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Chapter 1 of 9

Water Demand & Quality

In the SSC JE Civil syllabus under Environmental Engineering · 2 parts

Water Demand & Population Forecasting · Water Quality — Characteristics, Analysis, Standards & Water-borne Diseases

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📑 Contents (16 sections)

Part 1 of 2

Water Demand & Population Forecasting

Last reviewed 16 Sept 2026 · 7 min read

Planning a water supply scheme

A public water supply scheme must supply enough water of acceptable quality at adequate pressure to the population at the end of the design period. The first steps are to estimate the per capita demand, the future population and the variations in demand.

Types of water demand

Demand	Includes
Domestic	Drinking, cooking, bathing, washing clothes and utensils, flushing toilets, gardening — usually the largest share in towns
Industrial	Factories, processing units (varies greatly with type of industry)
Institutional and commercial	Schools, hospitals, hotels, offices, restaurants, railway and bus stations
Public (civic) use	Street washing, sewer flushing, public parks, fountains
Fire demand	Fire fighting (small annual quantity but high rate)
Losses and wastage	Leakage from mains and fittings, unauthorised connections, meter errors — part of non-revenue water

Per capita demand norms (CPHEEO)

Classification of town/city	Recommended maximum water supply (litres per capita per day)
Towns with piped water supply but without sewerage	70
Cities with piped water supply where sewerage exists or is planned	135
Metropolitan and mega cities with piped supply and existing/planned sewerage	150
Towns supplied through public stand posts	40

These figures exclude unaccounted-for water; an allowance (commonly up to about 15%) is added for losses.
Rural household tap connections under the Jal Jeevan Mission are planned for 55 lpcd.

Factors affecting per capita demand

Size of the city (larger cities — higher demand); climate (hot, dry — higher); living standards and habits of people; industrial and commercial activity; pressure in the distribution system (higher pressure — more wastage); quality of water; metering and cost (metering reduces demand); sewerage system (flush toilets increase demand); policy on continuous or intermittent supply; efficiency of the water works administration (leak control).

Fire demand

FormulaFire demand formulas (

P

= population in thousands)

Formula	Fire demand
Kuichling	$Q = 3182 P$ (litres/min)
Freeman	$Q = 1136 (\frac{P}{10} + 10)$ (litres/min)
National Board of Fire Underwriters (central congested areas)	$Q = 4637 P (1 - 0.01 P)$ (litres/min)
Buston	$Q = 5663 P$ (litres/min)
Indian practice (CPHEEO)	$Q = 100 P$ (kilolitres/day)

Fire hydrants are provided at intervals along mains, and the distribution system (especially storage) is designed to meet fire flow at adequate pressure.

Variations in demand

Water demand varies:

Seasonally — higher in summer.
Daily — with days of the week and festivals.
Hourly — two peaks in most towns (morning and evening), minimum at night.

FormulaPeak demands (common rule-of-thumb values)

Maximum daily demand ≈ 1.8 × average daily demand
Maximum hourly demand ≈ 1.5 × maximum daily demand = 2.7 × average daily demand
Goodrich's formula: $p = 180 t^{- 0.10}$ — percentage of the annual average demand for a period of $t$ days (e.g. about 180% for a day, 148% for a week)

Peak factors for distribution systems (CPHEEO):

Population	Peak factor
Up to 50 000	3.0
50 000 to 2 lakh	2.5
Above 2 lakh	2.0
Rural water supply schemes	3.0

Design flows for components

Component	Designed for
Source, intake, raw water pumps and mains	Maximum daily demand (often average daily for sources with storage)
Treatment plant	Maximum daily demand
Service reservoirs	Hourly fluctuations (balancing), fire and emergency storage
Distribution system	Maximum hourly demand (or maximum daily + fire demand, whichever is greater)

Design period

The design period is the number of years for which a component is designed to be adequate. It depends on the useful life of the component, ease of future expansion, rate of population growth, cost and interest rates, and the performance of the component in early years (under-loading).

Design periods recommended by CPHEEO (typical):

Component	Design period (years)
Storage by dams	50
Intake structures, conveyance mains, distribution system, trunk mains	30
Pumping machinery	15
Water treatment units	15
Service (clear water) reservoirs	15
Land acquisition for future extensions	30

The design period is counted from the expected year of completion of the project, not from the start of design.

Population forecasting

The future population is estimated from past census data (India's census is decennial).

FormulaMathematical methods (

P_{0}

= latest population,

n

= number of decades)

1. Arithmetic increase method — constant increase per decade (old, large, saturated cities):

P_{n} = P_{0} + n \overset{x}{ˉ}

$\overset{x}{ˉ}$ = average increase per decade.

2. Geometric increase method — constant percentage growth (young, rapidly growing cities):

P_{n} = P_{0} (1 + r)^{n}

$r$ = geometric mean of the decadal growth rates $= (r_{1} r_{2} \dots r_{t})^{1/ t}$ . Gives the highest estimate.

3. Incremental increase method — combines arithmetic and geometric trends (average cities):

P_{n} = P_{0} + n \overset{x}{ˉ} + \frac{n ( n + 1 )}{2} \overset{y}{ˉ}

$\overset{y}{ˉ}$ = average of the increments (change in decadal increases).

4. Decreasing rate of growth method — the percentage growth rate decreases steadily; the average decrease in rate is subtracted from the latest rate each decade.

5. Logistic curve (S-curve) method:

P = \frac{P _{s}}{1 + m e ^{n t}}, P_{s} = \frac{2 P _{0} P _{1} P _{2} - P _{1}^{2} ( P _{0} + P _{2} )}{P _{0} P _{2} - P _{1}^{2}}

using three census populations $P_{0}$ , $P_{1}$ , $P_{2}$ at equal time intervals; $P_{s}$ = saturation population.

Other methods:

Simple graphical method — extend the population–time curve by eye.
Comparative graphical method — the city's growth is assumed to follow that of larger, similar cities when they were of the same population.
Master plan / zoning method — population based on planned densities of zones in the city's master plan.
Ratio and correlation method — city population as a ratio of the state or national population forecast.

Arithmetic increase generally gives low estimates, geometric increase high estimates, and incremental increase intermediate values.

Worked examples

Worked ExampleExample 1 — population forecasts

Census populations of a town are 40 000 (1981), 50 000 (1991), 62 000 (2001) and 76 000 (2011). Estimate the 2041 population by the arithmetic, geometric and incremental increase methods.

Solution. Decadal increases: 10 000, 12 000, 14 000 → $\overset{x}{ˉ}$ = 12 000; increments: 2000, 2000 → $\overset{y}{ˉ}$ = 2000; $n$ = 3.

Arithmetic: $P = 76000 + 3 \times 12000 = 112000$

Incremental: $P = 76000 + 36000 + \frac{3 \times 4}{2} \times 2000 = 124000$

Geometric: rates 25%, 24%, 22.58% → $r = (0.25 \times 0.24 \times 0.2258)^{1/3} = 0.2384$ $P = 76000 \times 1.238 4^{3} = 76000 \times 1.899 = 144300$ (approx.)

Worked ExampleExample 2 — fire demand

Find the fire demand for a city of 1 lakh population by Kuichling's and Freeman's formulas and the CPHEEO formula.

Solution. $P$ = 100 (thousands) Kuichling: $3182100 = 31820 L/min$ Freeman: $1136 (10 + 10) = 22720 L/min$ CPHEEO: $100100 = 1000 kL/day$

Worked ExampleExample 3 — design demands

A city with sewerage has a design population of 1 lakh. Find the average daily demand, maximum daily demand and maximum hourly demand (use 135 lpcd).

Solution. Average $= 100000 \times 135 = 13.5 \times 1 0^{6}$ L/day = 13.5 MLD Maximum daily $= 1.8 \times 13.5 = 24.3 MLD$ Maximum hourly rate $= 2.7 \times 13.5 = 36.45 MLD$ (as a rate)

Frequently tested points

CPHEEO: 70 lpcd (no sewerage), 135 lpcd (with sewerage), 150 lpcd (metro), 40 lpcd (stand posts); JJM rural 55 lpcd.
Metering and lower pressure reduce per capita demand; flush toilets increase it.
Kuichling $3182 P$ ; Freeman $1136 (P /10 + 10)$ ; CPHEEO $100 P$ kL/day.
Max daily = 1.8 × average; max hourly = 2.7 × average; Goodrich $p = 180 t^{- 0.1}$ .
Peak factor: 3.0 (≤ 50 000), 2.5 (50 000–2 lakh), 2.0 (> 2 lakh).
Design periods: dams 50 y; intake, mains, distribution 30 y; pumps, treatment plants, service reservoirs 15 y.
Arithmetic (old cities, lowest), geometric (young cities, highest), incremental (intermediate).
Treatment plant designed for maximum daily demand; distribution for maximum hourly demand.

Common MistakeCommon mistakes

Using the arithmetic mean of growth rates in the geometric method (use the geometric mean).
Taking $n$ in years instead of decades.
Designing distribution pipes for average daily demand.

Revision SummaryChapter summary

Water demand combines domestic, industrial, commercial, public and fire uses plus losses.
CPHEEO norms and local factors fix per capita demand.
Demand varies seasonally, daily and hourly; peak factors size different components.
Design periods depend on component life and expandability.
Population is forecast by arithmetic, geometric, incremental, logistic and graphical methods.

Part 2 of 2

Water Quality — Characteristics, Analysis, Standards & Water-borne Diseases

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.

FormulaThreshold odour number

T O N = \frac{A + B}{A}

$A$ = volume of sample (mL); $B$ = 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

p H = - lo g_{10} [H^{+}]

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.

FormulaForms of alkalinity (mg/L as CaCO₃)

Result of titration	OH⁻	CO₃²⁻	HCO₃⁻
$P = 0$	0	0	$T$
$P < T /2$	0	$2 P$	$T - 2 P$
$P = T /2$	0	$2 P$	0
$P > T /2$	$2 P - T$	$2 (T - P)$	0
$P = T$	$T$	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.

FormulaHardness as CaCO₃

Hardness (mg/L as CaCO_{3}) = \sum (ion concentration, mg/L) \times \frac{50}{equivalent weight of ion}

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.)

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

Worked ExampleExample 1 — hardness

A water sample contains 60 mg/L Ca²⁺ and 24 mg/L Mg²⁺. Find the total hardness.

Solution. $60 \times \frac{50}{20} + 24 \times \frac{50}{12.15} = 150 + 98.8 = 248.8 mg/L as CaC O_{3}$ → hard water.

Worked ExampleExample 2 — alkalinity forms

Titration gives phenolphthalein alkalinity 20 mg/L and total alkalinity 100 mg/L (as CaCO₃). Find the forms.

Solution. $P < T /2$ → OH⁻ = 0; CO₃²⁻ = $2 P$ = 40 mg/L; HCO₃⁻ = $T - 2 P$ = 60 mg/L

Worked ExampleExample 3 — carbonate and non-carbonate hardness

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

Worked ExampleExample 4 — threshold odour number

Odour is just detectable when 20 mL of sample is diluted to 200 mL with odour-free water. Find TON.

Solution. $T O N = \frac{20 + 180}{20} = 10$

Frequently tested points

Turbidity measured in NTU by nephelometer; colour in Hazen (Pt–Co) units.
$T O N = (A + B) / A$ .
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.

Common MistakeCommon mistakes

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.

Revision SummaryChapter summary

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.

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Water Demand & Population Forecasting

Planning a water supply scheme

Types of water demand

Per capita demand norms (CPHEEO)

Factors affecting per capita demand

Fire demand

Variations in demand

Design flows for components

Design period

Population forecasting

Worked examples

Frequently tested points

Water Quality — Characteristics, Analysis, Standards & Water-borne Diseases

Impurities in water

Physical characteristics

Turbidity

Colour

Taste and odour

Temperature and specific conductance

Chemical characteristics

pH

Total solids

Alkalinity

Hardness

Chlorides

Nitrogen compounds

Other constituents

Bacteriological quality

Tests

Drinking water standards — IS 10500:2012 (selected parameters)

Water-borne and water-related diseases

Worked examples

Frequently tested points

Finished reading? Test yourself.