Water technology and treatment notes — Unit 2

Water is the most critical industrial solvent. However, natural water contains dissolved salts that make it unsuitable for direct use in boilers, cooling systems, or domestic washing. The most significant issue is 'Hardness'.

⇒What is Hard Water?

Hard water does not produce lather easily with soap; instead, it forms a sticky white precipitate called 'scum'. This is caused by the presence of dissolved salts of Calcium (Ca^2+) and Magnesium (Mg^2+), and to a lesser extent, Iron and Manganese.

// Soap is Sodium Stearate (soluble)
2 C17H35COONa (Soap) + CaCl2 (Hardness) → 

// Forms Calcium Stearate (insoluble scum)
(C17H35COO)2Ca ↓ + 2 NaCl

⇒Types of Hardness

Hardness is caused by multiple different salts. To make calculations uniform, the concentration of all hardness-causing salts is converted and expressed in terms of a single standard reference: Calcium Carbonate (CaCO3) equivalents.

⇒Why use CaCO3 as the standard?

• Its molecular weight is exactly 100, which makes mathematical calculations exceptionally easy.
• It is the most insoluble salt that precipitates out during water treatment processes.

CaCO3 Eq. = ( Mass of hardness producing salt / Molecular weight of salt ) × 100

// Example: If water has 162 mg/L of Ca(HCO3)2
// M.W. of Ca(HCO3)2 = 162
CaCO3 Eq. = (162 / 162) * 100 = 100 mg/L

⇒Units of Hardness

The most accurate method to estimate the hardness of water in a laboratory is the Complexometric Titration method using EDTA.

⇒What is EDTA?

EDTA stands for Ethylene Diamine Tetra Acetic acid. It is a hexadentate ligand. It surrounds metal ions like Ca^2+ and Mg^2+ to form highly stable, soluble, colorless complexes. We use its disodium salt because pure EDTA is insoluble in water.

⇒The Indicator & Buffer

• Indicator: Eriochrome Black-T (EBT). It is blue in its free state, but turns wine-red when complexed with Ca/Mg.
• Buffer: An ammonia-ammonium chloride buffer (NH4OH + NH4Cl) is added to maintain the pH exactly around 9 to 10. This specific pH is critical because the complexes are stable only in this alkaline range.

⇒The Chemical Mechanism

// Step 1: EBT is added to hard water. Forms unstable wine-red complex.
Ca^2+ / Mg^2+  +  EBT (Blue)  →  [Ca-EBT / Mg-EBT] (Wine-Red complex)

// Step 2: Titration with EDTA begins. EDTA replaces EBT because EDTA forms a STRONGER complex.
[Ca-EBT] (Wine-Red)  +  EDTA  →  [Ca-EDTA] (Colorless, strong)  +  EBT (Blue)

// The moment all Ca/Mg is seized by EDTA, the EBT is set free.
// Endpoint: Color abruptly changes from Wine-Red to pure Blue.

Industries use boilers to generate high-pressure steam. If untreated hard water is fed continuously into boilers, the continuous evaporation leaves behind salts, concentration increases, and severe operational hazards occur.

⇒Scale and Sludge Formation

⇒Disadvantages of Scales

• Wastage of Fuel: Scales act as thermal insulators. Heat from the furnace cannot reach the water efficiently, increasing fuel consumption drastically.
• Boiler Explosion: Because heat transfer is blocked, the metal boiler walls overheat, soften, and bulge. If the scale suddenly cracks, water rushes onto the super-heated metal, instantly forming massive steam pressure that can explode the boiler.
• Decrease in Efficiency: Narrowing of boiler pipes due to scale blocks the flow of water and steam.

⇒Priming and Foaming

When a boiler produces steam very rapidly, tiny droplets of liquid water get carried along with the steam. This 'wet steam' phenomenon is called Carry Over. It damages steam turbines.

⇒Caustic Embrittlement

A highly dangerous form of inter-crystalline corrosion that makes boiler metal brittle and prone to failure.

// Boiler water often contains Na2CO3 (used for softening).
// At high temperatures and pressure, it hydrolyzes:
Na2CO3 + H2O → 2 NaOH + CO2

// The water becomes highly caustic (basic) due to NaOH.
// This caustic water seeps into tiny cracks and rivets of the boiler.
// The water evaporates in the cracks, leaving hyper-concentrated NaOH.
// The concentrated NaOH attacks the iron:
Fe + 2 NaOH → Na2FeO2 (Sodium ferroate) + H2

This electrochemical attack dissolves the iron at the stress points, eventually causing the boiler to rupture. Prevented by adding Sodium Sulfate or Lignin.

Water softening is the process of removing hardness-causing salts. It is divided into Internal Treatment (done inside the boiler) and External Treatment (done before water enters the boiler).

⇒Internal Treatment Methods (Conditioning)

Instead of removing the salts entirely, chemicals are added directly to the boiler water to force the scale-forming salts to precipitate as soft, loose sludge instead of hard scale.

• Colloidal Conditioning: Adding organic substances like kerosene, tannin, or agar-agar. They coat the precipitated scale particles, preventing them from adhering to the walls, keeping them suspended as sludge.
• Calgon Conditioning: The most common method. Sodium hexametaphosphate (Calgon) is added. It reacts with Calcium to form a highly soluble complex compound, entirely preventing scale precipitation.
• Carbonate Conditioning: Adding Na2CO3 in low-pressure boilers to force Calcium to precipitate as soft Calcium Carbonate sludge rather than hard Calcium Sulfate scale.
• Phosphate Conditioning: In high-pressure boilers, Na3PO4 is added to react with hardness salts to form soft Calcium Phosphate sludge.

External treatment purifies the water before it ever touches the boiler. One classical method is the Zeolite (or Permutit) process.

⇒What is a Zeolite?

Zeolites are hydrated sodium alumino-silicates. They have a porous, cage-like structure. Their chemical formula is broadly Na2O·Al2O3·xSiO2·yH2O. For simplicity, we write it as Na2Ze.

⇒The Softening Process

Hard water is passed through a bed of zeolite sand. The sodium ions (Na+) in the zeolite are loosely held. They are exchanged for the hardness-causing Calcium (Ca^2+) and Magnesium (Mg^2+) ions in the water.

Na2Ze + Ca(HCO3)2 → CaZe + 2 NaHCO3
Na2Ze + MgSO4    → MgZe + Na2SO4

// The Ca and Mg get trapped inside the Zeolite.
// The outgoing water contains harmless Sodium salts.

⇒Regeneration

Eventually, the zeolite bed becomes completely exhausted (all Na+ is replaced by Ca/Mg). It is regenerated by flushing it with a concentrated brine solution (10% NaCl).

CaZe + 2 NaCl → Na2Ze + CaCl2 (Washed down the drain)

While Zeolite process only replaces Ca/Mg with Sodium, the water still contains dissolved salts. For high-pressure boilers, we need completely demineralized (Deionized) water. We use synthetic Ion Exchange Resins.

⇒1. Cation Exchange Column

Water is passed through a resin containing active Hydrogen ions (H+), represented as RH2. It absorbs ALL cations (Ca^2+, Mg^2+, Na+) and releases H+.

RH2 + CaCl2 → RCa + 2 H+
RH2 + MgSO4 → RMg + 2 H+

⇒2. Anion Exchange Column

The water then passes through a resin containing active Hydroxyl ions (OH-), represented as R'(OH)2. It absorbs ALL anions (Cl-, SO4^2-, HCO3-) and releases OH-.

R'(OH)2 + 2 Cl- → R'Cl2 + 2 OH-
R'(OH)2 + SO4^2- → R'SO4 + 2 OH-

⇒The Ultimate Neutralization

The H+ released from the first column and the OH- released from the second column combine to form pure water.

H+ + OH- → H2O

The result is 100% pure demineralized water. The cation resin is regenerated with dilute HCl, and the anion resin with dilute NaOH.

Brackish water or seawater contains dangerously high levels of dissolved salts (mostly NaCl). Removing these salts to make the water potable is called Desalination. The most prominent modern technique is Reverse Osmosis (RO).

⇒Understanding Osmosis

If you separate pure water and salty water with a semi-permeable membrane, pure water naturally flows into the salty side to dilute it. This natural driving force is called Osmotic Pressure.

⇒The Reverse Osmosis Process

If a hydrostatic pressure GREATER than the osmotic pressure is applied to the salty water side, the natural flow is reversed. Pure solvent (water) is forced out of the salty water, through the membrane, leaving the concentrated salts behind.

• Membranes used: Cellulose acetate, polymethacrylate, or polyamide polymers.
• Pressure required: Extremely high, ranging from 15 to 40 kg/cm².
• Advantages: Removes ionic as well as non-ionic, colloidal, and organic impurities. Very low energy consumption compared to thermal distillation. The lifespan of the membrane is about 2-3 years.

Removing hardness and salts makes water suitable for boilers, but for domestic drinking purposes, the water must be free from pathogenic (disease-causing) microorganisms. This is called Disinfection or Sterilization.

⇒Common Methods

• Boiling: Effective for domestic use, but not viable for municipal water supplies.
• Ozonization: Passing Ozone gas (O3) through water. It is highly unstable and breaks down to release nascent oxygen, an incredibly powerful oxidizing agent that kills bacteria instantly. No chemical residue is left behind, but it is expensive.
• UV Radiation: UV light destroys the DNA of microorganisms. Highly effective but only works if the water is perfectly clear (no turbidity to block the light).

⇒Chlorination

The most widely used municipal method. Chlorine gas, bleaching powder, or chloramines are added to water. Chlorine reacts with water to form Hypochlorous acid (HOCl), which is the actual bacteria-killing agent.

Cl2 + H2O → HCl + HOCl (Hypochlorous Acid)
HOCl + Bacteria → Inactivated Bacteria

Simply dumping chlorine into water is inefficient. Water contains organic impurities and ammonia, which consume chlorine before it can kill bacteria. We must calculate the exact amount of chlorine needed.

⇒The Chlorination Curve

• Stage 1: As initial chlorine is added, it reacts with reducing compounds (like H2S). The residual (free) chlorine remains zero.
• Stage 2: Further addition causes chlorine to react with ammonia to form chloramines. Residual chlorine starts appearing.
• Stage 3: As more chlorine is added, the chloramines and organic compounds are forcefully oxidized and destroyed. Strangely, the residual chlorine actually DECREASES during this stage.
• The Break Point: The exact point where all impurities, ammonia, and bacteria have been completely destroyed. The demand is met.
• Stage 4: Any chlorine added AFTER the break point remains in the water strictly as free, unreacted chlorine. This ensures residual protection against future contamination.

Alkalinity is the measure of water's capacity to neutralize acids. It is primarily caused by the presence of Hydroxide (OH-), Carbonate (CO3^2-), and Bicarbonate (HCO3-) ions.

⇒Determination of Alkalinity

Alkalinity is determined by titrating the water sample against a standard acid (like N/50 H2SO4) using two different indicators in sequence.

⇒Calculation Conditions

By comparing the volume of acid used for the P endpoint versus the total M endpoint, we can mathematically deduce exactly which ions are present.

• If P = 0 : Only HCO3- is present.
• If P = 1/2 M : Only CO3^2- is present.
• If P = M : Only OH- is present.
• If P > 1/2 M : Both OH- and CO3^2- are present.
• If P < 1/2 M : Both CO3^2- and HCO3- are present.

When domestic sewage or industrial waste enters a water body, it pollutes it. We measure the severity of this pollution by calculating how much oxygen is required to break down the waste.

⇒Biological Oxygen Demand (BOD)

BOD is the amount of dissolved oxygen required by aerobic microorganisms to biologically decompose the organic matter present in 1 liter of water over a period of 5 days at 20°C.

• It measures ONLY the bio-degradable organic matter.
• A high BOD indicates highly polluted water, as microbes consume all the oxygen, starving aquatic life.
• Clean drinking water has a BOD < 1 ppm.

⇒Chemical Oxygen Demand (COD)

COD is the amount of oxygen required to chemically oxidize ALL organic matter (both bio-degradable AND non-biodegradable) using a strong chemical oxidizing agent like Potassium Dichromate (K2Cr2O7) in an acidic medium.

Rule of thumb: COD is ALWAYS greater than or equal to BOD.
Because COD measures everything, while BOD only measures what bacteria can eat.

COD testing is much faster (takes ~3 hours) compared to BOD testing (takes 5 days).

⇒End-of-Unit Verification

• Differentiate between temporary and permanent hardness, including the specific salts that cause them.
• Calculate the CaCO3 equivalent for a given mass of a hardness-causing salt.
• Explain the chemistry of the EDTA titration method, including the roles of EBT and the ammonia buffer.
• Distinguish between Scale and Sludge in boilers, and list three hazards of scale formation.
• Describe the mechanism of Caustic Embrittlement and how to prevent it.
• Explain Calgon conditioning as an internal treatment method.
• Draw a flow diagram for the Ion Exchange Resin process and explain how it produces demineralized water.
• Explain the principle of Reverse Osmosis and its application in desalination.
• Draw and interpret the Break Point Chlorination curve.
• Calculate the presence of OH-, CO3^2-, and HCO3- based on Phenolphthalein (P) and Methyl Orange (M) titrimetry values.
• Explain why COD values of a water sample are always higher than its BOD values.