← Environmental Engineering · UPSC ESE Civil

Chapter 9 of 9

Sewerage & Sanitation

In the UPSC ESE Civil syllabus under Environmental Engineering · 2 parts

Sewerage Systems, Sewer Appurtenances & Pumping of Sewage · Building Sanitation & Plumbing

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

📑 Contents (21 sections)

Part 1 of 2

Sewerage Systems, Sewer Appurtenances & Pumping of Sewage

Last reviewed 16 Sept 2026 · 10 min read

Terminology

Term	Meaning
Sewage (wastewater)	Liquid waste from toilets, kitchens, baths and industries, conveyed in sewers
Sullage	Wastewater from kitchens, baths and wash basins — without human excreta (grey water)
Black water	Wastewater from toilets containing excreta
Storm water	Runoff from rainfall
Sewer	Pipe or conduit carrying sewage
Sewerage	The entire system — collection, conveyance, treatment and disposal of sewage
Dry weather flow (DWF)	Sewage flow in sewers in the absence of storm water (domestic + industrial + infiltration)
Refuse	General term for all wastes — garbage, rubbish, sewage, storm water
Garbage	Dry refuse — decomposable food waste, paper, rags

Systems of sanitation

Conservancy system	Water-carriage system
Night soil collected separately (dry latrines) and carried by people/carts; sullage and storm water in open drains	Excreta and wastewater carried by water through closed sewers
Cheap initially but insanitary, foul smells, health risks; involves manual handling	Hygienic, compact buildings, no nuisance; needs adequate water and high capital cost
Obsolete; manual scavenging is prohibited in India	Standard for towns and cities

Sewerage systems

System	Description	Merits	Demerits
Separate system	Two sets of sewers — one for sanitary sewage, another for storm water	Smaller sanitary sewers; less load on treatment plant; storm water discharged directly to streams; pumping cost low	Two sets of pipes — costlier in congested streets; sanitary sewers may not be self-cleansing (small flows) needing flushing
Combined system	Single sewer carries both sewage and storm water	One set of pipes; sewers self-cleansing (large flows); easy house connections	Large sewers; heavy load on pumps and treatment in rains; combined sewer overflows pollute streams
Partially separate system	Part of storm water (e.g. from roofs and courtyards) admitted to sanitary sewers; the rest in storm drains	Sewers reasonably self-cleansing; house drainage simple	Increased pumping and treatment in monsoon

Choice: the separate system suits Indian conditions (short intense monsoon, long dry periods) — storm water can go to natural drains while sanitary sewers remain small; the combined system suits places with evenly distributed rainfall and flat areas where pumping is needed anyway.

Patterns of collection systems

Perpendicular pattern — sewers perpendicular to a stream; shortest lengths; for storm water or combined sewers discharging to rivers (untreated — not suitable for sanitary sewage now).
Interceptor pattern — an interceptor sewer along the river collects branch sewers and carries sewage to a treatment plant.
Radial pattern — sewers radiate outward to the periphery (land disposal around the town).
Fan pattern — all sewers converge to a single outlet/trunk sewer.
Zone pattern — area divided into zones with separate interceptors (hilly or large towns).

Sewer materials

Material	Features
Salt-glazed stoneware (vitrified clay)	Highly resistant to corrosion by acids and H₂S; smooth; brittle; small diameters — house and lateral sewers
Reinforced cement concrete (RCC)	Most common for medium and large sewers; strong, durable; attacked by H₂S-derived acid at the crown if septic (use sulphate-resisting cement, linings); non-pressure (NP) classes as per IS 458
Brick masonry	Large trunk sewers of any shape (older cities)
Cast iron / ductile iron	Pressure mains (rising mains), crossings under roads, rail, rivers, and where leakage must be avoided
Steel	Force mains, outfalls, crossings; needs corrosion protection
PVC, HDPE, double-wall corrugated (DWC) pipes	Light, corrosion-free, smooth, long lengths with fewer joints; increasingly used
Asbestos cement	Formerly used; light, smooth; attacked by acids and H₂S
GRP	Corrosion-resistant large-diameter pipes

Sewer shapes

Circular — most common: minimum perimeter for a given area, easy to make, strong, hydraulically efficient.
Egg-shaped (ovoid) — narrow invert gives higher velocity at low flows — good for combined sewers with widely varying flows; costlier and less stable.
Horseshoe, semi-elliptical, basket-handle, U-shaped, rectangular — large sewers, tunnels, limited headroom.

Sewer appurtenances

Manholes

Openings to allow inspection, cleaning and repair of sewers.

Provided at every change of direction, gradient or diameter, at junctions, at the head of sewers, and at regular intervals on straight runs (spacing increases with sewer size — from about 30 m for small sewers to much larger intervals for big trunk sewers).
Types by depth: shallow manholes (inspection chambers, up to about 0.9 m deep), normal manholes (about 1.5 m), deep manholes (greater depth, with steps/ladders and a larger working chamber).
Parts: access shaft, working chamber, bottom benching with channels shaped to the sewer, cover and frame (cast iron or ductile iron/ FRP), steps.

Drop manhole — used when an incoming branch sewer is considerably higher than the outgoing sewer (drops of more than about 0.6 m); a vertical drop pipe outside (or inside) the manhole brings the sewage down so that it does not splash and damage the benching or endanger workers.

Other appurtenances

Appurtenance	Purpose
Lamp hole	Small vertical pipe from sewer to ground with a cover; a lamp lowered into it allows checking obstructions from the next manhole — where a manhole is not feasible
Clean-out	Inclined pipe connected to the sewer at its upper end for inserting rods/flushing
Street inlets (gullies)	Openings in kerbs or gutters admitting storm water; with gratings
Catch basins	Chambers at street inlets that retain grit and debris before water enters the sewer (can breed mosquitoes if not cleaned)
Flushing tanks	At the heads of sewers with low flows; periodically release a large volume to flush deposits (automatic tanks work with siphons)
Grease and oil traps	Remove grease and oil from wastewater of hotels, restaurants, garages before it enters sewers
Inverted siphon (depressed sewer)	Sewer dipping below the hydraulic gradient to pass under a river, railway or valley; flows full under pressure; multiple pipes for varying flows; needs self-cleansing velocity and cleaning provisions
Storm regulators (overflows)	Divert excess flow of combined sewers to storm outfalls — leaping weir, side-flow weir, siphon spillway
Ventilating shafts / columns	Release sewer gases
House connection	Connects building drainage to the sewer through an inspection chamber and a gully/intercepting trap

Part 2 of 2

Building Sanitation & Plumbing

Last reviewed 16 Sept 2026 · 11 min read

Principles of house drainage

Drains should be laid outside the building as far as possible, along boundary walls, and connected to the public sewer through an intercepting trap where required.
Pipes laid at self-cleansing gradients, in straight lines between inspection points, with inspection chambers at every change of direction, gradient and junction.
All fittings connected through traps with an adequate water seal to exclude foul gases.
The system must be ventilated — soil and waste pipes extended above the roof as vent pipes.
Pipes must be watertight and tested; joints durable; drains of adequate size (no over-sizing that prevents self-cleansing).
Rainwater pipes should not normally be connected to soil pipes; sullage and rainwater may go through gully traps.
The drainage system should be accessible for cleaning and not pass under living rooms where avoidable.

Types of pipes in building plumbing

Pipe	Carries
Soil pipe	Human excreta from water closets and urinals (black water)
Waste pipe	Sullage from wash basins, sinks, baths and floor traps (grey water)
Vent pipe	Air — ventilates the system, releases foul gases above the roof
Anti-siphonage pipe	Air to the traps of fittings to prevent loss of seal by siphonage
Rainwater pipe	Roof rainwater to the storm drain or harvesting system

Soil pipes are commonly at least 100 mm in diameter; vent stacks are carried above the roof well clear of windows and openings.

Plumbing systems

System	Description	Remarks
Two-pipe system	Separate soil pipe and waste pipe stacks, each with its own vent pipe (four pipes in total when anti-siphonage pipes are provided)	Most expensive and space-consuming; safest; soil and waste kept apart until the inspection chamber
One-pipe system	A single stack carries both soil and waste; a separate vent pipe and anti-siphonage connections to all traps	Fewer pipes than two-pipe; traps need deep seals
Single-stack system	One pipe carries soil and waste and also acts as the vent; no separate vent pipe	Cheapest and simplest; needs careful design — deep-seal traps, short branches, fittings close to the stack; suited to low-rise buildings
One-pipe partially ventilated system	Single stack for soil and waste; a relief vent pipe ventilates only the traps of water closets	Compromise between one-pipe and single-stack; common in multi-storey buildings

Traps

A trap is a fitting that holds a body of water (water seal) to prevent foul gases from entering the building, while allowing wastewater to flow.

The depth of water seal (vertical distance between the dip and the crown weir) is generally at least about 50 mm; deeper seals (about 75 mm) are used in single-stack systems.
A good trap is self-cleansing, has no internal projections, and has an access (clean-out) for cleaning.

Types by shape

P-trap — outlet horizontal (like the letter P lying) — for water closets and wash basins discharging horizontally.
Q-trap — outlet inclined at about 45° — upper floors.
S-trap — outlet vertical — WCs on ground floors discharging into a floor drain.

Types by use

Trap	Use
Floor trap (nahni trap)	Collects wastewater from bathroom and kitchen floors; grated inlet
Gully trap	Outside the building; receives sullage from kitchens and baths and rainwater; deep water seal; prevents gases from the drain and entry of rats and cockroaches
Intercepting trap (interceptor)	At the junction of the house drain and the municipal sewer; prevents sewer gases from entering the house drain; has a cleaning eye
Bottle trap	Under wash basins and sinks — compact, accessible
Grease trap	Hotels, restaurants, canteens — separates grease which floats and is removed
Silt/sand trap	Garages and workshops — retains grit and oil

Loss of water seal

Cause	Mechanism
Self-siphonage	Water discharging from a fitting flows full-bore in the waste pipe and sucks out its own trap seal
Induced siphonage	Discharge from another fitting in a common pipe creates suction that pulls out the seal of a trap connected to it
Back pressure (compression)	Air compressed ahead of a descending water plug blows the seal back into the fitting
Evaporation	Seal dries up in rarely used fittings
Capillary action	Thread or hair in the trap wicks water out
Wind effect / momentum	Strong winds across vent tops or sudden discharges disturb the seal

Prevention: anti-siphonage (vent) pipes connected near the trap crown, adequate pipe sizes, deep-seal traps, correct gradients and lengths of branches, regular use of fittings.

Sanitary fittings

Water closets: Indian type (squatting pan with footrests; Orissa pan), European type (pedestal/wash-down or siphonic), Anglo-Indian type; wall-hung types.
Flushing cisterns — valveless siphonic cisterns (bell type, flushing by siphon action) and dual-flush cisterns to save water; flush valves for direct flushing from a pressurised supply.
Urinals — bowl type, slab/stall type; automatic flushing; waterless urinals.
Wash basins, kitchen sinks, bath tubs, showers, bidets, drinking fountains.
Fixtures are made of vitreous china (sanitary ware), stainless steel, enamelled cast iron, plastics.

Building drains

Size and gradient

The load on drains is expressed in fixture units (discharge value of each fitting relative to a wash basin) for sizing stacks and branches.

FormulaMaguire's rule (self-cleansing gradients for house drains)

Drain diameter	Gradient
100 mm (4 inch)	1 in 40
150 mm (6 inch)	1 in 60
225 mm (9 inch)	1 in 90

Roughly: gradient $= 1$ in $(diameter in inches \times 10)$ .

Inspection chambers and manholes

Provided at junctions, changes of direction or gradient and at intervals, with benching and a cover; depth and size depend on the invert level.

Testing of house drains

Water test — the section is plugged at the lower end and filled with water to a head; the level must not fall appreciably over the test period.
Smoke test — smoke is pumped in; leaks show as smoke (also detects faulty traps above ground).
Air test — air pressure; drop in manometer reading indicates leakage.
Ball test — a ball slightly smaller than the pipe rolled through to check obstruction and alignment.
Mirror test — light visible through the pipe from one inspection chamber to the next.

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