Variability of trace organic chemical concentrations in raw wastewater at three distinct sewershed scales

Abstract

The site-specific daily fluctuations and scale-dependent variability of influent water quality, particularly concentrations of trace organic chemicals (TOrCs), have not yet been well described. In this study, raw wastewater from three distinct sewershed scales was sampled including a centralized wastewater treatment facility in Boulder, Colorado (population ~125,000) and two decentralized wastewater catchments in Golden, Colorado (clustered system population 400, and septic system population 32). Each site was sampled hourly for 26 h and samples were subsequently analyzed in triplicate for 32 TOrCs using liquid chromatography with tandem mass spectrometry and stable isotope dilution. Detection frequency (DF) of the various TOrCs was positively correlated with sewershed size with the greatest DF of the targeted TOrCs at the Boulder site and with decreasing DF with decreasing sewershed size. Site-specific fluctuations were both scale and compound-specific. The 11 TOrCs detected greater than 75% of the time across all three sites were used to further investigate and quantify variability and to develop a statistical model to investigate the flow-dependence and time-dependence of TOrC variability. Sewershed scale was inversely correlated to variability with coefficients of variation ranging from 0.24 to 0.96, 0.39 to 2.22, and 0.32 to 3.93 for the Boulder, cluster, and septic sites, respectively. A significant linear relationship was observed between concentration and flow and concentration and the concentration at prior time points for most TOrCs at the Boulder site. This suggests less variable influent concentrations result from dispersion and mixing in the conveyance system and a larger number of discrete inputs. A notable exception was the chlorinated flame retardant TCPP, which is likely associated with a high concentration, low-flow industrial input. A significant linear relationship between flow and concentration and sequential time points was not common at the decentralized sites. Scientists and engineers developing decentralized treatment systems must consider a larger range of influent qualities, particularly with respect to TOrCs.