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Correcting Inflow & Infiltration in Sanitary Sewer Systems

Sanitary sewers are designed to transport wastewater to treatment facilities. When water from other sources enters the system, known as Inflow & Infiltration (I/I), the efficiency of the wastewater treatment plant decreases as this increasing volume of waste hits the system.

Listen in as our water resource engineers discuss the significance of I/I and examine techniques used to identify and eliminate the problem.

Podcast Agenda

  • Understanding the Difference Between Inflow & Infiltration  (0:34)
  • Inflow & Infiltration Increases Operating Costs  (1:35)
  • Determining if your System has Inflow & Infiltration Issues  (2:08)
  • Identifying if your Sewer has Inflow or Infiltration  (3:34)
  • Budgeting & Identifying Funding Options for Sanitary Sewer Rehabilitation  (5:38)
  • Common Sewer Investigative Techniques  (9:34)
  • Common Infrastructure Improvement Methods  (11:52)
  • Snyder & Associates’ Experience Solving Inflow & Infiltration Issues  (14:55)
  • Thinking Big During Capital Improvement Planning to Include I/I Needs  (17:30)
  • Inflow & Infiltration Success Stories  (19:49)


Lindsay Beaman, PE

Business Unit Leader

Lindsay Beaman, PE

Business Unit Leader

Water System Analysis & Planning, Design & Construction, Wastewater Treatment Design, Municipal Engineering

Wes Farrand, PE

Civil Engineer

Wes Farrand, PE

Civil Engineer

Combined Sewer Planning & Design, Hydrologic & Hydraulic Analysis, Water Main design

Emily Wicoff, PE

Civil Engineer

Emily Wicoff, PE

Civil Engineer

Water & Wastewater Planning and Design, Environmental Compliance, Site Design, Funding Procurement, Permitting, Public Engagement

Full Transcript

Lindsay Beaman (0:20)

Hello and welcome to today’s podcast. I am talking with Emily and Wes about inflow and infiltration. First off, Emily, can you tell me what is inflow and infiltration? I think we sometimes call that I/I. Can you tell me what those mean?

Understanding the Difference Between Inflow & Infiltration

Emily Wicoff (0:34)

Yes. So, I/I is when stormwater or groundwater enters into city pipes and infrastructure that’s intended only for sanitary sewer wastewater. Inflow occurs when, say a house or building sump pump or tile drain discharges to the sanitary system, which results in an influx or increase of stormwater during rain events.

Infiltration, on the other hand, is when aging pipe infrastructure results in groundwater entering the sanitary sewer pipes and manholes through cracks or other leak points in the system. Similar to inflow, groundwater levels rise when it rains and the soil becomes saturated. So you see an increased infiltration flow during rain events as well.

So why does this matter? Well, the sanitary sewer pipes and receiving treatment facilities are typically sized for sanitary sewer flow only. When the flow increases above what the system was designed for several undesirable things can start to happen.

Inflow & Infiltration Increases Operating Costs

Lindsay Beaman (1:35)

 Since residents don’t typically have a sewer bill, I mean, there’s still that excess sewage going somewhere and someone is paying to make sure it’s managed, correct?

Wes Farrand (1:44)

Yeah, a lot of systems have pump stations, and increased flow is going to require that pump to run more often, which is going to require more power, more wear and tear on the system, might require repair and maintenance sooner, costs increase there. And a lot of times that’s buried costs cause the average citizen won’t see it, but it is going to be reflected in the operating budget of the system and that operating budget has to be made up with either taxes, sewer user fees, or things like that at some point.

Determining if your System has Inflow & Infiltration Issues

Lindsay Beaman (2:08)

Okay. Emily, can you tell me how do we know that we have an I/I problem or how do we define the scope of this?

Emily Wicoff (2:16)

Well, typically communities become aware of an I/I problem if they have a pipe that backs up or a manhole that overflows and they get calls about that flooding. Often, they can notice that they have a lift station that is constantly running or they might get a spike inflow at the wastewater treatment plant during or directly following a rain event.

Wes Farrand (2:36)

I’d add to that too, if I may, that sometimes you may have an issue and the community may not even realize it. It’s just part of how it’s always been operating. When you design, you design for mostly just your sanitary sewer flows but there is a factor in there for some peaking that you’re going to get, no matter how tight you build the sanitary sewer system. But there’s excessive peaking that can come, especially with older systems, that is beyond that design level anticipated peak flow. Sometimes you might notice, like Emily said, in one of those immediately tangible ways, but other times it might take a study with flow meters or other ways to measure flow during the dry season, base-level flows, and then compare that to what kind of flows happen during wet weather events to see what that peaking factor is. And there are some guidelines on what is a typical peaking factor versus what’s an excessive peaking factor that would indicate unacceptable levels of I/I.

Lindsay Beaman (3:22)

That’s a good answer, especially when you think about a lot of the rainfall events we’ve seemed to have over the last decade. I think a lot of system operators and managers can say they can reflect some correlations there. So, that’s really good advice. Other than knowing you have a problem, how do you go about identifying where it’s coming from? You might be able to see it overflowing a manhole, but that’s not the exact manhole’s problem, correct Emily?

Identifying if your Sewer has Inflow or Infiltration 

Emily Wicoff (3:45)

Correct. So it’s actually very difficult to know what portion of I/I would be from inflow and what portion is from infiltration. However, there are definitely some standard common ways to identify problem locations. Typical methods might be flow metering, smoke testing, dye testing, and CCTV.

Wes Farrand (4:04)

It comes down to sometimes what the, as Emily mentioned before, the difference between what is inflow and what is infiltration part of the I/I. Sometimes with flow metering,  you can divide up a system into different sections and monitor the flow during dry seasons and wet seasons and you can kind of see a correlation between those peak flow times when the flow in the sewer is elevated, whether it comes immediately after a storm event. So, you can overlay those graphs and that information onto a rainfall. And if you see a real significant peak, right after the rainfall or right with the rainfall, then there’s a good chance that that’s an inflow type impact where it’s the floor drains or sump pumps or open manholes. We’ve seen that in the past where there are manholes that run down through a creek drainage way and there are manhole covers that are off and the creek just goes right down directly into the sanitary sewer.

Then also the data can show the opposite or the other side of that, which is the infiltration side, that if that peak is more drawn out, delayed from the rainfall event, it can indicate a lot of times what would be primarily groundwater inflow coming through the cracks in the pipe, or even sometimes through sump pumps as they’re delayed as the groundwater comes up around homes and gets into those footing drain systems.

A lot of times it’s a good idea to do a comprehensive look at your I/I both to look at, do we have footing drains in a certain neighborhood, is the pipe an old pipe? So, is it going to have a chance of a lot of infiltration coming in the cracks, or is a new pipe that’s fairly tight? We’ve done studies where it’s just going to look for open manholes, things like that. So, a lot of times it’s going to take a comprehensive look at all these different ways to pinpoint where the issue really is and that helps go to the next step, which is how do you fix it.

Budgeting & Identifying Funding Options for Sanitary Sewer Rehabilitation 

Lindsay Beaman (5:38)

I think that we addressed early on the making of the dent in this to bring down the I/I. The purpose of that is to save money in the long run on downstream infrastructure, but you’re going to have to spend to save. When do you start to spend to save? Are there timing implications? When does it make sense maybe to start looking at some of these things?

Wes Farrand (5:55)

From my perspective, it makes sense to look at them if you’re seeing wide swings in your sewer flow. If you’re seeing lots of high peaks during rainfall or during wet seasons then it makes sense to start looking at something like an I/I study or evaluation to just see what kind of costs impacts that’s having on your system.

Lindsay Beaman (6:13)

So there’s no time like the present.

Wes Farrand (6:15)

Yeah, exactly. No time like the present. I think any community that has old sewers it’s worth at least a desktop look at what should we expect to see for sewer flow versus what are we seeing and does that point to a potential I/I problem. And that is pretty easy to do upfront and more often than not most places have some level of I/I issue that’s worth taking a look at or at least evaluating whether it makes sense to like you said, spend a little money now to save a lot of money down the road.

Lindsay Beaman (6:42)

And since you’re talking about money Wes, is there money available to help begin this review?

Wes Farrand (6:47)

The SRF, state revolving fund program does consider sewer rehabilitation projects as an eligible cost for their loan program. It’s a reduced rate loan that helps bring costs down quite a bit, for a lot of communities. There are probably, I’m not exactly familiar with all the funding options out there, but I think a CDBG or Committee Development Block Grant program out there that will help some to a certain level.

Emily Wicoff (7:06)

I’ll add two things to that. Sanitary sewers are one of those “out of sight out of mind” things that understandably are often not budgeted for routinely until there’s a problem that has to be addressed. It is worth considering, identifying a yearly budget for I/I maintenance and even in a small community, a few thousand dollars would go a long way to inspect part of their system and to understand if they have any inflow and infiltration problem areas. I wanted to add that for example, the Missouri Rural Water Association will actually go out and do smoke testing for free for a community. And a lot of the small communities have and are taking advantage of that service.

Something that’s really important for communities to note is that when they start looking at their aging infrastructure or any upgrades that they know they have to do, it can seem overwhelming, especially for a smaller community. I really want to emphasize that there are several funding programs available for all of this work, whether in the preliminary analysis stage or through detailed design and construction. There are low-interest loans plus grant programs that many communities are eligible for. And it’s definitely worth looking at and at Snyder & Associates we’re very familiar with working with these funding agencies and can step a community through that process and introduce them to the right personnel to talk to and see what they qualify for.

Bottom line, it does cost something to routinely repair and maintain systems, but it can also cost quite a bit not to. So here’s something to think about. Let’s assume you have a pipe leak that generates an infiltration flow of just one gallon per minute and let’s say that you know it costs you $7 for every thousand gallons of wastewater you treat. Over a year that is equal to over $3,500 for that one leak and if you have one leak you likely have many more. Now, obviously, a leak’s flow rate is going to vary through a year and each community’s treatment costs will vary, but you get the idea. So why not spend the money in the most beneficial way?

Common Sewer Investigative Techniques 

Lindsay Beaman (9:15)

Wow. That’s really impressive to hear Emily. Thank you for sharing that. I think what I’m hearing is that a lot of communities probably need more education as to why we should be looking at this regularly, so we can identify how much we could be saving in the future or daily. That’s an amazing assessment to make there. 

Emily, could you give me some more input on when you’re talking about CCTV, smoke testing, dye testing. What are some of those basic principles and what are you looking for?

Smoke Testing

Emily Wicoff (9:45)

Yes. Okay. Smoke testing would be when you set up a blower at a manhole and pump pressurized non-toxic smoke into the sewer lines. The smoke is going to follow the path of any leaks in the system, and it will come out and reveal I/I locations, such as cracks in the sewer pipe themselves, faulty service lines with drains, damaged or faulty manholes, or even cross-connections between like a storm sewer and sanitary sewer.

Dye Testing

Dye testing can be used to identify leaks also, and often is used to confirm smoke testing results because smoke testing you’ll see the smoke come up, but you won’t know all the details of what’s going on below ground. So with dye testing, colored water is pumped through the ground or into a stormwater system and suspicions are confirmed when the dye is observed in the sanitary sewer system, perhaps passing through a manhole downstream of where the dye was pumped.

CCTV Inspection

To briefly touch on the CCTV, which is Closed Circuit Television inspection. So with this, a small camera is run down the pipe and it takes videos and pictures so you can see exactly what’s going on down there. Using CCTV is actually always a prerequisite for pipelining rehabilitation because you know exactly what you’re working with before you go in.

Lindsay Beaman (10:54)

Awesome. So it sounds like we’ve collectively have identified sources of our I/I, whether that be direct connections or some accidental stormwater getting into the system. What do we do to mitigate the I/I?

Emily Wicoff  (11:10)

So typically you need to identify a plan that addresses the inflow component of I/I and another plan to address the infiltration component.

Infiltration Rehabilitation Tactics

For infiltration, you will want to complete those field investigations, which may include CCTV. Once you have identified the leak locations, common rehabilitation includes:

  • replacing manhole covers,
  • grouting or lining manholes, and
  • lining or replacing pipe segments.

Inflow Rehabilitation Tactics

For the inflow component, common corrections can be to relocate sump pump discharge locations so they don’t send stormwater into the sanitary sewer system. Or in the case of foundation drains, you would disconnect them from the sanitary sewer and install a sump pump to discharge elsewhere.

Common Infrastructure Improvement Methods 

Lindsay Beaman (11:52)

We’ve identified some I/I issues within the collection system. What are some infrastructure improvements we can make to tackle some of these situations?

Grout Leaky Joints with Chemical Grout

Wes Farrand (12:01)

I think one of the most common that we have at least especially in central Iowa and the Midwest area here is we have a lot of clay tile sewers that have a lot of leaky joints. They’re not real watertight sewers because it wasn’t a big issue when they were put in back in the forties, fifties, sixties timeframe. One of the primary causes of infiltration in sewers is all those joints, especially associated with a high groundwater table where the water can flow right in.

There are some trenchless rehabilitation techniques and things that can be done inside of the sewer and inside of the sewer system to stem that infiltration to slow it down, to get rid of a lot of it. I don’t know that it’s realistic to expect to get rid of all of the infiltration, just because there’s a lot of little holes in the system everywhere. But what you’re looking to do is to make a dent in it to really knock it down in order of magnitude or more.

Some of the things that are commonly done – you can grout the joints with a chemical grout, that is pumped through a system into each of the joints and kind of seals up the joint itself. And then the soil surrounding the joint stems the water flow coming in through those joints.

Sealing, Grouting & Lining Service Laterals

Another potential location for water to come into a sewer system is at the service taps. There are even some sealing techniques, grouting, and lining that can be done up into service laterals. A lot of the infiltration comes from those service laterals, which are often forgotten about and cause some complications for getting that done. Sometimes it’s considered the homeowner’s problem, sometimes it’s considered the city’s problem, or the city’s ownership. Sometimes the service laterals are considered owned by the city, sometimes it’s considered owned by the property owner. There could be some complications in how to address that and sealing up those service laterals, which can be a major source of infiltration if you think about each service lateral in a community times however many homes.

CIPP is a common lining method to seal up sewers, although its impact or influence on I/I is less than some of the other rehabilitation techniques because it does allow water to still travel through the service tap connections between the host pipe and the liner. But it can be done with a couple of added components like in-seals or maybe in conjunction with grouting to help seal up those pipes that have lots of joints and lots of leaky cracks and infiltration points along the sewer line.

Lindsay Beaman (14:07)

Yeah, that’s great to hear. It sounds like you have identified a lot of options for what you said, making a dent in the ultimate issue.

Wes Farrand (14:16)

I’ll add on to Emily. A little bit ago, she was talking about some inspection or a regular inspection program. The I/I doesn’t have to be a standalone target of your inspection. Obviously, if you’re maintaining your system really well, you’re going to be doing some kind of inspection just in general for the condition of your sewer system, whether it’s root intrusions or broken pipes, collapsing pipes, things like that. Those inspections and the evaluation will go hand-in-hand, you want to know if your pipe is collapsing and if it’s collapsing, there’s a good chance there’s I/I to go along with it. The solution can be twofold there, you can be addressing maybe a structural deficiency in the system and at the same time also addressing some of the I/I in the system as well.

Snyder & Associates’ Experience Solving Inflow & Infiltration Issues

Lindsay Beaman (14:55)

You both sound like you’ve had quite a bit of experience in helping with I/I problems addressing the situation. Can you explain to me some of what Snyder & Associates as a firm has been doing related to these projects, project setup, maybe evaluating the need, and all the way through construction?

Emily Wicoff (15:11)

So out of our Missouri office, we have a number of local communities that have had to do wastewater treatment facility upgrades due to more stringent permit limits. So what happens is when you start studying the existing infrastructure during the design of that treatment facility upgrade we do flow testing. We look at how much flow the plant is expected to receive and we look at, okay during rainfall events, what kind of I/I do we have to account for at the treatment plant?

Inevitably, in all of the communities with older systems, there is high I/I. Oftentimes, what we do is if there is extra funding available in the project, we allocate a portion to start completing CCTV testing of the system and if at all possible start doing lining and rehab of the piping system. At the same time, we talk to the communities about the inflow component of I/I, and many times they begin planning to start talking to their residents.

Wes Farrand (16:16)

Yeah, I would concur with Emily. A lot of times it does come out of the treatment system design or the overall facility planning for an upgrade to a treatment system because that is where a lot of that work gets done anyway. As you’re designing that treatment you need to know how much water is coming into it and that lends itself to doing some of that measuring and evaluation. So a lot of times the sewer rehab or I/I program will stem directly from that if at all possible.

We’ve also done projects that just straight come out of, “Hey, we know we have an I/I problem and we want to address it even though our plant is running fine.” So we’ve come at it from both directions. As you mentioned, Lindsey, we’ve done a lot of it just primarily because that is a big issue in a lot of our small communities, especially with old sewer systems. It’s a major issue and so it gets addressed a lot. We’ve done everything from just a standalone I/I evaluation study all the way to a complete system facility plan study, which includes a treatment plant, like Emily mentioned, as well as an evaluation of flows and I/I.

We’ve taken that then to the next step, a sewer rehabilitation project or program to address the I/I and maybe structural deficiencies as well. Then we’ve done the construction services along with that as well because a lot of small communities, they don’t have the staff or the skills to manage a construction program like that. Also along with the financial assistance, the applications, the procuring of some of the state loan programs or grant programs we can assist with that as well.

Thinking Big During Capital Improvement Planning to Include I/I Needs 

Lindsay Beaman (17:30)

We talked about the need for upgrading our sewer system to meet I/I needs. It’s probably something to consider recommending to any of our clients that if you’re doing any sort of infrastructure upgrade on a street and you’re going to be opening that street up, it’s also very relevant to be looking at the sewer system at that time.

Wes Farrand (17:47)

Definitely, I think if any community is doing a large reconstruction, especially in the street. Because a lot of times, if you can’t do a trenchless solution where you’re going into to rehab the pipe without tearing up the street if you do have major collapses or failures or other issues in the street program going on to reconstruct street above it, that is the time to do it because you’ve already got the street tore up and the access is much, much easier.

Lindsay Beaman (18:14)

That’s good to discuss because I think a lot of times we hear how sewer projects become street projects and wastewater treatment plant projects become slip lining projects. And it’s not out of the lack of need. These things are all related to each other. And that’s where if you had that like Emily suggested the annual look-see within your system, you might have a better awareness of what is coming. So you’re not going to be caught off guard all the time.

Wes Farrand (18:37)

I encourage everybody I talk to, especially in small communities, that if you don’t have a regular televising program or inspection program of some sort to evaluate your system, you’re missing out because then if you don’t know what state you’re sewers in you don’t know where you might see some significant savings and you’re going to miss out on opportunities.

Especially, if you’re doing a street program and you have maybe third street and fourth street both needs some work and your CIP planning, you’re trying to figure out which one you should do and you just pick one out of the hat and it turns out that you pick third street, but fourth street really has major sewer issues. Well, if you knew that, that can weigh your decision and play into what might take a higher priority because you’re going to see some cost savings by doubling up on projects like that.

Lindsay Beaman (19:18)

That all ties nicely into a lot of the discussion we’ve had on previous podcasts about capital improvement planning. And it sounds like through regular televising, or maybe even including investigations within a capital plan, then you’ll have a better idea of like Wes said, what streets need to be addressed. And is it more than just a pavement matter and what the wastewater plant might be doing in five years? It sounds like that ties nicely into a regular conversation, capital improvement planning, looking ahead, and being aware of your city infrastructure. Do any of you have any closing thoughts on any recent projects that you really had a success story to tell?

Inflow & Infiltration Success Stories

Wes Farrand (19:53)

We had a small community where we were looking at doing some sewer rehabilitation and through the televising program discovered a water line that had been bored through the sewer, had subsequently corroded, and was pumping directly into the sanitary sewer. That investigation and the subsequent repair of that cut the city’s water use and as well as their flow to their treatment plant by a third. So, amazing impact that would not have been caught unless for the investigation and the review and evaluation of that project.

Another success that we ran into with a sewer rehabilitation program that was for I/I, as well as structural deficiencies in a small community, we had planned out the worst parts of the city to do some rehab. And with the city’s available budget, we were able to do half of the city’s sewer system with a rehabilitation program. And it just so happened that when we got our bids back that the pricing from the contractor was very competitive and the city was able to add in additional lining length. So the city now has about three-quarters of effectively a brand new system and had tremendous impacts on the I/I downstream. Whether they got lucky or it just worked out in their favor, it was a success for how that one worked out and it made a big difference in their I/I to their pump station and up to their plant.

Lindsay Beaman (21:05)

I think a lot of communities can see being aware of their I/I issues as part of their insurance policy because when it rains if you have an operator that can’t sleep or leave town because rain threatens to back up into someone’s basement, that’s the kind of risk that as a community you don’t need to put on your staff and your maintenance crews to be concerned about all the time. Mitigate those risks for your benefit.

Wes Farrand (21:30

Yeah, there are definitely overtime costs associated with that as well as staff retention I think. If you’ve got a system that your staff can never take a break or have a vacation or holiday, it’s hard to keep good people with stuff like that too, something to consider.

Emily Wicoff  (21:44)

Very recently we had a smaller community in Missouri that some leftover budget for a wastewater treatment plant upgrade was used to televise a large portion of the sanitary sewer infrastructure. Well, some of the main lines leading to the plant were found to be filled with debris like over halfway full and since being cleaned out, several locations where they were having manhole overflows had been mitigated. And so that’s a nice result of that CCTV effort.

Wes Farrand (22:17)

It happens more often than not where you can see an immediate effect of stuff like this. Hey, look, remember that manhole that we always had to deal with. We haven’t had to in the last few storms. I don’t think that’s an uncommon occurrence.

Lindsay Beaman (22:30)

Thanks, Wes and Emily for joining me today, I learned a lot about inflow and infiltration and what our clients need to be aware of for planning ahead, prevention, maintenance, reducing elimination, and just overall the financial side of things, as well as the public outreach and the services Snyder & Associates has to offer. It was really great talking to you both today.

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