One of the thorniest issues about investigating sanitary sewer inflow and infiltration (I/I) is the unavoidable need to access private property, whether the method is as innocuous as asking residents about their gutter and sump pump hook-ups or as extreme as physically getting into their basements.
In answer to public response, many utilities have struggled to develop ordinances that strike the right balance between individual property rights and the municipality's responsibility to correct excessive inflow and infiltration that threatens the entire community's wastewater infrastructure.
A new methodology is being deployed to zero in on problem areas while greatly reducing the need to implement widespread and more expensive traditional sanitary system evaluation survey (SSES) services. Based on a device called a Micromonitor, the four-step process outlined below has the potential to drive down project costs and the number of private-property investigations by more efficiently locating potential sources of leaks and other system deficiencies.
Step one: Regional flow monitoring
Inflow and infiltration investigations typically begin by monitoring sanitary sewer flow in larger regional basins over a series of months or even years. It's rather expensive — monitoring a single location for a six-month wet season can range from $6,000 to $10,000. Baseline flow data from dry and wet seasons is collected to develop predicted base rates and rank the basins for the quantity of inflow and infiltration per house or per length of pipe.
While this identifies basins that contain problem areas, it generally doesn't provide the discrete level of information needed to pinpoint the location, or the actual cause, of the high infiltration rates. Any investigations would have to be basinwide, essentially affecting all residents equally, not just those with I/I sources. The new approach adds two more steps to focus the impact and reduce the costs.
Step two: Micromonitoring
Micromonitoring follows the basinwide blanket approach of regional flow monitoring: It identifies sewer segments with very high inflow and infiltration to be investigated via highly focused wet-weather technologies. The process defines areas within the collection system where conventional flow meters typically don't perform well, such as small, upstream collection-system areas contributing low base flows.
| WHAT'S A ‘MICROMONITOR'? Measure flows down to 1.0 gpm without a confined space permit. Standard monitoring equipment can be unreliable in very-low-flow situations because debris sometimes obstructs the equipment, leading to inaccurate measurements. Designed and developed by Stantec Consulting Services Inc., Micromonitors are fabricated fiberglass weir inserts installed behind standard area-velocity probes. The weir insert has a defined rating curve. At very low levels the weir is used as a primary device. If flow exceeds the limit of the weir's rating curve, the continuity equation is used to calculate the flow from the level-velocity data. The addition of the weir conditions the flow over the probe to prevent obstruction by debris, enabling the Micromonitor to measure flows down to 1.0 gallons per minute (gpm) — generally in low-flow sewer segments like those with only a few houses. The device is installed with a street-level insertion tool, eliminating issues related to Occupational Safety & Health Administration confined space entry (CSE) requirements. Thus the Micromonitors efficiently adapt existing equipment and open up a new approach for pinpointing inflow and infiltration in upstream segments. Without the need for permitted installation, rapid deployment is the hallmark of the micromonitoring approach. One person can remove Micromonitors from at least eight sites, download the data, change the batteries, recalibrate the meter, and install Micromonitors in eight new sites in one workday. In-house micromonitoring can easily reduce costs to a couple hundred dollars per point measured, much less expensive than regional flow monitoring to find out how the entire collection system's performing. The process also identifies areas with the worst inflow and infiltration, limiting further investigation via intrusive techniques to only the homes in those areas. |
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 ([email protected]) is a senior associate with Stantec Consulting Services Inc. in Cincinnati and an instructor for the company's flow monitoring training programs. Kamalesh ([email protected]) leads the engineering team of Stantec's Collection System Flow Monitoring Group.
