AWWA ACE63019

The environmental presence of antimicrobial compounds at sub-inhibitory concentrations may lead to the evolution of antimicrobial-resistant bacteria, and chronic exposure to antibiotics at trace levels in drinking water may contribute to increased allergies against antibiotics (Kummerer 2001). Removal of the antimicrobial parent compound is readily achieved by oxidation processes, but little is known about the antimicrobial activity of oxidation intermediates that are formed in the process. Therefore, the overall objective of this research is the systematic characterization of a sequential photochemical/biological oxidation process for the removal of the antimicrobial compounds sulfamethoxazole (SMX) and sulfamethazine (SMZ) from natural water. Specific objectives for the phase of the research described here included: quantification of SMX and SMZ photolysis and UV/H₂O₂ oxidation rates; and, assessment of residual antimicrobial activity of photooxidation intermediates. The sulfonamides SMX and SMZ were selected as model antimicrobial compounds. The effects of the following factors on SMX and SMZ oxidation and mineralization rates were evaluated: pH; H₂O₂ concentration; and, presence/absence of natural organic matter (NOM). Batch photolysis and UV/H₂O₂ oxidation experiments were conducted by recirculating water through an annular ultraviolet (UV) light reactor. The 25-W low pressure lamp had an emission wavelength of 254 nm. SMX and SMZ concentrations were monitored as a function of time and quantified by HPLC. Total organic carbon (TOC) analyses were performed to assess the extent of SMX/SMZ mineralization. Experiments were conducted in ultrapure water (DI water) buffered at pH values that were at least two pH units above and below the pKa of the sulfonamide group; thus, the degradation of the neutral and anionic forms of SMX and SMZ were evaluated. Tests conducted in the presence of NOM were performed by spiking SMX and SMZ into Lake Wheeler water (Raleigh, North Carolina) that had been filtered through a 0.45-µm filter. The TOC of filtered Lake Wheeler water was 5.3 mg/L, the pH was 7.6, and the total alkalinity was 20 mg/L as CaCO₃. To test whether photooxidation intermediates exhibit antimicrobial activity, a method based on the concept of minimum inhibitory concentrations (MICs) was implemented. For sulfonamides, the MIC is recorded as the lowest concentration that inhibits 80 % of the growth that would be observed in the absence of antimicrobial compound addition. The antimicrobial activity was determined by adapting the macrodilution method described by Andrews (2001) and the Clinical and Laboratory Standards Institute (CLSI) Standard M7-A6. The Enterobacteriaceae organism E. coli ATCC® 25922 was acquired to conduct MIC tests. In MIC tests, E. coli cells were added to Iso-Sensitest broth to yield 1x106 cells/mL. Then, 1 mL of this inoculum was added to 8-mL tubes containing 1 mL of water spiked with different concentrations of antimicrobial agent or 1 mL of antimicrobial-spiked water following exposure to different photolysis or UV/H₂O₂ oxidation times. Also, positive controls were prepared without the addition of antimicrobial agent. After incubating for 8 hours at 37ºC, the optical density of each sample was measured at 600 nm. To obtain an absorbance <0.6, samples were diluted (2 mL sample + 5 mL broth). Includes 4 references, tables, figures.