Smoke tests in search of sewer infiltration and inflow

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.

This is what we call sewer infiltration and inflow. Infiltration generally occurs through the introduction of groundwater into pipes, on a continuous or intermittent basis, depending on the level of the groundwater table. Water generally enters through defective pipes or manhole joints, cracks, etc. This phenomenon is therefore generally more important in the spring when the water table rises, following the spring melt. Flow monitoring at different manholes along the sewer line, or even camera inspections, are usually used to locate problems.

When the flow rates react very quickly following intense precipitation (e.g. summer storm), the network is then subject to one or more sources of surface water, whether it’s by connections to catch basins, gutters, flat roof drains, channeled ditches, etc. In this type of situation, the smoke test technique becomes a very effective tool for locating potential sources of surface water introduction. Smoke is injected via manholes in the sanitary sewer network, and any release of smoke on the surface can represent a potential source of sewer infiltration and inflow. These sources are photographed and mapped, with the anticipation of possible corrective work. In preparation for smoke testing, it is important to notify all users and the fire protection service, given the risk of smoke being introduced into buildings through faulty plumbing joints, floor drains, etc. Although the smoke generated is not toxic, it is indeed very important to inform occupants of the precautions keep at a limit the introduction of smoke and reduce the inconvenience that follows.

Note that smoke tests can also make it possible to locate, under certain conditions, foundation drains connected to the network which can represent indirect but important sewer infiltrations and inflows. However, sometimes the nature and thickness of the soil covering the drains does not allow the smoke to spread up to the surface. It is then necessary to opt instead for the dye or camera testing technique.

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.

The Guide méthodologique pour la recherche et l’élimination des raccordements inversés dans les réseaux de collecte d’eaux usées municipals describes the procedure to carry out the detection. After having identified and located the storm drains, Avizo can support you in this process, generally including the following activities:

• Inspection and sampling of drains in dry weather
• Smoke tests in the rainwater drainage system
• Dye tests on sanitary devices in targeted buildings

Inspection and sampling of drains allows the targeting of drains with upstream wastewater connections. During a dry weather inspection, a sample is taken for laboratory analysis (usually fecal coliforms). In the absence of flow, the investigation will be carried out by adding a mesh and carrying out a new inspection a few days later to assess the evidence of reversed connections (toilet paper, feces).

The Guide then recommends carrying out flow monitoring at different manholes along the sewer line, by visual inspections from manhole to manhole, to determine the origin of the wastewater. From experience, it can be laborious to proceed this way, particularly if the number of buildings connected to the network is large, but also because of the natural flow coming from the groundwater. A very effective and inexpensive technique used to limit the number of buildings to be examined is to carry out smoke tests in the rainwater drainage system. Any release of smoke from a plumbing vent in a building then indicates a very high potential for reverse connections, which should then be tested with dye. The same applies to all buildings for which it is not possible to see the vents. Using a pod or drone can make it easier to observe smoke releases from vents.

The final stage of screening involves performing dye tests. This technique inevitably requires entering the buildings, preferably after notifying the occupants and agreeing on an appointment to carry out 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.