Partitioning, persistence, and accumulation in digested sludge of the topical antiseptic triclocarban during wastewater treatment

Abstract

The topical antiseptic agent triclocarban (TCC) is a common additive in many antimicrobial household consumables, including soaps and other personal care products. Long-term usage of the mass-produced compound and a lack of understanding of its fate during sewage treatment motivated the present mass balance analysis conducted at a typical U.S. activated sludge wastewater treatment plant featuring a design capacity of 680 million liters per day. Using automated samplers and grab sampling, the mass of TCC contained in influent, effluent, and digested sludge was monitored by isotope dilution liquid chromatography (tandem) mass spectrometry. The average mass of TCC (mean +/- standard deviation) entering and exiting the plant in influent (6.1 +/- 2.0 microg/L) and effluent (0.17 +/- 0.03 microg/ L) was 3737 +/- 694 and 127 +/- 6 g/d, respectively, indicating an aqueous-phase removal efficiency of 97 +/- 1%. Tertiary treatment by chlorination and sand filtration provided no detectable benefit to the overall removal. Due to strong sorption of TCC to wastewater particulate matter (78 +/- 11% sorbed), the majority of the TCC mass was sequestered into sludge in the primary and secondary clarifiers of the plant. Anaerobic digestion for 19 days did not promote TCC transformation, resulting in an accumulation of the antiseptic compound in dewatered, digested municipal sludge to levels of 51 +/- 15 mg/kg dry weight (2815 +/- 917 g/d). In addition to the biocide mass passing through the plant contained in the effluent (3 +/- 1%), 76 +/- 30% of the TCC input entering the plant underwent no net transformation and instead partitioned into and accumulated in municipal sludge. Based on the rate of beneficial reuse of sludge produced by this facility (95%), which exceeds the national average (63%), study results suggest that approximately three-quarters of the mass of TCC disposed of by consumers in the sewershed of the plant ultimately is released into the environment by application of municipal sludge (biosolids) on land used in part for agriculture.

Figure 1

Flow diagram showing the process train for wastewater and sludge handling in the full-scale activated sludge treatment plant studied. Numbers indicate the four sampling locations used.

Figure 2

Mass estimates, flow rates and fate of TCC during full-scale activated sludge wastewater treatment. Substantial fluctuations of TCC mass in influent were observed over a 24-h time period (A; upper panel). Flow rates peaked in the morning hours of the day of sampling due in part to a heavy rainfall event (3.5 cm). Chemical analysis of aqueous and particulate fractions of raw wastewater showed that 76 ± 11% of the mass of TCC was sorbed to particles (A; lower panel). When analyzing 24-hour composite influent samples collected over the course of a week, fluctuations in TCC loading were observed to be less pronounced (B). The hourly mass of TCC passing through the plant in tertiary effluent also showed some fluctuation and averaged 5.3 g/h (C). The mass of TCC found in liquid and solid samples (expressed in units of ppb_w/v and ppb_w/w, respectively) demonstrated that elimination of TCC from the aqueous phase during the treatment process resulted in an accumulation of the compound in digested sludge (D). Error bars indicate the higher of two independent experimental measurements (A) or the standard deviation of 3 or more measurements for each observation reported (B, D); see SI for further information.

Figure 3

Fate of TCC in the activated sludge wastewater treatment plant. The mass balance presented considers all wastewater and sludge processing steps performed, i.e., pretreatment, primary, secondary and tertiary treatment of wastewater, as well as digestion and dewatering of wastewater residuals. Only about 21% of the TCC mass input was observed to undergo transformation. The majority of the load accumulated in sludge (76 ± 30%) and a small fraction passed through the plant (3 ± 1%).

Figure 4

TCC can undergo various fates during processing in wastewater treatment plants (WWTPs). The primary route observed in this study was via sorption of the compound to sludge, followed by persistence and accumulation in this matrix despite additional treatment of this material.

Figure 5

Fate of sewage sludge produced in the study plant. The majority of municipal sludge produced was subject to beneficial reuse. Minor amounts were deposited in landfills or incinerated (Source of 2004 data: Maryland Department of the Environment).