The design of a sewage system allows for multiple homes and businesses to dispose of wastewater through properly connected sanitary features. With proper maintenance, these systems can handle as many as 200 homes on a single 8-inch sewage pipe. However, sewer lines can become compromised due to inflow and infiltration in the system. (To learn more about sewer systems, check out The Complex World of Sewer Networks.)

What Are Inflow and Infiltration?

Both inflow and infiltration are the addition of waste or groundwater into a sewage system. While both contribute to the problem, these issues have slightly different means of entry from each other.

Inflow

Inflow is stormwater that enters a sewer through points directly connected to the sewage lines. Examples of these connections include sump pumps, drains, downspouts and streams.

Often the entry points are improperly connected to a sanitary fixture such as a bathtub drain. Joining a pipe or sump pump to an existing fixture may be illegal in some municipalities. Improperly or illegally connected lines can add as much as 7,000 gallons of water per day to a sewage system.

Infiltration

With infiltration, groundwater enters the sewage system through cracks or leaks in the lines. Cracks in the system allow the entrance of groundwater whenever the soil becomes saturated, or the water table lies above the sanitary waste system.

Infiltration occurs through age-related deterioration, poor design, improper installation or maintenance errors. Damage to lines from external factors also allows for penetration into the system. Sewage line design allows for 20 to 50 years of use. If not regularly inspected or maintained, cracks can form undetected.

Maintenance workers distinguish inflow and infiltration from sewage by calling infiltrated water “clear water,” even if the water is dirty. The clear water distinction is merely a way of stating it does not have sewage in it.

How Do Inflow and Infiltration Affect Sewage Systems?

The purpose of sanitary systems is to transport wastewater from showers, toilets, sinks and dishwashers. Water from these lines must be specially treated at wastewater facilities. Groundwater should flow through stormwater lines. These lines do not necessarily require the same wastewater treatment before discharging their contents into the environment.

When inflow and infiltration occur in the sewage system, the lines become overwhelmed by the additional clear water. It fills the sewer. Once at capacity, the sewer overflows – flooding basements, streets and sidewalks. Communities downstream from the overflowing sewer system are often most affected. Due to their location, they must accommodate not only the flow from their sewage system but also the overflow from all communities upstream.

Overflowing sewage systems are a public health risk, as not only does clear water, but also waste containing bacterial pathogens back up onto the street. The wastewater can also back up into creeks, rivers, lakes and streams, contaminating water supplies and endangering wildlife.

In addition to the potential health risks associated with infiltration, the additional water within the lines makes it more difficult for wastewater treatment plants to properly treat the volume that comes through the plant. As a result, poorly or untreated waste is ejected into the environment.

Overflow due to infiltration and inflow is costly. Cities must pay to correct the problems. There are also costs associated with cleanup and potential lawsuits from affected homeowners. There may also be fines related to damage caused by overflow if the issue is not corrected.

Signs of Infiltration and Inflow

Unfortunately, if cities do not regularly maintain and inspect their sewer systems, the signs of inflow and infiltration may not be evident until an overflow event occurs. Such an example would be the backup of pipes, flooding commercial and residential basements. Water flooding streets from manholes is another indicator.

Wastewater treatment plants may see an uptick in the amount of water passing through the system after a rain or snow event. Continually running or starting and stopping in a sump pump during one of these events or immediately following rain or snowmelt is another indicator.

Detecting Inflow and Infiltration

There are techniques cities can use to identify inflow and infiltration before it becomes a problem. (To learn more about underground pipe inspection, see Your Comprehensive Guide to Condition Assessment of Buried Pipelines.)

Manhole Inspections

Periodic manhole inspections help to identify joint leaks or cracks from root infiltration. Cities should pay particularly close attention to manholes in lower-lying areas, such as ditches or wetlands, as these tend to get submerged in rain events.

Smoke Detection and Dye Testing

In residential areas, cities may use a technique known as smoke detection to locate cracks. During this test, cities pump a nontoxic smoke into the line, pressurizing it. The pressurized smoke follows the path of the pipes and highlights leaks in the sewage system.

Along with smoke testing, some areas use dye testing, often as a way to confirm smoke testing. With dye testing, workers pump water with a nontoxic dye into the stormwater system. If there is a leak in the sewage system, the color will confirm it.

Line Inspections and Electroscans

One of the best ways to find cracks in sewage systems is through a pipe inspection using closed-circuit television, or CCTV. Using a small camera to travel through the pipes, workers can see the interior of the lines on a CCTV while remaining topside. The camera rotates 360° to give a full view of the inside of the pipe.

Alongside camera inspection, workers sometimes employ a low voltage electrical current through nonmetal pipes. This procedure is known as an electroscan. The current measures variations in the electricity to detect pipe defects.

Trenchless Techniques to Rehabilitate Inflow and Infiltration Problems

To combat inflow issues, fixing and maintaining the joints to the sewage line along with ensuring the lines are properly connected is the only way to ensure the problem is solved. Rehabilitating pipelines is the only way to ensure infiltration issues cease.

Chemical Grouting

Using robotic technology, workers can seal the crack in a pipeline with grout. The robot maneuvers through the pipe. Once it reaches the location of the damage, the outer layer expands, pushing grout into the cracks and sealing them.

Cured-in-Place Pipe

Cured-in-place pipe (CIPP) is a liner permeated with resin. Workers insert it into the affected line and pull it into place. Once in place, the liner hardens, forming a new pipe. The old pipe helps to support the new coating, which blocks infiltration and inflow from outside sources.

Unlike other lining methods, CIPP does not need to have the annular space filled. The liner expands to fill the space. CIPP does not diminish the pipe size by as much as other lining options, although there is some narrowing.

Sliplining

Sliplining is similar to CIPP. Workers slide a liner into the existing pipe. This liner is much narrower than the existing pipeline, and the annular space between the liner and existing pipe needs to be filled with grout.

Sliplining can be as one continuous pipe, segmental or spiral wound. Each method requires lateral pieces to reconnect by remote cutter or excavation. Sliplining, unlike CIPP, cannot go around bends and corners in the pipe. Workers must use separate liners and then connect them to go around curves.

Sliplining methods do require an excavation pit, as many projects cannot use existing manholes as an access point.

Pipe Bursting

Some pipelines are so compromised that rehabilitation is not an option. Workers must install new lines. The use of pipe bursting to break up the old lines and establish new ones make it the easiest method of installing new lines.

With pipe bursting, workers pull a bursting head through the existing line. The head breaks the pipe, pushing the pieces into the surrounding soil or out the exit trench. The new pipeline is pulled into place behind the head.

Inflow and infiltration are common problems facing city sewage systems. Without proper inspection and maintenance, ground and stormwater can invade the sewer system, causing potential overflow and hazardous health conditions. That is why it is so important to regularly inspect pipes and perform maintenance, as well as repairing any damage when it is discovered.