Coagulants

Subject: Science and Tech

Context: Rise in turbidity in rivers due to Uttarakhand flood saw use of coagulants to treat turbidity.

Concept:

To address the turbidity in Delhi’s water sources coagulants are being added temporarily, so that they can be filtered.
Coagulants are particles that causes particles in liquid to clump together and form clot which can be filtered out.
Coagulation with extracts from natural and renewable vegetation has been widely practiced throughout history.
Extracts from the seeds of the Moringa oleifera tree, widely present in Africa, the Middle East, and the Indian subcontinent, can be used. Strychnos potatorum (also known as the clearing nut or nirmali tree), is found in India, Sri Lanka and Myanmar and can be used to treat water. Prickly pear cactus is prevalent and traditionally used in Latin America. Lava beans are other example of natural coagulant.
Alum is one of the most widely used coagulants in the water treatment industry.
Organic coagulants are generally used for solid & liquid separation and sludge generation. Ex- PolyAMINEs and PolyDADMACs; Melamine Formaldehydes and Tannins
Inorganic coagulants: cost-effective and applicable for a broad variety of water and wastewater. Inorganic coagulants are particularly effective on raw water with low turbidity and will often treat this type of water when organic coagulants cannot. Ex- Alum, Aluminum Chloride, Polyaluminum Chloride (PAC) & Aluminum Chlorohydrate (ACH), Ferric Sulfate & Ferrous Sulfate, Ferric Chloride

How does coagulation work to remove turbidity?

Coagulants contain significant quantities of water-soluble proteins, which carry an overall positive charge when in solution.
The proteins bind to the predominantly negatively charged particles that cause turbidity (e.g., sand, silt, clay).
Coagulation happens when the positively and negatively charged particles are chemically attracted.
They can then accumulate (a process called flocculation) to form larger and heavier particles called flocs.
The flocs can be settled out or removed by filtration. Bacteria and viruses can attach themselves to the suspended particles in water that cause turbidity.
Therefore, reducing turbidity levels through coagulation may also improve the microbiological quality of water.