Coagulation and Flocculation Principles | Civil Engineering Optional Notes for UPSC PDF Download

Water Treatment Processes

Suspended particles in water, such as fine silt, bacteria, and organic material, cannot be entirely removed by simple settling due to their small size and low weight. To address this, a chemical process known as coagulation is used. During coagulation, chemicals (coagulants) like aluminum sulfate (alum), ferric sulfate, or sodium aluminate are added to water. These coagulants cause the non-settling particles to clump together into larger, heavier masses called floc, which can then settle more easily.

Coagulation is typically performed in two stages:

1. Rapid Mixing: This stage ensures the even distribution of coagulants throughout the water and completes the chemical reaction. Rapid mixing is often done in a flash-mix tank with about one minute of detention time.

2. Slow Mixing (Flocculation): Gentle agitation is used to promote particle collisions and enhance floc growth. Flocculation occurs in a tank with wooden paddle-type mixers that rotate slowly, providing at least a half-hour of detention time.

After flocculation, the water moves to sedimentation tanks where the floc can settle. Some small water-treatment plants combine coagulation and sedimentation in a single unit called a solids-contact tank.

Filtration

Even after coagulation and flocculation, sedimentation may not remove all suspended impurities, leaving noticeable turbidity in the water. Filtration is then used to further clean the water. This process involves passing the water through a bed of porous, granular material (like sand) to trap suspended particles.
There are two main types of sand filters:

1. Slow Sand Filters: These require a large surface area and are difficult to clean.

2. Rapid Filters: These are more commonly used and consist of a dual-media filter with a layer of anthracite coal above a layer of fine sand, sometimes with an additional layer of garnet for in-depth filtration. When clogged, rapid filters are cleaned by backwashing, where clean water is forced upward through the filter media to remove trapped impurities.

• Pressure Filters: Enclosed in a steel tank, used mainly for industrial applications.
• Diatomaceous Earth Filters: Use a natural powder-like material and are often used for swimming pools and industrial applications.
• Microstrainers: Stainless-steel wire cloth mounted on a revolving drum, used mainly for removing algae from surface water supplies.

Disinfection

Disinfection is the final process in drinking-water treatment and is crucial for destroying pathogenic bacteria to prevent waterborne diseases. Methods of disinfection include:

• Chlorine or Chlorine Compounds: Widely used due to their effectiveness and residual protection.
• Ozone: A powerful disinfectant that does not leave harmful residues.
• Ultraviolet (UV) Radiation: Effective in inactivating microorganisms without adding chemicals to the water.

Each of these processes plays a critical role in ensuring that water is safe for consumption and free from harmful impurities and pathogens.