efficacy: Up to 50 times more effective than chlorine.
2. No Residual Chemicals: Leaves no harmful residuals in treated water.
3. Broad Spectrum: Effective against bacteria, viruses, and protozoa (e.g., Giardia and Cryptosporidium).
4. Reduction of Taste and Odor: Eliminates unpleasant tastes and odors caused by organic compounds.
Process Implementation:
1. Ozone Generation: Produced onsite using corona discharge or UV light to convert O₂ into O₃.
2. Dissolution in Water: Ozone is introduced through venturi injectors or bubble diffusers.
3. Contact Time: Water is held in a contact chamber to allow ozone to react with pathogens.
4. PostTreatment Filtration: Remaining ozone is removed via aeration or activated carbon filtration.
5. Monitoring and Control: Continuous monitoring of ozone concentration ensures effective disinfection.
Considerations:
1. Environmental Factors: pH, temperature, and organic matter influence ozone's effectiveness.
2. Safety Precautions: Ozone can be harmful at high concentrations, so proper handling and safety measures
are required.
Methods of Disinfection in Water Treatment
1. Chlorination
Description: Adding chlorine or chlorine compounds to water.
Mechanism: Chlorine forms hypochlorous acid (HOCl) that kills microorganisms.
Advantages:
Effective against a broad range of pathogens.
Provides residual disinfection during distribution.
Low cost and easy to implement.
Disadvantages:
May form harmful byproducts (e.g., trihalomethanes).
Some microorganisms (e.g., Cryptosporidium) are chlorineresistant.
2. Ultraviolet (UV) Radiation
Description: Uses UV light to inactivate microorganisms.
Mechanism: UV light damages DNA/RNA, preventing reproduction.
Advantages:
No chemical residues or taste/odor changes.
Effective against chlorine resistant organisms.
Fast treatment process.
Disadvantages:
No residual disinfection; re contamination risk.
Requires clear water for effective disinfection.
3. Ozonation
Description: Uses ozone (O₃) as an oxidizing agent for disinfection.
Mechanism: Ozone disrupts microorganism cell walls and metabolic processes.
Advantages:
Highly effective against many pathogens.
No harmful residuals; ozone decomposes to oxygen.
Rapid action with shorter contact times.
Disadvantages:
Ozone is unstable and requires onsite generation.
No residual disinfection.
4. Chlorine Dioxide
Description: A chemical disinfectant effective over a wide pH range.
Mechanism: Acts as a strong oxidizing agent to kill microorganisms.
Advantages:
Fewer taste/odor issues compared to chlorine.
Effective against various pathogens, including Cryptosporidium.
Provides some residual disinfection.
Disadvantages:
Requires careful handling.
Less widely available.
5. Heat (Boiling)
Description: Uses heat to kill pathogens by thermal inactivation.
Mechanism: High temperatures denature proteins and disrupt cell functions.
Advantages:
Simple, effective, and chemical free.
Kills all types of pathogens.
Disadvantages:
Energy intensive and impractical for large water volumes.
No residual protection.
6. Filtration
Description: Physically removes pathogens using filters.
Mechanism: Filters trap particles, sediments, and some microorganisms.
Advantages:
Removes particles, sediments, and some pathogens.
Improves taste and odor by removing organic materials.
Disadvantages:
May not remove all pathogens unless combined with other methods.
Explain Break-point Chlorination with the help of graph:
Definition: The point at which enough chlorine has been added to water to satisfy all chlorine demands,
resulting in free available chlorine (FAC) for disinfection.
Free chlorine: Combination of Cl₂, HOCl, and OCl⁻.
Combined chlorine: Mostly chloramines (monochloramine, dichloramine, trichloramine, and organic
chloramines) formed when ammonia reacts with free chlorine.
Total chlorine: Sum of free and combined chlorine.
Chlorine Residuals vs. Added Chlorine Dose (Graphic Summary)
Zone I:
Free chlorine reacts with transition metals (e.g., iron, manganese).
Total chlorine residual increases minimally.
Zone II:
Free chlorine reacts with ammonia to form monochloramine.
Total chlorine residual increases as chloramines form.
Zone III:
Total chlorine residual drops as free chlorine oxidizes monochloramine to dichloramine and
trichloramine.
Trichloramine evaporates away.
Chlorine dose overcomes oxidant demand (breakpoint).
Zone IV:
Chlorine residual increases linearly after the breakpoint.
Free chlorine residual is present for disinfection.
Factors Affecting the Curve:
1. Temperature
2. pH
3. Organic species present
Breakpoint Chlorination:
Definition: The point where enough chlorine is added to satisfy all chlorine demands, allowing free available
chlorine (FAC) to accumulate and effectively disinfect water.
Key Concepts:
1.Chlorine Demand:
Chlorine reacts with contaminants like ammonia and organic matter, consuming chlorine before it can
disinfect.
2.Combined Chlorine:
Formed when chlorine reacts with ammonia or nitrogen compounds. It is less effective as a disinfectant.
3.Achieving Break point:
After adding sufficient chlorine (1015 times the ammonia concentration), ammonia and nitrogen
compounds are oxidized, and free chlorine starts to form.
4.Free Available Chlorine (FAC):
The active disinfectant formed after break point chlorination, available for effective disinfection.
Process Overview:
Chlorine Addition:
Gradually add chlorine until initial chlorine demand is met.
Monitoring:
Measure total chlorine and free available chlorine to detect when break point is reached.
Post Breakpoint:
Maintain an adequate level of FAC for disinfection and avoid excess combined chlorine.
Residual Management:
Ensure a proper free chlorine residual for ongoing disinfection.
Advantages of Break point Chlorination:
1. Effective Disinfection: Ensures pathogens are killed by creating a measurable FAC residual.
2. Control of Combined Chlorine: Reduces undesirable chloramines.
3. Improved Water Quality: Ensures contaminants are oxidized before water reaches distribution systems.
Disinfection:
“The process of removing, deactivating, or killing pathogenic microorganisms in water or on surfaces to ensure
safety for human health and the environment. It is essential in water treatment to prevent waterborne
diseases.”
Ideal Characteristics of Disinfection:
Effectiveness against Pathogens:
Should kill or inactivate a wide range of microorganisms (bacteria, viruses, protozoa, fungi).
Rapid Action:
Quick enough to minimize contact time while ensuring complete pathogen elimination.
Residual Activity:
Should leave a residual effect in water to prevent re contamination during storage and
distribution.
Safety:
Must be safe for human health and the environment with minimal toxic byproducts.
Compatibility with Water Quality:
Effective across varying water conditions (pH, turbidity, organic matter).
Cost Effectiveness:
Economically viable, considering both operational and disinfectant costs.
Ease of Use:
Simple to implement and maintain in various settings (large-scale plants to small scale emergency use).
Minimal Impact on Taste and Odor:
Should not affect the taste or odor of the treated water.
Regulatory Compliance:
Must meet local and international health regulations and standards.
Characteristics of Chemical Disinfection
Effectiveness: Kills a wide range of pathogens (bacteria, viruses, etc.).
Speed: Works quickly to disinfect.
Residual Effect: Leaves a lasting disinfectant in the water (e.g., chlorine).
Safety: Safe for humans and the environment, with minimal harmful byproducts.
Compatibility: Effective in various water qualities (pH, turbidity).
Cost Effective: Affordable for large scale use.
Ease of Handling: Simple to store and apply.
Taste and Odor: Does not negatively affect water taste or smell.
Regulatory Compliance: Meets health and safety standards.
By Products: Minimizes harmful byproducts.
Post-Chlorination in Water Treatment:
Purpose:
1. Kills remaining microorganisms in treated water.
2. Ensures a residual chlorine level to protect against re-contamination during distribution.
Process:
Chlorine added after primary and secondary treatments.
Water held in contact tank for 20-30 minutes for proper disinfection.
Chlorine Dosage:
Aimed at achieving 0.1 to 0.2 mg/L residual chlorine.
Avoid levels above 2.0 mg/L to prevent infrastructure damage.
Monitoring:
Continuous monitoring of residual chlorine to meet safety standards.
Adjustments made based on incoming water quality.
Benefits:
Reduces waterborne diseases.
Maintains water quality during transport and storage.
Considerations:
Can produce disinfection by-products (DBPs) when reacting with organic matter.
Managing organic content in source water is key.
