Which of the following gives decreasing order of sewer size (in terms of diameter)?

Question

Prepp Team · Accepted Answer

Understanding Sewer Sizes and Network Structure

Sewer systems are designed to collect wastewater from homes and businesses and transport it to treatment facilities. The network of sewers is structured like a tree, with smaller pipes collecting wastewater from individual buildings and larger pipes carrying the combined flow from many smaller pipes.

This structure naturally dictates the size of the pipes. As more wastewater flows into a pipe from upstream connections, the pipe downstream needs to be larger to accommodate the increased volume. Therefore, the size (diameter) of sewers generally increases as they get closer to the point of discharge or treatment.

Types of Sewers and Their Role in the Network

Let's look at the common types of sewers mentioned and their typical position in the collection system:

House Sewer: This is the smallest pipe, connecting a single building (house, store, etc.) to the public sewer system. It carries wastewater from one source only.
Lateral Sewer: These sewers collect wastewater from several house sewers on a street or block. They are larger than house sewers as they carry the combined flow from multiple buildings.
Main Sewer (or Trunk Sewer): Lateral sewers connect to main sewers. Main sewers serve larger areas, collecting flow from many lateral sewers. They are significantly larger than lateral sewers due to the accumulated volume of wastewater.
Outfall Sewer: This is the largest sewer pipe in the network. It collects flow from one or more main sewers and carries it away from the collection area, typically transporting it to a wastewater treatment plant or a final discharge point. It handles the combined flow of the entire drainage area it serves.

Determining the Decreasing Order of Sewer Diameter

Based on the flow of wastewater through the sewer network and the increasing volume as pipes combine flows, the size of the pipes must increase in the direction of flow.

The flow starts at the house sewer, moves to the lateral sewer, then to the main sewer, and finally to the outfall sewer.

Therefore, the diameter increases in the order:

House sewer < Lateral sewer < Main sewer < Outfall sewer

To find the decreasing order of sewer size (diameter), we simply reverse this sequence:

Outfall sewer > Main sewer > Lateral sewer > House sewer

This sequence represents the pipes from the largest diameter to the smallest diameter.

Summary of Sewer Sizes

The decreasing order of sewer size (in terms of diameter) is:

Outfall sewer (Largest diameter)
Main sewer
Lateral sewer
House sewer (Smallest diameter)

Sewer Type	Typical Role	Relative Size (Diameter)
House Sewer	Connects single building to public sewer	Smallest
Lateral Sewer	Collects from multiple house sewers	Larger than house sewer
Main Sewer	Collects from multiple lateral sewers	Larger than lateral sewer
Outfall Sewer	Carries collected flow to treatment/discharge	Largest

Revision Table: Sewer Network Structure

Sewer Type	Collects From	Connects To
House Sewer	Single building fixtures	Lateral sewer
Lateral Sewer	Multiple house sewers	Main sewer
Main Sewer	Multiple lateral sewers	Outfall sewer
Outfall Sewer	Multiple main sewers	Treatment plant or outfall point

Additional Information on Sewer Systems

Understanding the different types of sewers is crucial in wastewater engineering. Beyond the size, other factors like slope, material, and flow velocity are important for proper system function. Sewer networks can be designed as gravity systems, relying on gravity for flow due to proper slopes, or pressurized systems, using pumps.

Sewer systems are broadly classified into sanitary sewers (carrying only domestic and industrial wastewater) and storm sewers (carrying only rainwater runoff). Sometimes, combined sewers exist which carry both, but these are less common in modern construction due to environmental concerns.