Inflow & InfiltrationIntermediate 24 min read

Inflow & Infiltration: Process Control Basics

A closer look at the specific sources of inflow and infiltration, the field methods operators use to locate them — smoke testing, dye testing, and basic flow monitoring — and the routine inspection and disconnection programs that keep I&I in check.

A Class 1 operator can tell you what inflow and infiltration are. A Class 2 operator is expected to go further — to know the specific sources well enough to find them, and to understand the basic field tools (flow monitoring, smoke testing, dye testing) that turn a suspicion of I&I into a documented, locatable defect a rehabilitation crew can actually fix. This guide builds that working knowledge.

Common Sources of Inflow, in Detail

Inflow sources are almost always improper connections — points where someone, at some point, tied a source of surface or storm water into the sanitary sewer, either intentionally (often decades ago, before it was understood to be a problem) or by mistake. The most common sources an operator will encounter include:

Illegally connected roof drains and downspouts — instead of discharging to grade or to a storm system, the downspout ties directly into

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