Biochar-based filtration of treated wastewater effectively removes trimethoprim but may introduce antimicrobial resistance determinants over time

Authors: PHAN, DUC - University Of California, Riverside; BHATTACHARJEE, ANANDA - University Of South Florida; Schmidt, Michael; Ibekwe, Abasiofiok; Skaggs, Todd; Ashworth, Daniel

Submitted to: Journal of Water Process Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2025
Publication Date: 12/17/2025
Citation: Phan, D., Bhattacharjee, A., Schmidt, M.P., Ibekwe, A.M., Skaggs, T.H., Ashworth, D.J. 2025. Biochar-based filtration of treated wastewater effectively removes trimethoprim but may introduce antimicrobial resistance determinants over time. Journal of Water Process Engineering. 81. Article 109262. https://doi.org/10.1016/j.jwpe.2025.109262.
DOI: https://doi.org/10.1016/j.jwpe.2025.109262

Interpretive Summary: Treated municipal wastewater (TMW) is the effluent from wastewater treatment plants. It can be used as a source of agricultural irrigation water, which is especially useful in regions where water is scarce. However, this water may contain contaminants such as antibiotics, which are not removed during the treamtent process. These antibiotics can be a problem if the water is used in agriculture because they can cause the development of antibiotic resistance in soil bacteria; this resitance may then spread through the food chain to humans and impact health. In this study we filtered the water through columns of sand or sand mixed with biochar (pyrolyzed dairy manure) to remove the antibiotic trimethoprim (TMP). We found that sand/boichar filtration was highly effective in removing TMP (>94% removal) when compared with sand alone (20-40% removal) due to the compound becoming adsorbed onto the surfaces of the biochar. However, the system also released bacteria and antibiotic resistance genes into the water during treatment, raising concerns about the microbiological quality of the water. This indicates that operational management of such systems should include processes such as backwashing to remove bacterial films. Nevertheless, microcosm studies indicated that when the filtered water was used to irrigate soil, the TMP concentration of the soil was decreased and certain ARGs were also reduced.

Technical Abstract: Using treated municipal wastewater (TMW) for agricultural irrigation can help reduce stress on freshwater resources. However, the use of TMW could expose agricultural environments to antibiotics and antibiotic resistance genes (ARGs), ultimately impacting human health. In this study, we investigated two filtration systems, conventional sand and sand mixed with 2% (w/w) dairy manure biochar, for the removal of the common antibiotic trimethoprim (TMP) from synthetic TMW. While the performance of the sand filtration system ultimately declined to 20–40% TMP removal efficiency, the sand/biochar filtration system maintained >94% TMP removal efficiency throughout the study. Bacterial cells (indicated by the 16S rRNA gene) and ARGs were detected in the effluent of both systems after several weeks of operation, raising concerns about the microbiological quality of the water for long-term use. ubsequent microcosm studies indicated that sand/biochar filtration reduced the presence of TMP in the soil following irrigation. Moreover, the system also effectively reduced the development of several ARGs that need a short period of time to develop (e.g., ttgA, ttgB), but did not significantly impact ARGs that need a longer time to develop. We note that since long-term operation of the sand/biochar system could introduce ARGs into soil, processes such as backwashing and biochar regeneration may be needed to optimize the filtration system. This study offers insights into a potential strategy for mitigating antibiotic dissemination and antibiotic resistance development following TMW-irrigation and suggests the importance of appropriate design and operational management of such systems.