Water Treatment & Desalination – Key Vocabulary

Vocabulary flashcards covering impurities, hardness, analytical tests, sewage treatment stages, potable water, and desalination processes.

Impurities in Natural Water

Any foreign substances—organic, inorganic, dissolved, or suspended—that make natural water impure.

Dissolved Gases (in water)

Gases like CO₂ and O₂ naturally present; NH₃, H₂S, SOₓ cause bad taste and odour.

Suspended Impurities

Insoluble particles (clay, silica, wood debris, etc.) removable by settling or filtration.

Microscopic Matter

Bacteria, algae, fungi responsible for many water-borne diseases.

Dissolved Mineral Salts

Soluble carbonates, bicarbonates, chlorides, sulphates of Ca²⁺, Mg²⁺, Fe²⁺, Pb²⁺, etc.

Colloidal Impurities

Very fine particles such as proteins, amino acids, ferric hydroxide and clay that remain dispersed.

Hard Water

Water that does not lather readily with soap because of Ca²⁺/Mg²⁺ ions.

Soft Water

Water that lathers easily with soap owing to low Ca²⁺/Mg²⁺ content.

Temporary (Carbonate) Hardness

Hardness caused by Ca²⁺/Mg²⁺ bicarbonates; removable by boiling.

Permanent (Non-carbonate) Hardness

Hardness due to Ca²⁺/Mg²⁺ chlorides & sulphates; not removed by boiling.

Total Hardness

Sum of temporary and permanent hardness, usually expressed as mg L⁻¹ CaCO₃.

EDTA Method

Complexometric titration using EDTA to determine total hardness of water.

Eriochrome Black-T

Wine-red metal-ion indicator used at pH 10 in EDTA hardness titrations.

Biological Oxygen Demand (BOD)

O₂ required by microbes to oxidise biodegradable material in 1 L water over 5 days at 20 °C.

Chemical Oxygen Demand (COD)

O₂ equivalent consumed in chemical oxidation (K₂Cr₂O₇/H₂SO₄) of all oxidisable substances in 1 L wastewater.

Ag₂SO₄ (in COD test)

Catalyst that enhances oxidation of straight-chain organics and aromatic compounds.

HgSO₄ (in COD test)

Reagent that binds Cl⁻ ions to prevent their interference during COD determination.

Primary Sewage Treatment

Physical operations—screening, grit removal, skimming, sedimentation—that remove settleable solids & some BOD.

Screening

Passage of sewage through bar or mesh screens to trap large floating debris.

Grit Chamber (Presedimentation)

Tank where flow velocity is reduced so sand & silt settle out.

Microstraining

Removal of small algae and plankton via a rotating drum fitted with fine stainless-steel mesh.

Skimming Tank

Unit where oil and grease float to surface and are mechanically removed.

Sedimentation Tank

Clarifier where suspended solids settle; coagulants like alum aid colloid removal.

Pre-aeration

Air agitation before primary treatment to freshen sewage, lower BOD, and aid grease removal.

Pre-chlorination

Addition of chlorine before primary treatment to control odour and slow organic decay.

Secondary Treatment

Biological oxidation of dissolved/colloidal organics, typically via activated sludge.

Activated Sludge Process

Aerated mixture of wastewater and microbial flocs that oxidises organics; followed by settling.

Tertiary Treatment

Advanced steps (nutrient removal, adsorption, disinfection) producing high-quality effluent.

Phosphorus Removal

Precipitation of phosphate as Ca₃(PO₄)₂ using lime or other agents.

Nitrogen Removal

Microbial nitrification followed by denitrification to convert nitrogen compounds to N₂ gas.

Disinfection

Final pathogen kill step using chlorine, chloramines, ozone, or UV light.

Potable Water

Water safe and acceptable for drinking—odorless, pathogen-free, and low in toxic compounds.

Reverse Osmosis (RO)

Desalination technique where pressure > osmotic pressure forces water through semi-permeable membrane, leaving salts behind.

Osmotic Pressure

Driving force causing natural water flow across a membrane toward higher solute concentration.

Electrodialysis

Desalination method moving ions through alternating cation/anion-exchange membranes under an electric field.

Ion-Selective Membrane

Semi-permeable barrier permitting passage of only cations or anions, used in electrodialysis cells.

RO Membrane Materials

Common films include cellulose acetate, polymethacrylate, and polyamide polymers.

RO Advantage

Removes a wide spectrum of ions, microorganisms, and organics, yielding high-purity water.

RO Disadvantage

High reject water ratio—up to 90 L waste for 5 L product in some systems.