NEW Water had a problem. A slimy one.The interplant force main between its Green Bay and De Pere facilities was getting slimed up. A noted decline in pumping capacity and rise in pumping pressures was worrisome. One of the community's greatest luxuries - flushing and forgetting - was being jeopardized. "We weren't sure exactly what the problem was - but we knew it was bad," said Brian Vander Loop, Field Services Manager for NEW Water, the brand of the Green Bay Metropolitan Sewerage District. "We were pumping at 54% of the system's capacity." Engineers were consulted. They recommended taking system pressures at each air relief structure of the 10-inch-diameter force main. That data indicated there were no obstructions in the force main - but the data did reveal that whatever was clogging things up was happening consistently throughout the entire length of the force main - 7.1 miles.
Staff at NEW Water, which treats on average 38 million gallons per day, dewatered a couple of segments of the force main in order to televise within the pipe. The resulting video showed a slimy layer gripping to the walls of the force main. (See accompanying photo.) Treatment and Field Services teamed up to analyze different methods to combat the slime: poly pigging, chemical treatment with hydrogen peroxide, ferric chloride, caustic to ozone and oxygen generators. The design engineer ruled out poly pigging because it was believed a pig could not pass through existing square port plug valves. The estimated cost was $330,000 to construct pig launchers, pig catchers, and to replace 18 square port plug valves with full port plug valves, plus annual chemical costs. NEW Water staff eliminated ferric chloride chemical treatment, owing to past experience with pipe wall scaling. In addition, sodium hypochlorite was eliminated due to its potential to shock and kill the biological process at the Green Bay Facility, which employs microorganisms to treat water. All told, these annual chemical costs are $48,000. Together, construction costs avoided, and chemical savings totaled $378,000.
"In addition to the engineering cost estimate, we had to add onto that the costs for chemical treatment, which can rack up pretty quick," Vander Loop said. Next, NEW Water tested hydrogen peroxide chemical treatment, along with ozone/oxygen generator technology to remove the slime. The best results achieved were 674 gallons per minute (gpm) with pumping pressures near 85 pounds per square inch (psi). Staff determined that a more cost-effective approach needed to be visited to solve the problem. NEW Water contacted a company experienced in poly pigging pressurized pipelines. After research, review, and analysis, it was determined to try poly pigging the force main. NEW Water employed a three-phased approach to the project.
Phase One: Will the Pig Fit?
During this phase, staff designed and installed a pig launcher to prove that a low-density swab poly pig could pass through four square, port-plug valves. The section of force main tested was about 1,100 feet long, and the catcher consisted of a large fishing net that captured the pig in the primary effluent channels. This successful phase proved that it would be possible for the pig to pass through square port valves. This phase also identified the need to design and install a poly pigcatching device. Staff salvaged and repurposed an unused check valve, installed it with the flange-side down, and inserted a screen device to stop the pig from passing through to the primary effluent channels. "Staff employed creativity and elbowgrease," Vander Loop said. "They literally salvaged some materials in our facility, designed a contraption from scratch, and installed it. Their ingenuity equated to a nice cost savings."
Phase Two: Journey Beneath the Fox River
This phase would determine whether the 10-inch force main could be isolated, dewatered, and contained at an air relief structure without overflow to the environment. Staff would insert a low-density swab poly pig through the six-inch by 10-inch air relief tee and into the force main. "We were quite concerned with pig velocity and potential for debris accumulation in the belly of the Fox River crossing force main segment," Vander Loop said. To combat this, staff devised a work plan to drain and contain wastewater at three air relief structure sites. Tracking indicated a gain of 40 gpm and a 2 psi decline in discharge pumping pressure. The pig had successfully travelled through nine square port plug valves and one check valve.
Phase Three: The Dangers of a Slug Load
Staff determined that it was too risky to use just one pig for the remaining 86.5% portion of the force main needing to be cleaned - the potential for a slug load, or massive influx of slime, was too great. "We were worried about 7.1 miles of slime coming at us at once," Vander Loop said. "It could've thrown our biological treatment process out of whack at the plant. That's why we broke it up into three phases." For the sub phases, staff identified two high points along the force main that could be isolated, drained, and wastewater contained to insert a low-density swab pig. Over the course of a month, two swab poly pigs were successfully launched from each of the three sites, which yielded decreases in pumping pressure, and increases in pumping capacity by 25%, 40%, and 47% respectively.
Collaboration, Communication, and a Little Comic Relief
Vander Loop credits staff's effective communication, persistence, and collaboration as the cause for the success of the project. "NEW Water's team effort and outside-the-box thinking really paid off - both monetarily, and environmentally," Vander Loop said. "Plus, 'The Pig's Tale' makes for a great story!"
A Pig's Tale: The Video
The "Interplant Forcemain" between De Pere Green Bay was getting clogged. After testing, data collection, and a final analysis, NEW Water staff employed an innovative idea: a non-chemical, environmentally friendly one that saved $378,000: The Poly Pig.
This cylindrical, polyurethane, $65 plug would travel 7.1 miles, like a squeegee on a windshield, scraping away slime and clearing the path for wastewater to journey to Quincy Street to be cleaned and returned to the water of Green Bay. This video (optimized for viewing on a PC) highlights the story.