How one city pinpointed the single biggest direct source of inflow and infiltration.

To northern California residents, Cotati, Calif.—located an hour north of San Francisco—is known for its annual jazz, blues, and arts festival in June and a two-day accordion festival in late August. To infrastructure professionals, the city also is known as one that runs its civil infrastructure with the discipline and proactive outlook of a private business enterprise.

Cotati's Water and Sewer Department retained the California firm Winzler & Kelly Consulting Engineers with V&A Consulting Engineers as a subconsultant to complete a master plan of its collection system, including an inflow and infiltration (I/I) study and reduction program that takes advantage of flow monitoring (FM).

The consulting firms partitioned the collection system into 10 basins and conducted dry- and wet-weather flow monitoring, for a total project cost of about $66,000. The data were analyzed to determine which basins contributed the most ground-water infiltration, rainfall-dependent infiltration, and stormwater inflow into the collection system.

When the analysis showed that Basin 9 had a significantly large source of stormwater inflow during the larger storm events, the basin was investigated using several techniques, including smoke testing, low-flow reconnaissance investigations, interviews, and closed-caption TV pipe inspections. Two months of investigation pinpointed the source of the offending inflow: a heavy-water-use business that had three outside floor drains situated adjacent to the property and directly connected to the sanitary sewer system.

At a low point within the watershed basin, the property flooded repeatedly during the larger storm events of the winter season, sometimes resulting in more than 24 inches of standing water. More than 500,000 gallons a day were flushed into the system via the floor drains. The connection of the floor drains to the sanitary sewer was later confirmed using a tracing dye test, and the characteristics of a flood event were consistent with the FM data.

By correcting this single defect, the city eliminated the biggest direct source of I/I in its wastewater system.