Determining Base Infiltration in Sewers: A Comparison of Empirical Methods and Verification Results

This paper addresses three empirical methods used to determine degree of Base Infiltration (BI) in 45 isolated sewer basins throughout the Orange County Sanitation District (OCSD) collection system. These include a common estimation method called night-time "Wastewater Production", a second method called "Minimum flow Factor", and a third empirical method called the "Stevens/ Schutzbach" equation. These empirical methods were tested against a chemical parameter verification method that involves regressing hourly parameter concentrations (Chemical Oxygen Demand - COD, etc.) with sewage flow rates. Results to date indicate that the "Minimum flow Factor" method and a slightly modified version of the "Wastewater Production" method provide more accurate estimates of BI in basins yielding flows comprised of more than 25% BI. Alternatively, the "Stevens/Schutzbach" empirical method provides good estimations of BI in basins yielding BI flows less than 25% and is also far more stable in such basins (i.e., less sensitive to errors in minimum night-time flow measurements).