Chlorine (Cl)

1. Basic Information

Chlorine is a halogen element discovered by Carl Wilhelm Scheele in 1774. In its pure form, chlorine is a yellowish-green diatomic gas with a sharp odor.

2. Physical and Chemical Properties

Chlorine has a melting point of -101°C and a boiling point of -34.6°C. Chlorine gas is 2.5 times heavier than air. Chlorine is highly reactive and reacts easily with almost all other elements. In aqueous solution, chlorine forms hypochlorous acid (HClO) which is a strong oxidizer.

Chlorine dissolves easily in water, forming a solution called "chlorine water". At high concentrations, this solution is acidic and highly oxidative. Chlorine can also form various inorganic compounds such as chloride salts and chlorinated organic compounds.

3. Presence in Water and Health Effects

Chlorine is rarely found in free form in nature. In water, chlorine is usually present in the form of chloride ions (Cl-) or dissolved chlorine compounds such as sodium chloride (table salt). Seawater contains about 1.9% chloride ions.

Exposure to small amounts of chlorine can cause irritation to the eyes, nose and throat. Higher exposure can cause coughing, shortness of breath and even fluid retention in the lungs. However, these health effects generally do not occur at chlorine levels commonly found in treated drinking water.

4. Water Treatment Applications and Removal Methods

Chlorine is the most commonly used disinfectant in drinking water and wastewater treatment. Some of the main applications include:

• Disinfection of drinking water to remove pathogens
• Oxidation of iron and manganese in groundwater
• Ammonia removal through breakpoint chlorination
• Control of algae and biofilm growth in distribution systems

To remove excess chlorine from water, some methods that can be used include:

• Aeration or stripping
• Adsorption with activated carbon
• Chemical reduction using sodium sulfite or hydrogen peroxide
• Membrane filtration such as reverse osmosis

To remove chloride ions, ion exchange methods using strong base anion exchange resins can be used. However, due to the low selectivity of chloride, other ions with higher selectivity must be removed first.

5. Industrial Use in Water Treatment

In addition to disinfection, chlorine is also used in various industrial water treatment applications, including:

• Bleaching of paper pulp and textiles
• Synthesis of chlorinated organic chemicals
• Cooling water treatment in power plants
• Sanitization in the food and beverage processing industry

6. Case Studies and Real-World Applications

Examples of application of chlorine technology in water treatment:

1. New York City, USA: Uses a large-scale chlorination system to treat more than 3.8 billion liters of drinking water per day, serving more than 9 million residents.

2. Singapore: Implemented a chloramination system (combination of chlorine and ammonia) to maintain water quality in an extensive distribution network, reducing the formation of disinfection by-products.

3. Perth, Australia: Using chlorine dioxide as an alternative to chlorine to solve taste and odor problems, and reduce the formation of trihalomethanes.

7. Regulatory Guidelines and Standards

The World Health Organization (WHO) recommends a maximum limit of free chlorine in drinking water of 5 mg/L. In Indonesia, based on Permenkes No. 492/Menkes/Per/IV/2010, the maximum level of free chlorine allowed in drinking water is 5 mg/L.

For disinfection by-products, the WHO recommended maximum limit of total trihalomethanes is 0.1 mg/L. Some countries have stricter standards, for example the European Union sets a limit of 0.1 mg/L for each trihalomethane compound.

8. Environmental Impact and Sustainability Considerations

While effective as a disinfectant, the use of chlorine in water treatment has some environmental impacts that need to be considered:

• Formation of disinfection by-products such as trihalomethanes and haloacetic acid, which are potentially carcinogenic
• Toxicity to aquatic organisms if chlorinated water is discharged directly into water bodies
• Potential formation of persistent organochlorine compounds in the environment

To improve sustainability, some approaches that can be applied include:

• Optimization of chlorim doses to reduce the formation of by-products
• Use of alternative disinfection technologies such as UV or ozone
• Implementation of chloramination systems to minimize the formation of trihalomethanes
• Treatment of chlorine-containing wastewater before discharge into the environment

9. Future Trends and Research

Some research and development directions related to the use of chlorine in water treatment include:

• Development of real-time sensors for free and bound chlorine monitoring
• Studies on the mechanism of formation and control of new disinfection by-products
• Integration of chlorination technology with membrane-based water treatment systems
• Development of hybrid disinfection methods that combine chlorine with other technologies such as UV or ultrasonic
• Research on the impact of long-term chronic exposure to chlorination by-products in low concentrations

10. Fun Facts Related to Water Treatment

• Chlorine was the first widely used water disinfectant, starting in the early 20th century
• The use of chlorine in drinking water treatment has reduced the incidence of typhoid disease in the US by 99% between 1900 and 1960
• The distinctive "swimming pool" odor is actually not caused by chlorine, but rather by chloramine compounds formed when chlorine reacts with organic contaminants
• Some microorganisms, such as Cryptosporidium, are relatively resistant to chlorination and require additional treatment methods
• Astronauts on the International Space Station use a water treatment system that incorporates iodine and silver ions as an alternative to chlorine for disinfection