Benefits and Risks of Using Chlorine for Water Disinfection

Clean water is a very important basic human need. However, raw water often contains various pathogenic microorganisms that can cause disease. Therefore, the disinfection process is a crucial stage in drinking water treatment to ensure its safety for human consumption. One of the most commonly used disinfection methods is chlorination or the use of chlorine. Chlorine has been widely used for more than a century to kill harmful microbes in water. While effective, the use of chlorine also comes with some risks that need to be considered. This article will comprehensively discuss the benefits and risks of using chlorine for water disinfection.

Introduction

Chlorine was first used as a drinking water disinfectant in the early 20th century and has since played a major role in reducing the spread of waterborne diseases. Its effective ability to kill a wide variety of pathogenic microorganisms has made chlorine the first choice in water treatment processes in many countries. In addition, its persistent nature allows chlorine to provide residual protection in water distribution systems.

The chlorination process involves adding chlorine in the form of chlorine gas (Cl2) or hypochlorite solution to water. When added to water, chlorine reacts to form hypochlorous acid (HOCl) which is the primary disinfecting agent. The chemical reaction that occurs is as follows:

Cl₂ + H₂O⇌ HOCl + H⁺ + Cl^-

Hypochlorous acid can then further dissociate into hypochlorite ions (OCl-) depending on the pH of the water:

HOCl ⇌H⁺ + OCl^-

The effectiveness of chlorine as a disinfectant is strongly influenced by the pH of the water. At low pH (<7.5), HOCl which is the strongest disinfectant is more dominant. While at high pH (> 7.5), (OCl^-) which is less effective as a disinfectant becomes more dominant. Therefore, pH regulation becomes an important factor in the chlorination process.

In addition to pH, several factors are important in the chlorination process.

In addition to pH, some other factors that affect the effectiveness of chlorination include:

• Dose of chlorine added
• Contact time between chlorine and water
• Water temperature
• Water turbidity
• The presence of organic and inorganic compounds in water
• Type and number of microorganisms to be inactivated

Understanding these factors is essential to optimize the chlorination process and ensure effective disinfection. Water treatment operators need to consider raw water characteristics and disinfection targets in determining the appropriate chlorine dosage.

Benefits of Using Chlorine for Water Disinfection

The use of chlorine as a water disinfectant has several advantages that make it a top choice in many water treatment facilities:

1. High Effectiveness

Chlorine is effective at killing a wide variety of pathogenic microorganisms including bacteria, viruses, and protozoa. Its ability to inactivate E. coli which is considered more resistant than most other pathogenic bacteria makes chlorine a reliable disinfectant. Although some pathogenic viruses are believed to be more resistant to chlorination than E. coli, chlorine is still capable of inactivating most harmful microorganisms in water at the right dosage.

2. Residual Effects

One of the main advantages of chlorine is its ability to provide residual effects in the water distribution system. After the initial disinfection process at the treatment plant, the residual free chlorine present in the water will continue to protect the water from microbial contamination during its journey through the distribution pipes to the consumer. This is critical to prevent the regrowth of microorganisms and protect water quality up to the point of use.

3. Relatively Low Cost

Compared to other disinfection methods such as ozonation or UV, chlorination has lower operational costs. Chlorine is available in various forms such as chlorine gas, sodium hypochlorite, or calcium hypochlorite at relatively affordable prices. In addition, chlorine dosing equipment is also quite simple and easy to operate.

4. Easy to Apply and Monitor

The chlorination process is relatively easy to apply for both small and large scale systems. Chlorine dosage can be easily adjusted as needed. Residual chlorine measurements can also be made quickly and accurately using simple colorimetric methods, allowing for effective routine monitoring of water quality.

5. Additional Benefits

In addition to being a disinfectant, chlorine also provides several additional benefits in water treatment such as:

• Oxidizes dissolved iron and manganese making them easier to remove
• Removes odors and tastes caused by algae or other organic compounds
• Assists the coagulation and flocculation process
• Prevents algae growth in the treatment unit

With these advantages, it is not surprising that chlorine is still the main choice for water disinfection in many countries. However, the use of chlorine also has some risks that need to be considered.

Risks and Challenges of Using Chlorine

While effective as a disinfectant, the use of chlorine also has some risks and challenges to be aware of:

1. Formation of Disinfection By-Products (DBPs)

One of the main concerns regarding the use of chlorine is the formation of disinfection by-products (DBPs). DBPs are formed when chlorine reacts with natural organic matter (NOM) found in water. Trihalomethanes (THMs) and Haloacetic Acids (HAAs) are the two main groups of DBPs that are of concern due to their potential long-term health effects.

Some epidemiologic studies have shown that chlorine can be harmful to the environment.

Some epidemiologic studies suggest a correlation between long-term exposure to DBPs and an increased risk of bladder and colon cancer, as well as reproductive problems. Although a cause-and-effect relationship has not been fully proven, many countries have set maximum limits on the levels of DBPs in drinking water.

To address this issue, some countries have set maximum limits for DBPs in drinking water.

To address this issue, some strategies that can be implemented include:

• Reducing NOM precursors through optimized coagulation and filtration processes
• Using chlorine dioxide or chloramine as an alternative disinfectant
• Implement advanced treatment processes such as activated carbon adsorption or membrane filtration

The choice of strategy depends on the characteristics of the raw water and the water quality target to be achieved. Calgon's coal-based activated carbon, for example, can be used to reduce precursors of DBPs as well as adsorb DBPs that have already formed.

2. Limitations against Some Pathogens

While effective against most microorganisms, chlorine has limitations in inactivating certain pathogens. Cryptosporidium parvum, a protozoan that can cause severe diarrhea, is highly resistant to chlorine. At normal doses and contact times, chlorine is not effective at killing Cryptosporidium oocysts.

To overcome this, additional treatment processes such as filtration or UV disinfection are required. Ultraviolet disinfection systems can be used as an additional barrier against Cryptosporidium and other chlorine-resistant pathogens.

3. Taste and Odor Issues

The use of chlorine often leads to complaints from consumers regarding the taste and odor of water. Especially at high doses, chlorine can produce an undesirable taste and odor. In addition, the reaction of chlorine with certain organic compounds can also produce unpleasant odors.

To address this issue, some approaches that can be taken include:

• Optimizing chlorine dosage to achieve effective disinfection with minimal residual chlorine
• Applying dechlorination process after sufficient contact time
• Using chloramines that produce a more acceptable taste and odor

4. Safety Risks

The use of chlorine gas has a high safety risk due to its toxic nature. Chlorine gas leakage can endanger workers and the community around the water treatment plant. Therefore, strict safety systems and handling procedures are required when using chlorine gas.

As a safer alternative to chlorine gas, chlorine gas can be used as an alternative.

As a safer alternative, many facilities are turning to sodium hypochlorite or on-site chlorine generation systems. Accurate and reliable dosing pumps are essential to ensure precise and consistent chlorine dosing.

5. Corrosion Potential

Chlorine is corrosive to some types of metals. Long-term use of chlorine can accelerate corrosion of metal pipes and equipment in water distribution systems. This can lead to leaks, water contamination, and increased maintenance costs.

To minimize the impact of corrosion, some steps that can be taken include:

• Controlling water pH to reduce corrosiveness
• Using corrosion inhibitors
• Choosing corrosion-resistant materials for distribution system components

Corrosion-resistant automatic valves can be an option for water treatment systems that use chlorine.

Optimizing the Chlorination Process

To maximize the benefits and minimize the risks of using chlorine, chlorination process optimization is required. Some important aspects that need to be considered include:

1. Optimal Dosage Determination

The right chlorine dosage is essential to ensure effective disinfection while minimizing the formation of DBPs. The concept of breakpoint chlorination can be used to determine the optimal dose. At the breakpoint point, all easily oxidizable compounds have reacted and free chlorine begins to form.

Generally, the chlorine dosage is set to produce residual free chlorine of about 0.5-1.0 mg/L after 30 minutes of contact time. However, the specific dosage depends on the water characteristics and the disinfection target. The use of a analyzer of pH and conductivity can help monitor key parameters that affect the effectiveness of chlorination.

2. pH regulation

The pH of water greatly affects the effectiveness of chlorine. At low pH, HOCl which is the most powerful disinfectant is more dominant. Conversely, at high pH, the less effective OCl^- becomes more dominant. Generally, the optimal pH for chlorination is between 6.5-7.5.

If pH adjustment is required, media such as Calcite and Corosex can be used to increase the pH of water that is too low.

3. Sufficient Contact Time

Sufficient contact time is required for chlorine to effectively react with the target microorganisms. Generally, a minimum contact time of 30 minutes is required before water is distributed to consumers. The use of baffled contact tanks or plug-flow pipes can help ensure adequate contact time.

4. Routine Monitoring

Routine monitoring of residual chlorine, pH, and other water quality parameters is essential to ensure the effectiveness of the chlorination process. Residual chlorine measurement can be done by simple colorimetric methods or using an online analyzer for continuous monitoring.

5. Control of DBPs Precursors

Reducing DBPs precursors such as NOM before the chlorination process can help minimize the formation of DBPs. Some methods that can be applied include:

• Optimization of coagulation-flocculation process
• Use of advanced oxidation processes such as ozonation
• Application of membrane technology such as ultrafiltration membranes

Alternatives and Enabling Technologies

While chlorine is still the primary choice for water disinfection, several alternatives and enabling technologies can be considered to overcome chlorine's limitations:

1. Chloramine

Chloramine is formed from the reaction between chlorine and ammonia. Compared to free chlorine, chloramine is more stable and produces fewer DBPs. Chloramine also produces a more acceptable taste and odor. However, chloramine is a weaker disinfectant and thus requires a longer contact time.

2. Chlorine Dioxide

Chlorine dioxide is a strong disinfectant and does not form THMs. Chlorine dioxide is effective over a wider pH range than chlorine. However, its use is more complex and expensive than regular chlorine.

3. Ozonation

Ozone is a strong oxidant that is effective in killing various microorganisms including Cryptosporidium. Ozone can also help reduce the taste, odor, and color of water. However, ozone does not provide a residual effect so it needs to be combined with a secondary disinfectant such as chlorine.

4. UV Disinfection

UV radiation effectively inactivates various microorganisms including Cryptosporidium and Giardia. UV disinfection does not produce DBPs and does not affect the taste of water. However, as with ozone, UV does not provide residual effects. UV systems can be used as an additional barrier in multi-barrier water treatment.

5. Membrane Technology

Membrane technologies such as ultrafiltration and reverse osmosis can remove a variety of contaminants including microorganisms. Membranes can reduce the need for chemical disinfectants, but still require secondary disinfection to provide a residual effect. Reverse osmosis membranes from DuPont Filmtec for example, can produce very high quality water.

Conclusion

The use of chlorine for water disinfection has proven effective in controlling waterborne diseases for more than a century. Its strong ability to kill a wide range of pathogenic microorganisms and the residual effects it provides make chlorine the first choice in many water treatment facilities. However, several challenges such as the formation of DBPs and limitations against certain pathogens need to be addressed.

Optimization of the chlorination process through proper dosing, pH regulation, and regular monitoring is essential to maximize the benefits while minimizing the risks of chlorine use. Combination with other technologies such as UV disinfection or filtration membranes can provide more comprehensive multi-barrier protection.

Choosing the right disinfection method is important to maximize the benefits while minimizing the risks of chlorine use.

The choice of an appropriate disinfection method should consider various factors including raw water characteristics, water quality targets, available infrastructure, and technical and economic aspects. In many cases, the use of optimized chlorine is still an effective and economical option to ensure the safety of drinking water.

With a good understanding of the benefits of optimized chlorine disinfection, the choice of disinfection method should be made.

With a good understanding of the benefits and risks of chlorine use, and the implementation of best practices in the chlorination process, we can continue to utilize chlorine as a primary disinfectant while minimizing its negative impacts. While innovations in water treatment technologies continue to evolve, chlorine is predicted to continue to play an important role in ensuring the safety of drinking water in the future.

Questions and Answers

1. Why is chlorine still the top choice for water disinfection despite its risks?

Chlorine remains the top choice for water disinfection for several reasons:

• High effectiveness in killing various types of pathogenic microorganisms
• Ability to provide residual effects in the distribution system
• Relatively low cost compared to other disinfection methods
• Easy to apply and monitor
• A well-established technology with a long track record

Although it has some risks such as the formation of DBPs, these risks can be minimized through process optimization and the application of supporting technologies. The benefits of chlorine in ensuring drinking water safety are still considered to outweigh the risks.

2. How to solve the problem of DBPs formation in the chlorination process?

Some strategies that can be applied to overcome the problem of DBPs formation include:

• Reducing NOM precursors through optimized coagulation and filtration processes
• Using advanced oxidation processes such as ozonation to decompose DBPs precursors
• Apply membrane technology such as ultrafiltration or nanofiltration to remove NOM
• Optimizing chlorine dosage and pH to minimize DBPs formation
• Use alternative disinfectants such as chloramines or chlorine dioxide that produce less DBPs
• Implement advanced treatment processes such as activated carbon adsorption to remove DBPs that have formed

A combination of several methods can provide optimal results in controlling the formation of DBPs without sacrificing the effectiveness of disinfection.

3. Can membrane technologies such as reverse osmosis replace the role of chlorine in water disinfection?

Membrane technologies such as reverse osmosis are indeed very effective in removing various contaminants including microorganisms. However, membranes cannot completely replace the role of chlorine in water disinfection for several reasons:

• Membranes do not provide residual effects in the distribution system
• The investment and operational costs of membranes are still relatively high for large-scale applications
• Membranes require good pretreatment to prevent fouling
• There is a risk of microorganism regrowth after the membrane process

Therefore, membrane technology is more appropriately positioned as an additional barrier in multi-barrier water treatment systems. The combination of membranes with lower chlorine doses can provide more comprehensive protection while minimizing the formation of DBPs.

References

1. Binnie, C., & Kimber, M. (2013). Basic water treatment (5th ed.). ICE Publishing. p. 202-207.

2. Spellman, F. R. (2013). Handbook of water and wastewater treatment plant operations (3rd ed.). CRC Press. p. 673.

3. Fair, G. M., Geyer, J. C., & Okun, D. A. (1971). Elements of water supply and wastewater disposal (2nd ed.). John Wiley & Sons.