Sand and gravel purify water primarily by allowing suspended particles to stick to their surfaces through cohesive and adhesive forces.
The Adhesion Process in Water Purification
When water passes through a layer of sand or gravel, it contains various small particles suspended within it, such as silt, clay, organic matter, and microorganisms. While mechanical filtration (straining particles larger than the gaps between sand/gravel grains) is a known process, the reference specifically highlights the role of adhesion.
Based on the provided information:
The cohesive and adhesive forces between suspended particles and sand and gravel enable particle adhesion to the sand and gravel surface. As a result, purify the water.
This means the purification happens because these forces cause the particles floating in the water to cling to the surface of the sand and gravel grains they come into contact with.
Understanding Cohesive and Adhesive Forces
- Cohesive Forces: These are the attractive forces between molecules of the same substance. For example, water molecules sticking to other water molecules.
- Adhesive Forces: These are the attractive forces between molecules of different substances. For example, water molecules sticking to a surface like glass, sand, or gravel.
In the context of sand and gravel filters:
- Adhesion plays a crucial role as the suspended particles (which have their own properties and surface charges) are attracted to and stick onto the surface of the sand and gravel grains.
- Cohesion can also be involved, helping to hold clusters of particles together or influencing how water interacts with the particle and the filter medium.
Role in Water Purification
The adhesion of suspended particles to the sand and gravel surface effectively removes these particles from the water stream. As water flows through the filter layer, more and more particles adhere to the sand and gravel, leaving the water cleaner.
Key aspects of this process:
- Physical Removal: Particles are physically trapped and held onto the filter material.
- Clarity Improvement: Removing suspended solids makes the water clearer and more aesthetically pleasing.
- Reduction of Contaminants: Many pollutants, including some bacteria and organic matter, are often attached to or are themselves suspended particles, so their removal helps reduce overall contamination levels.
This mechanism, where particles adhere to the filter media surface due to molecular forces, is a significant way sand and gravel beds contribute to water purification, specifically targeting suspended solids.