The I/I Cycle

Whenever I discuss the removal of Inflow and Infiltration, I can’t help but think of the encouraging comments I received from one of my early mentors.  He used to say “Tony, I know what your I/I plan is, you want to keep flushing money down the toilet and hope it sticks to the pipe and stops the leaks.”  Unfortunately, although this sentiment may not be expressed as eloquently by others, it is often shared. As stewards of the community’s collection system it is important to continually educate the public and other staff members. Everyone wants to see a return on their investment, and traditionally rehabilitation programs have been sold under the premise that they would see lower flows at the plant.  Yes, there are “mother loads” out there that can be found to reduce extraneous flows, but more often than not, a successful I/I program is one that does not use plant flows as the only measure of their success but can begin to think of I/I removal as a never-ending maintenance cycle.

The Four Steps

Flow Monitoring

As a general rule of thumb long term monitoring should be conducted about every 100,000 linear feet of gravity system.  After establishing a base line and determining priorities. These initial basins need to broken down with temporary flow monitors at approximately 25,000 ft intervals.

Prioritize Mini Basins

If possible, acquire data from 3 rain events of approximately 1”.  Initial rankings can be made by simply subtracting a typical dry 48-hour period from a wet 48-hour period, remembering to use net flows for ranking purposes.


Now that you know which mini system should give you the biggest “bang for the buck” when it comes to I/I removal, it’s time to get out there and find out some specifics.

The Sanitary System Evaluation Survey normally consists of:

  • Physical Inspection
  • Smoke Testing
  • Flow Isolations
  • CCTV Inspection


Issuing a bond to perform work that is not going to generate additional revenue should make everyone a little squeamish.  The rehabilitation of old and failing collection systems has become its own industry over the past two decades and the plethora of available new technologies require the Collection System Manager to constantly sharpen the saw.  With the information gleaned from the first three phases of the cycle, the system manager should have enough to data to provide accurate quantities for bidding rehab projects such as CIPP, Manhole Repair/Coating, and Point Repairs.   Knowing the location and quantity of work will allow the Owner to receive more competitive unit prices.

Volumes could be written on each of the four steps, and if the interest is there we can cover of steps in greater detail during future issues.  The important thing to remember is be systematic in your approach, think of I/I removal as a maintenance/ asset management program, and finally, like all circles the cycle never ends


~ contributed by James A. Sneed, P.E.