Drop of Knowledge – Taming Inflow and Infiltration in Sanitary Sewer Systems

March 3, 2020 | Blog

Joseph Lawrie, Great Lakes Community Action Partnership (GLCAP)

The effects of rainfall are easily seen on our streets, walkways, and green spaces.  Storm surges, no matter the size, activate ground water flows through existing pathways.  Heavy rain makes daily transportation more difficult and more dangerous.  What is often unseen are the effects of this same rainwater on our sanitary sewer systems.  Once the ponds and puddles drain away from the surface, where does the water go?  Some may end up channeled to water bodies by storm sewer collection systems through catch basins, bio-swales, and detention ponds.  Some is absorbed into the ground to filter through the soil and sediment back to the ground water table.  Ultimately, all of this water continues its never-ending path through the water cycle.  However, before being returned to nature, some of this water finds its way into our sanitary sewer systems which can cause negative impacts on resource-strapped communities.  This is known in the utility industry as inflow and infiltration (I&I), and it affects nearly all wastewater collection systems across the country. 

Modern sewer systems are built with a specific purpose – either to channel storm water safely to natural waterways or to collect wastewater and bring it to a facility for treatment.  In the past, these two purposes were often served by a single sewerage system.  In fact, sewer piping, which serves as the backbone of any sewer system, long pre-dates the invention of the wastewater treatment plant.  Therein lies the problem.  A treatment plant is only capable of treating a fixed volume of water; the weather is unpredictable and capable of overwhelming designed capacity at a moment’s notice.  A wastewater plant that cannot treat the full volume of water brought to it by the collection system can overflow and allow untreated waste to enter natural water bodies.  To effectively treat wastewater, natural ground water and stormwater must be kept separated from soiled wastewater in the sanitary sewer system.  Preventing rainfall from finding its way into sanitary sewer pipes is difficult to attain in practice. 

No sanitary sewer system is completely water-tight, and there are numerous ways by which stormwater can stealthily enter wastewater collection systems.  Rainwater can directly enter sewer main lines through cracks and deteriorated joints.  Anywhere two pipes are connected, or where a pipe ties into a manhole, there is a potential vulnerability.  Materials used to seal pipe joints and fittings often have a shorter useful life than the pipe itself, eroding away while the pipe remains structurally sound.  Rainwater also enters through piped-in connections either unintentionally or on purpose.  This is seen when yard drains, sump pump discharges, and roof downspouts are allowed to drain into the sanitary sewer lateral (the section of pipe serving a single building) which leads to the main sewer line.  Since I&I is occurring below grade, its effect is seen at the wastewater plant, not where it occurs. This makes pinpointing locations of I&I difficult.  Locating and eliminating I&I is an art that combines several tools to study potential sources.

While there are many tools used in the field to attack sources of I&I, an effective removal campaign starts on paper.  Knowing the layout of the collection system, the boundaries of each sub shed, and additional, non-utility data is the best way to determine where to start hunting.  A good map of the sewer system is a must.  For any tool to be effective it is imperative to know what is connected and where.  A good workflow is to use a GIS layer including sewer main lines, manholes, lift stations, and treatment plants.  Stacking this information with flood plains and hydric soils gives an indication of areas that are prone to the saturation of ground water, which can drain into the sanitary sewer.  RCAP has used this method to narrow down target areas in rural communities, where it is not feasible to study the entire collection system at once.  After studying the details of the sewer system, tools and resources can be focused on the highest risk areas and yield the best results. 

One such study was carried out by RCAP in the Village of Sparta, Ohio.  After creating a GIS database from existing mapping, the collaborative team of RCAP and village staff broke out the small collection system into three main sewer sheds.  Each of these sheds was then tested using a smoke machine.  By staying within the bounds of each individual sewer shed, the village and RCAP systematically investigated over several days.  While smoke testing it is important to record all observations of visible smoke seen exiting the sewer system.  In a properly plumbed home (or any structure) the smoke follows the path that sewer gases would in order to vent out of the sewer.  The smoke should move through the main line, up laterals, and pass through homes out of the roof vent without ever escaping into the building’s interior.  In this study, and in general, observing smoke coming from a roof vent is a good sign.  However, just because a roof vent smokes does not mean that there aren’t any other problems occurring simultaneously.  It is imperative to continue blowing smoke into the sewer long enough to allow the field crew to walk each property looking for signs of smoke from the roofs and the grounds in the entire search area.  Where smoke comes from outside the home during this test indicates that this area is often a contributor to I&I.  Examples include downspouts, yard drains, and defective laterals.  In some cases, the entire lateral line can be seen by smoke escaping cracks and joints all the way to the main.  It is highly important to document the exact location of these observations so that corrective action can be taken.  Smoke observed, or reported, escaping into homes or businesses should also be noted.  These are generally not large contributors to I&I but pose a health hazard to the resident.  Examples of sources within buildings include dry traps in floor drains, uncapped clean outs, and defective vent pipes.  While these issues are not the main interest of an I&I study, they are an indication that dangerous sewer gases could enter a building and cause harm to the occupants.

After the smoke cleared in Sparta, the village and RCAP identified only a few apparent sources of I&I.  Being a relatively new system, Sparta had a plastic main line pipe with good joints, and no major breaches were found.  The village had also undergone an inspection program that removed downspouts from the sanitary sewer.  The biggest potential sources uncovered included unsealed manhole lids and castings that were in drainage ways.  These were suspected of contributing to I&I during wet weather events.  No downspouts were found to be connected, although several uncapped clean outs were found in yards.  Several homes and buildings had smoke in their interiors, which was reported to the home owners with potential repair recommendations. 

This project, and many others, serve to illustrate that often I&I occurs because of the accumulation of many small sources instead of a few large holes in the system.  Operating a small rural wastewater plant is challenging due to limited capacity, but studying the collection system in detail – and examining and documenting both public and private property sources – is critical to removing significant amounts of I&I.