Trunk Sewer

Published: | Updated: September 17, 2020;

Definition - What does Trunk Sewer mean?

The trunk sewer is the sewer line that receives wastewater flow from the collector sewer and conveys this wastewater either to an intercepting sewer or a treatment plant. The trunk sewer forms a significant part of a sewer network in large population or industrial centers.

Small diameter pipes, also known as lateral sewers, carry wastewater from residential and commercial buildings to the collector sewer, which flows into the larger trunk sewer for onward transmission.

The trunk sewer is sometimes referred to as the main sewer, although in other instances, tributary sewer lines that feed the trunk sewer are called the main sewers. Figure 1 shows one arrangement of the trunk sewer within the sewer network in a neighborhood.

In this case, the lateral lines carry wastewater to the local main sewer line, which then flows to the regional trunk line. Manholes form an essential part of this system as they are necessary for access to undertake inspection and repairs.

Figure 1: Arrangement of a Sewer Network in a Development (Source: Iowa Public Services)

The trunk sewer size is determined by the anticipated design flow for the required service area and the hydraulic characteristics of the sewer piping network. The minimum diameter of the trunk sewer is typically 12 inches. The design flow is found through sewer computer modeling and the study of the assumed flow rates in the development.

Trenchlesspedia explains Trunk Sewer

The trunk sewer is generally located within the public right of way or easement and must be adequately maintained to meet operational requirements. Continuous operation of the trunk sewer and other sewer network components is achieved through routine inspection and cleaning exercises. When developing a maintenance plan, factors considered include:

  • Age.
  • Diameter.
  • The material of the sewer line.
  • Location and accessibility.
  • Subsurface conditions.
  • Corrosion potential.

Inspection of the sewer network can be done in several ways, including closed-circuit television (CCTV), cameras, visual inspection, and lamping inspection. The use of CCTV inspections ranks as the most common and effective method.

CCTV inspections are cost-effective with the equipment successfully able to evaluate trunk sewer lines up to 48 inches. CCTV is beneficial in providing a comprehensive assessment of the state of the pipe walls. The video footage generated from this process is instrumental for future reference.

In Washington DC, the B Street-New Jersey trunk sewer was inspected by CCTV in 2004, with many defects found. Although the flaws in the interior tunnel were minor, previous experience showed that deterioration in the tunnel sidewalls was possible. In this case, non-destructive testing (NDT) determined the entire tunnel thickness condition.

CCTV, combined with NDT, yielded information on the trunk sewer wall thickness, areas of deterioration, and strengths at significant locations. Data from these investigations is critical to structural engineers in deciding remedial action and maintenance plans for the trunk sewer.

Another critical facet of maintenance of the sewer network is cleaning. Standard cleaning methods employ mechanical means such as rodding, hydraulic activities, including jetting and flushing, and the installation of silt traps and sand interceptors. Public education is also vital in keeping the pipes in the sewer network clean as pollution, and improper waste disposal can lead to clogging.

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