Diagnosis of sanitary sewer networks: infiltration, catchment, reversed connection and condition of pipes and manholes

Detection of infiltration, parasitic water intake and reversed connections in sewer networks

Our wastewater treatment networks convey surprising quantities of rainwater that should not end up there, causing overloads during heavy rains and during the spring melt.

In addition to the surface capture, there are infiltrations by the groundwater.

Smoke tests of sanitary sewers

Smoke tests can locate sources of surface water intake that are connected to sanitary sewer systems, when they should be connected to stormwater sewers or discharged on the surface.

The technique verifies the conformity of connections on long sections of sewers in a very short time and at relatively low cost.

Smoke tests are particularly effective in detecting the capture of:

  • Street sumps
  • Gutters
  • Flat roof drains
  • Yard drains
  • Ditches.

Depending on the nature of the soil, they can also help locate certain inadequate foundation drain connections.

Inspection of sump pumps and foundation drains

Although foundation drainage should ideally be done on the surface, at a good distance from buildings, indirect collection from foundation drains is often an important factor explaining the presence of parasitic water in sewer systems.

Inspecting sump-pump connections with tracers is a very effective method for searching and locating  parasitic water collection from foundation drains.

It is possible to determine where the water collected by the foundation drains is discharged.

It requires the collaboration of the owner of the building, because an inspector will have to enter the building.

The inspector will put a non-staining dye and some water into the pump’s intake well, while another inspector observes the discharge outside the building, whether on the surface or in a storm or sanitary sewer manhole.

Photos are taken with the inspection as evidence for the production of the report.

This technique should be considered when there is a high flow rate in the sanitary sewer system in the spring or for several hours, or even several days, following heavy rain.

A preliminary evaluation of flow rates can be made from the operating times of the pumps at the pumping stations or by the prior performance of continuous flow measurements in the network.

Continuous sewer flow and rainfall measurements

This method is used to identify and to quantify the effect of precipitation and the level of the groundwater table on the flow rates to the sewer network (domestic or unitary) and to deduce the infiltration rate, the domestic flow and the abstraction flow.

It is also possible to determine whether it is a direct or an indirect capture.

Alain Bédard
Alain Bédard
Team Leader

A member in good standing of the Ordre des ingénieurs du Québec since 1994, Mr. Alain Bédard has participated in several hundred flow measurement and wastewater sampling campaigns, for industrial effluents and municipal sewer networks. His many years in the field, combined with the compilation and analysis of results, as well as the management of project teams, make him a key asset for your characterization and measurement projects, for your sewer network diagnosis or for your flow measurement system verification.

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Contact the sewer network diagnosis specialists at Avizo Consulting

Assessment of pipes and manholes by video camera

Infiltration through pipe joints and manholes can constitute a significant continuous parasitic water supply, often indicating a poorly maintained network. Television inspections are very useful to locate these defects and evaluate the state of a section of pipes.

However, they do not allow the quantification of the infiltration problem, namely the daily flow rates and volumes of parasitic water infiltration carried by a section of the network.

Instantaneous flow measurements at night in the sewer network

The flow rate of a pipe leading to a manhole can be measured with the flow rates raised by sections technique at night.

By repeating the measurements from one manhole to the other, it is possible to calculate the additional contribution of a section of pipes to the infiltration of parasitic water.

These measurements must be taken at a time when there are few users on the network, between midnight and 6 a.m, for instance.

The instantaneous night flow measurement should be considered when a high flow is observed in the network in the spring or when little difference is observed between the day and night flows.

A preliminary evaluation of the flow rates can be made from the operating times of the pumps at the pumping stations or by a campaign of continuous flow measurements in the network.

The most commonly used technique involves the use of portable weirs. The flow can be read directly on the graduated scale of the weir but only after the flow has stabilized.

Alternatively, volumetric tests will be carried out for pipes with a low flow, less than 1.67 l/s, falling.

Due to its lower accuracy, a portable H/V flowmeter will be used as a last resort, when the other techniques are not adequate.

Detection of reverse connections to the stormwater sewer network

Reverse connections can be a significant source of pollution to wetlands, streams and lakes, and have the potential to affect drinking water supplies, aquatic habitat, recreational and sporting activities, aesthetics of the environment, the health and safety of citizens.

The release of contaminants into the environment through these reverse connections is prohibited by the Environment Quality Act. Sewage system operators are responsible for preventing and disposing of these spills appropriately.

After identifying the storm drains containing discharges of domestic origin, and summarily locating the pipes containing one or more reversed connections, the next step could be to carry out tracer tests to confirm the cases of reversed connections.

When many buildings are connected to the section, it may be advantageous to carry out smoke tests on the stormwater network beforehand.

Smoke test to the stormwater sewer systems to identify suspicious connections

The smoke test aims to detect the emission of smoke from plumbing vents, usually visible on the roofs of buildings indicating reversed connections causing wastewater discharges to the stormwater network.

This quick and relatively inexpensive technique allows suspicious connections to be identified over long stretches of network, without having to individually test all connections.

Any building where smoke from one or more plumbing vents has been observed will be considered suspicious. Suspicious connections must be confirmed by carrying out dye tests.

When smoke testing is carried out in an area with many flat-roofed buildings, it is generally advantageous to use a bucket truck so that the vents can be observed from up high, thereby reducing the number of dye tests necessary to confirm suscpicious connections.

Tracer test in the sanitary elements of buildings to confirm reversed connections

When smoke tests have been used as a first step in identifying suspicious connections or when few buildings are connected to a section of sewer, it is advisable to carry out tracer tests to validate the conformity of the connection to the sanitary sewer.

The tracer test technique inevitably requires entering the buildings, preferably after notifying the occupants and agreeing on an appointment to perform the tests.

The inspector injects a dye that does not stain in the toilets of a building. He then activates the flush, opens the taps of a lavatory or bath, in order to increase the flow to the sewer thus reducing the duration of the test.

Another inspector, in the street, will have opened the sanitary and storm sewer manholes serving the building being tested.

The presence of the dye in the stormwater sewer will indicate a non-compliant connection, while the presence of dye in the sanitary sewer will indicate a compliant connection.

The test will be repeated with sanitary elements connected to different plumbing groups, with different colors of dyes, in order to establish whether the non-compliance of the connection is total or partial.

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