Against the Current

With its wastewater plant at capacity, an Alaska community is metering flows while upgrading its collection system to keep I&I in check
Against the Current

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The resort community of Girdwood, Alaska, receives more than 200 inches of snow annually. That’s good news for tourists who enjoy skiing on the slopes of the Chugach Mountains surrounding the town.

But during spring runoff, it’s bad news for the wastewater treatment plant, which is approaching capacity. While inspection and maintenance keep the collection system in top shape, the Anchorage Water & Wastewater Utility (AWWU) is conducting an extensive flowmeter study to determine how inflow and infiltration (I&I) from stormwater and snowmelt can best be reduced to meet EPA guidelines.

The study is critical to finalizing the design for an expansion of the treatment plant, to be completed over the next few years. Although the community lies about 40 miles from Anchorage, it falls under the jurisdiction of the Municipality of Anchorage, and its water and wastewater infrastructure is operated by the AWWU.

“We have an activated sludge plant at capacity right now,” says Tom Winkler, project engineer with the AWWU. “Now that it’s reached the end of its useful life at 30 years, we’re going to build a new plant alongside it. We chose a membrane bioreactor (MBR) system for treatment, but MBR is not very good at a wide variation in flow.

“The water is also very cold, and that affects the flux rate of the membrane. So essentially, the less water we need to treat, the fewer membranes will be required and the less expensive the plant will be. The question is whether it’s more cost-effective to expand the plant or upgrade the collection system.”

The mountains around Girdwood tend to direct snowmelt into the community. The valley also gets significantly higher rainfall than the surrounding area, approaching 80 inches annually. Wastewater flows in Girdwood average about 500,000 gpd, depending on tourist activity. When rain falls on melting snow, the system can be inundated with up to 2 mgd.

 

Measuring flows

“We’ve initiated a flowmeter study of the area to identify the leakier parts of the collection system,” says Winkler. “We’ve also placed two rain gauges so we can see if there’s a correlation between storm events and higher I&I.”

The AWWU contracted Stephl Engineering of Anchorage, a firm specializing in collection systems and trenchless technology consulting, to approach the issue from several directions. The project includes:

• Designing and evaluating the flowmeter project.

• Ongoing evaluation of I&I in the sanitary sewer system.

• Rainfall monitoring.

• CCTV inspection of services and sewer mains.

• Collection system rehabilitation.

“Much of what’s happening is occurring below the surface,” says company owner Matt Stephl, P.E. “At times the collection system becomes totally submerged, and we think the groundwater levels may be rising by as much as six to eight feet. All of the surface water eventually ends up at the treatment plant. It’s the volume of the water, not the condition of the water, that’s the challenge. Currently, all they can do at the plant is move the water through at a faster pace.”

In 2006, the utility had tried a study using flowmeters with submerged sensors. “The pipes around the ski resort area run at steep angles, with sewer flows at very low depth,” says Stephl. “The high water velocity and shallow flows presented tough conditions for those monitors. We had planned a more comprehensive study using a meter with more advanced technology for 2009.”

 

Better data delivery

Stephl contacted local Hach flowmeter representative Bob Feltman with Northwest Pump & Equipment about the Girdwood project. The two discussed the advantages of using the Flo-Dar flowmeter and of the utility contracting for Data Delivery Service (DDS) instead of buying the equipment outright.

Stephl pushed for the DDS option because the meters would only be employed for the length of the study. “Matt also thought it would be advantageous for AWWU not to be involved in the installation and maintenance, but he was fairly certain the utility would want to purchase the equipment,” says Feltman.

“Eventually, through Matt’s gentle persuasion, we got the job put out for bid, and AWWU contracted for six Flo-Dar meters and two Sigma rain gauges. They were installed in August 2009 in a DDS contract.” That contract has since been expanded to include two more flowmeters.

The Girdwood collection system runs about 110,000 feet and consists mostly of 8- to 12-inch pipes. Stephl divided the system into six roughly equal basin areas to determine which ones might be the greatest I&I contributors during high-runoff events.

“What we’re trying to do is slowly identify the basins, working toward finding the sources of I&I,” says Stephl. “Some basins react quicker to storm events, while others have a fairly uniform base groundwater flow. We’ll move the meters around within the basins to continue to try to pinpoint the I&I to smaller areas.”

The Flo-Dar meters read the flow area with a scanning device, then transmit the data via cell phone through broadcast towers erected at each meter site. The information is then transmitted to users through a Web interface.

Ian Morrison, Hach flow field service manager II, was involved in the installation. “We used directional Yagi antennas on 10-foot poles for each site,” he says. “This setup allowed us to position the antennas for optimal reception and keep them above the snow line.” Contractor Frawner Corporation fabricated and installed the antenna masts.

 

Instant data collection

“All six DDS meters have excellent cell reception, and we had no loss of data at all in an area that is notorious for poor cell phone service,” says Stephl. “We can have our treatment plant operator call us on the phone and tell us we have a runoff event going on, and treatment plant flows are going up, and I can look at the meter data right away.

“So far, the flow data has been good, and the data all looks consistent. Unlike the submerged-style sensors that were used previously, the Flo-Dar radar velocity sensor accurately relays water velocities. We correlate the rainfall input with the flow times and try to figure out if we have a bigger inflow problem or a bigger infiltration problem.

“With a dynamic population based on tourism, it’s very difficult to figure out a true sanitary sewer base flow, so the meters helped us figure that out as well.”

Stephl Engineering is now taking the raw sewer data and comparing it against the flow during individual storms, which in the area typically last two to three days. Storms in April last even longer. These rains, in tandem with snowmelt, can quickly submerge the entire collection system.

The rain gauges testify to the unusual microclimate of Girdwood. The gauge placed at the base of one of the mountains collects 50 percent more precipitation than the one at the wastewater plant a short distance away. The data is broken down to try to determine where the water is entering the system and whether it’s coming from the surface or groundwater.

 

I&I pilot program

Although the AWWU conducts routine maintenance on the wastewater system, the flowmeter study has helped it concentrate on the basin exhibiting the greatest I&I. A project carried out in 2010 was a test case for the effectiveness of reducing I&I flows at the source: It helped show how much water could be diverted from the treatment plant, measured against baseline data collected from the meters.

“We decided to find and repair all of the deficiencies in the cell that encompasses that sub-drainage system,” says Winkler. The utility carried out closed-circuit camera inspections of all sewer mains in the cell, then repaired any problems uncovered.

The collection system consists largely of ductile iron pipe, and some of it is relatively new. The Great Alaskan Earthquake of 1964 destroyed some of the original settlement, which was then relocated further up the valley.

The wastewater infrastructure for the ski resort, which was not relocated, was built in the 1970s. The mains are buried deep beneath the community’s frost line — water mains are buried to 10 feet and sewer lines to eight feet. In winter, the ground must be thawed with steam probes before repairs can be made.

 

Dig and replace

“With the mains at that depth, open-cut repairs are expensive,” says Stephl. “So some techniques that are cost-effective in other states will not be around Anchorage. We do a lot of cured-in-place work and inversion lining. Pipe bursting is also fine in certain applications, but it’s not cost-effective when it involves service connections, because we’ll have to dig down to establish those anyway.”

The utility repaired a number of leaks and holes in the mains by installing external wrap-around waterproof repair clamps. “We also found dozens of joints in the service lines that were leaking,” says Winkler. “The leaks were usually because of frost-jacking. We repaired those leaks by digging and replacing the entire service pipe from the sewer main in the street to the property line. Groundwater was also entering the sewer through the cleanouts on the sewer main. The leaking cleanouts were removed and replaced with new manholes.”

The repair and rehab program is paying off. “Just on repairs carried out during the summer on about 5,000 feet of sewer main, we’ve knocked off about 22,000 gallons of wastewater per day from entering the collection system,” says Winkler. The utility is also testing manholes and repairing defects as part of a 10-year program that began in 2005.

The new Girdwood Wastewater Treatment Plant will be designed with an initial capacity to handle as much water as the Girdwood collection system sends it, but as little as possible considering efforts to reduce flow and control I&I. “We want to act on I&I so that we can adapt the design capacity to reflect the results of our control efforts,” says Winkler. As 2010 ended, Stephl Engineering was designing a repair and rehab blitz on a second basin, this one closer to one of the mountains.

“The AWWU will televise the sewer system and conduct a comprehensive repair project on the sewer mains, services and manholes in that basin,” says Stephl. “We’re happy with the result of the program on the first basin in 2010. Moving on to the next basin, we’ll see if we can repeat those results in 2011.”



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