by James A. Sneed. P.E.

The last article discussed the four major steps involved in establishing a successful, sustainable inflow and infiltration (I&I) reduction program.  It is important to remind public officials that this process is cyclic and should continue as part of any good O&M program.

Most system operators know when they have a problem based solely on the increase in flows at the treatment plant or lagoon.  The issue is identifying where to begin looking for the collection system defects and focusing rehabilitation efforts in areas that will have the most immediate impact on the reduction of unnecessary flow to the plant.

Flow monitoring is a topic that could fill books. The goal of this article is to give some basic insights that should prove useful whether contracting out the task or purchasing some monitors for in-house use.

Site Selection

As a general rule of thumb, long-term monitoring should be placed about every 100,000 linear feet of gravity system (or on each major trunk line).  After establishing a baseline and determining priorities, initial basins need to be isolated with temporary flow monitors at approximately 25,000 feet intervals. To select the actual sites for monitor installation, make sure the following criteria are addressed:

  • Traffic/safety concerns
  • Hydraulics – avoid 90s or other obstructions
  • If possible, avoid manholes with service lines
  • Consider pump station influences
  • Avoid sites with silt or grease build-up to reduce sensor maintenance
  • When possible, straight through lines with flow depth of greater than 2 inches and velocities greater than 1.5 feet/sec will produce better results

When to Monitor

Ideally, pro-active collection system managers have a long-term network of flow meters that not only produce solid baseline flow data, but also act as in an early warning system to detect changes in the system.

Budget constraints may make this impractical for smaller systems.  In this case, the operator needs to know the historical rainfall patterns and conduct the monitoring over three months during the rainy season.  The goal is to capture three, approximately 1-inch rain events during the monitoring period. This will produce the most useful hydrographs for evaluating system response. The following criteria will help ensure good rain events are selected for evaluation:

  • The event should be preceded by dry weather; it must not be overly influenced by previous rainfalls
  • Look for equal amounts of rainfall over the entire basin
  • Use events that do not surcharge the system

Tips for Success

Whether you are purchasing portable meters for use by internal staff or you have decided to contract out the study, preparation will go a long way towards success of the project.

  1. Know your system, look for the most current maps, do field verification and understand the footage in the basin you are monitoring.
  2. Choose locations with the best possible hydraulics.
  3. Research the historical weather patterns, choose the three months that give you the best chance for multiple 1-inch rain events.

Again, this is the first step in a process that will help focus your collection system rehabilitation.