SEWER: New Ways of Seeing

Laser and sonar technologies give the King County Wastewater Treatment Division two more effective tools for assessing sewer pipe condition

A few years ago, a sewer inspection contractor approached the King County Wastewater Treatment Division in Seattle, Wash., with an offer. The contractor had expanded its capabilities to include sonar inspection in tough situations and wanted to show off its new technology.

“They wanted to do a demonstration for us,” recalls Lee Miller, lead facilities inspector for the division, part of the King County Department of Natural Resources and Parks. Officials knew just the place: a century-old siphon.

Sonar made the difference. That first inspection uncovered a debris blockage that was the source of the problem in the siphon, and that led to a sonar contract for King County. And it didn’t stop there. Having learned about sonar, the division has tried out another high-tech inspection tool — laser profiling — with comparable success.

It’s all part of a strong, proactive inspection program. While sonar and laser are the two most advanced technologies the division uses, its practices reflect a progressive outlook in other ways. The division was an early adopter of CCTV inspection and in the last decade has brought much of that inspection in-house.

Attacking pollution

The King County Wastewater Treatment Division (METRO) was created after a citizen vote in 1958 to create a regional wastewater conveyance and treatment system. The move responded to worsening pollution in Lake Washington and the Puget Sound.

Small wastewater treatment plants throughout the county were shut down, and two large ones were built, with trunk lines and interceptors to convey waste to them. The county’s 34 wholesale customers are the cities and water and sewer districts within the county — a population of about 1.6 million.

“We started video-inspecting our lines in 1974,” Miller says. At the beginning and until 2002, the CCTV work was contracted. “In 2002, we purchased our own CCTV truck, and we started doing our own CCTV work in 2003.”

Today the county owns an inspection van with three OZ III cameras from CUES Inc., mounted on various crawlers, the largest a Mud Master. “We also use this same camera equipment on boats we float through our bigger sewers,” Miller says.

The county has an established inspection schedule to ensure that all 350 miles of lines for which it is responsible get inspected on a seven- to 10-year cycle. The vast majority of lines, from 18 inches to 144 inches, are concrete pipes, but about 10 percent are newer materials, such as HDPE, glass fiber-reinforced polymer mortar, and PVC.

Trying out sonar

When Sonar Solutions International Inc. (now Pipe Experts LLC) offered King County WTD a chance to see what sonar inspection could do, the Montlake Siphon was an obvious demonstration site.

The siphon, built in 1909 and running under the Lake Washington Ship Canal, had become the site of recurring combined sewer overflows. “Generally it would overflow two or three times a year,” Miller says. “Between 2002 and 2003 it had overflowed 11 times.”

The problem was obvious, but the cause was not, and conventional CCTV inspection wasn’t much help. The siphon is 500 feet long with two shafts 100 feet deep. “It’s completely full of sewage all the time,” says Miller. “We could get a CCTV camera in there, but we couldn’t see anything.”

An initial sonar inspection of one shaft found a blockage at the bottom. A crew cleaned it out, and Sonar Solutions then did a thorough sonar inspection of the whole siphon. The inspection used a 2.2-MHz rotating-head sonar system, accompanied by a low-light underwater CCTV camera.

Both transmitted their information via a fiber-optic cable as they were pulled through the siphon by a tag line attached to a winch at the downstream end. For the vertical shafts, the sonar head’s cable was directed from the top downward using a mounted sheave wheel.

The sonar, positioned underwater, sends out an acoustic pulse in a narrow cone. Software then interprets the sound’s echo, and the data is converted into X-Y-Z coordinates. The result can produce a variety of images. None of them look like a conventional video or photograph, but all can be useful once operators are trained to read them properly.

Immediate results

In real-time inspection, personnel can see a silhouette-like outline of the pipe’s cross section showing obstructions where they are present. Software can also create three-dimensional images looking ahead into the pipe, with a grid imposed on the image to help more clearly show even small irregularities in the pipe surface. Data can even be used to construct angle or side view drawings that might include color coding for various pipe conditions and contour lines to show sediment buildup.

In one segment of the horizontal siphon tunnel, the inspection found an obstruction measured at 32.1 percent of the pipe. After the contractor cleaned and reinspected the siphon, the obstruction was reduced to less than 8 percent.

“The pipe was cleaned as well as possible,” Miller reports. “There was still some debris that we were unable to remove. We felt that what was left in the pipe would not cause us any more problems.”

CSOs returned to their usual frequency of two or three per year. Additionally, the rock box at the siphon’s entrance was put on a cleaning schedule of every two years.

The county was sold on sonar. “We figured this would be a good technology to expand our inspection toolbox,” says Miller. “We have a list of 85 sites that we haven’t been able to inspect. It’s a really big to-do list.” It includes 20 siphons, five pressure sewers, and 51 force mains.

The county is working down the list at about three sites per year. Sometimes the findings are good news, as with some recent pressure-sewer inspections that found the lines to be in good shape. “We wouldn’t have known that because we wouldn’t have been able to do it with our regular CCTV inspection,” Miller says.

Laser profiling

The county’s use of laser inspection tools grew out of another project that at first was a standard TV job, but was too big for the in-house CCTV team. “Our TV truck has only a limited length — 2,200 feet of cable,” Miller says. “The tunnels we needed to inspect were all 4,000 feet or longer, up to a little bit over 16,000 feet.”

The county’s CCTV equipment still uses wire cable, which further limits how far the camera transporter can go. For the longer runs, fiber optics was the way to go, and that meant the county had to issue an RFP for the work.

“Two proposals came back,” says Miller. “The proposal we took had an interesting twist to it. They said they could do CCTV, sonar, and laser profiling of our pipes at the same time. It ended up being a much lower cost than the other proposal for just doing CCTV.”

The county accepted the proposal and in 2006 conducted a laser inspection on sewage tunnels with significant pipe deterioration from hydrogen sulfide exposure. The laser view allowed a detailed view of the pipe profiles. “You could see all the defects,” Miller says. “It gave more of a 3-D depiction of the pipe.”

The vendor, RedZone Robotics Inc. of Pittsburgh, Pa., used a sonar unit first to give a reading below the surface of the liquid flowing in the pipe. “We were able to see if there was any sediment in the pipe,” Miller says. Laser and CCTV inspections followed. The CCTV camera was mounted on a tracked transporter backed by 6,000 feet of cable.

Cutting edge

As with sonar, the laser inspection doesn’t produce a photo-like image, but more of an abstract diagram that indicates the shape of the pipe and the condition of the inside surface. Some of the images almost look like a photo that has been subjected to special effects to change the coloration.

The laser inspection is useful in two ways, Miller says. First, it can uncover eccentricity in the pipe that changes its profile from perfectly circular to slightly egg-shaped. The district uses it in that way to inspect new lines to make sure they are true and don’t have bellies.

Second, it can report pipe conditions in far more detail than a simple visual inspection via CCTV. Of specific interest, laser profiling uncovers the pitting in concrete characteristic of hydrogen sulfide exposure.

The district used laser inspection again in 2007, when it inspected two tunnels totaling about 30,000 feet, and in 2008, when another 12,000 feet was up for profiling. Certain lines where there has been chronic hydrogen sulfide erosion are designated for inspection every three years to monitor their condition.

As technologically advanced as sonar and laser may be, in the end it’s not about just having the latest gadgetry. “I’d like to think we’re on the cutting edge with our inspection,” Miller says. But there’s a bigger reason for that: “Like everybody else in the U.S., we’re dealing with an aging infrastructure. This helps us keep on top of that before we have one of those catastrophic failures.”



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