Kentucky's Sanitation District 1 (SD1) was facing a 2007 U.S. EPA consent decree to address combined and sanitary sewer overflows. The district needed a better means of identifying inflow and infiltration sources.

Among the many issues facing SD1 — which covers 220 square miles and 30 municipalities — was the prospect of investing millions of dollars in manhole, pipeline and lateral rehabilitation. One particularly vexing challenge was the cost, time and intrusiveness associated with identifying infiltration and inflow into the system.

In the fall of 2011, SD1 agreed to participate in a unique micromonitoring program developed by Stantec Consulting Services Inc. The engineering firm was initially engaged to design a pilot test of one basin, employing its recently developed custom-made micromonitoring device and exploratory program.

The team targeted a single drainage basin where regional flow monitoring results had shown excessive I&I rates. SD1 had already lined every manhole and mainline, but I&I remained too high and rehabilitation of approximately 129 laterals was scheduled. With budgeted costs of about $4,500 per lateral, the next stage of work on this one basin was anticipated to cost about $580,500.

The first round of micromonitoring tested eight reaches of the basin and revealed that 50 percent of the tested reaches had very low I&I and could be eliminated from further investigation. By excising these reaches from future rehabilitation, SD1 saved about $240,000, investment in micromonitoring.

As the micromonitoring program in this pilot basin progressed, Stantec was able to zero in on just 12 homes where gutters or driveway drains were connected to the sewers, representing the predominate source of I&I for the entire basin. The results were in keeping with Stantec's findings on more than 200 sites where the micromonitoring program has been implemented for 10 other utilities. In some cases, more than 80 percent of the project area was eliminated from further testing.

SD1 has since commissioned micromonitoring on seven additional basins with more planned for the next four years.

Before the micromonitoring program

Before SD1 began implementing the micromonitoring program, its approach to identifying I&I was difficult, time-consuming and often intrusive for ratepayers. Moreover, the effort was not always very effective.

Consider this all-too-familiar scenario: A utility sets out to identify where the largest source of I&I is coming into a sanitary sewer system using traditional flow metering, smoke and dye, and CCTV methods in a neighborhood of 400 homes in a sub-watershed. They will often need to eliminate every storm drain for cross-connections, every house downspout, foundation drain, area drain, etc. At the end of the day, they may confirm that 25 percent of the homes have downspouts connected to one or two area drains. This means great effort and expense is expended to discover that 75 percent of homes had no inflow connections and were not contributing to the problem.

The micromonitoring program allows utilities to identify the largest sources of I&I over the course of just a few rain events and then concentrate more intrusive investigation efforts on those areas.

Solving the problem

Once the source of the I&I is identified, options for removing these connections depend on whether they emanate from public or private sources. If the sources are due to SD1 assets, such as defective manholes, pipes, lateral connections, storm sewer or catch basin connections, the problems can be solved through rehabilitation, replacement or rerouting.

Micromonitoring helps to reduce the number of customers asked to make a change, limiting the amount of interaction while significantly refining the scope of the area where efforts should be concentrated.

Moreover, micromonitoring can distinguish whether the problem is an infiltration issue such as a defective pipe, manhole or lateral, or an inflow issue such as a downspout, area drain or storm sewer cross-connection. By distinguishing and identifying whether the source of the water is inflow or infiltration, investigation efforts and rehabilitation methods can be more precisely planned and budgeted. While impossible to eliminate all sensitive interactions with private property owners, micromonitoring can dramatically reduce the number of private property owners who are impacted by intrusive investigations and potentially costly repairs.

As a result of this pilot micromonitoring program, SD1 has refocused and refined its overall strategy for I&I investigations. The utility now employs micromonitoring before the start of the majority of private source removal projects. This quickly and efficiently narrows down the area for SSES investigation work. In addition, the data from micromonitoring has helped with SD1's asset management program and is being used to determine where rehabilitation and replacement will be successful for proactive asset renewal.

Understanding how it works

Conventional flow monitoring equipment can be unreliable in very low-flow situations where debris can obstruct the equipment. The micromonitor permits accurate measurement of flows within small upstream collection system areas contributing to low base flows, locations where conventional flowmeters typically do not collect usable data.

Pipe segments that flow into the trunk line are targeted and monitored for only one or two storm events. The micromonitors are placed at strategic locations within a project area and a conventional flowmeter is placed at the downstream end to provide a regional confirmation. The micromonitors are then moved to locations in smaller sub-basins within the project area after a storm event that produces an I&I response at the downstream regional meter. The ability to make rapid decisions based on a single storm response significantly reduces the cost of collecting data from each location and offers the opportunity to further hone in on segments with an identified I&I response.

The micromonitor device is a modified version of a standard area-velocity flow probe with the addition of a fabricated fiberglass weir insert installed behind the probe on a single stainless ring. The weir structure conditions the flow over the probe to prevent obstruction by debris, enabling the micromonitor to measure flows down to 1 gallon per minute, generally in low-flow sewer segments such as those with only a few houses.

Advantages

The cost-effectiveness is driven by the shorter monitoring duration needed at each location. Flow data from just one storm is enough to tell if a section of pipe has an I&I problem. It is designed for rapid deployment; no confined-space entry and no detailed site assessments are required. The micromonitors are versatile in nature; they function independent of pipe hydraulics such as offset joints, root balls and drop pipes, and can be applied to sewer segments with fewer than 50 houses where conventional flow monitoring will not yield usable data.
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 program employed by SD1 in Kentucky provides a useful and proven approach.

A patent for the micromonitor is pending with the U.S. Patent and Trademark Office.

About the Authors

Rich McGillis is the senior manager of Collection Systems for Kentucky Sanitation District 1.
John Barton, P.E., Ph.D., is a senior associate with Stantec in Cincinnati, Ohio, and an instructor for the company's flow monitoring program.