Chlorine: A Comprehensive Guide to its Role in Water Treatment

Chlorine is one of the most widely utilized disinfectants in water treatment due to its broad-spectrum efficacy against most microorganisms and its cost-effectiveness. This guide details its discovery, properties, production, and critical applications in disinfection.

Discovery of Chlorine

Chlorine gas (Cl₂) was first prepared in its pure form by the Swedish chemist Carl Wilhelm Scheele in 1774. He achieved this by heating manganese dioxide (MnO₂) with hydrochloric acid (HCl). The reaction is represented as:

MnO₂ + 4HCl -> MnCl₂ + Cl₂ + 2H₂O

Scheele observed that chlorine gas was water-soluble and possessed bleaching capabilities for various materials. In 1810, Sir Humphry Davy, an English chemist, confirmed chlorine as an element and named it after the Greek word 'chloros', meaning yellow-greenish, describing its characteristic color.

Properties of Chlorine

Chlorine (Cl₂) is a highly reactive element belonging to the halogen group, which also includes fluorine (F), bromine (Br), iodine (I), and astatine (At). Halogens readily react with other elements, particularly metals, to form soluble salts.

A chlorine atom contains 17 electrons. Its outermost shell has seven electrons, leaving space for one additional electron. This characteristic leads to the formation of either free, charged ions or covalent bonds, completing its outer electron shell.

Chlorine can form both highly stable compounds, such as sodium chloride (NaCl, common salt), and highly reactive ones, like hydrogen chloride (HCl). When hydrogen chloride dissolves in water, it forms hydrochloric acid, which is known for its strong corrosive properties, even capable of corroding stainless steel under concentrated conditions.

Transport and Storage

Given its reactive and corrosive nature, strict safety precautions are essential during the transport, storage, and use of chlorine.

Aqueous chlorine solutions must be protected from sunlight. Ultraviolet (UV) radiation from sunlight accelerates the breakdown of hypochlorous acid (HOCl) molecules. This process involves the breakdown of water molecules, releasing electrons that reduce the chlorine atom in hypochlorous acid to chloride (Cl⁻), while an oxygen atom is released and converted into an oxygen molecule:

2HOCl -> 2H⁺ + 2Cl⁻ + O₂

Chlorine Production Methods

Chlorine is primarily produced from chloride compounds through electrolytic or chemical oxidation processes.

Electrolytic Production

Electrolysis of brine (a solution of dissolved salts, typically sodium chloride, in water) is the most common method. A powerful direct current is passed through an electrolytic cell, converting chloride ions into chlorine atoms. Simultaneously, sodium hydroxide (NaOH) and hydrogen gas (H₂) are formed at the cathode. Separation of these products is crucial due to the aggressive reactivity between hydrogen and chlorine gas.

Three main electrolytic methods are employed:

1. Diaphragm Cell Method: This method uses a diaphragm to prevent product mixing. A positive titanium pole and a negative steel pole are separated by a porous diaphragm that allows fluid flow but keeps gases separate. The countercurrent principle prevents hydroxide ions from reaching the positive pole, though some chlorine ions can pass, leading to slight sodium hydroxide contamination.

   • Anode reaction (+ pole): 2Cl⁻ -> Cl₂ + 2e⁻
   • Cathode reaction (- pole): 2H₂O + 2e⁻ -> 2OH⁻ + H₂

2. Mercury Cell Method: This method uses a mercury electrode, yielding purer reaction products. A positive titanium pole and a flowing mercury negative pole are used. At the negative pole, sodium ions (Na⁺) react to form sodium amalgams. These amalgams then react with water in a second reaction vessel to produce sodium hydroxide and hydrogen, keeping the hydrogen gas separate from the chlorine gas formed at the positive pole.

   • Electrolysis cell anode (+ pole): 2Cl⁻ -> Cl₂ + 2e⁻
   • Electrolysis cell cathode (- pole): Na⁺ + e⁻ -> Na
   • Second reaction vessel: 2Na + 2H₂O -> 2Na⁺ + 2OH⁻ + H₂

3. Membrane Method: Similar to the diaphragm method, but a selective membrane only allows positive ions to pass, resulting in a relatively pure form of sodium hydroxide.

European Production Breakdown: Approximately 60% of European chlorine production uses mercury electrolysis, while 20% uses the diaphragm process, and 20% uses the membrane process. The mercury electrolysis process directly yields a 50 mass-% sodium hydroxide solution, whereas the membrane and diaphragm processes require steam evaporation to achieve this concentration.

Chemical Oxidation

Chlorine can also be produced by oxidizing hydrogen chloride with atmospheric oxygen, known as the 'Deacon process', using copper(II) chloride (CuCl₂) as a catalyst:

4HCl + O₂ -> 2H₂O + 2Cl₂

For laboratory-scale production, hydrochloric acid can be oxidized with manganese dioxide:

MnO₂ + 4HCl -> MnCl₂ + 2H₂O + Cl₂

When gaseous chlorine is added to water, a hydrolysis reaction occurs:

Cl₂ + H₂O = H⁺ + Cl⁻ + HOCl

Applications of Chlorine

Chlorine is extensively applied due to its high reactivity, forming compounds with many substances and facilitating bonds between otherwise unreactive substances. When chlorine bonds with carbon-containing substances, organic compounds like plastics, solvents, and oils are formed. It often replaces hydrogen atoms in substitution reactions within molecules.

Medical Science: Chlorine is vital in medicine, used as a disinfectant and a component of many medicines and medical herbs. Chloroform (CHCl₃), a chlorine-containing compound, was an early anesthetic.

Chemical Industry: Tens of thousands of chlorine-containing products exist, including glues, paints, solvents, foam rubbers, car bumpers, food additives, pesticides, and antifreeze. Polyvinyl chloride (PVC) is a widely used chlorine-containing plastic found in drainpipes, insulation, flooring, windows, and bottles.

Industrial Use Distribution:

• Organic Chemicals (e.g., plastics): Approximately 65% of industrialized chlorine.
• Bleach and Disinfectants: About 20%.
• Inorganic Compounds: The remaining portion, used to produce compounds with elements like zinc (Zn), iron (Fe), and titanium (Ti).

Chlorine as a Disinfectant

Chlorine has been used for over two centuries to deactivate pathogenic microorganisms in drinking water, swimming pools, wastewater, household disinfection, and textile bleaching. Its widespread adoption significantly contributed to extending human life expectancy by controlling waterborne diseases.

Chlorine as a Bleach

Bleach typically consists of chlorine gas dissolved in an alkaline solution, such as sodium hydroxide (NaOH). This forms hypochlorite ions (OCl⁻) through an autoredux reaction, creating sodium hypochlorite (NaOCl), an effective and stable disinfectant.

Caution: Bleach must not be combined with acids, as this destabilizes hypochlorite, releasing poisonous chlorine gas.

Bleaching powder (CaOCl₂), a solid produced by directing chlorine through calcium hydroxide (CaOH), offers easier application in medical settings and also serves as a bleach. When dissolved, it reacts with water to form hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻).

Mechanism of Chlorine Disinfection

Chlorine disinfects by breaking chemical bonds within microorganisms, such as enzymes. When chlorine interacts with enzymes, one or more hydrogen atoms can be replaced by chlorine. This alteration changes the enzyme's shape or causes it to break apart, leading to cell or bacterial death.

When chlorine is added to water, hypochlorous acid (HOCl) forms:

Cl₂ + H₂O -> HOCl + H⁺ + Cl⁻

Depending on the pH, hypochlorous acid partially dissociates into hypochlorite ions (OCl⁻):

HOCl + H₂O -> H₃O⁺ + OCl⁻

Hypochlorite ions can further decompose:

OCl⁻ -> Cl⁻ + O (atomic oxygen)

Free chlorine, composed of hypochlorous acid (HOCl, electrically neutral) and hypochlorite ions (OCl⁻, electrically negative), is responsible for disinfection. Hypochlorous acid is significantly more reactive and a stronger disinfectant than hypochlorite ions (80-100% more effective). The disinfecting properties arise from the oxidizing power of atomic oxygen and chlorine substitution reactions.

The neutral hypochlorous acid can more effectively penetrate the negatively charged cell walls and protective layers of pathogenic microorganisms compared to the negatively charged hypochlorite ion, leading to their inactivation or reproductive failure.

The effectiveness of chlorine disinfection is heavily influenced by the water's pH. Optimal disinfection occurs when the pH is between 5.5 and 7.5. At pH 6, hypochlorous acid constitutes approximately 80% of free chlorine, with hypochlorite ions making up 20%. At pH 8, this ratio is reversed. At pH 7.5, the concentrations of hypochlorous acid and hypochlorite ions are roughly equal.

Free and Bound Active Chlorine

When chlorine is added to water, it first reacts with dissolved organic and inorganic compounds. The amount of chlorine consumed in these initial reactions is termed "chlorine demand." Chlorine that has reacted in this way is no longer available for disinfection.

Chlorine can also react with ammonia (NH₃) to form chloramines, which are compounds containing chlorine, nitrogen, and hydrogen. These are referred to as "bound active chlorine compounds." While they contribute to disinfection, they react much more slowly than "free active chlorine" (hypochlorous acid and hypochlorite ions).

Chlorine Dosing

Chlorine dosing must account for the water's chlorine demand. The dose must be sufficient to ensure a residual amount of chlorine remains for effective disinfection. Chlorine demand is influenced by the organic matter content, pH, contact time, and temperature of the water. Chlorine reactions with organic matter can also produce disinfection byproducts (DBPs), such as trihalomethanes (THMs) and halogenated acetic acids (HAAs).

Dosing Methods:

• Ordinary Chlorination: Simple addition of chlorine without prior treatment.
• Pre- and Post-chlorination: Adding chlorine before and after other treatment stages.
• Rechlorination: Adding chlorine to treated water at various points in the distribution system to maintain disinfection.

Breakpoint Chlorination

Breakpoint chlorination involves continuously adding chlorine until the chlorine demand is met, and all ammonia is oxidized, leaving only free chlorine residual. This method is effective for disinfection and also aids in taste and odor control. Achieving the breakpoint often requires "superchlorination," using chlorine concentrations significantly exceeding the typical 1 mg/L (1 ppm) needed for basic disinfection.

Applied Chlorine Concentrations

Chlorine gas, stored as a fluid gas in 10 bar pressure vessels, is highly water-soluble (approximately 3 liters of chlorine gas per 1 liter of water). While only 0.2-0.4 mg/L (0.2-0.4 ppm) is required to kill bacteria, typically higher concentrations are added to account for chlorine demand.

For large municipal and industrial water treatment plants, chlorine gas is commonly used. Smaller applications often utilize calcium or sodium hypochlorite.

Factors Affecting Chlorine Disinfection Effectiveness

The efficacy of chlorine disinfection is determined by:

• Chlorine concentrations
• Contact time
• Temperature
• pH
• Number and types of microorganisms
• Concentrations of organic matter in the water

Disinfection Time for Fecal Pollutants with Chlorinated Water

The following table illustrates typical disinfection times for various pathogens with chlorinated water containing a free chlorine concentration of 1 mg/L (1 ppm), at pH 7.5 and 25 °C (77 °F):

Note: Cryptosporidium is highly resistant to chlorine disinfection.

Health Effects of Chlorine

The human body's response to chlorine exposure depends on the concentration in the air, duration, and frequency of exposure, as well as individual health and environmental conditions. Short-term inhalation of small amounts can cause respiratory issues like coughing, chest pains, and fluid accumulation in the lungs. Skin and eye irritations are also possible.

Pure chlorine is highly toxic, with even small amounts being deadly. Historically, chlorine gas was used as a chemical weapon during World War I due to its density (denser than air), forming toxic fumes that settle near the ground. It primarily affects mucous membranes (nose, throat, eyes) by dissolving them, allowing chlorine to enter the bloodstream. Inhaling significant amounts can lead to fluid filling the lungs, resembling drowning.

Legislation and Standards for Chlorine in Water

European Union (EU)

The European Drinking Water Directive 98/83/EC does not specify guidelines for chlorine levels.

World Health Organization (WHO)

The WHO Guidelines for Drinking-water Quality recommend a residual chlorine concentration of 2-3 mg/L (2-3 ppm) for satisfactory disinfection. The maximum permissible amount of chlorine is 5 mg/L (5 ppm). For effective disinfection, the free chlorine residual should exceed 0.5 mg/L (0.5 ppm) after at least 30 minutes of contact time at a pH of 8 or less.

United States of America (USA)

The national drinking water standards set a maximum residual chlorine level of 4 mg/L (4 ppm). While chlorine remains the primary disinfectant due to its cost-effectiveness, the use of chlorine gas in wastewater treatment has decreased. This shift is largely due to concerns over disinfection byproducts (e.g., THMs) and the regulatory burden associated with managing toxic chemicals under the Clean Air Act (CAA) Risk Management Plan (RMP) by the EPA. Consequently, many wastewater treatment plants have transitioned from chlorine gas to sodium hypochlorite.

AquaChain Engineering Tip

When utilizing chlorine for disinfection, diligently monitor and adjust the water's pH. Maintaining the pH between 5.5 and 7.5 ensures a higher proportion of hypochlorous acid (HOCl) over hypochlorite ions (OCl⁻), leading to significantly more effective pathogen inactivation. This direct control over HOCl concentration is crucial for optimizing disinfection performance and minimizing contact time requirements.

For more information on water disinfection, refer to our guide on Drinking Water Preparation.

Frequently Asked Questions

Q1: What is the primary difference between hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻) in disinfection?

A1: Hypochlorous acid (HOCl) is electrically neutral and significantly more effective (80-100% stronger) as a disinfectant than hypochlorite ions (OCl⁻), which are negatively charged. HOCl can more easily penetrate the negatively charged cell walls of microorganisms, making it a superior oxidizing agent for pathogen inactivation.

Q2: Why is breakpoint chlorination used in water treatment?

A2: Breakpoint chlorination ensures that sufficient chlorine is added to satisfy the water's total chlorine demand, oxidize all ammonia, and leave a measurable "free chlorine" residual. This process not only guarantees effective disinfection but also helps control taste and odor issues in the water.

Q3: What are the main concerns regarding disinfection byproducts (DBPs) when using chlorine?

A3: The main concern with DBPs is the formation of compounds like trihalomethanes (THMs) and halogenated acetic acids (HAAs) when chlorine reacts with organic matter in the water. These DBPs can have potential health risks, leading to regulations and efforts to minimize their formation or shift towards alternative disinfectants in certain applications.