Launch Slideshow

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Low-impact inflow/infiltration investigation

Low-impact inflow/infiltration investigation

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    Six Micromonitors placed throughout 4.7 miles of sanitary sewer showed significant inflow and infiltration was limited to two of six sub-basins. As a result, Clayton County Water Authority managers limited investigative efforts to homes only within those areas. Photo: Stantec

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    A Micromonitor is ready to be installed. An early concept for the device was developed by David Saylor, president of Indiana's South Haven Utilities Water Works. Photos: Stantec

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    Close-up of a Micromonitor inside a controlled flow flume. Measurements of the level and flow under controlled conditions develop the low-flow rating curve for the weir as a primary device before it's installed in a sewer pipe.

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    Stantec Consulting Services Inc. engineers demonstrate a Micromonitor installation.

All 8-inch pipes in the highly ranked I/I basins that flow into the trunk line are targeted and monitored for only one or two storm events. Micromonitors are then moved to smaller sub-basins within the identified project area after each storm event that produces an inflow and infiltration response at the downstream regional meter. This rapid deployment across multiple sites significantly reduces the cost of collecting the data from each location and more quickly hones in on problematic segments.

Results from pilot programs show that, after one or two storms, engineers can rapidly start to eliminate large sections of sub-basins as potential sources of inflow and infiltration.

Step three: Wet weather CCTV

Pipe segments with high inflow and infiltration are scheduled for focused investigation by wet weather CCTV — that is, deploying CCTV inspections during or immediately after a storm. Televised investigations are typically conducted for asset management and I/I investigations. Most televising is done in dry weather to investigate defects and staining that might or might not indicate a significant source of inflow and infiltration.

The cost for wet weather CCTV, when defects and laterals are gushing clear water, is likely to be more than the $3 per foot for dry weather work. This is partially offset by covering footages at greater speeds, but much more than offset by identifying individual private property sources.

Targeting just the most critical sections for wet weather CCTV requires that crews have other work assigned to them on dry days so they can focus on televising when rain begins. (This probably works best for in-house crews rather than contracted work.) Unlike regular inspection, wet weather work must be completed quickly, moving the camera through the line at speeds much higher than the 30 feet per minute recommended by the Pipeline Assessment and Certification Program code. Other aspects of the code should likewise be suspended because the goal is to move fast, find the inflow and infiltration source, and obtain good shots of the defects or gushing laterals.

Step four: Approach selected property owners

Once specific sources or laterals contributing I/I are identified during wet weather CCTV, the project team can work with individual property owners to address the issues involved by investigating basements, launching lateral cameras, testing sump pumps, and so forth.

While invasive, these highly targeted evaluations inconvenience far fewer residents.

Results of two pilot programs

Georgia's Clayton County Water Authority (CCWA) collects, conveys, and treats wastewater from 275,000 residents. Excessive wet weather flows at one of three treatment plants prompted managers to evaluate inflow and infiltration removal as a way to reduce hydraulic capacity and thereby (hopefully) avoid or delay facility expansion.

Subsequent to basinwide flow monitoring, they requested a pilot basin to test the effectiveness of micromonitoring in reducing overall evaluation costs. Micromonitor developer Stantec Consulting Services Inc. isolated direct inflow sources to one sub-basin comprising just 18% of the entire basin. In addition, one additional sub-basin was targeted for investigation because micromonitoring showed higher-than-average infiltration.

Of the six sub-basins tested within this pilot basin, only two had significant I/I. That meant authority managers were able to rule out the other basins for further traditional testing. They've since commissioned micromonitoring on four additional basins in the collection system that were identified during the basinwide flow monitoring.

“When we priced out the SSES approach using traditional line item prices, we saw the micromonitoring program could greatly reduce costs,” says Engineering Coordinator Keith Watkins. “With micromonitoring we're not doing detailed flow monitoring, we're just trying to identify spikes and flows. As long as you have a couple of good rain showers, you can identify your problem areas.”

The approach also reduces time spent getting unnecessary permissions and access from property owners. “You can focus on the areas that need it and don't need to be intrusive in areas that don't,” he says.

The City of Florence, Ky., Department of Public Works partnered with Stantec for a research study that used Micromonitors to rapidly assess inflow and infiltration locations. During spring 2011, seven of the devices were installed in sub-basins known to have high I/I rates. Of the 26 sections tested, eight had high rates and warranted further investigation, including wet weather CCTV. Based on the results of the televised inspection, to be conducted in spring 2012, individual homeowners will be approached for I/I reduction.

For water and sewer managers seeking ways to stretch budgets further, receive actionable data faster, and limit inconvenience to property owners during inflow and infiltration investigations, the Micromonitor may well provide a useful tool.

—Barton (john.barton@stantec.com) is a senior associate with Stantec Consulting Services Inc. in Cincinnati and an instructor for the company's flow monitoring training programs. Kamalesh (joseph.kamalesh@stantec.com) leads the engineering team of Stantec's Collection System Flow Monitoring Group.