Improved Quantification of Stormwater Dilution in Wastewater Treatment Plant Influent Using Organic Mater Fluorescence

Abstract

Improved Quantification of Stormwater Dilution in Wastewater Treatment Plant Influent using Organic Mater Fluorescence. Wastewater treatment plants (WWTPs) are designed to process residential and industrial sewer flow as influent; however, they often receive addition water during and after rainfall through inflow and infiltration (I&I). These unplanned inputs dilute the influent, reduce treatment efficiency, and increase operational costs. The objective of this study was to use organic mater fluorescence in a source apportionment model (FluorMod) to detect and quantify I&I contributions to WWTP influent. A reference library of 142 excitation-emission matrices (EEMs) was developed, representing six potential sources: wastewater, rainwater, groundwater, river water, soil leachate, and street runoff. Samples from ten municipal WWTPs across North Carolina were analyzed, including facilities spanning both the Coastal Plain and Piedmont regions. Each sources’ contributions to each unknown sample were estimated using FluorMod and, coupled with descriptive statistics, Pearson correlation, and time-series analysis, we gained insight into the variability and dynamics of I&I. Two Coastal Plain WWTPs showed median influent dilution by groundwater exceeding 83%, while others showed variable contributions from both rainwater and groundwater, with an overall median dilution of 16% and 9% respectively. Several Piedmont WWTPs showed median dilution values near 0%, only exhibiting episodic dilution events following substantial five-day antecedent precipitation (AP5). In several systems, the number of contributing I&I sources increased with higher AP5, consistent with rainfall activating additional inflow and infiltration pathways. Higher and chronic groundwater dilution occurred in the Coastal Plain relative to the Piedmont. However, the observed variability in I&I dynamics suggests that other factors including, but not limited to, sewer infrastructure conditions, hydraulic connectivity, soil saturation, geologic setting, and the spatial distribution of rainfall, likely play significant roles in modulating dilution behavior. Results for each plant were disseminated in two outreach efforts. First, reports that summarized the project scope, methods, and time series results were shared with each plant shared. Second, offers for an online discussion of results and deeper explanation of the FluorMod approach were made, culminating in three Zoom meeting held prior to the end of the project. Project results demonstrate that fluorescence-based source apportionment is a practical, cost-effective tool for identifying and quantifying I&I contributions from multiple sources. Future applications extend to exfiltration of sewage and other potential pollution sources that carry a fluorescence signature. The project outcomes represent a substantial innovative method to corroborate results from other I&I methods and investigate I&I issues in sewersheds at scale. FluorMod’s adaptability and accessibility make it a promising diagnostic for WWTP operators and may complement conventional field techniques to support more targeted infrastructure assessments, long-term monitoring, and investment planning to address I&I challenges.