The coagulant ‘alum’ used for treatment of water is also known as:
Aluminium sulphate
Understanding Coagulation in Water Treatment
Water treatment involves several processes to remove impurities and make water safe for consumption. One crucial step is coagulation and flocculation. Coagulation is the process of adding chemicals to the water to neutralize negative charges on suspended particles, allowing them to clump together. These clumps are called flocs, which can then be easily removed by sedimentation or filtration.
A widely used chemical for coagulation in water treatment is 'alum'.
What is Alum? Identifying the Chemical Name
'Alum' is a general term for a group of hydrated double salts, usually consisting of an alkali metal or ammonium and a trivalent metal like aluminium, chromium, or iron. However, in the context of water treatment, when people refer to 'alum', they are most commonly talking about a specific compound used as a primary coagulant.
Examining the Options for Alum in Water Treatment
Let's look at the options provided and see which one correctly identifies the chemical commonly known as 'alum' in water treatment:
- Sodium aluminate: This is a chemical compound used in water treatment, often as an alkali additive, and can act as a coagulant, but it is not typically referred to as the main 'alum' used for primary coagulation. Its formula is <tex>\text{NaAlO}_2</tex>.
- Aluminium sulphate: This is a chemical compound with the formula <tex>\text{Al}_2(\text{SO}_4)_3</tex>. When hydrated, it is often written as <tex>\text{Al}_2(\text{SO}_4)_3 \cdot n\text{H}_2\text{O}</tex>. This compound is the most common type of 'alum' used as a coagulant in water treatment plants worldwide. It is effective in removing suspended solids, turbidity, and colour from water.
- Ferric chloride: This is another type of coagulant used in water treatment, with the formula <tex>\text{FeCl}_3</tex>. It uses iron (ferric) instead of aluminium as the trivalent metal. While also a coagulant, it is distinct from the typical 'alum'.
- Ferric sulphate: Similar to ferric chloride, ferric sulphate (<tex>\text{Fe}_2(\text{SO}_4)_3</tex>) is an iron-based coagulant. It is also used in water treatment but is not the substance commonly meant by the term 'alum'.
Based on common usage and the chemical properties required for primary coagulation using 'alum' in water treatment, Aluminium sulphate is the correct identification.
Mechanism of Alum Coagulation (Aluminium Sulphate)
When aluminium sulphate is added to water, it reacts with the alkalinity in the water (often bicarbonate) to form insoluble aluminium hydroxide precipitates, <tex>\text{Al(OH)}_3</tex>. This reaction can be simplified as:
<tex>\text{Al}_2(\text{SO}_4)_3 + 6\text{HCO}_3^- \rightarrow 2\text{Al(OH)}_3\downarrow + 6\text{CO}_2 + 3\text{SO}_4^{2-}</tex>
The aluminium hydroxide precipitate is gelatinous and positively charged in slightly acidic to neutral pH ranges. These positively charged precipitates and the soluble aluminium ions effectively neutralize the negative charges on the suspended particles (like clay, silt, organic matter), allowing them to aggregate into larger, settleable or filterable flocs.
| Chemical Name | Formula | Typical Use in Water Treatment | Commonly Called 'Alum'? |
|---|---|---|---|
| Aluminium sulphate | <tex>\text{Al}_2(\text{SO}_4)_3</tex> | Primary coagulant | Yes (most common meaning of 'alum') |
| Sodium aluminate | <tex>\text{NaAlO}_2</tex> | Coagulant aid, alkalinity source | No |
| Ferric chloride | <tex>\text{FeCl}_3</tex> | Primary coagulant | No |
| Ferric sulphate | <tex>\text{Fe}_2(\text{SO}_4)_3</tex> | Primary coagulant | No |
Therefore, the coagulant 'alum' commonly used for treatment of water is Aluminium sulphate.
Revision Table: Water Treatment Coagulants
| Coagulant Type | Key Metal Ion | Common Examples | Notes |
|---|---|---|---|
| Aluminium-based | Al<sup>3+</sup> | Aluminium Sulphate (Alum), Polyaluminium Chloride (PAC), Aluminium Chlorohydrate (ACH) | Widely used, effective over moderate pH range |
| Iron-based | Fe<sup>3+</sup> (Ferric) or Fe<sup>2+</sup> (Ferrous) | Ferric Chloride, Ferric Sulphate, Ferrous Sulphate | Effective over a wider pH range, can sometimes add colour |
| Synthetic Polymers | Organic chains | Polyacrylamides, Polyamines | Often used as coagulant aids or primary coagulants, can be tailored for specific applications |
Additional Information: Types of Alum
While Aluminium sulphate is the most common substance referred to as 'alum' in water treatment, the term 'alum' can technically refer to a class of compounds with the general formula <tex>\text{XAl(SO}_4)_2 \cdot 12\text{H}_2\text{O}</tex> where X is a univalent cation like Potassium (K<sup>+</sup>), Sodium (Na<sup>+</sup>), or Ammonium (NH<sub>4</sub><sup>+</sup>). Examples include:
- Potassium alum (<tex>\text{KAl(SO}_4)_2 \cdot 12\text{H}_2\text{O}</tex>)
- Sodium alum (<tex>\text{NaAl(SO}_4)_2 \cdot 12\text{H}_2\text{O}</tex>)
- Ammonium alum (<tex>\text{NH}_4\text{Al(SO}_4)_2 \cdot 12\text{H}_2\text{O}</tex>)
These traditional alums are also acidic and can function as coagulants, but Aluminium sulphate (filter alum or papermaker's alum) is generally preferred for large-scale water treatment due to its effectiveness and cost-efficiency.
Important Questions from Water Treatment - Teaching
Which out of the following does NOT help in disinfecting water?
The best method for controlling taste and odor problems in water is through ______ process.
________ is the most recent innovation in desalting processes.
A town has an existing horizontal flow sedimentation tank with an overflow rate of 17 m 3/day/m 2, and it is desirable to remove particles that have settling velocity of 0.1 mm/second. Assuming the tank is an ideal sedimentation tank, the percentage of particle’s removal would be approximately equal to:
Activated carbon is used for:
