Inflow & Infiltration Reduction
What is Inflow/Infiltration?
The City of Naperville maintains two separate and distinct systems of pipes to move wastewater throughout the City. Rainwater and groundwater are typically routed to area rivers and streams through a network of storm drains and pipes called a stormwater system. Sanitary wastewater from bathrooms, sinks, kitchens and other plumbing is moved through a separate network of pipes and pumps (called a sanitary system) to the Springbrook Water Reclamation Center (SWRC), where it is treated and disinfected before being discharged to the DuPage River.
Each of these systems are designed to accommodate certain levels of flow to maintain optimal operation. Inflow and Infiltration (I&I) occurs when rainwater or groundwater enter the wrong system – the sanitary wastewater system – and overburden it.
Infiltration occurs when groundwater enters the sanitary system through defects in pipes and manholes. Cracks in pipes, leaking joints and unsealed manholes can all allow groundwater to infiltrate into the sanitary sewer system. Water entering the sanitary sewer system through inappropriate connections is referred to as Inflow. Examples of Inflow are sump pumps, roof drains, yard drains that are connected to sanitary pipes instead of the stormwater system.
Why is it a problem?
Every wastewater system experiences problems with I&I, particularly as pipes and manholes age. In sanitary sewer systems where I&I is particularly bad, rainwater and groundwater that has entered the system can easily overwhelm the ability of pipes and pumps to move water downstream. This can cause wastewater to back-up and, in some instances, flow back into residences. Excess wastewater in the system also causes pumps and wastewater treatment equipment at the SWRC to run for extended periods of time, which reduces the efficiency of the equipment and shortens their life spans. When wastewater equipment needs to be replaced more frequently, it can negatively affect residents’ wastewater rates.
Inflow and infiltration are most pronounced during periods of wet weather. The graphs below show two separate week-long snapshots of wastewater flow at the same location in the system during one wet week and one dry week. During the week of wet conditions, as rainfall increased, wastewater flow also increased as a result of rainwater entering the sanitary system. Conversely, during the week with dry conditions, wastewater flow remained constant.
Identifying I&I Issues
As wastewater systems throughout the country continue to age and infrastructure approaches its useful life expectancy, wastewater utilities are exploring different methods of identifying and reducing locations with I&I.
Identification techniques include visual inspection or one done with a Closed Circuit Television (CCTV) truck. A CCTV truck is equipped with camera equipment attached to a robotic crawler, which is driven through sewer pipes to inspect for any issues. The crawler relays video to the CCTV truck, where it is recorded, and an operator documents any defects with sewer inspection software. This is an invaluable tool for identifying sources of infiltration that otherwise would not be found.
Another common identification technique is smoke testing. This involves placing non-toxic smoke inside wastewater pipes and seeing where it becomes visible above ground. Visible smoke above ground reveals the location of inappropriate connections to the wastewater pipes (like if the smoke starts coming out of a yard drain) or defects in the system itself. For this reason, smoke testing is good at identifying not only inflow, but infiltration as well.
The use of flow meters, devices that measure the volume of water passing a given point, can also be useful to identify general areas where I&I is occurring. In a perfect system, there should be no noticeable difference in the volume of water in the sanitary system when weather conditions are dry versus wet. Flow meters that register large differences between the volume of dry weather wastewater flow and wet weather flow are a good sign that I&I is occurring upstream of the flow meter’s location.
Reduction of I&I can take many forms. Inflow resulting from inappropriate connections to the sanitary system can often be resolved by working with property owners to ensure that groundwater and rainwater are directed to the stormwater system instead of the sanitary system. Infiltration resulting from defects in sanitary sewer system pipes or manholes requires a different set of techniques for reduction. The two primary methods for reducing infiltration and rehabilitating the infrastructure are the grouting and/or lining of pipes and manholes.
One of the most effective methods for reducing infiltration is through the application of non-toxic chemical grout. The chemicals come in gel form and flow similar to water while being applied. The gel then hardens or "cures" into a consistency similar to rubber.
Grout is applied to sewer pipes by a piece of remote-controlled equipment called a "packer." A mainline packer is lowered into a manhole, placed in a sewer pipe and pulled into place. Grout is then pumped through hoses down to the packer. The grout is forced into a crack or joint and travels through to the exterior of the pipe. Once the grout comes in contact with the soil surrounding the pipe, it cures and forms a watertight collar sealing that point from any further groundwater infiltration.
Another effective method in addressing infiltration is through lining sewer pipe. CIPP or "Cured-In-Place-Pipe" allows existing sewer pipe to be rehabilitated without any digging.
The process involves inserting a flexible felt liner into an existing or "host" sewer pipe, where the liner is cured and forms a new, structurally-improved pipe within the existing pipe.
Prior to installation, the felt liner must be prepared by being injected with resin, which is what allows the liner to form to the host sewer pipe. Liners are usually installed one sewer pipe at a time, or from an upstream manhole to the next downstream manhole. The liner starts off inverted (like a pair of rolled socks) and is fed into the sewer pipe through a manhole. It is then pushed with air pressure or water through the pipe to the next manhole. While being pushed through, the liner is un-inverted. Once the liner has reached the next manhole, the curing process can start.
The resin in the liner is heated using hot water, steam, ambient temperature or UV light, accelerating the chemical process that hardens the resin. The resin must reach a certain temperature for an extended length of time to fully cure. If not fully cured, the liner can sag, creating a defective liner.
Once the liner has fully cured, a new "pipe within a pipe" is formed. A key factor in making CIPP so effective is that the new pipe is tight-fitting and jointless, which seals off any points of entry for infiltration. CIPP-lined pipe also has a life span of 50-100 years, reduces costs needed to maintain the pipe and improves the ability of wastewater to flow through the pipe.
What is Naperville doing to combat I&I?
Since the early 1990's, Naperville has committed significant resources (more than $3 million annually) towards the goal of reducing I&I using the techniques mentioned above. City staff take a holistic approach when planning and completing rehab projects, taking care to match the proper technique with the existing field conditions and desired outcome. Planning for rehabilitation begins with flow monitoring from I&I studies, which help to form a general 10-year guide for rehabilitation.
Generally mainline pipes are the first to be rehabilitated, followed by manholes, and finally individual service lines. The number of assets scheduled to be rehabbed per year are prioritized and can vary based on a number of factors including asset condition and budget. Assets in the same geographic area are all rehabbed before moving on to other areas. All assets in a geographic area can be completed in as little as two years or up to several years-time. Assets that are candidates for grouting typically receive grout prior to any lining in the area.
Lining Qualified Sanitary Sewer Pipe
"Cured-In-Place-Pipe" (CIPP) lining is primarily performed on sanitary sewer pipe constructed of clay or truss. These materials are fragile and qualify for rehabilitation because they are susceptible to leaking joints and invading roots. Other pipe materials are only lined in circumstances where it is more cost efficient to line the pipe rather than perform a different type of rehab.
Clay pipe was an industry standard in the post-WWII building boom. Just under 1.25 million linear feet of qualifying pipe was installed in Naperville between 1960-1990. In the late 1980's, the use of polyvinyl chloride (PVC) pipe and improved construction practices reduced the need for rehabilitation for relatively new development.
The graph below shows the total lineal footage of pipe qualified to be rehabbed (in black) compared with the total lineal footage that has been rehabilitated to date (in blue). Due to the low likelihood of any additional clay or truss pipe being installed, the qualified install length will most likely remain stagnant. Each year that rehab work is completed, the rehab length (blue line) will increase, reducing the amount of sewer pipe left to be rehabbed.
Manholes are typically constructed of multiple pre-cast concrete segments with their joints sealed. Older manholes were often constructed of brick and mortar. After construction, manholes are vacuum tested to ensure they are sealed properly, but over time the materials degrade and leaks occur. As with pipes, manholes can be grouted or lined. The process of grouting manholes differs from sewer pipe in that a packer is not used. Instead, a worker can apply grout with hand-held equipment connected to the grout truck. Grout is very beneficial when applied to manholes as it fills voids and stabilizes soil around the exterior of the structure.
Rehabbing Sewer Laterals
Naperville also addresses I&I through the rehabilitation of sanitary sewer laterals. Laterals are smaller diameter pipes that extend from the sewer main to a building. Even though these pipes are privately owned, maintenance tasks are divided between property owner and the city depending on the nature of the work. Laterals can be grouted and lined just like manholes and sewer mains. Naperville typically performs rehabilitation work on laterals from the sewer main to a sanitary cleanout. This cleanout is installed and used as an access point at ground level (pictured left) during the rehab process on the lateral.
Naperville's proactive approach to combating I&I has yielded positive results. Since 1998, the population of Naperville has increased by roughly 30,000 people, or about 20%. During that same period, however, the average amount of water treated at the Springbrook Water Reclamation Center has decreased by roughly 18%. Reducing the amount of I&I also allows Naperville to remain in compliance with Illinois Environmental Protection Agency (IEPA) regulations limiting sanitary sewer overflows and water quality discharged to area waterways.