A comparison of radio frequency and microwave heating methods for enhancing the removal and biotransformation of endocrine-disrupting triclocarban during advanced anaerobic digestion

Abstract

Anaerobic digestion (AD) has very limited effectiveness against the removal of many emerging contaminants, including the pervasive antimicrobial triclocarban (TCC). This is the first study to compare two thermal pretreatment methods to evaluate the fate of persistent TCC and its transformation/degradation by-products during advanced AD. Two electromagnetic heating methods are employed: one uses an innovative radio frequency (RF) heating system at 13.56 MHz, while the other uses microwave (MW) heating at 2450 MHz. The biodegradation was assessed by biochemical methane potential (BMP) assays utilizing spiked secondary sludge. Overall, advanced digestion processes, combined with RF and MW pretreatments, were highly effective in increasing TCC elimination (up to 65 %) from digested sludge after 60 days of incubation, compared to conventional digestion (without pretreatment), which achieved only up to 20 % elimination. The contribution of RF and MW pretreatments to the total TCC removal efficiencies of the overall advanced digestion process was 26 ± 1 % and 22 ± 1 %, respectively. After pretreatment, during thermophilic anaerobic digestion, 34 ± 3 % and 30 ± 2 % additional TCC removals were achieved from the RF- and MW-pretreated samples, respectively. Significantly higher TCC removal and biotransformation occurred in advanced BMPs incubated at thermophilic temperatures compared to mesophilic temperatures. The chlorinated TCC was converted to less toxic carbanilide during digestion, reducing the environmental risk of TCC through the dechlorination mechanism. Considering the feasibility of RF and MW heating systems at full scale, the RF heating technology combined with anaerobic sludge digestion can be preferred to manage the municipal sludge contaminated with antimicrobials.