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Simple Method for Estimating I/I Using Treatment Plant Flow Monitoring Reports - A Self Help Tool for Operators

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The purpose of this paper is to illustrate a simple, low cost method for making an initial estimate of I/I (Inflow and Infiltration) in a sewage collection system when other types of flow monitoring are not available. Additionally, the method includes an estimate of correlation between I/I and rainfall, and the 95% confidence range. This method may be applied by a system operator or city engineer, and uses existing data, with the non-proprietary analytical tools available on an Excel spreadsheet. The method relates the incremental increase of influent flow (a day's flow which represents an increase above the flow observed two days previously), with the cumulative rainfall depth over two days.

The validity of the method was tested by comparing the daily results (simulating data from a POTW DMR) with hourly flow data collected for the year 2004 at a monitoring site with a dry weather flow of 0.497 mgd. The data transformation method described in this paper resulted in an estimate of 24-hour I/I which was 13% lower than the 5-year design storm rate estimated using a rigorous application of the standardized approach for hourly data. However, the R2 value R2 value (the statistical measure of per cent of variation of influent flow attributable to the 24-hour ainfall) improved by 25% above the R resulting from using the daily numbers without transformation. When this method was applied to eleven year's of data for nine municipal treatment plants, the correlation coefficient R2 averaged about 61%. This represented a four-fold improvement over a day-to-day comparison of rainfall and flow.

Keywords: Discharge Monitoring Reports; I/I; I/I Analysis; Inflow-Infiltration

Document Type: Research Article


Publication date: 2009-01-01

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