Wastewater Infiltration and Inflow


Infiltration and Inflow

Infiltration and inflow are the technical terms referring to rainwater and/or groundwater that enter the sanitary sewer system through such sources as cracked pipes, leaky manholes, or improperly connected storm drains and roof gutter downspouts. Most infiltration comes from groundwater and most inflow comes from rainwater.

Why are infiltration and inflow big problems?

In addition to causing sewer spills, the additional flow from infiltration and inflow results in the need for larger sewers and treatment plants. This raises the sewer fees that residents and businesses must pay the government or private sewer agency to build, operate and maintain the sewers and wastewater treatment plants.

Sewer systems (sewer pipes and pumping stations) are designed to handle sewage flows from houses and businesses plus some additional flow from infiltration and inflow. Sewage flow rates used to design sewers have been developed over the years based on information obtained from water usage within the household and workplace. The exact volume of groundwater and rainwater (infiltration and inflow) entering the system, however, varies with location and is virtually impossible to predict. Infiltration and inflow entering the system can be much higher than the system's capacity when there is too much leakage due to infiltration from deteriorated sewer pipes or significant sources of rainwater inflow.

The infiltration and inflow that enters the sewer system is transported to wastewater treatment plants along with the sewage. The groundwater and/or rainwater mixed with the sewage can double and even triple the design capacity of the treatment plant. Like the sewer system, the treatment plants are generally designed and constructed to accommodate the expected sewage flows plus some infiltration and inflow, but not large volumes of groundwater and rainwater.

When large volumes of infiltration and inflow increase the wastewater flow, the sewer system is overwhelmed to the point where a sewage spill can occur. The extra flow from infiltration and inflow simply causes the sewer system capacity to be exceeded. Sewage spills pose a public health risk due to increased probability of human contact with harmful pathogens as the sewage runs down the street to the storm drains, the streams, and eventually our recreational waters. Devastating backups of sewage into homes can also occur. In addition to causing sewage spills, the high flows can also affect the ability of the treatment plant to adequately treat the wastewater.

How does this affect the sewer fees that everyone pays?

In many cases, the City will deal with heavy infiltration and inflow by increasing the size of the sewer pipes, pumping stations, and treatment plants.

Constructing large sewer lines to handle high infiltration and inflow is very expensive and has its own problems associated with it. For example, large sewer pipes tend to result in sluggish flow during normal low dry weather flows. This causes the organic matter to putrefy and generate gases that are both odorous and corrosive to the sewer pipes. The corrosive gases shorten the life of the sewer lines and manholes, which increases your sewer bill even more!

At the sewage treatment plant, high infiltration and inflow can result in a significant amount of money being spent to construct facilities that are rarely used. Sewer users pay for the higher maintenance costs as well as the added construction costs.

Who is responsible for the infiltration and inflow problem?

Although infiltration of groundwater is a concern, the large jump in flow caused by inflow of rainwater has the greatest impact on a sewer system. Through extensive studies on sewers in the U.S., it has been found that the greatest contribution of inflow comes from private property. Common inflow sources include direct connections from rain gutter downspouts, basement floor drains and drain tile, outdoor drains, and pool/pond overflow pipes connected to the sewer lines. Uncapped cleanouts and broken house sewer laterals also cause excessive rainwater to enter the sewer system.

Although these inflow connections at your home may alleviate the inconvenience of yard flooding and puddles, they have significant impacts to the sewer system, the sewer rates, and public health. It has been estimated that as much as 40% of the total infiltration and inflow is contributed by the "private" side of the sewer. The individual sewer user plays a significant role in minimizing sewer fees, promoting proper functioning of the sewer system (reducing spills), and protecting the environment.

What can you do to prevent and reduce infiltration and inflow?

The following are important actions that sewer users can take to help reduce infiltration and inflow:

- Inspect the rain gutters on your house to see if the downspout connects to a sewer line. Such connections are illegal. If the gutter downspouts are connected to the sewer line, have them disconnected. The large amount of water from the roof can cause a sewage spill. The rainwater needs to be directed onto your lawn and/or to the storm drain system.

- Look for and check your sewer clean out. The cleanout is usually a small pipe, about 4-inches in diameter, outside your house that is used to access the sewer lateral for cleaning. You will normally find it near the house (where the sewer lateral comes out) and/or near the property line (where the sewer lateral connects to the City’s sewer lateral). Make sure the cap to the cleanout pipe is not missing and has not been damaged (such as by a lawn mower). Replace missing caps so that rainwater cannot get into the sewer line. If it is the city’s, call the sewer division and they will replace it. Kids love to throw rocks, toys and other nasty things down an uncapped cleanout! By keeping the cleanout capped, you can also prevent unpleasant sewer odors and gases from escaping.

  •   Check to see that outdoor patio, deck or yard drains are not connected to the sewer. Also, be sure that pool or pond overflow drains are not connected to the sewer. These connections are not allowed by the plumbing code. You may want to call your plumber to assist you in checking your connection. You can also try calling the city’s sewer division for assistance since they often have personnel that can trace lines and have a strong interest in keeping rainwater out of the sewers. If you are voluntarily taking steps to find and correct the problem, it is unlikely that you will be fined for the illegal connection(s).
  • If you live in a low area with a high water table, and/or experience a lot of settlement on your property, you may want to have your sewer line checked for cracks, separated joints, or "sags" that could cause entry of rainwater or clogging problems. Many plumbers now have miniature video cameras that can be sent down your line to check if the line has any significant damage or other problems.
  • Avoid planting trees and shrubs over or near the sewer laterals. This also applies to sewer mains that may be in yard easements. Roots can enter and damage sewers. This allows groundwater and rainwater to enter the sewer and also causes costly ongoing problems with sewer clogging, backups and spills.
  • If you have a basement sump pump to pump out groundwater or rainwater leakage, be sure that it does not connect to your sewer pipes or to a sink or floor drain in your basement. This would be another source of unwanted excess flows that can overload the sewer system.
  •   If your area is experiencing flooding, NEVER try to drain the areas by removing the sewer manhole covers in the street or covers from your cleanouts. The huge amount of flow that would enter the sewer system will definitely cause a problem downstream. Notify the City if you observe or know of someone doing this.