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 |