NMSU Student Investigating Processes for Enhanced Removal of Superbugs and Their Genes from Wastewater

By Catherine Ortega Klett, NM WRRI Program Manager

Xiaoxiao Cheng is a graduate student working on her PhD in the NMSU Department of Civil Engineering. She is also a recipient of a 2018 NM WRRI Student Water Research Grant entitled: Developing New Strategies to Mitigate Antimicrobial Resistance for Safe Water Reuse.

Water resources management in the arid and semiarid southwestern United States has been a significant challenge due to limited fresh water supplies and chronic droughts. Water scarcity has a huge impact on food production since agriculture accounts for 70 percent of global freshwater withdrawals. Wastewater has long been identified as a potential water resource for agriculture. However, the massive use of antibiotics for human and veterinary purposes has unfortunately accelerated the evolution of antimicrobial resistance in bacteria, which has resulted in increasingly contaminated wastewater. It is now effectively an environmental reservoir of antibiotic resistant bacteria (ARB) as well as antibiotic resistant genes (ARGs). Making matters worse, conventional wastewater treatment does not include a process to eliminate phages (virus particles that infect bacteria), which may therefore also contain ARGs. Moreover, the highest relative abundance of ARGs has been observed in the surviving bacteria in sludge effluent, indicating that conventional biological wastewater treatment processes may actually be promoting antibiotic resistance. The overall objective of this study is to develop new strategies to mitigate the ARB and ARGs in the effluent of wastewater treatment plants (WWTP), in order to minimize their dissemination into the environment and agricultural ecosystems. In this effort, Xiaoxiao is working in collaboration with her faculty advisor Dr. Yanyan Zhang, Assistant Professor, also of the Civil Engineering Department at NMSU.

As a first step in this study, Xiaoxiao and Dr. Zhang are investigating the performance of the Las Cruces WWTP by targeting 18 ARGs for concentration measurements. This is carried out using the amazing technique of quantitative polymerase chain reaction (qPCR), whereby many copies of a specific DNA segment can be made so that enough material is present for analysis. In addition, conventional Agar plate techniques are being used to count the ARB in water samples.

Following this quantitative assessment of the superbug contamination problem, two mitigation methodologies are being explored. The first is a stage of filtration using modified granular activated carbon (GAC) to deplete the count of ARB and phages from treated water before discharge. GACs have relatively high sorption capacity for organic compounds because of their porous structure and high specific surface area. In their previous studies, Xiaoxiao and Dr. Zhang have shown that GAC has excellent capacity for bacteria and phage removal via adsorption. Also, the use of an iron-impregnated modification of this GAC that can enhance the immobilization of phages is also being investigated.

In addition to ARB and phages in contaminated water, free DNA-containing ARGs are also a major concern. Therefore, a second stage of contamination mitigation is being explored, and this involves the trapping of ARGs by adsorption onto clay minerals. Many factors that affect the degree of adsorption are being studied, including the types of clay minerals, dosages, the types of ARGs, ion bonding strengths, and so on, in order to optimize this approach of clay-based removal of ARGs through coagulation with clay particles.

Expected results and significance: With the advantages of economic feasibility and environmental sustainability, the innovative activated-carbon filter can remove ARB and phage encoding ARGs from reclaimed water, while clay-based coagulation processes can eliminate ARGs. These processes can be easily scaled up and applied by management agencies and industries for onsite treatment to minimize the dissemination of antibiotic resistance into the environment and agricultural ecosystems. The results of this study will provide valuable information to stakeholders and policymakers for transforming wastewater into an alternative water resource to augment local water supplies for irrigation, livestock watering, and aquaculture. The proposed research also addresses the issue of reducing the environmental risks resulting from the phenomenon of antibiotic resistance developing during wastewater management, reuse and disposal.

Xiaoxiao expects to graduate with a doctoral degree in civil engineering in July 2021. She received a bachelor’s degree from Beijing University of Chemical Technology and a master’s degree from Shihezi University. Recently she said, “I would like to become a researcher in the wastewater treatment field because this is a big issue for human health and the whole environment. I will put my passion into it.” Xiaoxiao says that the NM WRRI student grant has been helpful and particularly enjoyed meeting other students conducting water-related research at the NM WRRI annual water conference in October. She indicated, “I met a student from New Mexico Tech and her advisor who are also working on antibiotic resistance. We talked and were so glad to get to know each other.”

All recipients of 2018 NM WRRI Student Water Research Grants will provide completion reports that will be posted on the institute’s website in July 2019.