An Eye on I/I

Across the United States, public works budgets are strained to the max while essential infrastructure ages in place. Not surprisingly, those charged with maintaining water systems are on the lookout for solutions that can help them pinpoint their most problematic areas with speed and precision. A new tool has emerged that is helping these managers reduce the cost of water and sewer line rehabilitation by applying a simple maxim: If it ain?t broke, don?t fix it.

Dubbed the micromonitor by its developer, Stantec, the device measures water flows in very low flow upstream pipe sections. Used in conjunction with regional flow monitoring, the careful deployment of micromonitors can quickly zero in on the most serious sources of infiltration/inflow (I/I) in underground pipe systems. This means that major areas of a basin can be ruled out from more detailed and intrusive investigations and unnecessary repairs. The avoided costs for rehabilitation can be significant. In Kentucky, one sanitary sewer district estimates that more than $5 million in potential rehabilitation has been averted due to its micromonitoring program.

MicromonitorsA New Tool in the Toolbox

The micromonitor is an adapted version of a standard flow monitor. The flow-monitoring sensor is installed in front of a fiberglass weir insert. At low subcritical flows, the micromonitor acts as a weir with a primary rating curve. At higher supercritical flows, it offers no obstruction, and the sensor calculates the flow directly using the continuity equation (flow = area ? velocity).

Micromonitoring focuses on small pipe segments to isolate I/I sources and quickly identify sewer pipes with no I/I. To achieve this goal, micromonitors are deployed in individual pipe reaches for one or two storm events. The result is targeted area instead of system-wide solutions. If a sewer line does not show any I/I response to a storm event, it is immediately removed from the investigation and rehabilitation program. The sewer system evaluation system (SSES) investigations are then focused on other segments with high I/I to further locate the ultimate source.

Municipalities often discover problems inherent with traditional flow monitors when it comes to monitoring low flows in small pipe sections ? the very areas where I/I issues all too often reside. These constraints are generally due to debris fouling and to the depth limitations of standard equipment. When a micromonitor is installed downstream of the probe, a flow level is maintained about on inch deeper. This allows an essentially unobstructed path for the debris on either side of the probe.

A graph from micromonitoring data clearly shows the flow pickups during rain events.

A Penny Saved

Stantec has completed a number of micromonitoring programs over the past several years and interest is growing as public works officials see the primary benefits of this versatile device to:

  • Identify significant I/I sources in small sewers;
  • Measure in low flow conditions (thus minor wet weather events);
  • Minimize confined space entry (CSE); monitors can be installed by a one-man crew;
  • Work where traditional monitors do not; and
  • Be deployed, moved and removed quickly.

In Milford Center, Ohio, the relatively new sanitary sewer system, comprised of PVC pipe and precast manholes, was showing high I/I. During the wet season, costs to treat the flow generated from 364 customers increased by three times. Smoke testing and manhole inspections conducted during the rain events failed to reveal the source of I/I. Officials were faced with two choices: move on to CCTV the entire system, or deploy a micromonitoring program. The cost to CCTV the system?s 30,000 lf of pipe was estimated at $60,000 more than twice the costs of the micromonitoring approach.

Here the micromonitors were able to isolate inflow sources to three sewer pipes in an alley between two streets. Infiltration sources were mapped to three sewer pipes along a creek. In the end, I/I sources were isolated to just 1,900 ft of the 30,000 ft of pipe that comprised the village?s system.

In Logan, Ohio, the sanitary sewer system had not been surveyed in nearly 30 years. The outdated collection system of nearly 100 percent vitrified clay was surcharging in several locations during heavy rains. I/I was contributing to basement flooding, overflows and inflated operating costs for the city?s newly expanded wastewater treatment plant. Officials wanted to identify the source of the bottlenecks and remove as much I/I from the system as possible, without exhausting their capital budget. Finally, they wanted to update system mapping to reflect actual field conditions. This situation called for a new approach.

A micromonitor ready for install with no CSE.Traditional Approach

  • Identify I/I Related Problems
  • Regional Flow Monitoring /Basin Selection
  • Detailed SSES
  • Remediation Recommendations

Customized Approach

  • Identify I/I Related Problems
  • Regional Flow Monitoring /Basin Selection
  • Focused SSES
    • Micromonitoring
    • CCTV
    • Smoke and dye testing and other SSES activities as needed
  • Remediation Recommendations

The Stantec team recommended and implemented this new approach with a three-step process:

Step 1: The team completed the regional model, dividing the system into 11 district sewer basins. As a result of this regional flow monitoring effort, three basins were eliminated from further study.

Step 2: Micromonitors were installed in the eight basins that had been identified the worst contributors of I/I/. Sixty-seven sites were monitored with 10 to 15 micromonitor devices over three months during wet weather events. Using this method 132,000 ft of sewer pipe was monitored; 9,047 ft were isolated as the excessive I/I and 26,706 ft were isolated as moderate I/I sources. As a result, detailed SSEE was focused on only those areas, about one third of the overall pipe system. The real payoff for the city came not in the sewers found to contribute excessive I/I, but rather in those that do not contribute excessive I/I and were eliminated from consideration for additional SSES work.

Step 3: The next stage involved closed circuit television inspection of only the identified problem areas.

The I/I data was incorporated into a complete system analysis including system mapping and verification, lift station run time analysis, a comparison of water use records with flow monitoring and micromonitoring data, and interviews with city staff to gain historical knowledge of the problem areas. As a result of study:

  • Total pipe footage excluded by regional flow monitoring: 45,700 ft.
  • Total pipe footage excluded by micromonitoring: 86,290 ft.
  • Documented savings: $263,980 in avoided CCTV work alone.

Over the past several years, this new micromonitoring approach to sanitary sewer investigations has repeatedly been shown to speed identification of discrete problem areas at a lower cost while limiting the need for confined space entry and intrusive investigation techniques. By focusing rehabilitation dollars on only those areas which need it, more resources are available for priority areas: a win-win situation for public works officials.

Gary D. Silcott Jr., P.E., is a principal in Stantec?s water practice in Logan, Ohio.

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