Chemistry Notes

Water Technology & Quality Parameters

Part - A Answers

1. Alkalinity

Definition: Alkalinity of water is a measure of its acid-neutralising ability. This is naturally imparted to waters by the presence of hydroxides, carbonates, and bicarbonates.

Significance: Very high values of alkalinity are harmful to aquatic organisms. In industrial applications, alkalinity in boiler feed water causes the caustic embrittlement of pipes.

2. Calgon Conditioning

Calgon conditioning is an internal boiler treatment method that prevents scale formation by adding sodium hexa meta phosphate, known as Calgon or $Na_2[Na_4(PO_3)_6]$.

This substance interacts with calcium ions to form a highly soluble complex, which prevents scale-forming salts from precipitating without creating sludge disposal issues.

Example Reaction:
$$Ca^{2+} + Na_2[Na_4(PO_3)_6] \rightarrow Na_2[CaNa_2(PO_3)_6] + 2Na^+$$
3. Priming vs Foaming
  • Priming: The process of producing wet steam (steam containing liquid water droplets). Caused by high steam velocity, sudden boiling, high water levels, or poor design.
  • Foaming: The formation of stable bubbles above the surface of the water. Primarily caused by the presence of oil, grease, or finely divided particles.
4. Hardness

Definition: The property of water that prevents it from producing lather with soap.

  • Temporary: Caused by bicarbonates of Ca and Mg.
  • Permanent: Caused by chlorides and sulphates of Ca and Mg.
5. Total Hardness Calculation

Total hardness is the sum of temporary and permanent hardness.

Using the EDTA method:

$$\text{Total Hardness (ppm)} = \frac{V_2}{V_1} \times 1000$$
  • $V_1$: Volume of EDTA consumed by standard hard water.
  • $V_2$: Volume of EDTA consumed by the sample hard water.

1. Demineralization Process

Introduction & Principle

This process removes almost all the ions (both anions and cations) present in hard water. It is carried out using ion exchange resins, which are long chain, cross-linked, insoluble organic polymers with a microporous structure.

Water coming out of this process is known as demineralised (or deionised) water. It is different from soft water, as soft water may still contain non-hardness producing ions like $Na^+$ and $K^+$, whereas demineralised water is completely free from both anions and cations.

The Process

Ion Exchange Process

Hard Water In (Ca²⁺, Mg²⁺, Na⁺, etc.) Cation Exchanger (R-H Resin) Releases H⁺ Cation-Free (H⁺, Cl⁻, SO₄²⁻) Anion Exchanger (R'-OH Resin) Releases OH⁻ Demineralized Water Out (H⁺ + OH⁻ → H₂O)
Schematic representation of the two-stage ion exchange process.
1

Cation Exchanger

Hard water is first passed through a cation exchange column containing acidic functional groups. This resin ($R-H$) absorbs all the cations like $Ca^{2+}$, $Mg^{2+}$, and $Na^+$ present in the water.

$$R-H + Na^+ \rightarrow R-Na + H^+$$
$$2R-H + Ca^{2+} \rightarrow R_2-Ca + 2H^+$$
2

Anion Exchanger

The cation-free water is then passed through an anion exchange column containing basic functional groups. This resin ($R'-OH$) absorbs all the anions like $Cl^-$, $SO_4^{2-}$, and $HCO_3^-$.

$$R'-OH + Cl^- \rightarrow R'-Cl + OH^-$$
$$2R'-OH + SO_4^{2-} \rightarrow R'_2-SO_4 + 2OH^-$$

The released $H^+$ and $OH^-$ ions combine to form water ($H_2O$).

2. Reverse Osmosis and Applications

Principle

  • Osmosis: When two solutions of different concentrations are separated by a semi-permeable membrane, water (the solvent) naturally flows from the region of lower concentration to higher concentration due to osmotic pressure.
  • Reverse Osmosis (RO): If a hydrostatic pressure greater than the osmotic pressure is applied on the higher concentration side, the solvent flow is reversed. Water flows from the higher concentration to the lower concentration, effectively separating pure water from salt water.

RO is also known as super-filtration, and the semi-permeable membranes used are typically made of cellulose acetate or cellulose butyrate.

Reverse Osmosis Process

Semi-Permeable Membrane Applied Pressure (P > π) Salt Water (High Concentration) Pure Water (Low Concentration) Water Flow Fresh Water
Diagram illustrating the flow of water through a semi-permeable membrane under pressure.

Applications and Advantages

  • Desalination: RO is primarily used to desalinate brackish water and sea water, making it available as drinking water. It is widely used for this due to its low capital cost, simplicity, and low operating costs.
  • Impurity Removal: It is highly effective as it removes ionic, non-ionic, and colloidal impurities from the water.
  • Durability: The life time of the RO membrane is high, and if needed, it can be replaced within a few minutes.

3. Disadvantages of Hardwater

Hard water causes numerous issues across both domestic and industrial applications.

Domestic Purposes

  • Washing: Leads to a significant wastage of soap as hard water forms a white precipitate instead of lather.
  • Drinking: Can lead to health issues like kidney stones, which are calcium oxalate crystals.
  • Cooking: Results in the wastage of fuel.

Industrial Purposes

  • Boilers: Hard water decreases boiler efficiency and can sometimes lead to explosions. It causes severe boiler troubles including the formation of scale (hard deposits), sludge (loose deposits), priming (wet steam), foaming (unbreakable bubbles), caustic embrittlement, and boiler corrosion.
  • Textile and Dyeing: It negatively affects the overall quality of the dye.
  • Concrete Making: It decreases the structural strength of the concrete.
  • Sugar Industry: It interferes with and affects the crystallization process of the sugar.
  • Beverages: Temporary hardness-producing substances are alkaline and neutralize fruit acids, which negatively affects the taste of the beverages.
  • Pharmaceuticals: Affects the quality of medicine by causing the formation of unwanted products.
  • Paper Industry: It decreases the brightness and glossiness of the paper while increasing its ash content.
  • Laundries: Causes a massive wastage of soaps and detergents.

Water Quality Parameters

Colour

A shade imparted by organic materials (algae, tannins) or inorganic materials (iron, manganese). A yellowish tinge often indicates iron. Removed by coagulation, settling, adsorption, and filtration.

Tastes and Odours

Taste is flavour; odour is smell caused by volatilized compounds (decaying vegetation, sulphides). Highly objectionable for food industries. Removed by activated carbon, aeration, or chlorination.

Turbidity

Reduction of clarity due to suspended impurities (clay, silt, oils). Affects taste, odour, and disinfection. Removed through coagulation, settling, and filtering.

pH

Defined as $pH = -\log[H^+]$. Measures hydrogen ion concentration. Recommended range for drinking water is 6.5 to 8.5. Sudden changes indicate pollution.

Alkalinity

Measure of acid-neutralising ability (hydroxides, carbonates, bicarbonates). High alkalinity harms aquatic life and causes caustic embrittlement. Removed by adding limited HCl.

Total Dissolved Solids (TDS)

Measures all inorganic and organic substances. >300 ppm tastes salty. High TDS causes scaling in pipes and boilers.

Chemical Oxygen Demand (COD)

Oxygen required to chemically oxidise all impurities using acidified $K_2Cr_2O_7$. Determined in 3 hours. Measures both biologically oxidisable and inert matter.

Biological Oxygen Demand (BOD)

Oxygen required by bacteria to biologically oxidise organic matter at $20^\circ C$ for 5 days. <3 ppm is pure; >4 ppm is polluted.

Hardness

Prevents lather formation. Forms scales in boilers. Categorized into temporary (bicarbonates) and permanent (chlorides/sulphates).