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Chapter 3 of 6

Sewerage & Wastewater Treatment

In the GATE Civil syllabus under Environmental Engineering · 2 parts

Design of Sewers & Storm Water Drains · Sewage Treatment

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (19 sections)

Part 1 of 2

Design of Sewers & Storm Water Drains

Last reviewed 16 Sept 2026 · 7 min read

Quantity of sanitary sewage

FormulaSanitary sewage flow

Average sewage flow \approx 0.80 \times water supply rate \times population

(About 80% of the water supplied reaches the sewers — CPHEEO; the rest is lost in gardening, evaporation, consumption.)

Add infiltration of groundwater into sewers (depends on water table, pipe material, joints) and industrial or institutional flows.

Variation of flow

Sewage flow varies hourly (peaks a little after water-demand peaks, lag in the sewers), daily and seasonally.

Peak factors for sewers (CPHEEO, based on contributory population):

Population	Peak factor
Up to 20 000	3.0
20 000 to 50 000	2.5
50 000 to 7.5 lakh	2.25
Above 7.5 lakh	2.0

FormulaHarmon's formula

Peak factor = 1 + \frac{14}{4 + P}

( $P$ = population in thousands.)

Sewers are designed for the peak flow at the end of the design period and checked for self-cleansing velocity at the minimum (present) flow.
Minimum flow is commonly taken as a fraction (about one-third to one-half) of the average flow.
Design period for sewers is long (about 30 years), since relaying is costly; pumping machinery and treatment units shorter (about 15 years).

Quantity of storm water

FormulaRational method

Q = \frac{C i A}{360}

$Q$ in m³/s; $i$ = rainfall intensity (mm/h) for a duration equal to the time of concentration; $A$ in hectares; $C$ = runoff (impermeability) coefficient.

Composite coefficient: $C = \frac{\sum C _{j} A _{j}}{\sum A _{j}}$

Runoff coefficients (indicative)

Surface	$C$
Roofs (watertight)	0.70 – 0.95
Asphalt and concrete pavements	0.80 – 0.95
Brick/stone pavements	0.70 – 0.85
Gravel roads	0.25 – 0.60
Lawns, gardens, parks	0.05 – 0.30
Wooded areas	0.01 – 0.20

Time of concentration

t_{c} = t_{i} + t_{f}

Inlet time $t_{i}$ — time for overland flow to reach the first inlet (commonly 5–30 min depending on slope and surface).
Time of flow $t_{f}$ — travel time in the drain = length ÷ velocity.

Rainfall intensity

Intensity–duration–frequency relations are used where available. Classical formulas (British Ministry of Health, $i$ in mm/h, $t$ in min):

i = \frac{760}{t + 10} (5 \leq t \leq 20 min), i = \frac{1020}{t + 10} (20 < t \leq 100 min)

The design frequency (return period) is chosen by the importance of the area — higher for commercial centres and critical areas than for residential streets.

Hydraulic design of sewers

FormulaManning's formula

V = \frac{1}{n} R^{2/3} S^{1/2}, Q = A V

For a circular sewer flowing full: $R = D /4$ .

Typical $n$ : about 0.013 for concrete and stoneware sewers; about 0.010–0.011 for smooth plastic pipes; about 0.015–0.017 for brick sewers.

Velocity limits

Self-cleansing velocity — the minimum velocity that prevents deposition of solids. Commonly adopted minimum values: about 0.6 m/s for sanitary sewers at peak flow (with a check at present peak flow) and about 0.75–1.0 m/s for storm and combined sewers (grit is heavier).
Camp's (Shields) formula:

V_{s} = \frac{8 β}{f} (S_{s} - 1) g d

( $β$ ≈ 0.04 for starting motion, up to 0.8 for complete cleansing of sticky material; $f$ = Darcy friction factor; $S_{s}$ , $d$ = specific gravity and size of particles.)

Non-scouring (maximum) velocity — to prevent erosion of the sewer: about 2.5–3 m/s for concrete and stoneware (higher for cast iron); steep ground needs drop manholes to keep velocities down.

Minimum size and depth

A minimum sewer diameter (commonly 150 mm for laterals, larger in big cities) is used to avoid choking.
Sewers are laid deep enough to receive house connections from basements/ground floors and to be below water mains; minimum cover protects against traffic loads.

Part 2 of 2

Sewage Treatment

Last reviewed 16 Sept 2026 · 11 min read

Objectives and levels of treatment

Sewage treatment removes suspended solids, biodegradable organic matter (BOD), pathogens and nutrients so that the effluent can be safely discharged or reused.

Level	Units	Removes
Preliminary	Screens, comminutors, grit chambers, skimming tanks	Large floating matter, grit, oil and grease — protects pumps and later units
Primary	Primary sedimentation tanks	Settleable organic and inorganic solids — roughly 50–70% of SS and 25–40% of BOD
Secondary (biological)	Trickling filters, activated sludge, ponds, UASB with secondary clarifiers	Dissolved and colloidal organic matter — overall BOD removal about 85–95%
Tertiary / advanced	Nutrient removal, filtration, activated carbon, disinfection, membranes	Nitrogen, phosphorus, residual SS, pathogens, refractory organics — for sensitive waters or reuse

A typical flow sheet: screens → grit chamber → primary settling tank → aeration tank or trickling filter → secondary settling tank → disinfection → discharge/reuse, with sludge from settling tanks sent to thickening, digestion and dewatering.

Preliminary treatment

Screens

Screen	Clear openings (typical)	Purpose
Coarse (rack) screens	About 50 mm or more	Remove large floating objects
Medium screens	About 20–50 mm	General screening at pumping stations and plants
Fine screens	Less than about 10 mm (down to a few mm)	Remove smaller solids; reduce load on later units

Bars are inclined (commonly about 30–60° to the horizontal) and cleaned manually or mechanically.
Velocity through screens is kept moderate — high enough to avoid deposition in the channel, low enough not to force material through.
Comminutors (macerators/shredders) cut solids in the flow instead of removing them.

FormulaHead loss through bar screens

h_{L} = 0.0729 (V^{2} - v^{2})

$V$ = velocity through the screen openings, $v$ = approach velocity (m/s); $h_{L}$ in m.

Grit chambers

Remove heavy inorganic particles — sand, gravel, cinders (typically particles of about 0.2 mm and larger, specific gravity about 2.65) — which cause abrasion of pumps and accumulate in digesters.

Horizontal-flow (velocity-controlled) grit chambers: the horizontal velocity is held near 0.3 m/s at all flows — fast enough to keep lighter organic solids in suspension, slow enough to let grit settle. A proportional flow weir (Sutro weir) or Parshall flume at the outlet maintains constant velocity as flow varies.
Theoretical length $L = \frac{H v _{h}}{v _{s}}$ , increased for turbulence and inlet/outlet effects.
Aerated grit chambers — spiral flow created by air; also remove grease and pre-aerate sewage.
Detritus tanks — square tanks with mechanical scrapers.

Skimming tanks

Remove oil, grease, soap and floating fats that would form scum and interfere with treatment. Air is blown in to help grease rise; the floating scum is skimmed off. Grease traps serve individual buildings.

Primary sedimentation

Flocculent (Type II) settling of organic suspended solids.
Typical design (indicative): detention time about 1.5–2.5 h; surface overflow rate of the order of 25–50 m³/m²/day (at average and peak flows respectively); side water depth about 2.5–3.5 m.
Rectangular (horizontal flow) or circular (radial flow) tanks with sludge scrapers and scum removal.
Primary sludge is removed regularly to prevent septic conditions.
Chemically assisted primary treatment — coagulants improve removal.

Principles of biological treatment

Microorganisms (mainly bacteria) use organic matter as food, converting it to CO₂, water and new cells (biomass). The biomass is then separated by settling.

Attached growth (fixed film)	Suspended growth
Microorganisms grow as a film on media (stones, plastic)	Microorganisms kept in suspension as flocs by mixing and aeration
Trickling filter, rotating biological contactor, biofilters	Activated sludge process, oxidation ditch, aerated lagoon, SBR

Under aerobic conditions, bacteria need oxygen; under anaerobic conditions (UASB, digesters), organics are converted to methane and CO₂.

Trickling filters

A bed of coarse media (crushed stone about 25–75 mm, or plastic media) over which settled sewage is sprinkled by a rotary distributor. A biological slime layer (zoogleal film) grows on the media; as sewage trickles over it, organic matter is adsorbed and oxidised. Air enters through the under-drains by natural draft.

The film thickens and periodically sloughs off; the sloughed solids (humus) are removed in a secondary settling tank.
Standard (low-rate) filter — low hydraulic loading (about 1–4 m³/m²/day), no recirculation, deep beds; good BOD removal (about 80–90%) and some nitrification; intermittent dosing.
High-rate filter — much higher hydraulic loading (about 10–40 m³/m²/day) with recirculation of effluent; smaller area; continuous dosing; BOD removal of a single stage somewhat lower (about 65–85%); two-stage filters for higher efficiency.

FormulaNRC formula (single-stage / first-stage filter, SI units)

E = \frac{100}{1 + 0.4432 \frac{W}{V F}}

$E$ = BOD removal efficiency (%) of filter and its secondary clarifier; $W$ = BOD load applied (kg/day); $V$ = volume of filter media (m³); $F$ = recirculation factor:

F = \frac{1 + R}{( 1 + 0.1 R ) ^{2}}, R = \frac{Q _{R}}{Q}

Operational problems: ponding (clogging of voids by excessive slime or fine media), filter flies (Psychoda) in low-rate filters, odours (anaerobic conditions), ice formation in cold climates. Recirculation and flooding the filter help control flies and ponding.

Rotating biological contactors (RBC)

Large closely spaced plastic discs mounted on a horizontal shaft, about 40% submerged and slowly rotated; biofilm on the discs alternately contacts sewage and air. Compact, low energy; sensitive to shaft/mechanical failures.

Activated sludge process (ASP)

Settled sewage is mixed with return activated sludge in an aeration tank where air (or oxygen) is supplied; microorganisms form flocs that oxidise organic matter. The mixed liquor flows to a secondary clarifier; settled sludge is partly returned to the aeration tank and the excess (waste activated sludge) is removed.

FormulaASP design parameters

Hydraulic retention time: $θ = \frac{V}{Q}$
Volumetric (organic) loading: $\frac{Q S _{0}}{V}$ (kg BOD/m³/day)
Food-to-microorganism ratio: $\frac{F}{M} = \frac{Q S _{0}}{V X}$ ( $X$ = MLSS or MLVSS)
Mean cell residence time (sludge age, SRT):

θ_{c} = \frac{V X}{Q _{w} X _{r} + ( Q - Q _{w} ) X _{e}} \approx \frac{V X}{Q _{w} X _{r}}

Sludge volume index: $S V I = \frac{settled sludge volume in 30 min (mL/L) \times 1000}{MLSS (mg/L)}$ (mL/g)
Return sludge concentration: $X_{r} \approx \frac{1 0 ^{6}}{S V I}$ (mg/L)
Return ratio: $\frac{Q _{r}}{Q} = \frac{X}{X _{r} - X}$

Typical ranges (conventional ASP, indicative): MLSS about 1500–3000 mg/L; F/M about 0.2–0.4 per day; SRT about 5–15 days; HRT about 4–8 hours; SVI 50–150 mL/g indicates good settling (above about 200 — bulking); BOD removal about 85–95%.

Modifications

Process	Feature
Conventional (plug flow)	Long narrow tanks; high oxygen demand at inlet
Tapered aeration	More air at the inlet, less towards the outlet — matches oxygen demand
Step aeration	Sewage fed at several points along the tank — spreads load
Complete mix	Uniform conditions throughout — resists shock loads and toxic inflows
Contact stabilisation	Short contact tank followed by re-aeration (stabilisation) of return sludge
Extended aeration	Long HRT (about 18–36 h), low F/M, long SRT (about 20–30 days) — little excess sludge, well-stabilised; for small communities
High-rate aeration	High F/M, short HRT — partial treatment
Oxidation ditch (Pasveer ditch)	Extended aeration in an oval channel with cage rotors/brush aerators; simple and robust; widely used for small towns

Oxygen supply: diffused air systems (fine or coarse bubble diffusers) or mechanical surface aerators.

Operational problems

Sludge bulking — poor settling due to filamentous organisms (low DO, low F/M, nutrient deficiency, septic sewage) or viscous bulking; sludge is lost with the effluent.
Rising sludge — denitrification in the secondary clarifier releases nitrogen gas that floats sludge; remedy — faster sludge removal, reduced SRT.
Foaming/scum — from detergents or filamentous Nocardia.
Pin floc — very long SRT, small flocs that do not settle.

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