Amogh N P
✦ In loving memory of Amogh N P — Architect · Designer · Visionary ✦
A sewage treatment plant with circular aeration and clarifier tanks — the activated sludge process in action.
Unit IIBuilding Services - I

Sewage Treatment & Refuse Disposal

Carrying it away, and cleaning it up — the other half of water.

≈ 35 min + studio task

What comes in must go out. Sewage is treated in stages — primary treatment physically settles the solids, secondary treatment uses micro-organisms to consume the dissolved organics — measured by the BOD it removes. And the city's solid refuse must be collected, conveyed and disposed. Learn the two-stage treatment, suspended versus attached growth, and the refuse routes.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Design of Structures I:

1
CO2 · Understand

Distinguish primary (physical) from secondary (biological) sewage treatment.

2
CO2 · Analyse

Compare the activated sludge process with the trickling filter (suspended vs attached growth).

3
CO2 · Understand

Explain BOD as the measure of organic load and the role of aeration.

4
CO2 · Understand

Describe the collection, conveyance and disposal of town refuse.

Primary then secondary

Cleaning the sewage

Primary treatment is physical (settles solids); secondary is biological (microbes eat the dissolved organics). The activated sludge process is suspended growth (~90–95% BOD); the trickling filter is attached growth (~75–90%).[1, 2]

Sewage treatment — primary then secondary Primary (physical) Secondary (biological) screen grit 1° settle aeration / biofilm 2° settle solids settled out first; microbes then eat the dissolved organics (BOD ↓)
DiagramSewage treatment in two stages: primary physical treatment then secondary biological treatment
Activated sludge process aeration tank air + microbes clarifier clean out return sludge (keeps the culture alive)
DiagramThe activated sludge process: aeration tank, secondary clarifier, and return sludge recirculated

Physical — settle it

Primary treatment is physical: screening removes rags, a grit chamber drops sand, and primary sedimentation settles the suspended solids (~60% of SS, ~35% of BOD). It does NOT remove dissolved or colloidal organic matter — that needs the next stage.[1]

Collect, convey, dispose

Disposing of refuse

Town refuse is segregated at source, collected and conveyed, then disposed by sanitary landfill (engineered burial), composting (organics → soil) or incineration (controlled burning).[6]

Town refuse — collect, segregate, dispose segregated refuse Sanitary landfill Composting Incineration
DiagramThree routes for town refuse: sanitary landfill, composting, and incineration

Collect, convey, dispose

Town refuse must be collected (segregated at source into wet, dry, sanitary and hazardous), conveyed (bins → transfer station → trucks), and disposed of by a chosen method. Segregation at source is required by the Solid Waste Management Rules.[6]

The contrasts

At a glance

AspectOneThe other
Two stagesPrimary: physical, settleable solidsSecondary: biological, dissolved organics
Two biological processesActivated sludge: suspended growth, ~90–95% BODTrickling filter: attached growth, ~75–90% BOD
Growth typeSuspended: biomass floats in the tankAttached: biofilm grows on media
Two disposal routesLandfill: engineered burial of residueComposting: organics → soil conditioner
Composting methodIndore: aerobic, ~8–12 weeksBangalore: anaerobic, ~4–5 months
Vocabulary

Key terms

Primary treatment

Physical removal of settleable suspended solids (screening, grit, sedimentation).

Secondary treatment

Biological removal of dissolved and colloidal organics by micro-organisms.

Activated sludge process

Suspended-growth treatment: aeration tank + clarifier + return sludge (~90–95% BOD).

Trickling filter

Attached-growth treatment: sewage trickled over a media biofilm (~75–90% BOD).

BOD

Biochemical oxygen demand — the oxygen microbes need to degrade the organics; the load measure.

Aeration

Supplying oxygen so aerobic microbes can oxidise the organic matter.

Sanitary landfill

Engineered, lined burial of residual refuse with leachate and gas control.

Composting

Aerobic or anaerobic conversion of organic refuse into a soil conditioner.

Apply it

Studio task

For a 200-flat residential complex, sketch the sewage path from the WC to the on-site STP and say which secondary process you would specify and why. Then propose how the treated water and the segregated wet refuse could be reused on site.

Check your understanding

Self-assessment

1. Primary sewage treatment mainly removes —

2. The activated sludge process is a —

3. BOD is a measure of —

In a nutshell

Recap

Sewage is treated in two stages: primary (physical — settles solids) then secondary (biological — micro-organisms consume dissolved organics).
Activated sludge is suspended growth (~90–95% BOD); the trickling filter is attached growth (~75–90% BOD); BOD measures the organic load.
A building's STP treats sewage on site, often for reuse in flushing and landscape.
Town refuse is collected (segregated), conveyed and disposed by sanitary landfill, composting or incineration.
The evidence

References & further reading

  1. [1]Primary vs secondary wastewater treatment (environmental engineering references).
  2. [2]Activated sludge process and trickling filter — suspended vs attached growth, BOD removal (Rao & Datta; standard texts).
  3. [6]Methods of solid-waste disposal — sanitary landfill, composting (Indore/Bangalore), incineration; SWM Rules 2016 (segregation).

Further reading

  • M.N. Rao & A.K. Datta, Waste Water Treatment.
  • S.C. Rangwala, Water Supply and Sanitary Engineering.
  • B.C. Punmia et al., Waste Water Engineering.

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.