Unit 1: Water and its Treatment - Subjective Questions

Hardness of water is the characteristic of water that prevents the lathering of soap. It is caused by the presence of dissolved salts of calcium (), magnesium (), and other heavy metal ions.

Types of Hardness:

• Temporary (Carbonate) Hardness:

  • Caused by the presence of bicarbonates of calcium and magnesium, i.e., and .
  • It can be removed simply by boiling the water.

• Permanent (Non-carbonate) Hardness:

  • Caused by the presence of chlorides and sulphates of calcium and magnesium, i.e., , , , .
  • It cannot be removed by boiling and requires chemical treatment.

Total Hardness = Temporary Hardness + Permanent Hardness

Hardness is always expressed in terms of equivalent amount of because its molecular weight (100) and equivalent weight (50) are convenient whole numbers.

Units of Hardness:

• Parts per million (ppm): Number of parts of equivalent hardness per parts of water.
• Milligrams per litre (mg/L): Number of milligrams of equivalent hardness per litre of water.
• Clarke's degree (): Number of grains of equivalent hardness per gallon (70,000 grains) of water.
• Degree French (): Number of parts of equivalent hardness per parts of water.

Relationship between units:

Since 1 mg/L of in mg (1 litre) of water equals 1 ppm, mg/L and ppm are numerically equal.

The EDTA (Ethylene Diamine Tetra Acetic Acid) method is a complexometric titration used to determine the total hardness of water.

Principle:

• EDTA (usually its disodium salt) forms stable complexes with and ions.
• The titration is carried out at pH 9-10 maintained using an ammonia buffer ().
• Eriochrome Black-T (EBT) is used as the indicator.

Reactions:

• First, the metal ions form an unstable wine-red complex with the indicator:
  
• On adding EDTA, it takes the metal ions from the weak complex forming a stable metal-EDTA complex, releasing free EBT which is blue:
  

End Point: Change of colour from wine-red to blue.

Advantages: More accurate, gives sharp end point, and convenient compared to older methods.

Alkalinity of water is its capacity to neutralize a strong acid. It is due to the presence of hydroxide (), carbonate (), and bicarbonate () ions.

Types of Alkalinity:

• Due to only
• Due to only
• Due to only
• Due to and together
• Due to and together

Note: and cannot exist together as they react to form carbonate.

Determination: Alkalinity is measured by titrating with standard acid using:

• Phenolphthalein (P) indicator (endpoint pH 8.3)
• Methyl orange (M) indicator (endpoint pH 4.5)

Significance:

• Helps in determining the dose of chemicals required for water softening.
• Excess alkalinity causes caustic embrittlement and scale formation in boilers.
• Important parameter for controlling corrosion and water quality.

Alkalinity is determined by titrating the water sample with standard acid (/HCl) using two indicators.

Titration steps:

• Titrate with phenolphthalein → volume = P (neutralizes all and half of ).
• Continue titration with methyl orange → total volume = M (neutralizes remaining and all ).

Reactions:

Interpretation Table:

This allows precise identification of both the type and amount of alkalinity present.

Boiler feed water is the water supplied to a boiler for the generation of steam. It must be free from hardness-causing salts, dissolved gases, and suspended impurities.

Problems caused by impure boiler feed water:

• Scale and Sludge Formation:

  • Sludge is a loose, slimy precipitate (e.g., , ) that reduces efficiency.
  • Scale is a hard, adherent deposit (e.g., , ) causing overheating and poor heat transfer.

• Priming and Foaming:

  • Priming is the carrying over of water droplets with steam.
  • Foaming is the formation of stable bubbles due to oils and impurities.

• Boiler Corrosion:

  • Caused by dissolved , , and acids leading to metal deterioration.

• Caustic Embrittlement:

  • Inter-crystalline cracking of boiler metal due to high concentration of (alkalinity).

These problems reduce boiler efficiency, increase fuel cost, and may cause boiler explosions.

During steam generation, water gets concentrated and dissolved salts deposit on boiler surfaces.

Sludge: A loose, slimy, non-adherent precipitate formed by soluble salts like , , .

Scale: A hard, adherent deposit that sticks firmly to the boiler surface, formed by , , , and silica.

Distinction between Scale and Sludge:

Prevention Methods:

• Using softened water (external treatment).
• Internal treatment (adding phosphates, carbonates, calgon).
• Blow-down operation to remove concentrated impurities.
• Mechanical or chemical scale removal.

In the lime-soda process, calculated amounts of lime () and soda () are added to precipitate the hardness-causing ions as insoluble and .

Reactions:

Removal of temporary hardness:

Removal of permanent hardness:

Cold vs Hot Lime-Soda Process:

Zeolite is a hydrated sodium alumino-silicate, represented as (written as ).

Principle: Zeolite exchanges its sodium ions with the hardness-causing and ions in water.

Softening Reactions:

Regeneration: When exhausted, zeolite is regenerated by treating with 10% NaCl (brine) solution:

Advantages:

• Produces water of very low hardness (~10 ppm).
• Equipment is compact and easy to operate.
• No sludge formation.

Limitations:

• Cannot be used for turbid or acidic water.
• Does not remove anions or dissolved solids.
• Coloured water containing and cannot be treated.

The ion-exchange process removes all cations and anions from water using synthetic resins, producing demineralized (deionized) water.

Types of Resins:

• Cation exchange resin (RH): Contains acidic groups (, ) that exchange ions.
• Anion exchange resin (ROH): Contains basic groups (, quaternary ammonium) that exchange ions.

Softening Reactions:

Cation exchanger:

Anion exchanger:

The released and combine to form water:

Regeneration:

• Exhausted cation resin is regenerated with dilute /HCl.
• Exhausted anion resin is regenerated with dilute .

Advantages: Produces water of very high purity (hardness ~0-2 ppm), suitable for high-pressure boilers.

Limitations: Costly, and turbid water must be pre-treated.

Membrane processes use a semi-permeable membrane to separate dissolved solids, ions, and particles from water under a driving force (pressure or concentration).

Common Membrane Processes:

• Reverse Osmosis (RO)
• Ultrafiltration (UF)
• Electrodialysis (ED)

Reverse Osmosis (RO):

• In normal osmosis, solvent flows from a dilute to a concentrated solution across a semi-permeable membrane.
• In reverse osmosis, applying pressure greater than the osmotic pressure on the concentrated (impure) side forces pure water to flow in the reverse direction, leaving behind dissolved salts.

Working: Pressure of about 15-40 kg/cm² is applied on the saline/impure water. Pure water passes through the membrane while impurities are retained and discharged.

Advantages of RO:

• Removes ionic, non-ionic, colloidal, and organic impurities.
• Simple, low maintenance, and reliable.
• Membrane life is long and can be replaced easily.
• Used for desalination of sea/brackish water to produce potable water.

Osmosis is the natural flow of solvent molecules from a region of low solute concentration to high solute concentration through a semi-permeable membrane.

Reverse Osmosis is the process in which solvent is forced from a region of high concentration to low concentration by applying external pressure greater than the osmotic pressure.

Distinction:

Role of Osmotic Pressure ():

• Osmotic pressure is the pressure required to stop the flow of solvent during osmosis.
• In RO, the applied pressure must exceed so that water flows in the reverse direction.
• where = molar concentration, = gas constant, = temperature.
• Higher salt concentration means higher , requiring greater applied pressure.

Chlorination is the process of adding chlorine to water for disinfection (destroying disease-causing microorganisms).

Mechanism: When chlorine is added to water, it forms hypochlorous acid (HOCl):

Hypochlorous acid is a powerful germicide that penetrates and destroys the enzymes of microorganisms, killing them.

Break-point Chlorination:

• It refers to the addition of chlorine to water until the chlorine demand is fully satisfied.
• Initially, chlorine reacts with reducing agents, organic matter, and ammonia (forming chloramines).
• At the break-point, all these are oxidized and further addition results in free residual chlorine.
• This ensures complete disinfection and removal of taste/odour causing compounds.

Advantages:

• Effective and economical.
• Provides residual protection against recontamination.

Disadvantage: Excess chlorine gives bad taste and odour.

Drinking (potable) water must be safe, free from harmful microorganisms and toxic chemicals, and pleasant in taste.

Important Specifications (as per WHO / BIS):

• Physical Parameters:

  • Colour: Should be nil (< 5 Hazen units).
  • Turbidity: Less than 5-10 NTU.
  • Taste and Odour: Should be agreeable.
  • Temperature: Cool and palatable.

• Chemical Parameters:

  • pH: Between 6.5 and 8.5.
  • Total hardness: < 300 mg/L (as ).
  • Total Dissolved Solids (TDS): < 500 mg/L (max 2000).
  • Chlorides: < 250 mg/L.
  • Sulphates: < 200 mg/L.
  • Fluorides: < 1.0-1.5 mg/L.
  • Nitrates: < 45 mg/L.

• Biological Parameters:

  • Should be free from pathogenic bacteria.
  • Coliform count should be nil per 100 mL.

Water meeting these standards is considered safe and potable.

We convert each salt to its equivalent using:

Given molar masses: , , , .

Calculations:

• ppm (temporary)
• ppm (temporary)
• ppm (permanent)
• ppm (permanent)

Results:

• Temporary hardness = ppm
• Permanent hardness = ppm
• Total hardness = ppm

Lime formula:

Soda formula:

Given (as equivalent):

• Temporary hardness () = 25 ppm
• = 20 ppm (permanent Mg)
• = 15 ppm (permanent Ca)

Lime required (per litre):

(Temporary Ca + Permanent Mg both need lime)

For 50,000 L and 90% purity:

Soda required (per litre):

For 50,000 L and 98% purity:

Answer: Lime ≈ 1.85 kg, Soda ≈ 1.89 kg.

Caustic Embrittlement:

• It is a type of boiler corrosion caused by the presence of excess sodium carbonate/NaOH in boiler water.
• hydrolyzes to at high temperature and pressure:
  
• The concentrated seeps into cracks/joints and dissolves iron, forming sodium ferroate, causing inter-crystalline cracking (embrittlement).

Prevention:

• Use sodium phosphate instead of soda for softening.
• Add tannin, lignin, or sodium sulphate to block cracks.

Boiler Corrosion:

• Decay of boiler metal due to chemical/electrochemical attack.

Causes:

• Dissolved oxygen:
• Dissolved : forms carbonic acid .
• Acidic salts like hydrolyze to form HCl.

Prevention:

• Remove dissolved gases by deaeration or adding / hydrazine.
• Maintain proper alkalinity.
• Use corrosion inhibitors.

Both are external treatment methods but differ in mechanism and quality of water produced.

Conclusion: The ion-exchange process produces water of higher purity by removing all ions, whereas the zeolite process only removes hardness-causing cations by replacing them with .

Step 1: Standardize EDTA (find equivalent of EDTA)

Standard solution: 0.5 g in 500 mL → concentration = 1 mg/mL.

50 mL of standard = mg , consumed 45 mL EDTA.

Step 2: Total Hardness

50 mL hard water = 25 mL EDTA.

Step 3: Permanent Hardness (after boiling)

50 mL boiled water = 15 mL EDTA.

Step 4: Temporary Hardness

Answer: Total = 555.6 ppm, Permanent = 333.3 ppm, Temporary = 222.3 ppm.

Internal treatment (also called sequestration) involves adding chemicals directly to the boiler water to prevent scale and sludge formation by converting hardness into easily removable forms.

Methods of Internal Treatment:

• Colloidal Conditioning:

  • Adding organic substances like kerosene, tannin, agar-agar in low-pressure boilers.
  • They coat the precipitate as non-adherent, loose sludge that is removed by blow-down.

• Phosphate Conditioning:

  • Adding sodium phosphates in high-pressure boilers.
  • Reacts with hardness to form soft, non-adherent sludge of calcium/magnesium phosphate.

• Calgon Conditioning:

  • Adding Calgon (sodium hexametaphosphate, ).
  • It forms a soluble complex with calcium, preventing scale formation.

• Carbonate Conditioning:

  • Adding to convert into loose sludge.

These methods keep the boiler surface clean and improve efficiency.