Water is purified at a central water system through a combination of physical, chemical, and biological processes to remove contaminants and make it safe for consumption.
Here's a breakdown of the typical steps involved:
1. Intake and Screening:
- Raw water is drawn from a source such as a river, lake, or reservoir.
- Large debris like leaves, branches, and trash are removed using screens. This protects downstream equipment.
2. Coagulation and Flocculation:
- Coagulation: Chemicals like alum or ferric chloride are added to the water. These chemicals neutralize the electrical charges of tiny particles suspended in the water, causing them to clump together.
- Flocculation: The water is gently mixed to encourage the newly formed clumps (called flocs) to collide and form larger, heavier flocs.
3. Sedimentation:
- The water with the flocs flows into a sedimentation basin.
- Because they are heavier than water, the flocs settle to the bottom of the basin through gravity. This process significantly reduces the amount of suspended solids in the water.
4. Filtration:
- The clarified water then passes through filters composed of sand, gravel, and sometimes activated carbon.
- These filters remove smaller particles that did not settle during sedimentation, as well as microorganisms.
- Activated carbon filters can also remove organic compounds that cause taste and odor problems.
5. Disinfection:
- Disinfection is crucial for killing harmful bacteria, viruses, and other pathogens. Common disinfection methods include:
- Chlorination: Chlorine (in various forms) is a widely used disinfectant. It's effective and provides residual disinfection, meaning it continues to protect the water as it travels through the distribution system.
- Chloramination: Ammonia is added to chlorine to form chloramines, which provide longer-lasting disinfection with fewer disinfection byproducts than chlorine alone.
- Ozonation: Ozone is a powerful disinfectant that effectively kills a wide range of pathogens. However, it does not provide residual disinfection.
- Ultraviolet (UV) Radiation: UV light disrupts the DNA of microorganisms, rendering them unable to reproduce. It's effective and doesn't add chemicals to the water, but also provides no residual disinfection.
6. pH Adjustment:
- The pH of the water is adjusted to optimize disinfection and minimize corrosion in the distribution system.
7. Fluoridation (Optional):
- In many communities, fluoride is added to the water to help prevent tooth decay.
8. Distribution:
- The treated water is then pumped into the distribution system and delivered to homes and businesses.
Summary Table of Purification Processes:
| Process | Purpose | Method |
|---|---|---|
| Intake/Screening | Remove large debris | Physical barrier |
| Coagulation | Clump small particles together | Addition of chemicals (e.g., alum, ferric chloride) |
| Flocculation | Form larger, heavier clumps (flocs) | Gentle mixing |
| Sedimentation | Remove flocs | Gravity settling |
| Filtration | Remove remaining particles and microorganisms | Passage through sand, gravel, and activated carbon filters |
| Disinfection | Kill pathogens | Chlorination, chloramination, ozonation, UV radiation |
| pH Adjustment | Optimize disinfection and prevent corrosion | Addition of chemicals to adjust pH |
| Fluoridation | Prevent tooth decay (optional) | Addition of fluoride |
The specific processes used and the order in which they are applied can vary depending on the source water quality and the treatment goals of the water system. Regular monitoring and testing ensure that the treated water meets stringent safety standards.
