Smart Strategy and Planning Solve the Challenge of Interceptor Failure

Aurora tackles deteriorating interceptors with a new comprehensive asset management plan.

Smart Strategy and Planning Solve the Challenge of Interceptor Failure

Aurora Water principal engineer and infrastructure planner Steve Simon (left) with staff member Shiva Sapkota, Inliner project manager Joseph Moya, and staff member Kirk Skogen.

When a manhole cover is broken, you don’t even have to climb out of your service truck to see it. A suspected crack in an underground sewer line is a whole different matter. You can run a camera through the line, but when the cost of a complete inspection is prohibitive — especially for a line that seems intact — what are the alternatives?

This was the dilemma facing Aurora (Colorado) Water Operations and Engineering personnel in 2015. A sinkhole adjacent to busy Peoria Street in the Colorado city announced a leak from a sewer interceptor line buried 30 feet underground. A camera was inserted in the line at the point of the rupture and water department engineers got a peek at a short section of the 42-inch reinforced concrete pipe. It wasn’t a pretty picture.

Concrete had been eaten away by hydrogen sulfide. The corrosive gas produced by the wastewater flow had exposed some of the pipe’s steel reinforcement ribbing. A sub-sequent CCTV examination of the rest of the 40-year-old interceptor line showed the deterioration was pervasive.

Strategic decisions

The city was not guilty of having ignored its underground infrastructure. It had already launched a CIPP repair program to rehabilitate older sanitary sewer collectors where substantial pipeline defects were found. Nor were water department officials totally unaware of the weakened interceptor lines: The deterioration had been caught on camera inspections. Unfortunately, that information had not been forwarded in a timely way to engineers, a communications breakdown that was fixed.

The larger reinforced concrete interceptors were not being routinely monitored for the simple reason that they are active, high-flow lines. Aurora is home to 351,000 people spread over 154 square miles. Consequently, wastewater flows through the interceptors constantly and at fairly high levels. The flow masks the condition of the pipes below the waterline and precludes taking the pipes out of service for comprehensive inspection.

“It’s a strategic decision,” says Steve Simon, Aurora’s principal engineer for infrastructure planning. “Is it worth the investment to completely inspect a line? You can spend several hundreds of thousands of dollars just to decide if rehabilitation needs to be done. In some cases, investing to find out the condition of a pipe is justified. If an inspection and repair extends the life of a pipe tens of years, that can be justified. We feel pretty confident that it is not a prudent investment to expend that much money to inspect our 40-year-old interceptors.”

The cost of such inspection is so high in these instances because of the need to divert the wastewater flow so that the entire circumference of the pipe’s interior can be exposed to the camera’s eye. Though the greatest deterioration in a concrete sewer pipe occurs in the upper area where the hydrogen sulfide collects, the integrity of high-flow pipes also is weakened under the surface by turbulence. Therefore, to completely inspect a section of pipe, it must be taken out of service.

Bypassing a line is not a problem when a parallel line has been run nearby for redundancy, but most of Aurora’s large-diameter interceptors do not have parallel lines, so the expensive diversion of sewage through temporary lines is the only option. Simon says the cost of temporarily bypassing a length of interceptor pipe can range from $100,000 to $150,000 per mile, depending upon the amount of flow being diverted.

Setting a plan

Simon arrived in Aurora from a position in the San Diego County Water Authority in January 2015, just in time to experience the interceptor failure. As principal engineer — a new position — he was tasked with creating and implementing a comprehensive asset management program incorporating data systematically collected by the city during previous decades.

“A lot of good work had been done over time, and my role is to use that data,” Simon says. “It was pretty forward-thinking for a relatively young water system to create an asset management position now so that as our system continues to age, we don’t have to play catch-up and plan at the same time.”

Simon oversaw development of an organizationwide strategic plan and a specific strategy to systematically inspect the community’s wastewater lines. The plan relies upon accumulated data and available CCTV footage. From that, Simon and other department engineers extrapolate scenarios and prioritize interceptor relining work.

Specifically, the engineers stipulated that an interceptor needing urgent rehabilitation must:

  • Be at least 30 inches in diameter and constructed of reinforced concrete   
  • Be 40 years old or older
  • Carry at least 5 million gallons daily
  • Flow at least half full
  • Lack previous inspection data.

This “predictive rehabilitative” model guides the department’s interceptor inspection and repair regime. Simon concedes that the modeling is mostly theoretical, with the exception of occasional video evidence.

“I really enjoy this process, but it is guessing,” he says, acknowledging that some colleagues have suggested as much. “It is not like a more calculated approach. We are not able to really tell you the exact condition of a pipe and its remaining useful life, so we rely upon formulas.” 

What this means for the community’s aging interceptor lines is that over the next five years, 33,000 feet of concrete interceptor pipe will be systematically relined. The rehab program may have been launched just in time: Some of the pipe will approach the end of its 50-year engineered life span by the time it is reconditioned. The contracted cost of restoring the pipe is an estimated $7.5 million.

The targeted pipe is a small percentage of Aurora’s 60 miles of larger diameter wastewater interceptors. Most of the pipe is polyvinyl chloride, which has an estimated functional life of 75 years; the city is holding off serious inspection of it. Pipes that don’t meet the criteria for full-out attention now — for example, lines between 20 and 30 inches in diameter — will be inspected in the next few years using floating camera technology. In all, the city has 1,094 miles of wastewater pipe ranging from 6 to 48 inches in diameter.

Across the board

Aurora’s asset management tool is not restricted to sewer lines. After the strategic plan was implemented, Simon and his colleagues began to develop management plans for all water utilities services (water transmission and distribution, pumping, treatment, stormwater and so on).

“We envision 10 to 12 different plans,” Simon says. “Three are done. We’ll map two or three this year and two to three each year moving forward.” After 2021, the ongoing job will be to maintain the management protocols.

Simon says the city’s waterlines are generally in better condition, partly because of the less corrosive environment inside the pipes. The city has been aggressive, he says, in inspecting and maintaining critical water transmission lines so a rigorous repair program probably will not be necessary. The assets of the potable water division will be strategically monitored, of course, but the wastewater line upgrade is likely to be the city’s big-dollar project for the foreseeable future.

Better buy-in

The bottom line for any asset management program is ensuring that assets maintain defined levels of service. When that happens, all stakeholders in a community — from residents to city leadership to operational personnel — see the practical value of a pre-emptive strategy. A rate increase has not yet been necessary to fund Aurora’s strategic upgrade, but City Council members seem prepared to continue to fund both planning and operational elements.

“We haven’t had to ask for more money,” Simon says. “We have recurring budgets needed to do the work, and so far we have had support for them. Everyone on council seems to understand and appreciate the need for what we are doing.”

Some of that support, he believes, is rooted in the department’s decision to develop the asset management program in-house. Before Simon joined the city staff, there was an initiative undertaken to contract out the program’s development, but it stalled. “Deciding to do it in-house was, I think, a good thing. It was a better investment of time and money and, as important, there was better buy-in for what we came up with.

“I believe we have been really successful so far, but there still is much work to do.”

Building proficiency

Aurora, Colorado, is Denver’s largest neighbor. In square miles, Aurora is larger than the Mile High City. So did Aurora look to its big-city neighbor in 2015 for guidance on how to assess and upgrade its water utilities?

“No, we didn’t try to emulate Denver,” says Steve Simon, principal engineer of Aurora Water Operations. “It’s an older utility with a different set of infrastructure challenges. We didn’t go and look and try to exactly mimic what Denver or any other utility was doing.”

Rather, Simon and other Aurora officials started from scratch and worked up their own plan to strategically manage assets and resources. The plan was finalized in 2017. Planners, engineers, managers and field crews all contributed to it. The team effort succeeded well enough that Denver-area utilities officials now look to Aurora for guidance.

“In some situations, other utilities have contacted us about things in our experience that might help them. I would say that we are getting a reputation for being pretty proficient in managing our assets,” Simon says. “Experts? It feels like we are getting there.”

More evidence: Aurora’s asset management group gave asset management presentations at the American Water Works Association annual conference, the North American Society for Trenchless Technology No-Dig Show, and the American Society of Civil Engineers’ annual pipelines conference. Last year, Simon reached out and formed a Denver Metro Asset Management Council. Some 15 to 20 public sector water utilities now are corresponding and have met twice — most recently in May — to share experiences and hone their understanding of how to take care of assets.


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