The performance of a limited aerated anaerobic membrane bioreactor for treating synthetic black water spiked with common Indian antibiotics

Abstract

With increasing pressure on natural water resources, wastewater is gradually being considered as a potential source for potable water. Current WWTPs are designed for the removal of parameters like solids, nutrients, organic matter, and pathogens. For achieving a high-quality effluent, that enables reuse, it is important to also address the removal of micropollutants, particularly antibiotics, from the wastewater. Due to these antibiotics, antibiotic resistance spreads among the microorganisms and increases through various mechanisms. Antibiotics of sulfamethoxazole (SMX), trimethoprim (TMP), ciprofloxacin (CIP), and ampicillin (AMP) are known to be found abundantly in natural waters all across the globe. The abilities of an anaerobic membrane bioreactor (AnMBR) of maintaining high SRTs with low biomass losses help in treating wastewater containing antibiotics. A recently developed technique of adding limited aeration to AnMBR has the potential of removing recalcitrant antibiotics by improving the performance of the reactor. Hence, this research aims to study the removal mechanisms of the antibiotics (SMX, TMP) and the persistence of corresponding antibiotic resistance in AnMBR, followed by the effect of the antibiotics on the performance of the AnMBR. In addition, antibiotics CIP and AMP were tested via anaerobic batch tests to investigate the effect of the limited aeration on their removal.
After adding the antibiotics SMX and TMP to the reactor, no significant difference in COD and nutrients removal was observed. The biogas production was reduced slightly after the addition of SMX 150 µg/L initially, however, it increased back to the original state after few days. Total removal of SMX and TMP was 86% and 97% respectively in the reactor. Results showed that 85% of SMX and 94% of TMP were removed through biodegradation/biotransformation and 14% of SMX and only 3% of TMP were discharged through the effluent. From the adsorption batch tests conducted, it was observed that the linear adsorption isotherm fits well for TMP. With the increase in temperature, the adsorption potential of TMP was reduced with a Kd value of 1.234 L/g at 10˚C and 0.513 L/g at 37˚C. The removal of SMX was low through adsorption and high due to degradation and follows the first-order rate kinetics with a half-life of 1.71 days. After two weeks of SMX addition to the reactor, almost all the bacteria present in the effluent gained resistance either to TMP or SMX or both. Of all the ARGs measured in this study, the genes responsible for the resistance development were sul1 and sul2. The addition of antibiotics increased the presence of ARGs in the system. The correlation between the presence of sul1 and TMP resistant bacteria, and sul1 and SMX resistant bacteria was 0.91-0.93, indicating that the gene sul1 might be involved in multidrug resistance. ARGs sul1, sul2, and dfrA1 were removed respectively by 3.2 log, 3.6 log, and 7.3 log units by the membrane. In addition, the class 1 integrons and 16s rRNA were removed by 3 log and 3.2 log units respectively. Removal of CIP and AMP was found to be high with values of 82% and 84% respectively in limited aeration assisted anaerobic batch tests. The removal efficiencies of all antibiotics were more than 80% and independent of their initial concentrations in the selected range. The increase in the removal of CIP and AMP in comparison to literature points to a relation with the added limited aeration. Nevertheless, more studies need to be performed to establish this.