It's a common for a treatment system operator need to measure the flow of sewage in a wastewater collection system.  These flows are below grade and are usually under open channel (non-full pipe) conditions.  In some cases, existing manholes or vaults can be used.  Into these a flume or flow meter can sometimes be installed.  In other cases, a new access point (manhole or vault) is necessary.

When an existing manhole or vault is used to measure flows, the flows can usually only be small (lines up to 12") as the flume has to fit completely inside the structure. 

It is important to remember that it only the flume but also its end connections need to fit inside the existing structure.  Palmer-Bowlus flumes can be a good flume to use as they were specifically designed for use in-line with sewer pipes and conduits.

For larger flows / pipe sizes a new structure is usually required.  Here a metering manhole is frequently used as the flume integrated can extend out of the manhole barrel upstream/ downstream of the manhole.  There is no need to install an oversized vault to try and fit a flume and its end transitions into.

When a flume is used - either in an existing structure or in a metering manhole, the flow entering the flume must be both sub-critical and under non-full pipe conditions (open channel).  In sewer systems, these conditions are usually easy to meet. 

However, if the flow needs conditioning, an energy absorbing manhole can be placed upstream of the monitoring site.  An energy absorber is a plate that is perpendicular to the direction of flow that slows a flow stream and breaks up the velocity profile before it enters a flume.  As a prepackaged unit, an energy absorbing manhole can be placed immediately upstream of a metering manhole or flow monitoring vault.