Sanitary Sewer Systems: Rainfall Derived Infiltration and Inflow (RDII) Modeling

Rainfall derived infiltration and inflow (RDII) of extraneous stormwater and groundwater to sanitary sewers can unduly affect the capacity and operation of collection and treatment systems. In many cases, excessive RDII can be a significant cause of sanitary sewer overflows (SSOs) and basement backups. Many utilities are investing significant resources on reducing RDII through sewer...

RDII simulation models use simplified mathematical representations of complex systems and dynamic processes, which are depicted in Figure 1.

The Water Environment & Reuse Foundation, (WE&RF, 1999) provided an expanded review of RDII quantification methods in the literature looking back to as early as 1984. WE&RF grouped the methods into eight broad categories that included both deterministic and probabilistic methods. Empirically based deterministic simulation models are used most commonly in practice, as they can provide...

In many sanitary systems, RDII is problematic as it is the major component of wastewater flows observed under wet weather conditions. In northern climates, systems also may experience RDII type responses from snowmelt. The extent of these sources and their corresponding contributions vary greatly between systems and specific events.

Key applications of the calibrated-validated RDII model include the following:

While models are valuable tools in sewer system management, experienced practitioners recognize that RDII models are limited by inherent assumptions and conceptual representations of complex processes. One key consideration is the selection and application of proper RDII methods and modeling tools tailored to system conditions and modeling objectives.

Along with advancements in long-term data collection and management have come recent innovations in RDII modeling. These innovations have been focused largely on the improvement of techniques for defining and relating established model RDII parameters under a range of environmental and physical conditions over long periods of time on a continuous basis. A literature search will reveal a few case...

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