4.2 Water Treatment & Disinfection

Municipal Water Treatment Steps

Surface water, and sometimes groundwater under the direct influence of surface water, requires comprehensive treatment to ensure it is safe for human consumption. The standard conventional municipal water treatment process consists of several sequential steps designed to remove particulate matter, dissolved impurities, and pathogenic microorganisms.

Coagulation and Flocculation

The first step in removing suspended particles, such as clay, silt, and natural organic matter, is coagulation. These particles often carry a negative electrical charge, causing them to repel each other and remain suspended indefinitely.

• Coagulation: Chemical coagulants, most commonly aluminum sulfate (alum) or ferric chloride, are added to the raw water and rapidly mixed. These chemicals carry a positive charge, which neutralizes the negative charges on the suspended particles.
• Flocculation: Following the rapid mixing of coagulation, the water enters a flocculation basin where it is gently stirred. This slow mixing encourages the neutralized particles to collide and stick together, forming larger, heavier aggregates called "floc."

Sedimentation

After flocculation, the water flows into a sedimentation basin or clarifier. Here, the water moves very slowly, allowing the heavy floc particles to settle to the bottom by gravity.

• The settled material, known as sludge, is periodically removed from the bottom of the basin.
• The clarified water at the top of the basin proceeds to the next treatment step.
• Sedimentation effectively removes a significant portion of the suspended solids and associated pathogens, reducing the load on the subsequent filtration process.

Filtration

Filtration is a physical process that removes the remaining suspended particles, floc, and microorganisms that did not settle out during sedimentation.

• Rapid Sand Filters: The most common type of filter in conventional treatment. Water passes downwards through a bed of granular media, typically layers of anthracite coal, sand, and gravel. Particles are trapped in the pore spaces between the grains.
• Membrane Filters: Increasingly popular, these use semi-permeable membranes to physically block particles. Microfiltration and ultrafiltration can remove bacteria and protozoa, while nanofiltration and reverse osmosis can remove dissolved organic matter and even viruses.

Disinfection

The final and most critical step is disinfection, which destroys or inactivates pathogenic microorganisms.

• Chlorine: The most widely used disinfectant. It is highly effective against bacteria and viruses and provides a residual that protects the water as it travels through the distribution system.
• Chloramines: Formed by combining chlorine and ammonia. While a weaker disinfectant than free chlorine, chloramines are more stable, provide a longer-lasting residual, and produce fewer disinfection byproducts.
• Ozone (O3): A very powerful oxidant and disinfectant, highly effective against Cryptosporidium. However, it does not provide a lasting residual in the distribution system, so secondary disinfection (usually with chlorine or chloramines) is required.
• Ultraviolet (UV) Light: Physical disinfection that damages the DNA of microorganisms, preventing them from replicating. It is highly effective against protozoan cysts but, like ozone, provides no residual protection.

Disinfection Chemistry

Understanding chlorine chemistry is essential for environmental health specialists to evaluate the effectiveness of water treatment.

Free Chlorine vs. Combined Chlorine

When chlorine is added to pure water, it forms free available chlorine. However, if the water contains ammonia or nitrogenous organic compounds, the chlorine will react with them to form chloramines, known as combined chlorine.

• Total Chlorine = Free Chlorine + Combined Chlorine
• Free chlorine is a much more powerful and rapid disinfectant than combined chlorine. Regulatory standards often require maintaining a specific minimum free chlorine residual.

Hypochlorous Acid (HOCl) vs. Hypochlorite Ion (OCl-)

When chlorine gas or liquid bleach (sodium hypochlorite) dissolves in water, it forms two primary species: hypochlorous acid (HOCl) and the hypochlorite ion (OCl-). Together, these constitute free available chlorine.

• HOCl (Hypochlorous Acid): This is the active, highly potent killing agent. It is electrically neutral, allowing it to easily penetrate the cell walls of bacteria.
• OCl- (Hypochlorite Ion): This is a much weaker disinfectant (approximately 80-100 times less effective than HOCl) because its negative charge hinders its ability to penetrate microbial cell walls.

The Influence of pH

The ratio of HOCl to OCl- is entirely dependent on the pH of the water.

• At a lower pH (e.g., pH 6.5 - 7.0), HOCl dominates, meaning the disinfection power is strong.
• As the pH rises above 7.5, HOCl increasingly dissociates into the weaker OCl- ion.
• Therefore, maintaining an appropriate pH (typically slightly acidic to neutral) is crucial for maximizing chlorine disinfection efficiency.

Fluoridation

Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay (dental caries). It is considered one of the great public health achievements of the 20th century.

• Optimal Level: The current recommended optimal concentration for fluoride in drinking water is 0.7 mg/L. This level provides the maximum protection against tooth decay while minimizing the risk of dental fluorosis (a cosmetic discoloration of the teeth).
• Chemicals Used: Common compounds include sodium fluoride, fluorosilicic acid, and sodium fluorosilicate.

Disinfection Byproducts (DBPs)

While chlorination is vital for preventing waterborne diseases, it has a significant drawback: the formation of disinfection byproducts (DBPs).

• Formation: DBPs form when chemical disinfectants, primarily chlorine, react with naturally occurring organic matter (NOM), such as decaying leaves and vegetation, present in the source water.
• Key Regulated DBPs: The most prominent regulated DBPs are Trihalomethanes (THMs) and Haloacetic Acids (HAAs). Long-term exposure to high levels of THMs and HAAs is associated with an increased risk of cancer and potential reproductive issues.
• Control Strategies: To minimize DBP formation, water utilities employ strategies such as maximizing the removal of organic matter before applying chlorine (enhanced coagulation), using alternative disinfectants like chloramines or UV light, or modifying the point of chlorine application.