🧪 Unit 1 – Part A (2-Mark Q&A)

1. Why is Calgon conditioning better than phosphate conditioning?

Calgon conditioning is better because it forms a product that is highly soluble (dissolves easily) in water, meaning no solid waste (sludge or scale) is created.

2. Write the importance of break point chlorination?

Break point chlorination is important because it is a very good disinfectant and completely removes many undesirable qualities from the water.

Its main advantages are:

• It completely removes colors, odors, and bad taste from the water.
• It destroys the bacteria.
• It oxidizes all organic matter and other substances like Fe²⁺ and Mn²⁺.

3. What is brackish water?

Brackish water is:

• Water that has a "peculiar Salty" or "brackish taste".
• It contains dissolved salts.
• It has a specific concentration of dissolved solids: more than 1,000 ppm but less than 35,000 ppm.

4. Write a reason for caustic embrittlement in boiler

The reason for caustic embrittlement in a boiler is the presence of Na₂CO₃ (sodium carbonate) in the boiler water. This Na₂CO₃ decomposes in high-pressure boilers to form NaOH (sodium hydroxide), which then seeps into minute cracks and reacts with the boiler's iron material, causing it to crack and become brittle.

5. Give brief note on BOD and COD

Chemical Oxygen Demand (COD)
Definition: COD is the measure of the amount of oxygen required to chemically oxidize all the oxidizable impurities (organic and inorganic) present in the sewage, typically using an oxidizing agent like K₂Cr₂O₇.
Importance:

• It measures both biologically oxidizable and biologically inert organic matter.
• It is used to monitor water treatment plant efficiency.
• It helps assess the effects of discharged wastewater on the environment.

Biological Oxygen Demand (BOD)
Definition: BOD is defined as "the amount of free oxygen required by bacteria for the biological oxidation of the organic matter under aerobic conditions at 20°C for a period of 5 days".
Importance:

• It indicates the amount of decomposable organic matter present in the sewage.
• It helps determine the degree of pollution.
• A lesser BOD indicates better water quality.
• Water with BOD < 30 ppm is considered pure water, while water with BOD > 40 ppm is considered polluted water.

6. Name two salts that cause temporary hardness

The two salts that cause Temporary Hardness (also called Carbonate Hardness) are the bicarbonates of:

1. Calcium (Ca(HCO₃)₂)
2. Magnesium (Mg(HCO₃)₂)

7. How is exhausted ion exchange resins regenerated?

Exhausted ion exchange resins are regenerated by rinsing them with concentrated solutions to replace the ions they have absorbed:

• Cation Exchange Resin (RH): Regenerated by treating it with a dilute acid (HCl or H₂SO₄).
• Anion Exchange Resin (ROH): Regenerated by treating it with a dilute alkali (NaOH).

This process restores the resin to its active form so it can be used again.

8. Why is water softened by zeolite process unfit for use in boilers?

The water is unfit because the zeolite process replaces hardness-causing salts with sodium salts (Na₂SO₄, NaCl, etc.). When this water is boiled, the sodium carbonate (Na₂CO₃) decomposes to form sodium hydroxide (NaOH), which causes boiler corrosion and caustic embrittlement.

9. Bring out the difference between scale and sludge

The following table shows the differences:

10. Why do we express hardness of water in terms of calcium carbonate equivalent

Hardness is expressed in Calcium Carbonate (CaCO₃) equivalent for the following reasons:

1. Ease of Calculation: CaCO₃'s molecular weight is 100, which is a whole number, making calculations simple.
2. Insolubility Standard: CaCO₃ is considered the most insoluble salt.

11. What are zeolites?

Zeolites, also called Permutit, are naturally occurring or synthetically prepared hydrated sodium alumino silicates.

• They are represented by the general formula Na₂O Al₂O₃ xSiO₂ yH₂O.
• In the softening process, the sodium ions in the zeolite are loosely held and are easily replaced by the Ca²⁺ and Mg²⁺ ions present in hard water.

12. What happens when the water containing bicarbonate of Ca²⁺ and Mg²⁺ is boiled?

When water containing bicarbonates of Ca²⁺ and Mg²⁺ is boiled, the temporary hardness is removed because the soluble bicarbonates decompose into insoluble compounds.

• They break down to form Calcium Carbonate (CaCO₃) and Magnesium Hydroxide (Mg(OH)₂).
• These insoluble products precipitate (fall out) of the water as a solid, removing the hardness.

13. Why is water softened before using in boiler?

Water is softened before use in a boiler to prevent several serious problems that collectively are called Boiler Troubles.

These problems, caused by hardness-causing salts (mainly calcium and magnesium), are:

1. Scale and Sludge Formation: Hardness salts precipitate to form scale (a hard, insulating layer) and sludge (a loose deposit) on the boiler walls. This reduces heat transfer efficiency, wastes fuel, and can cause the boiler to overheat and potentially explode.
2. Caustic Embrittlement: Sodium carbonate in boiler water can decompose into caustic soda (NaOH), which concentrates in minute cracks and corrodes the boiler metal, leading to cracking.
3. Priming and Foaming: The presence of suspended solids, oil, and grease causes stable bubbles (foaming) and the production of wet steam (priming). This carries water and impurities over into the steam lines, which can damage turbines.
4. Boiler Corrosion: Dissolved gases like oxygen and CO₂, and salts like MgCl₂, cause the gradual decay of the boiler.

14. What is meant by caustic embrittlement? How it is prevented?

Caustic embrittlement is the intercrystalline cracking of boiler metal, typically at joints and rivets. It's caused by high concentrations of sodium hydroxide (NaOH), which is formed when sodium carbonate (Na₂CO₃) decomposes in high-pressure steam. This NaOH reacts with the iron in the boiler, causing it to crack.

Prevention

You can prevent it by:

1. Using Sodium Phosphate instead of Na₂CO₃ for water softening.
2. Adding agents like tannin or lignin to block the cracks.