When ground water and stormwater enter city collection systems – known as inflow and infiltration (I&I) – treatment plants become less efficient and systems become strained. This additional water can also cost cities money. Here are some ways to identify and address the issue.
Inflow happens when groundwater and stormwater seep into the sanitary sewer system through private and public defects within the collection system. Residential homes or businesses can contribute to the issue in a number of ways – downspouts, sump pumps, driveway drains, stairwells and even streams can contribute to the inflow. Public infrastructure can contribute when a storm sewer is cross connected with a wastewater system. Leaky or vented manhole covers also contribute.
Infiltration is when groundwater enters the sanitary sewer system through faulty pipes or manholes. These pipes might have cracks or leaks that let the water in. This can happen because of age, design, installation or maintenance issues or even tree root intrusion.
The infiltration within the 18 in. pipe shown below was caused by a crack leaking approximately 8 gallons per minute. This equates to 4,204,800 gallons per year for a cost of approximately $10,500.
I&I water is referred to as “clear water,” distinguishing it from sanitary sewage water. When clear water gets into the wastewater or sewer system, it gets treated. And when it gets treated it costs cities money. It also takes up valuable capacity within a collection system. This is a problem for cities whose collection system may already be working at maximum capacity. Clear water also takes up valuable capacity at the wastewater treatment facility.
SEH Wastewater Engineer Susan Danzl says, “Accommodating I&I at the treatment facility results in oversized, overpriced treatment systems and can cause inefficient treatment, add to treatment costs and potentially lead to illicit discharges into lakes and rivers because the treatment plant capacity has been reached.”
Cities must often decide between investing dollars to reduce I&I or limiting new residential or economic development. New development can strain collection systems already at their capacity, leading to further repairs. But, limiting residential or economic development opportunities stifles a city’s growth and sacrifices opportunities to capture tax revenue. Both can create a financial burden.
The following rendering shows common examples of inflow and infiltration:
According to SEH Water Resources Project Design Leader Spencer Cossalter, a single leak from a joint in a manhole or pipe can generate 7,200 gallons of water each day. After being treated at the wastewater plant, that water can translate into an annual cost of $6,500 for a city. Multiply this cost by the number of leaks across the sanitary sewer system and it could mean a hefty price tag for a city.
According to Susan, your city may have an I&I issue if you experience any of the following:
There are a number of strategies cities can undertake to find problem connections and leaks in order to reduce or stop I&I:
The first step in identifying I&I is to identify where problems are occurring and isolate the areas that have the worst I&I. Set up flow monitoring instruments in each of your city’s sewersheds. By monitoring the flows, you can better identify which areas may have problems.
Inspection of manholes can identify leaks from joints and provides the opportunity to review the drainage near a structure. Manholes can be located within a wetland or ditch that gets submerged during rain events. These manholes can contribute significant inflow and infiltration through leaky covers. Three-dimensional manhole scanning technology can inspect manholes and uncover these defects.
Fortunately, there are several actions you can take after a manhole inspection reveals I&I. Here are a few:
1. Replace the manhole covers.
a. Cost – approximately $200 per cover.
b. I&I reduction could be $350 per year or more if the covers are submerged during rain events.
2. Chemical grouting to seal up leaky joints.
a. Cost – $500 and up.
b. Reduction could be as much as $5,000 per year per manhole.
3. Line the manhole.
a. Cost – approximately $3,000.
b. Could reduce costs by $20,000 per year per manhole.
This method for identifying I&I typically involves residential areas. According to SEH Water Resources Lead Technician Paul Kubesh, "A blower is set up over a neighborhood manhole, and non-toxic smoke is pumped through the sewer line. The smoke is pressurized and follows the path of any leaks in the system, revealing the location of the inflow and infiltration." Sources of I&I can be cracks in the sewer pipes themselves, roof drains, cleanout caps inside homes, damaged or faulty manholes, or a cross-connection between a storm sewer and a sanitary sewer.
After smoke testing reveals I&I, there are a number of things you can do to reduce or eliminate it. However, depending on where the leaks are found, the fixes can fall on homeowners. These are things like disconnecting the roof drains and fixing the cleanout caps. If the testing reveals leaks in the city connections, you may have to eliminate the storm and sanitary cross connections. This can be costlier but yields great savings over time. A vent improperly connected to the sanitary sewer can produce more than 50 gallons per minute during a rainfall event. This can cost thousands of dollars each year in treatment costs and increases the potential for exceeding the capacity of the collection system.
Dye testing can be used to identify leaks and confirm smoke testing results and uses water mixed with a non-toxic dye. The colored water is pumped through the ground and stormwater system and appears in the sanitary sewer collection system where leaks occur.
Cities can peek inside their pipes through the use of CCTV. A small camera travels down the length of a pipe and produces visuals where an operator can identify leaks by way of a monitor viewed at the surface.
CCTV inspections can reveal the need for rehabilitating leaky pipes. Pipe rehabilitation options can include cured in place piping (CIPP) trenchless rehabilitation, chemical grouting to seal up leaks and open cut replacement of pipes.
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This inspection shows I&I as evidenced by mineral deposits and active flow from joints and a crack within the 12 in. reinforced concrete pipe.
In addition to CCTV, Electroscan can approximate I&I in pipe segments. The technology uses low-voltage electrical currents to assess sewer pipes. By sending a low-voltage electrical current through a non-metal pipe (water or sewer pipes made of brick, cement, concrete, plastic or pipe-lining resins), Electroscan measures the variation in the electricity passing through any pipe defects.
Related Content: Learn more about Electroscan in this video.
Residential sump pumps and foundation drain lines are sometimes improperly hooked up to the sanitary sewer laterals. A single sump pump can send more than 7,000 gallons of water to the system during a rainfall event. That’s about the same as the average daily flow from 18 homes!
According to Spencer, a properly constructed and maintained sanitary collection system will result in acceptable levels of I&I each year. These levels vary from city to city and system to system. As the system ages, it becomes more susceptible to surface and groundwater infiltration and increased maintenance problems and if it not addressed. This can lead to increasing collection and treatment costs over time.
“A single I&I defect caused by a joint leak in a manhole or collection pipe can result in treatment costs of up to $30,000 over the life of the wastewater treatment plant,” says Paul.
The best way to identify these connections is through a dedicated inspection program, which includes evaluation of the sewer ordinance.
In the City of Foley, Minnesota, the City inspected more than 800 properties and found 224 of the homes to have foundation drains connected to the City’s sanitary sewer lateral. Push cameras (CCTV) from the buildings located the pipes connected to the City main.
If the results from private property inspections indicate I&I, here are some solutions:
Sump pump discharge relocations
a. Cost – $100-$1,000.
b. Could reduce costs up to $500 per year depending on the amount of flow through the sump pump.
Foundation drain disconnections and sump pump installations
a. Cost – $500-$5,000.
b. Could reduce costs up to $1,000 per year depending on the amount of flow.
Service lateral lining and/or replacement
a. Cost – $1,000-$8,000.
b. Could reduce costs up to $750 per year depending on the amount of flow through the lateral joints and cracks.
Download this worksheet to help identify I&I
Quick Reference Guide
Case Study: City of Eagan, Minnesota
A significant rainfall event in Eagan resulted in an excess of peak flow allocations to the regional collection system and treatment plant owned and operated by the Metropolitan Council Environmental Services (MCES). The culprit of the excess flow was internal I&I. According to Russ Matthys, Eagan Director of Public Works, the MCES determined the City would need to spend $1.7 million over five years ($343,700 per year) to either mitigate their internal I&I or pay that amount to the MCES for the equivalent expansion of the Metropolitan Disposal System through an annual surcharge program. If excessive flows were to continue after a specified date, the annual surcharge would become a permanent demand charge.
After inspecting and addressing all I&I issues within City sewer pipes and manholes, staff realized they would also need to address private sewer systems. However, inspecting nearly 18,800 private connections would be no easy task. As the project would likely take seven to 10 years, the City hired SEH for a sump pump and service lateral inspection program for private systems.
With the assistance of SEH, Eagan was able to inspect 99%of private properties in less than five years. The inspections found that 5% of properties in Eagan had one or more factors contributing clear water (groundwater or rainwater) to the sanitary sewer system. The addition of clear water into the sanitary sewer system puts strain on equipment and infrastructure, and results in higher sewer rates for residents. As a result of these inspections, more than 750 required repairs were completed. Since the inception of the I&I program, Eagan has seen a decrease of more than 9% (given similar precipitation conditions) in wastewater being sent to its wastewater treatment facility. They were also able to reduce sewer rates for their customers. Eagan's annual wastewater rate increases continue to remain noticeably below the average wastewater rate increases of other metro cities.
I&I adds clear water to a city’s sewer system. This can cause back-ups, system strains and interruptions. Because the water is treated along with wastewater, the costs add up. Cities can make arrangements to correct the problem by proactively identifying I&I.
Spencer Cossalter, is an SEH water resources project design leader dedicated to identifying and eliminating sources of I&I. Contact Spencer
Susan Danzl, PE*, is an SEH wastewater engineer who helps cities maximize their treatment facility capabilities, saving time, money and resources.
Paul Kubesh is a water resources technician committed to helping cities save money through identifying and resolving I&I.