Inflow & InfiltrationBeginner 18 min read

Inflow & Infiltration: Operator Fundamentals

An introduction to inflow and infiltration in sanitary sewer collection systems — what each term means, why excess I&I threatens system capacity and treatment budgets, and the visible warning signs every entry-level collection operator should recognize.

Every time a hard rain or a fast spring snowmelt hits a Colorado sewer system, a pipe network sized and built to carry wastewater from toilets, sinks, and floor drains can suddenly find itself carrying stormwater and groundwater instead — sometimes two, three, or more times its normal flow within a matter of hours. This hidden flood has a name: inflow and infiltration, or I&I, and it is one of the most persistent, expensive, and preventable problems in wastewater collection. Learning to tell inflow apart from infiltration, and to recognize the warning signs of each, is one of the very first skills every collection system operator builds — because the people who work the system every day are usually the first ones with a chance to catch it.

What Is Inflow?

Inflow is stormwater or other surface water that enters the sanitary sewer directly and immediately through a man-m

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