A PROPOSAL FOR INDUSTRY-WIDE STANDARDIZATION OF I/I CALCULATIONS

Within the practice of environmental engineering, many types of data are collected and analyzed using widely accepted standard procedures. Examples of such procedures for water quality analyses are contained in “Standard Methods for the Examination of Water and Wastewater” (APHA, 1992). However, no such similar standard procedures are available for analysis of flow monitoring data, which have a comparable level of acceptance among engineers. As flow monitoring is used increasingly for verifying sewer capacity (EPA's CMOM program), and measuring I/I reduction, there is an increasing need for standardization (especially to compare results from various studies within a single sewage collection system). Yet, the challenge to our profession is to overcome the inertia inherent in analysts continuing to use many different “favorite” methods for calculating I/I. This paper presents a standardized approach for analysis of flow monitoring data for estimating I/I (inflow and infiltration) quantities and rates that will have wide applicability. The presentation of this approach and demonstration of its efficacy is intended to challenge practitioners to achieve a higher standard for I/I analysis. The procedures were designed to be objective (minimizing variation among human analysts) and thereby have reproducible results. This methodology is not proprietary and is described in sufficient detail for application by an engineer or experienced technician.The Metro Nashville Department of Water and Sewerage Services and its contractor agreed to implement this approach for establishing the baseline I/I measurements for a unique “performance-based” sewer renewal contract. This contract for the Whites Creek I/I Reduction Project (Metro 93-SC-34K) is one of the first in the USA that considers the effectiveness of I/I reduction in the schedule for compensation. In addition to “technical soundness”, the two major criteria for establishing the baseline I/I quantities were “objectivity” and “reproducibility”. The contract was issued in August 2000 by Metro Nashville and has a five-year duration. The contract requires that, “a projected I/I volume from a 5-year, 24-hour storm will be compared before and after rehabilitation.” The contract also provides for an incentive bonus based on this I/I reduction in monitored parts of the Whites Creek sewer-shed. Using the approach described in this paper, the baseline 24-hour I/I from the 5-year rainfall event was projected to be nearly 129,000 m3/d (34.039 mgd). The success of I/I reduction efforts under this contract will be represented by a decrease in this level, when measurements are repeated following the completion of sewer renewal.Statistical parameters (r- correlation coefficient, and 95% confidence intervals) were used as objective means to express the level of overall confidence in the data. The approach was tested by analyzing data from eight flow-monitoring sites for periods up to 38 months and using 247 rainfall events. Correlation coefficients “r” ranged from 0.82 to 0.94. Additionally, the 95% confidence limits ranged from 11% to 38% of the projected 24-hour I/I from a 5-year rainfall event (11.43-cm in 24 hours). Annual variations were observed at the long-term monitoring locations. Accordingly, it was recommended that the contract procedures allow for this annual variability in the calculation for the bonus.