When ‘I’ Does Not Equal ‘I’

Public Law 92-500, more commonly known as the Clean Water Act (CWA), was passed in October 1972, less than two years after the Environmental Protection Agency (EPA) was established. The Clean Water Act is the primary federal law addressing water pollution in the United States, and it became the principal funding mechanism, through the Construction Grants Program, for new and upgraded wastewater treatment plants and collection systems. The Clean Water Act is administered by the EPA in coordination with state governments.

Many regulations were established following passage of the CWA, some included in 40 CFR 35.927 published in February 1974. These regulations included the requirement that all applicants for construction grants must demonstrate that sewer systems discharging into the wastewater treatment works were not subject to excessive infiltration/inflow. “Excessive” was defined as quantities of infiltration/inflow that could be economically eliminated from a collection system by rehabilitation. This would be determined by a cost-effectiveness analysis that compared the costs for correcting the infiltration/inflow condition with the total costs for transportation and treatment of the infiltration/inflow.

Often included was a Sewer System Evaluation Survey (SSES), an examination of the collection system to determine the location, flow rate and cost of correcting the infiltration/inflow situation. The results of this “Infiltration/Inflow Analysis” and SSES were used to prepare a Facilities Plan which described the recommended treatment works and/or collection system improvements.

It is interesting to note that from the mid-1970s to the mid-1980s, when the accepted practice was to construct larger sewers and wastewater treatment plants, and sewer rehabilitation was not that well-know or accepted, often the results of infiltration/inflow analyses found that it was more cost-effective to transport and treat infiltration/inflow rather than to remove the extraneous flows from the collection system.


And so, the race for grant money was on. Consulting engineers, municipal officials and regulators quickly became conversant in the new language and regulations coming out of the CWA. Everyone immersed in the industry over the next several years generally understood the meaning of infiltration/inflow because it was well-defined in guidance documents produced by EPA, and many municipal officials and engineers dealt with it on a daily basis, completing infiltration/inflow analyses and SSES reports.

These regulations were reviewed regularly, and consulting engineers often included definitions of the new buzz words in facility plans when those plans were submitted to the client or regulators for review. The following definitions are from the EPA document Handbook for Sewer System Evaluation and Rehabilitation, EPA document 430/9-75-021, published in December 1975:

Infiltration: The water entering a sewer system and service connections from the ground, through such means as, but not limited to, defective pipes, pipe joints, connections, or manhole walls. Infiltration does not include, and is distinguished from, inflow.

Inflow: The water discharged into a sewer system, including service connections, from such sources as, but not limited to, roof leaders; cellar, yard and area drains; foundation drains; cooling water discharges; drains from springs and swampy areas; manhole covers; cross connections from storm sewers and combined sewers; catch basins; stormwaters; surface run-off; street wash waters; or drainage. Inflow does not include, and is distinguished from, infiltration.

Apparently, the EPA became weary of saying and/or typing infiltration/inflow, because in the foreword of the 1975 document it began using I/I for infiltration/inflow. From that point forward, the industry used I/I as the acronym for infiltration/inflow. However, those in the business, those working on I/I analyses, SSES reports or facility plans on a daily basis, understood that the first I doesn’t equal the second I, that infiltration and inflow were two completely different occurrences.

Over the last several years there has emerged a noticeable trend in the industry, one we often try to correct. The acronym I/I has seemed to take on a life of its own, one where I/I doesn’t necessarily mean infiltration/inflow, but rather any extraneous water that may find its way into a wastewater collection system. This can be noticed during casual conversation, product claims, magazine articles or even technical presentations at conferences.

One often hears that various sewer rehabilitation products remove or reduce I/I from sewers. Let’s face it, you can rehabilitate a sewer from manhole to manhole, seal the product at the manholes and lateral connections, and seriously reduce infiltration into that length of sewer. However, rehabilitating a length of sewer manhole to manhole does little, if anything, to reduce inflow.

There is the situation where the sewer may pass under a stream where the stream is leaking directly into the sewer. In this case, rehabilitating the sewer will reduce inflow, but this is a rare occurrence when compared to roof leaders, yard drains, leaking manhole covers, cross connections and so on that occur on a regular basis.

Getting it straight

We can be comfortable saying that mainline sewer rehabilitation products, if installed properly, can reduce I, but typically not I/I. There are products and techniques available that can reduce the first I, some that can reduce the second I, and combinations of products and techniques that can reduce I/I. I realize that there are grey areas, such as rainfall induced infiltration, but, in general, infiltration is one thing, inflow quite another.

It may have been easier if the EPA, those nearly 50 years ago, would have used different terms for infiltration/inflow – maybe something like groundwater/surface water, and used the acronym GW/SW. That would have been less likely to become confused. Or, maybe the EPA should have not started using the acronym I/I way back in 1975.

However, as they say, we are where we are. As an industry, we need to step back, learn from those that helped pioneer this sewer inspection, assessment and rehabilitation business, and remember that I does not equal I, that there are distinct differences between infiltration and inflow. Speaking a common language can only be beneficial to all those participating in this tremendous industry.

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