Amogh N P
✦ In loving memory of Amogh N P — Architect · Designer · Visionary ✦
A sewer manhole / inspection chamber under construction — access for cleaning and rodding the underground drain.
Unit IIConcept of Building Services

Sanitation, Sewerage & Drainage

Waste out, gases blocked, rain kept separate.

≈ 35 min + study task

Getting waste safely out of a building is as important as getting water in. The trick is keeping three flows apart and under control — foul waste behind a trap's water seal, clean rainwater on its own path, and sewer gases firmly outside.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Concept of Building Services:

1
CO2 · Understand

Distinguish soil, waste and vent pipes and explain the trap's water seal.

2
CO2 · Apply

Describe how building drainage connects to a sewer or to a septic tank and soak pit.

3
CO2 · Analyse

Explain why storm water and sewage are kept in separate systems.

4
CO6 · Understand

Outline rainwater disposal and rainwater harvesting from buildings.

Pipes, traps, systems

Building drainage

Soil pipes carry excreta, waste pipes carry used water, vent pipes carry air; every fixture sits behind a trap whose water seal blocks sewer gas. Where there is no public sewer, a septic tank and soak pit take over.[1, 2]

The trap and its water seal from fixture water seal (50–75 mm) to drain → sewer gas blocked ✕
DiagramA P-trap holding a water seal that blocks sewer gas while letting waste pass
Soil, waste and vent soil pipe (WC) WC waste pipe basin/bath vent pipe (air, above roof) to inspection chamber → sewer
DiagramSoil, waste and vent pipes in a building drainage diagram

Soil, waste & vent

A soil pipe carries WC and urinal discharge (excreta); a waste pipe carries used water from basins, baths and sinks (no excreta); a vent pipe carries no water — it admits air to balance pressure and protect the trap seals from siphonage.[1, 2]

Septic tank and soak pit scum clarified liquid sludge (settles & digests) septic tank sewage in effluent soak pit (percolation)
DiagramA septic tank settling and digesting sewage, with effluent flowing to a soak pit
A reinforced-concrete septic tank being built — the on-site chamber that settles and digests sewage.
PhotoA reinforced-concrete septic tank being built — the on-site chamber that settles and digests sewage.U.S. National Archives · Public domain · via Wikimedia Commons
Keep it separate

Rainwater and storm water

Clean rain runoff is drained or, better, harvested for recharge — and always kept separate from foul sewage so that monsoon flows never overload the treatment system.[2, 7]

Getting rain off the building

Rainwater is collected by gutters and khurras and carried down rainwater pipes to surface drains or to harvesting structures. Pipe size follows roof area and local rainfall intensity (a rule of thumb is roughly 1 cm² of pipe per 1 m² of roof).[7, 2]

A rooftop rainwater-harvesting recharge pit — roof runoff directed to replenish the ground water.
PhotoA rooftop rainwater-harvesting recharge pit — roof runoff directed to replenish the ground water.SuSanA Secretariat · CC BY 2.0 · via Wikimedia Commons
At a glance

Soil vs waste, storm vs sewage

AspectOneThe other
CarriesSoil pipe: WC/urinal excretaWaste pipe: basin/bath/sink water
Drainage liquidStorm: clean rain runoffSewage: foul + excreta
LayoutTwo-pipe: separate vented stacksSingle-stack: one stack, deep seals
On-site disposalSeptic tank: settle + digestSoak pit: percolate effluent
System typeSeparate: two networks (preferred)Combined: one network (monsoon risk)
Vocabulary

Key terms

Soil pipe

Carries WC and urinal discharge (human excreta).

Waste pipe

Carries used water from basins, baths and sinks (no excreta).

Vent pipe

Carries no water; admits air to protect trap seals from siphonage.

Trap / water seal

A U-bend holding a 50–75 mm water plug that blocks sewer gas.

Gully trap

A trapped chamber at the external wall connecting waste pipes to the underground drain.

Intercepting trap

A deep-seal trap in the last manhole isolating house drainage from the public sewer.

Septic tank

An on-site watertight tank where sewage settles and is anaerobically digested.

Soak pit

A pit that lets septic-tank effluent percolate into the soil.

Apply it

Study task

Sketch a single bathroom's drainage: show the WC on a soil pipe and the basin on a waste pipe, draw the trap and its water seal on each, and add the vent pipe. Then in two lines explain what the water seal does and why the vent is needed.

Check your understanding

Self-assessment

1. A trap in plumbing primarily —

2. A soil pipe carries —

3. Where there is no public sewer, household sewage is best disposed by a —

In a nutshell

Recap

Building drainage uses soil pipes (excreta), waste pipes (used water) and vent pipes (air) — each distinct.
Every fixture needs a trap whose water seal blocks sewer gas; floor, gully and intercepting traps serve specific points.
Drainage reaches the public sewer, or where none exists a septic tank plus soak pit handles it on site.
Storm water is kept separate from sewage and is increasingly harvested for recharge rather than wasted.
The evidence

References & further reading

  1. [1]S.C. Rangwala, Water Supply and Sanitary Engineering. Charotar Publishing House.
  2. [2]NBC 2016 — National Building Code of India, Part 9: Plumbing Services. Bureau of Indian Standards.
  3. [6]IS 2470 (Part 1):1985 — Code of Practice for Installation of Septic Tanks. Bureau of Indian Standards.
  4. [7]IS 2527:1984 — Code of Practice for Fixing Rainwater Gutters and Down-pipes. Bureau of Indian Standards.

Further reading

  • S.C. Rangwala, Water Supply and Sanitary Engineering.
  • G.S. Birdie & J.S. Birdie, Water Supply and Sanitary Engineering. Dhanpat Rai.
  • SP 35:1987 — Handbook on Water Supply and Drainage. Bureau of Indian Standards.

Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.