Last reviewed 16 Sept 2026 · 9 min read
Impurities in water
| Type | Examples | Effects |
|---|---|---|
| Suspended | Clay, silt, algae, bacteria, organic debris | Turbidity, colour, disease |
| Colloidal | Very fine clay, organic colour, some bacteria (size roughly 1 nm–1 µm) | Turbidity and colour; do not settle — need coagulation |
| Dissolved | Salts of Ca, Mg, Na (carbonates, bicarbonates, chlorides, sulphates), iron, manganese, fluoride, nitrate, gases (O₂, CO₂, H₂S), organic compounds | Hardness, alkalinity, taste, toxicity, corrosion |
Physical characteristics
Turbidity
Cloudiness caused by suspended and colloidal particles that scatter light.
- Measured by nephelometer (NTU — nephelometric turbidity units; the standard method), Jackson candle turbidimeter (older, for high turbidity) and Baylis turbidimeter.
- High turbidity shields microorganisms from disinfection.
Colour
Due to dissolved organic matter (humic substances from decaying vegetation), iron and manganese, or industrial wastes.
- True colour — after removing suspended matter; apparent colour — includes suspended matter.
- Measured in Hazen units (platinum–cobalt scale) with a tintometer or comparator.
Taste and odour
Caused by dissolved gases (H₂S — rotten egg), algae, decaying organic matter, industrial chemicals, excess chlorine.
= volume of sample (mL); = volume of odour-free dilution water (mL) at which odour is just detectable.
Temperature and specific conductance
- Temperature affects taste, viscosity, settling, chemical reactions and dissolved oxygen.
- Electrical conductivity is proportional to dissolved salts; approximately TDS (mg/L) ≈ 0.55 to 0.7 × EC (µS/cm).
Chemical characteristics
pH
Neutral = 7; acidic below 7; alkaline above 7. Low pH causes corrosion; high pH causes scale and reduces chlorine effectiveness. Measured by pH meter (electrometric) or indicators (colorimetric).
Total solids
- Total solids = dissolved + suspended solids (evaporation at 103–105 °C).
- Volatile solids — lost on ignition at 550 °C (approximate organic content); fixed solids — residue (inorganic).
- High TDS gives a salty taste and laxative effects.
Alkalinity
The acid-neutralising capacity of water, due to hydroxide (OH⁻), carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions; expressed as mg/L as CaCO₃. Measured by titration with sulphuric acid:
- Phenolphthalein alkalinity (P) — to pH ≈ 8.3.
- Total (methyl orange) alkalinity (T) — to pH ≈ 4.5.
| Result of titration | OH⁻ | CO₃²⁻ | HCO₃⁻ |
|---|---|---|---|
| 0 | 0 | ||
| 0 | |||
| 0 | 0 | ||
| 0 | |||
| 0 | 0 |
Alkalinity is needed for coagulation (alum consumes alkalinity) and buffers pH.
Hardness
The soap-consuming capacity of water, caused by multivalent cations, mainly calcium and magnesium (also Fe²⁺, Mn²⁺, Sr²⁺).
- Carbonate (temporary) hardness — associated with carbonate and bicarbonate alkalinity; removed by boiling (bicarbonates precipitate).
- Non-carbonate (permanent) hardness — associated with sulphates, chlorides and nitrates; not removed by boiling.
- Total hardness = carbonate + non-carbonate hardness.
- If total hardness > total alkalinity: carbonate hardness = alkalinity; otherwise carbonate hardness = total hardness.
Equivalent weights: Ca²⁺ = 20, Mg²⁺ = 12.15, CaCO₃ = 50.
Measured by EDTA titration (Eriochrome Black T indicator). Classification (approximate): soft < 75 mg/L; moderately hard 75–150; hard 150–300; very hard > 300 mg/L as CaCO₃. Hard water wastes soap, forms scale in boilers and pipes; very soft water can be corrosive.
Chlorides
From geological formations, sea water intrusion, sewage (human urine) and industrial wastes. Measured by Mohr's method (titration with silver nitrate using potassium chromate indicator). A sudden rise in chlorides may indicate sewage contamination.
Nitrogen compounds
| Form | Indicates |
|---|---|
| Free ammonia | Recent pollution (fresh organic matter decomposing) |
| Albuminoid (organic) ammonia | Quantity of nitrogenous organic matter not yet decomposed |
| Nitrites | Partly oxidised organic matter — pollution in progress (dangerous) |
| Nitrates | Fully oxidised — old pollution; excess causes methaemoglobinaemia |
Other constituents
- Fluoride — about 1 mg/L helps prevent dental caries; higher concentrations cause dental and skeletal fluorosis; measured by SPADNS colorimetric or ion-selective electrode.
- Iron and manganese — reddish-brown or black stains, metallic taste, growth of iron bacteria.
- Toxic substances — arsenic, lead, mercury, cadmium, chromium, cyanide, pesticides.
- Dissolved oxygen — indicates freshness; low DO in surface water indicates organic pollution.
- Sulphates — laxative effect at high concentration; H₂S odour under anaerobic conditions.
Bacteriological quality
Pathogens are too varied and too few to test directly, so indicator organisms are used.
- Coliform group — present in large numbers in human and animal intestines; easy to detect; survive somewhat longer than most pathogens.
- Escherichia coli (E. coli) / thermotolerant (faecal) coliforms — specific indicators of faecal contamination.
Tests
- Multiple tube fermentation (MPN) test — sample portions inoculated into lactose broth tubes:
- Presumptive test — gas formation within 24–48 h at 35–37 °C.
- Confirmed test — positive tubes transferred to brilliant green lactose bile broth.
- Completed test — further confirmation on agar and microscopy (Gram-negative, non-spore-forming rods).
- Results are expressed as Most Probable Number (MPN) per 100 mL from statistical tables.
- Membrane filter (MF) technique — a measured sample is filtered; the membrane is incubated on selective medium; colonies are counted directly per 100 mL. Quicker and more precise for low counts.
- Standard plate count — total bacteria (heterotrophic) on agar plates.
- Presence–absence tests and rapid enzymatic tests.
Drinking water standards — IS 10500:2012 (selected parameters)
| Parameter | Acceptable limit | Permissible limit in absence of alternate source |
|---|---|---|
| Colour (Hazen units) | 5 | 15 |
| Odour, taste | Agreeable | Agreeable |
| Turbidity (NTU) | 1 | 5 |
| pH | 6.5 – 8.5 | No relaxation |
| Total dissolved solids (mg/L) | 500 | 2000 |
| Total hardness as CaCO₃ (mg/L) | 200 | 600 |
| Total alkalinity as CaCO₃ (mg/L) | 200 | 600 |
| Calcium (mg/L) | 75 | 200 |
| Magnesium (mg/L) | 30 | 100 |
| Chloride (mg/L) | 250 | 1000 |
| Sulphate (mg/L) | 200 | 400 |
| Nitrate (mg/L as NO₃) | 45 | No relaxation |
| Fluoride (mg/L) | 1.0 | 1.5 |
| Residual free chlorine (mg/L) | 0.2 (minimum) | 1.0 |
| Manganese (mg/L) | 0.1 | 0.3 |
| Copper (mg/L) | 0.05 | 1.5 |
| Zinc (mg/L) | 5 | 15 |
| Arsenic (mg/L) | 0.01 | — |
| Lead (mg/L) | 0.01 | No relaxation |
| Mercury (mg/L) | 0.001 | No relaxation |
| Cadmium (mg/L) | 0.003 | No relaxation |
| E. coli / thermotolerant coliforms | Shall not be detectable in any 100 mL sample | — |
(Limits for some parameters have been amended since 2012; always check the latest amendment of the standard.)
Water-borne and water-related diseases
| Category | Diseases (causative agent) |
|---|---|
| Bacterial (water-borne) | Cholera (Vibrio cholerae), typhoid (Salmonella typhi), paratyphoid, bacillary dysentery (Shigella), gastroenteritis (E. coli strains) |
| Viral | Infectious hepatitis A and E, poliomyelitis, rotavirus diarrhoea |
| Protozoal | Amoebic dysentery (Entamoeba histolytica), giardiasis (Giardia), cryptosporidiosis |
| Helminthic (worms) | Guinea worm (dracunculiasis — India certified free), roundworm, schistosomiasis |
| Water-washed | Scabies, trachoma — due to lack of water for hygiene |
| Water-related vector-borne | Malaria, dengue, filariasis — mosquitoes breeding in water |
Chemical causes: fluorosis (excess fluoride); methaemoglobinaemia or "blue baby syndrome" (nitrate); arsenicosis and blackfoot disease (arsenic); Minamata disease (mercury); itai-itai disease (cadmium); plumbism (lead); goitre (iodine deficiency).
Worked examples
A water sample contains 60 mg/L Ca²⁺ and 24 mg/L Mg²⁺. Find the total hardness.
Solution. → hard water.
Titration gives phenolphthalein alkalinity 20 mg/L and total alkalinity 100 mg/L (as CaCO₃). Find the forms.
Solution. → OH⁻ = 0; CO₃²⁻ = = 40 mg/L; HCO₃⁻ = = 60 mg/L
Total hardness is 250 mg/L and total alkalinity 180 mg/L (as CaCO₃). Find the carbonate and non-carbonate hardness.
Solution. Hardness > alkalinity → carbonate hardness = 180 mg/L; non-carbonate = 250 − 180 = 70 mg/L
Odour is just detectable when 20 mL of sample is diluted to 200 mL with odour-free water. Find TON.
Solution.
Frequently tested points
- Turbidity measured in NTU by nephelometer; colour in Hazen (Pt–Co) units.
- .
- Alkalinity due to OH⁻, CO₃²⁻, HCO₃⁻; P and T titrations.
- Hardness due to Ca²⁺ and Mg²⁺; temporary = carbonate; EDTA method.
- Chlorides by Mohr's method (AgNO₃, K₂CrO₄).
- Nitrites → pollution in progress; nitrates → old pollution.
- Coliforms/E. coli are indicators; MPN (presumptive, confirmed, completed) and membrane filter tests.
- IS 10500: turbidity 1/5 NTU; pH 6.5–8.5; TDS 500/2000; hardness 200/600; chloride 250/1000; fluoride 1.0/1.5; nitrate 45; residual chlorine 0.2 minimum; E. coli nil in 100 mL.
- Cholera, typhoid — bacterial; hepatitis — viral; amoebic dysentery — protozoal; Minamata — mercury; itai-itai — cadmium; blackfoot — arsenic.
- Using equivalent weight 24 (atomic weight) for Mg in hardness calculations (it is 12.15).
- Saying temporary hardness is caused by chlorides and sulphates.
- Treating coliforms as the disease-causing organisms rather than indicators.
- Water contains suspended, colloidal and dissolved impurities.
- Physical tests cover turbidity, colour, odour and temperature; chemical tests cover pH, solids, alkalinity, hardness, chlorides, nitrogen, fluoride and toxic metals.
- Bacteriological safety is judged by coliform and E. coli tests (MPN, membrane filter).
- IS 10500 sets acceptable and permissible limits for drinking water.
- Contaminated water spreads bacterial, viral, protozoal and helminthic diseases and chemical poisoning.