No abstract

Interest in reusing treated wastewater drives efforts to eliminate antibiotics from water sources to prevent antibiotic resistance. Micro-aerated anaerobic membrane bioreactors (MA-AnMBR) promote wastewater reuse with high organic matter conversion to biogas, under a small footprint. However, the fates of antibiotics, antibiotic-resistant bacteria (ARB), and their antibiotic-resistance genes (ARGs) are not known in these systems. We studied the effects, conversions, and resistance induction, following the addition of 150 μg∙L-1 of two antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in a laboratory-scale MA-AnMBR. TMP and SMX were removed at 97 and 86%, indicating that micro-aeration did not hamper the removal of the antibiotics. These antibiotics only affected the pH and biogas composition of the process, with a significant change in pH from 7.8 to 7.5, and a decrease in biogas CH4 content from 84 to 78%. TMP was rapidly adsorbed onto the sludge and subsequently degraded during the long retention of the solids of 27 days. SMX adsorption was minimal, but the applied hydraulic retention time of 2.6 days was sufficiently long to biodegrade SMX. The levels of three ARGs (sul1 and sul2 for SMX, dfrA1) and one mobile genetic element biomarker (intI1) were analysed by qPCR, in combination with ARB tracked by plating. Additions of the antibiotics increased the relative abundances of all ARGs and intI1 in the MA-AnMBR sludge, with the sul2 gene folding 15 times after 310 days of operation. The MA-AnMBR was able to reduce the concentration of ARB in the permeate by 3 log.

Small-scale electrical power generation (<100 kW) from biogas plants to provide off-grid electricity is of growing interest. Currently, gas engines are used to meet this demand. Alternatively, more efficient small-scale solid oxide fuel cells (SOFCs) can be used to enhance electricity generation from small-scale biogas plants. Most electricity generators require a constant gas supply and high gas quality in terms of absence of impurities like H2S. Therefore, to efficiently use the biogas from existing decentralized anaerobic digesters for electricity production, higher quality and stable biogas flow must be guaranteed. The installation of a biogas upgrading and buffer system could be considered; however, the cost implication could be high at a small scale as compared to locally available alternatives such as co-digestion and improved digester operation. Therefore, this study initially describes relevant literature related to feedstock pre-treatment, co-digestion and user operational practices of small-scale digesters, which theoretically could lead to major improvements of anaerobic digestion process efficiency. The theoretical preamble is then coupled to the results of a field study, which demonstrated that many locally available resources and user practices constitute frugal innovations with potential to improve biogas quality and digester performance in off-grid settings.