Assessment of the contribution of triclosan to dioxin emissions from sludge incineration in the U.S. Using a mathematical model

Triclosan, a household antimicrobial, is present in raw and treated wastewater and in digested sewage sludge (biosolids) across the U.S. It was demonstrated to be converted to various dioxins upon chlorination and combustion, common processes in sewage treatment and biosolids disposal. In this study, a mathematical model was developed to assess triclosan's contribution to dioxin emissions resulting from sewage sludge incineration. Triclosan transformation rates were identified from the literature. Sludge incineration was identified as a probable pathway leading to dioxin formation because triclosan is exposed to chlorine during drinking water and wastewater treatment and accumulates in biosolids. Representative concentrations and transformation rates of triclosan, chlorinated triclosan, and dioxins were utilized to populate a mathematical model predicting the mass of dioxins formed from triclosan combustion. Analyses considered representative tetrachlorodibenzo-p-dioxin (TCDD) concentrations and toxic equivalency quantities (TEQ) based on established and theoretical (TEQ*) dioxin toxicity data. The model suggests that triclosan conversion to dioxins accounts for a significant fraction of the TCDD and TEQ burden from sludge incineration. Depending on triclosan concentrations in sludge, annual dioxin mass loads range were predicted to range from 2 to 168 g total TCDD/yr or 0.3 to 32 g I-TEQ*/yr, suggesting that triclosan contributes significantly to the total dioxin emissions in the U.S. This study is the first to quantitatively link triclosan-enriched sludge incineration to toxic dioxins in the U.S. Future work should seek to verify the modeling results obtained here.