Risk-Based Approach to Creating a Wastewater Asset Management Plan
With costly rehab decisions at stake, risk management is a tool used to prioritize wastewater collection system resources for rehabilitation. Preventative maintenance ensures sanitary sewers remain in proper working condition and helps prevent potential problems. Utilizing the data collected from inspections, our wastewater engineering team applies risk analysis methods to assess the health of networks and create an asset management plan for sewer rehabilitation.
Sewer systems are part of a city’s hidden, underappreciated infrastructure. Each day, this network of pipes enhances our quality of life and contributes to a community’s long-term sustainability, growth, and success. Every time you shower, brush, or flush, the sewer system is playing an essential part in maintaining public health.
Join Wes Farrand, one of our Project Engineers, as he talks through a risk-based approach to sewer system maintenance. Wes has presented this topic at multiple conferences and will discuss industry standards and practices while also sharing useful recommendations for mitigation.
- Functioning Sanitary Sewers (1:46)
- Sanitary Sewer Maintenance (2:26)
- Causes of Sewer Deterioration (4:42)
- Assessing Risk & Anticipating Failure (7:25)
- Sewer Inspection Technology (10:29)
- Data & Asset Management (13:23)
- Evaluation of Sewer Risk Assessment (14:58)
- Likelihood of Failure (LOF) (15:40)
- Consequences of Failure (COF) (17:18)
- Maintenance Planning Based Upon Calculated Risk (19:22)
- GIS Mapping for Sewer Assessments (20:10)
- Budget Forecasting & Capital Improvement Planning (22:45)
- Sewer Risk Summary (24:26)
Functioning Sanitary Sewers
Wes Farrand (0:18)
Today we’re talking about sewer system maintenance. I know it’s an exciting topic, and hopefully, it’ll be interesting, informative, and useful to everybody that’s listening.
As a water resources engineer, when people ask me what I do, I don’t like to get into too much of the details. I often tell them that I study and design things that most people take for granted until they break. Sewer systems are a prime example that illustrates that somewhat cheeky response.
There are various ways to approach maintaining these unseen sewer systems, but one we’re going to talk about today is a risk-based method. This is a methodology for determining the most economical use of maintenance resources based on risk. It’s commonly referred to as RBM. It’s not our invention. It’s out there. There are books on it everywhere. It’s not necessarily restricted to sewers. It’s used in a lot of different industries, but basically, it breaks down to a risk assessment of what you have and then planning your maintenance or what you’re going to do based on risk.
Obviously, most people would understand there are not enough resources to do everything. You can’t build a new sewer system every time something breaks. The challenge we face in almost every case is to allocate limited resources in some manner, and what we’re talking about today is allocating those resources based on risk exposure. The objectives that we’re looking at today are:
- Why it’s important to maintain your sewers
- Predicting failures in a sewer system
- How to stack the deck in your favor when it comes to those failures or potential failures, with the ultimate goal to assess and reduce risk and risk exposure
- Then to use that to plan for the future.
Sanitary sewers, a quick background, I’m sure probably all of us know, but it’s important for the principle of the risk management topic. Sewers are a sole conveyance; they are the critical path from the user’s homes to the treatment plant. There’s no other way that the wastewater is getting to the treatment plant, and there’s a quality of life expectation to that uninterrupted function. When people flush, they expect it to disappear. There’s no halfway point there. In the nature of sewers, they’re out of sight, so they’re easily forgotten. They’re usually taken for granted. I joke that some of my best work will never be seen again once that trench is back-filled, but a failure in that system becomes very visible very quickly as a number one priority.
Sanitary Sewer Maintenance (2:26)
What happens if we don’t maintain our sewers? Everything from the relatively minor reduction in capacity, blockages, collapsed pipes can eventually occur. All that can lead to basement backups, flooding, home damages. You can get sinkholes in the grade and roadways above the sewer, environmental damages that can come from overflows to adjacent creeks and streams, emergency situations like that can domino into much greater impacts, and there’s also a negative public image. You can do 99% great work, but one failure and public confidence in the infrastructure is seriously damaged. As with many things in life, the cost of reactive repairs in this area is typically much greater than proactive repairs. The age-old adage of a stitch in time is very applicable to sewer maintenance.
Good Sewer Maintenance (3:13)
Some examples of what we’re talking about. The good. This is a sanitary sewer that’s in relatively good condition. It’s not a new sewer. It’s an older clay pipe sewer, but it’s solid. It’s functioning well. It’s got a break in tap, but there’s no major I/I observed coming from it. There are no cracks in the pipe, no roots creeping into the joint. All in all, this is what a lot of communities wish to see.
Bad Sewer Maintenance (3:33)
The bad. This is a sewer that has been forgotten about and not maintained. The window for the stitch in time has long since passed. When we televised this line, the camera got to that void. You can kind of see on the right up there. It panned right and looked into a room, and the crawler could have turned into and taken a nap over there. The flow line’s gone, the wastewater here is flowing directly on dirt, and it’s probably not going to be long before this results in something major.
Ugly Sewer Maintenance (3:56)
Such as the ugly. This is where the unseen now becomes seen with major damages that is going to cost major money. This is not our picture, thankfully, but it’s a representation of the worst that could happen. A quick story of a near-miss of something like this in the photo, we had a recent storm sewer project where we were constructing a storm sewer adjacent to an existing sanitary sewer, and we had a trench wall collapse. The unique thing about this trench wall collapse was that the trench wall didn’t fall into the trench. It fell out of the trench. It exposed a void over the top of the sewer next to our storm sewer that a man could have walked into without any difficulty, all directly below a well-traveled residential street. We dodged the bullet on that one, thankfully discovered it, was able to repair it, but it could have easily turned into something like this had it gone longer.
Causes of Sewer Deterioration (4:42)
Now that we’ve seen some examples of failures, we’ll talk a bit about what causes them. Sanitary sewers are progressively decaying from the day we put them in the ground. Every sewer is different based on conditions, the pipe material, the type, but they are a decaying asset. They’re never going to be as shiny and clean as the day you put them in. Many systems especially in our region of the country, are reaching the end of that useful life, old clay pipes that were put in the 40s, 50s, and 60s. Those are getting to be 60, 70 years old now. While they’re a quality product, they’re starting to show their age. Over time they will decay, and they are decaying. The important takeaway is that we need to be aware of those deterioration causes so that we can plan accordingly.
MIC & Hydrogen Sulfide
One of the biggest causes of deterioration in sewers is hydrogen sulfide. Either directly emanating from the wastewater or compounded by microbial-induced corrosion. MIC, as it’s known, is the bacteria that grows on the pipe walls that increase the corrosion.
Industrial chemicals can also cause corrosion in our pipes. Things we don’t know are getting flushed. The picture on the left here shows a cast-iron gate or what was a cast-iron gate. It’s more of a rust gate. This is on a recent project where the hydrogen sulfide has corroded the iron in that gate, and it pretty much just crumbles in your hand. The picture on the right is a little bit more difficult, but you can kind of see the circumference of the pipe, it kind of looks like a mushroom. That shows where the normal water level of the pipe was, and where it’s expanded out on the upper portion of the pipe is where the corrosion of the pipe has lost about two inches of wall thickness—exposed all the rebar in that concrete pipe. This is actually after the lining, but it illustrates how that pipe wall disappears and the corrosion that hydrogen sulfide can cause.
The pipe traffic loadings, especially for shallow pipes. The natural freestyle cycle, soil movements, erosion, or cavitation of the flow in the pipe can cause movements, and anytime something moves, especially with old clay pipes that don’t like to move, they can result in cracks that eventually turned into collapsed sewers. Damages even from installation errors that harm the integrity of the pipe, break-in taps can damage pipe.
Other utilities, the picture on the left shows a gas line that was bored right through the top of the sewer pipe. Adjacent construction, even cleaning can cause damage to a pipe if you’ve got a sewer that has a lot of roots that are cleaned often. The equipment that cleans those roots can damage the inside of the pipe over time. There’s a lot of other things that who knows what we’ll run into. Material decay, iron pipes can have stray electrical currents that can cause corrosion, deposits, organic and inorganic deposits, root intrusions, or just a combination of everything above.
The most unique that we found was a picture on the right there. You can see a bunch of rebar, debris, and stuff hanging down into the pipe. When we did our investigation on what the cause of this was, we discovered that this location was directly below a piling for a bridge abutment that passed above it. When they put in the bridge, they drove the pile, and it broke through the top of the pipe, caused all the damage, and was presenting a blockage risk.
Assessing Risk & Anticipating Failure (7:25)
Where’s your next failure going to be? You know you can get this crystal ball on Amazon for 45 bucks. It’s got five-star reviews. Maybe that’ll work. Say it does work. What will the impacts of that failure be? Is that under the main street in town? Is it going to be an abutment of a bridge? Are people going to get sewer backups? Is it going to discharge into a waterway? Thinking about all those potential situations really puts the importance of proper sewer maintenance into perspective, but we know the crystal ball is not going to work. We can’t predict the failures, but there are ways that we can stack the deck in our favor.
Risk, by definition is being exposed to danger, harm, or loss. How does that relate to sewers? Well, a pipe failure is a loss, right? We want to reduce our risk of having a pipe failure. Because a pipe failure in a high-risk sewer is going to be accompanied by a large price tag. Emergency repairs are always the most expensive kind.
Knowing where those high-risk locations exist is key, but to know that you have to know your system, it’s important to have a baseline of your system health, which ties back to knowing where the good and bad areas are. A lot of operators they’re going to know in general where the good and bad areas of town are, and that’s very useful information. It’s important, though, however, to get a little bit more detailed on the data to use a risk management system so that you can apply it objectively to all parts of the system. Data is the most important thing to know your system. But you may not know you have an issue in an area unless you get data on it. That’s combined with looking at manholes, looking at sewer mains and other key facilities, siphoned crossing, pump stations, anything in the system that’s critical to conveying that flow.
For example, on a recent large diameter sewer assessment project, the client invested significant capital and inspected about 900 structures and over 60 miles of gravity force main and siphon work. Three phases of work over several years, but it was an important investment to them that paid off in the end. This is the extent of all those sewers that were looked at. Basically, everything in their system built before 2004. They had limited resources to align into, or to rehab problem areas.
How do you pick where the problem area is in a system that big? Well, we started with a manhole assessment. Those are the easiest things to see cause you can pop the lid and look down them. That’s really what it is. It’s a visual inspection, inspecting the interior. We always recommend using a NASSCO (National Association of Sanitary Sewer Companies) MACP (Manhole Assessment Certification Program). Basically, it’s a standardized format for determining what damages are and the severity of those damages. On that recent project, we only located about 661 of the 900 manholes. But, we did an interior assessment of all those different manholes, looking at structural and I/I components, categorized them in that matrix that you can see there of scoring on condition.
Then the next step was to go into sewer main inspections. Now, this really should be a regular inspection program. Unfortunately, that’s hard to do. It takes some capital, but it’s important to document the condition of the sewer, and conditions can change. Sewer videos from 10 years ago may not be representative of what is in there today. It’s important to do a regular, if you can, at least the problem areas and then to document so a comparison can be made, at least from last time.
Sewer Inspection Technology (10:29)
A couple of different technologies that are out there for doing the sewer main inspection: acoustic inspection, smoke testing, close circuit, and then a new one that we used on the large diameter project here recently was a multi-sensor, and I’ll talk about that here in a minute, too.
Acoustic is a relatively new technology that uses sound waves to indicate potential blockages or potential issues. We haven’t used it a whole lot, but its best application is for preliminary inspection. When you have a large quantity of sewers that need to be looked at and not enough financial resources to do them all, this is a little bit more cost-effective way to maybe identify those sewers that need to be looked at closer.
Smoke testing can identify public and private defects, illicit connections, I/I sources. It can be a gauge of leakiness. It’s not foolproof, but when you see smoke, you know there’s a connection and an issue.
CCTV is closed caption televising that’s the most commonly used, and it is the most useful. Obviously, if you can see it, it’s much better than the other data, for the most part. It’s a visual understanding and record of what the sewer looks like. NASSCO PACP coding is again another standardized system for coding damages and their severity of the damages. Then it usually results in a video and a report that can be used then as a basis for your data and then doing your risk assessment.
Multi-sensory inspection is the one I mentioned before. It’s a new technology, applied mostly towards large diameter sewers. It’s a floating platform where you can’t take the sewer out of service. It takes high definition, closed caption television for everything above the water surface, kind of like a Google streets for the inside of your sewer, if you will. It collects LIDAR data for determining the loss of pipe wall. If it’s supposed to be a 78-inch pipe, it’ll measure, and if it’s an 82-inch pipe, you know you’ve lost two inches of wall thickness around it. It has a laser for measuring ovality if there’s any squashing of the pipe due to overburden. It takes sonar data for everything below the water level, which is an estimate of sediment volumes, debris, and the damages below the water level. This is an example of the MSI output. It’s very data-heavy. You can see on the right it shows the wall loss in the yellow and has a roll-out view that shows the whole pipe all at once. Then you can see on the left the video plays as real-time data. It’s very handy for these large diameter sewers.
On our example project, we did MSI or CCTV inspection for about 57 miles of gravity, sewer, a whole bunch of force main, a whole bunch of siphon pipe. The small diameter stuff was a more typical assessment, but there was a lot of data that came with that. The question is what inspection to use. Well, it’s going to be unique to each system. You have small diameter pipes. You have large diameter pipes. That’s going to drive what technology is most worthwhile to you. Also, your goals. Is your goal just to look at the structural integrity of the sewer, I/I (Inflow and Infiltration), the reduced capacity of sewers, or as most likely the case, is it a combination of those things? But there can be other goals, too, so that can drive what inspection you want to use to get the data for your system.
Data & Asset Management (13:23)
So you’ve collected data. What are you going to do with it all? We could just stuff it in the filing cabinet like the picture shows there, and honestly, that’s not uncommon in a lot of areas, unfortunately. Maybe even the person that’s filing has a method to their madness, but they may not always be there, and it really isn’t useful in doing a risk assessment process. It’s important to have data management that’s done on purpose, stored in a common location, usually electronically on a server, and stored in a usable format. VHS tapes of 20 years ago aren’t very useful today, so having a usable electronic format and then have a system that’s planned and set up before you collect your data. It’ll be much easier down the road to put that data in and keep it where it should be. You’re going to spend good money on your data protect it with good data management practices.
Asset management is very similar to data management, but it kind of takes it a step further. This includes not just the data for the assets but knowing where those are assets are at, details about them, and the ability to view and review that information whenever and wherever. Really the backbone of an effective asset management program is a GIS system. For those in the audience that maybe aren’t familiar with GIS here’s a quick overview. It’s a database combined with spatial location data. As you can see in the visual there, it’s a sewer line. You can click on any of the data points. It’ll show you what it is. It’s a manhole, different data fields, and elevations. The customization of those data fields is infinite, you can put whatever information you want in there. It’s very useful to manage all the data that you collect on your system and have it easily accessible.
Evaluation of Sewer Risk Assessment (14:58)
So we’ve got our data. What repairs are needed now? This is really the big question when we talk about prioritizing and limited resources for sewer maintenance. Doing an evaluation of that collected information through a risk assessment is one way to do that. With the primary goal to reduce our exposure, we need to figure out which assets are high risk.
Risk assessment, when it comes to sewer maintenance. It’s a scoring method that identifies weak points of a sewer network through a matrix that assigns weights to different priority criteria. The components of this are the likelihood of failure, which is basically the condition of the asset or the pipe itself, but also the consequences of the failure. That really is what makes it a risk assessment, more than just a condition assessment.
Likelihood of Failure (LOF) (15:40)
The likelihood of failure is the asset’s physical condition. This is mostly based on that raw inspection data. Here we’re talking about the televising data, the LIDAR data, sonar data, whatever you get. PACP coding, this is where that standardized defect code is very handy. It makes it a little bit more subjective, and that data can be provided in a database format that can be easily manipulated electronically rather than manually.
In this case here, we’ve got the PACP ratings on the left, which kind of goes into the overall pipe rating. We also added in some pipe wall loss characteristics, the corrosion volume, the sediment volume in the bottom, all combined into a one to five rating system where five is the worst condition of the likelihood of failure. We take the raw data, and we assign a weighting criteria up to a hundred percent of what is our highest priority. In this case, the overall pipe rating was 70% of the criteria. The corrosion volume, or how much pipe wall loss had been, was 30% of it. The client, in particular for this one, didn’t have much concern with the sediment volume, so it was assigned to zero. This is customizable. You can make this whatever we want. We can add in different components in this matrix, but basically, it weights all these matrices and spits out our likelihood of failure rating at the end of the day.
So we could just stop there, that’s physical condition, those are going to be your worst pipes. The ones with the highest likelihood of failure they’re going to be the ones that need to be fixed. However, there are also financial limitations, and there may be pipes with an equal likelihood of failure. Which one do you pick? So this is where the risk exposure comes in, and this considers the system impacts as another factor to that risk, which is the consequences of failure.
Consequences of Failure (COF) (17:18)
It’s on a very similar one to five scoring as the likelihood of failure. This one is also customizable for different consequences of failure. The project we’ve been talking about identified one factor as critical crossings. Is it crossing the waterway? Is it underneath a bridge? Is it an arterial roadway that would be a major issue if it failed? How big is the pipe? Which kind of relates to, how many people does it serve? If it went down, how many people are affected by that pipe not being in service? The depth of the pipe, that factors into the constructability or the effort it would take to get down to it to do a fix on an emergency repair. The land use that’s over top of it. Is it in a residential area with tight roadways? Or is it out in the middle of a farm field, that way to factor?
As an example of how this applies, we’ve got two different pipe photos here, the one on the left pipe A, you can see the rebar from the concrete pipe starting to corrugate the wall of the pipe, which shows that you’ve lost about two inches of pipe wall thickness. Pipe B, you’re not seeing that rebar poking through yet, but there has been pipe wall thickness in this one. By likelihood of failure or condition pipe, A would seem to be the one to address.
However, when you take a look at where it is at in the world, pipe A is out in a remote area. Maybe not easily accessible, but much different impact than if pipe B had a catastrophic failure which is directly underneath a major roadway. One of the few bridges across the river in this area. The impacts to the street and the construction costs to do a major repair there would be significantly higher than pipe A would be. The risk factor weighs both of those in its assessment.
Example two, this is more on a smaller level. This process is scalable, we used it primarily on this recent project for a large diameter sewer system, but it could also be scaled down and used on a smaller system as well. Here, you’ve got red sewers that were identified as high risk. You’ve got some that are underneath a major roadway interchange to the North there. Then you’ve got others that are just a small diameter line that just serves a small portion of a neighborhood. They may have the same condition. If you have limited funds, it helps to have this assessment to allocate those funds to the most useful area.
Maintenance Planning Based Upon Calculated Risk (19:22)
Maintenance planning is the next step. We have the risk assessment where we identified the riskiest or the potential highest risk segments. We’re going to develop a program that addresses those using that risk information. This is going to be based on the calculated risk, which is the product of the likelihood of failure, and the consequence of failure with obviously the high-risk assets addressed and repaired first. In our case study, we had about 20% of the inspected sewers were classified as higher to highest risk. You can see that on this risk rating curve where the risk of the pipe was put on a chart. There are fewer of them, but you can see them on the left of that chart. The vast majority of the sewer system is in pretty decent condition, but it identifies where those riskiest pipes are. We know that if we’re going to apply limited dollars to our maintenance, we need to apply them in those segments that are on the left of that curve.
GIS Mapping for Sewer Assessments (20:10)
It’s not just as easy as picking off those segments, though. The next step to using your money wisely is to visualize those priority areas. This is where the GIS comes in very handy. We take the data from the GIS. I call it the little black box. We run it through the matrices of the likelihood of failure, consequences of failure. It kicks out a rating, and we take that rating, and we put it back into GIS and apply it to the segments with a color-coding so we can easily see in an area what the risk rating of those sewers are. This can be used then to stage your projects. As you can see there, the red is the high-priority risk sewers. It’s pretty easy to pick out which ones we need to address, but there are also some segments of orange that are in the middle there. Now, those sewers weren’t as bad a condition. However, you’re going to be staging a project. If you’re already going to be lining or rehabilitating the sewer on either side of that orange section. It’s most cost-effective to address that segment when you’re doing the others around it, then to have to come back with a separate project and re-mobilize into that area and address it at that time. You can kind of break up a reasonable project based on visualizing the extent of that on a map.
Here’s the project sewer risk map, this is all the 60 miles of sewers with their risk rating overlaid onto them. You can see that it kicks out some areas pretty quickly as low priority. Those on the newer sewers on the upper reaches would make sense. The older sewer down through the main part of the older part of town, we would expect to have a higher rating. That’s what we’re seeing, but there are some outliers that are high risk that would make sense in other areas to address as well. So visualizing it on the map really helps to break that down.
So in our case, it was identified to do two phases of rehab, a 78-inch to 90-inch sewer, totaling about $13 million estimated costs. Breaking that down into two phases. Again, the detail of this you’re getting down into, okay, phasing the construction we know where we want to go now we’ve got to figure out bypassing requirements. This one was driven mostly by bypassing. You can’t ask half the town to not flush. So you got to be able to bypass the sewer. Overlaying those risks ratings on the sewer with the visualization of where those are at and then applying it to the unique conditions of each system ultimately ends up in that planned project.
So here you can see the sewer rehabilitation curve with the orange and the red as those first two segments as the priority segments that are going to be addressed. It addresses a vast majority of those on that high end of the curve. It doesn’t get them all you’ll notice, those are outliers that didn’t fit into that staging area, but once these red and orange are addressed, they fall onto the flatter leg of the curve because they’ve been rehabilitated. Then the next phase of rehabilitation will address those that are now on the high end of that curve.
Budget Forecasting & Capital Improvement Planning (22:45)
Which leads us to the next step, which is budget forecasting. This is the capital improvement planning portion and where this information can really be beneficial in that CIP process because we know we have limited funds, we can’t do it all. You can take that curve and assign some dollars to those different segments and widen that box on the left until you maximize the dollars that you have available. Put the money that’s available towards the highest risk assets first. It’s a very good guidance tool that anybody financial advisors or the accountants can see very easily, what we’re doing and that we’re using limited money to the most effective means.
On the example project that was about 60,000 feet of sewer that was flagged as high risk. We broke that down into an eight-year program about 7,500 feet per year, budgeting about $5 million per year, to address all of that. So that after eight years of that program, all those high-risk assets would be addressed and rehabilitated. Plotting that on the chart, we were able to show that all right, the current CIP had about $25 million, $27 million worth of a budget allocated to it, but to get that full eight-year program was going to take about $45 million. This is a very useful tool then to take to the financial planning aspect of it, to show, “Hey, if we’re capped at 25 million, we’re only going to be able to address this number of sewers. If we want to address it all, we’re going to need to increase our funding or reallocate funding from elsewhere, as the case may be.” So, it’s a useful tool at the end to help explain financial needs. So that here, in the example projects case, they were able to take this to their finance committee, explain what they did, the data to back it up, and were able to justify additional CIP funds that were going to be allocated in future CIP planning to be able to do the remainder of the high-risk asset sewers.
Sewer Risk Summary: (24:26)
So our goal of cost savings, of proactive maintenance, it’s always important to know your system, getting that baseline knowledge of your sewer with data that you can use to manage purpose with a plan, and then using that risk assessment method to stack the deck in your favor to reduce your risk exposure, but also to apply your limited rehabilitation funds in the most cost-effective manner.
Thanks for the opportunity to share today, hopefully, it’s interesting, or at least maybe an idea that could be applied to an area in your community. Again, it’s not restricted to sewers. It could be applied to roadways. It could be scaled up to large systems, to small systems. The RBM isn’t our idea. There’s lots of books on the topic, but it is an idea that has worked very well on several projects. Especially when you’re taking a monumental undertaking, like the example project of 60-miles of sewer to get your head around that, break it down into manageable pieces, data, objective database that outputs usable results and eventually produces a good plan for moving forward, a plan that avoids that ugly if at all possible.