---
title: "Separate vs combined sewerage systems"
subject: "Sanitary Engineering"
chapter: 1
key: "semester_6/sanitary_engineering:separate-vs-combined-sewerage"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/separate-vs-combined-sewerage
site: BCT Engineering
---

# Separate vs combined sewerage systems

Sanitary Engineering · Chapter 1: Introduction

- Asked in: 2072 Magh, 4 marks; 2076 Baisakh, 2+2 marks; 2069 Bhadra, 4 marks; 2068 Bhadra (old course), 4 marks (asked 4 times)
- Video: [Combined vs Separated Sewer Systems](https://www.youtube.com/watch?v=j2cYAvGsALE) — Cincinnati MSD
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/separate-vs-combined-sewerage
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-1

## Question

Compare the separate, combined and partially separate systems of sewerage (in tabular form), giving the merits and demerits of each and the conditions favourable for the adoption of each.

## Short answer

Separate systems use two conduits (sewage and storm water), have higher initial cost but smaller sewers, steady treatment load, no overflow pollution, and suit heavy monsoon areas. Combined systems use one conduit, lower initial cost but large sewers, variable flow causing silting, overflow pollution risk, and suit moderate rainfall with steep terrain. Partially separate systems carry sewage plus some storm water, offering moderate cost and size, and suit areas with existing storm drains and moderate rainfall.

## Answer

#### Comparison of the three systems
| Point | Separate | Combined | Partially separate |
|---|---|---|---|
| Carries | Sewage only (storm water in separate drains) | Sewage and all storm water | Sewage and a part of storm water |
| Number of conduits | Two sets | One | One sewer plus surface drains |
| Size of sewer | Small | Large | Medium |
| Initial cost | Higher (two systems) | Lower | Medium |
| Flow variation | Small | Very large | Moderate |
| Self-cleansing in dry season | Good | Poor, silting likely | Fair |
| Load on treatment plant | Steady, small | Heavy in rain, needs overflow | Moderate |
| Pollution by overflow | None | Possible | Slight |
| Maintenance | Easy | Difficult (large sewers) | Moderate |

#### Separate system
**Merits:** smaller sewers; treatment plant is smaller and cheaper; no storm overflow pollution; storm water can go directly to streams; sewers flow at good velocity.
**Demerits:** two systems cost more; small sewers can block easily; storm drains may get contaminated by wrong connections.
**Favourable conditions:** heavy and long monsoon rain; dry-season sewage flow very small compared with the storm flow; flat terrain with little fall; a natural stream is near for storm water; sewage must be pumped.

#### Combined system
**Merits:** one pipe laid, so a single house connection and cheaper total cost; large sewers are self-flushed by storm water and are easy to enter for cleaning; only one system to maintain in narrow streets.
**Demerits:** large sewers are costly; the treatment plant must handle the large wet-weather flow or overflow pollutes the river; flow is low and silting occurs in dry weather; pumping is costly.
**Favourable conditions:** moderate, well-distributed rainfall; limited space for two sets of pipes; steep ground so that dry flow keeps sufficient velocity; storm drainage and sewage both needed at the same time as in a densely built-up area with an existing disposal point.

#### Partially separate system
**Merits:** moderate size of sewer; economical compared with the separate system; roof and courtyard runoff flushes the sewer.
**Demerits:** the sewer size is still larger than in the separate system; the plant sees some storm load; household connections are complicated, and silting may occur in the dry season.
**Favourable conditions:** when part of the storm water (from roofs and paved yards) can be admitted but street runoff must be carried separately, as in areas with moderate rainfall and where an existing storm drain exists.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Compared with the separate system, the combined system has
   A. smaller sewers and higher initial cost
   B. larger sewers but lower total initial cost because only one system is laid
   C. the same sewer size
   D. no cost advantage at all
   Answer: B. One large pipe instead of two sets gives lower initial cost, but the sewer itself is large. (Key point: Sewer size and cost)

2. What problem occurs in a combined sewer in the dry season?
   A. Overflow of the sewer
   B. Too high velocity
   C. Low flow and silting
   D. Backflow from the plant
   Answer: C. Dry-weather flow is very small compared with the sewer size, so solids settle. (Key point: Dry-season behaviour)

3. Which condition favours the separate system?
   A. Moderate, well-distributed rainfall and limited space
   B. Steep ground and no stream nearby
   C. Heavy and long monsoon rain with small dry-season sewage flow
   D. No need for pumping
   Answer: C. Heavy rain would make a combined sewer huge; separate sewers keep sewage flow in a small sewer. (Key point: Conditions for separate system)

4. In a partially separate system, the roof and courtyard runoff
   A. is carried by the sewage sewer and helps flush it
   B. is discharged to a stream
   C. is treated separately
   D. is stored in tanks
   Answer: A. This gives a medium sewer size that is flushed, but the plant sees some storm load. (Key point: Partially separate system)

## Related in Sanitary Engineering

- [Design discharge, separate and combined](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adesign-discharge-separate-combined-45000) — Sanitary Engineering, chapter 2
- [Incineration: merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asolid-waste-incineration) — Sanitary Engineering, chapter 10, asked 7×
- [Composting: methods, merits, demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asolid-waste-composting) — Sanitary Engineering, chapter 10, asked 5×
- [Sewer pipe materials: merits, demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-pipe-materials) — Sanitary Engineering, chapter 3
- [Sanitary landfill: merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asanitary-landfill-merits) — Sanitary Engineering, chapter 10, asked 4×
- [Wet weather discharge, combined sewer](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Awet-weather-discharge-small-town) — Sanitary Engineering, chapter 2

## Related in other subjects

- [Forward vs reverse engineering merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aforward-reverse-engineering-merits) — Object Oriented Analysis and Design, chapter 4, asked 3×
- [Combined cycle merits, demerits and designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-merits-designs) — Power Plant Equipment, chapter 8, asked 2×
- [Merits and demerits of active vs passive filters](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aactive-passive-merits-demerits) — Filter Design, chapter 6
- [Merits and demerits of flat-top sampling](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aflat-top-sampling-merits) — Communication Systems, chapter 4