Dissipation of sulfamethoxazole and trimethoprim during temporary storage of biosolids: A microcosm study

Little is known about the dissipation rate of microcontaminants in biosolids during storage and stabilization in stockpiles (unsaturated) or storage lagoons/tanks (saturated). The objective of this study was to characterize the dissipation in biosolids of two antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in microcosms under saturated and unsaturated conditions that simulate biosolids that are stockpiled on land or deposited in lagoons/tanks, respectively. The laboratory experiment was conducted at 22 °C using biosolids spiked at an initial nominal concentration of 10 mg kg^-1 for both antibiotics. Biosolids were sampled in triplicate at seven sampling times over a 42-d period. Concentrations of SMX and TMP in extracts prepared from biosolids were quantified using liquid chromatography with tandem mass spectrometry. Dissipation data fitted to a first-order kinetic model indicated that the time to 50% dissipation (DT₅₀) for SMX was significantly shorter in the unsaturated microcosms (2.8 d) than the saturated microcosms (4.4 d), while the DT₅₀ for TMP was significantly shorter in microcosms under saturated conditions (10 d) relative to unsaturated conditions (116 d). These results indicate that the reducing conditions that develop in biosolids deposited in lagoons or placed in storage tanks might be effective for enhancing the microbial degradation of antibiotics that are otherwise persistent under aerobic conditions (i.e., TMP), while also being effective for removing other antibiotics including those that dissipate relatively readily under aerobic conditions (i.e., SMX).