💧 Removal of Undissolved Suspended Solid Materials from Water
During rainy days, river water often becomes muddy due to undissolved suspended solid materials such as mud, sand, and sediment. This increases turbidity, which indicates the presence of insoluble matter in water. High turbidity not only affects water clarity but also impacts its usability.
To make water safe and clear, these suspended solids can be removed using three main methods:
Sedimentation
Filtration
Coagulation
⚙️ Sedimentation
Sedimentation is the process of allowing water to stand still in a settling tank (clarifier) so that suspended solids settle at the bottom by gravity.
Process: Solids settle slowly, forming sediment layers.
Limitations: It is a slow process and cannot remove all solids completely.
Enhancements: Adding coagulants like alum or polyelectrolytes speeds up settling by binding fine particles into larger flocs.
Applications: Commonly used in municipal water treatment plants before filtration.
🪨 Filtration
Filtration involves passing water through a filter bed made of sand, gravel, and charcoal.
Process: Suspended solids are trapped in the porous bed, and clear water is collected at the bottom.
Backwashing: After some time, the filter bed clogs. It is cleaned by reversing water flow (backwash) and using high-pressure air to agitate the bed.
Types of Filters:
Pressure Filters: Used in demineralisation (DM) plants; cleaned when differential pressure rises.
Activated Carbon Filters: Made from coconut shells, bamboo, or wood chips. They remove organic chemicals, chlorine, lead, and unpleasant odours.
🧪 Coagulation
Coagulation removes colloidal impurities that are too small to settle naturally.
Challenge: Colloidal particles carry similar charges, repelling each other and preventing aggregation.
Solution: Adding coagulants neutralizes charges, allowing particles to form larger flocs that settle easily.
Common Coagulants:
Aluminum sulphate (Alum) – widely used in clarifier tanks.
Polyelectrolytes – enhance floc formation.
Result: Faster settling and clearer water.
🚰 After Treatment
Drinking Water: Chlorination is added after suspended solids are removed to kill bacteria.
Industrial Use: This treated water is not suitable for steam generation because dissolved salts and minerals remain.
🌍 Hardness of Water
Even after suspended solids are removed, water may still contain dissolved salts and minerals, causing hardness.
Temporary Hardness: Caused by bicarbonates of calcium and magnesium. Can be removed by boiling.
Permanent Hardness: Caused by chlorides, sulphates, and nitrates of sodium, calcium, and magnesium. Small amounts of iron, aluminum, and manganese also contribute.
❓ FAQs
Q1: What is turbidity in water?
Ans- Turbidity is the cloudiness caused by undissolved suspended solids like mud, sand, and silt.
Q2: Which method is fastest for removing suspended solids?
Ans- Coagulation with alum is faster than simple sedimentation.
Q3: Why is filtration important after sedimentation?
Ans- Filtration removes fine particles that sedimentation cannot settle.
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