Contribution of wastewater to antimicrobial resistance: a review article

Publication summary

Wastewater treatment plants present myriad opportunities for bacteria, antibiotics, and antibiotic-resistance genes (ARGs) from different sources—hospitals, sewage, agriculture, and industrial sites—to interact before they are released into waterways, thus potentially furthering the evolution and transmission of drug-resistant pathogens. Because wastewater disinfection tactics were not developed specifically to reduce antimicrobial resistance (AMR), studies have found that significant amounts of bacteria, antibiotic residues, and ARGs remain in wastewater following treatment and are discharged into the environment. This review evaluates factors along the wastewater pathway that may contribute to the emergence or worsening of antibiotic-resistant bacteria, along with challenges and opportunities related to the effectiveness of wastewater treatment strategies and regulations on AMR in the environment.

Who this is for

• Environmental policymakers
• Wastewater treatment specialists
• AMR National Action Plan policymakers

Key findings

AMR and the Wastewater Treatment Plant Environment

Wastewater treatment plants are sites where bacteria (especially those prevalent in sewage, such as Enterococcus faecium, Klebsiella pneumoniae, and Enterobacter species), antibiotic residues, and ARGs in influents may mix with a highly nutritive environment before the resulting effluent is discharged into area waterways or reused for agricultural irrigation. These interactions, along with a host of other factors (acidity and temperature of the environment and the tanks, the presence of heavy metals and microplastics, the concentrations of antibiotic compounds, etc.), may promote the selection of resistant bacteria and encourage ARG transfer.

Studies have evaluated the ability of wastewater treatment plants around the world to remove bacterial residues and antimicrobial compounds, finding significant variation, yet confirming that plants generally cannot completely remove these contaminants from water before it is discharged into the environment. 

Diverse Factors Affecting AMR in Wastewater

Diverse sources of bacteria, antibiotic compounds, ARGs, and other contaminants that may foster the development of AMR illuminate why it is important to consider the entire wastewater pathway when developing possible solutions. Hospital effluents represent a significant source of bacterial and antibiotic residues. Antimicrobial compounds—such as triclosan, triclocarban, and benzalkonium chloride— used in over-the-counter cleaning products and disinfectants are also present in wastewater that flows into treatment plants. China, India, and South Korea have reported high concentrations of antibiotic residues from treated wastewater discharged from pharmaceutical manufacturing plants.

The presence of heavy metals—such as zinc, cobalt, and copper cations—may form metal-antibiotic complexes that do not biodegrade and lead to the selection of AMR, yet information on the presence and significance of these complexes in treatment plants is largely unavailable. Microplastic contaminants released in wastewater from numerous sources including manufacturing sites and households also remain fairly stable in the environment and have been shown to grow biofilms that can harbor ARGs.

Many countries use wastewater in agricultural irrigation, which is a beneficial strategy given food shortages and the need to conserve water. Reuse of wastewater in this way, especially when considering the inability of treatment plants to remove bacterial and antibacterial contaminants, however, may allow drug-resistant bacteria and ARGs to enter the soil, although little evidence is available to demonstrate that this transfer is occurring. 

Solutions to Preventing AMR in Wastewater

Because hospitals are such a significant source of bacterial residue, antibiotic compounds, and ARGs in wastewater, stronger regulations on pre-discharge water treatment and investment in monitoring systems are essential. Regulations should be informed by the development of thresholds for ARG concentrations, and hospitals should partner with research institutions, donors, or government agencies to develop monitoring strategies that will help to define these thresholds and standards for effluent discharge.

Countries that reuse wastewater for agricultural irrigation usually have quality standards that affect when wastewater can be used, yet the variation in standards across nations can create confusion and hinder trade of agricultural products. In 2023, the EU adopted minimum quality requirements for all member states that reuse wastewater for irrigation, demonstrating that wastewater quality standards and tactics to prevent water shortages can co-exist.

Although wastewater treatment plants use a variety of different disinfectant strategies, several of them—membrane filtration, coagulation, and advanced oxidation, to name a few—have been more successful at removing bacterial residues and ARGs. Disinfectant strategies require close monitoring, however, as the development of resistance in response to disinfection strategies, and particularly in response to byproducts of chlorination, may present a risk.

Although water represents one of the clearest links between human, animal, and environmental health, the development of evidence-informed strategies to prevent one of the largest One Health threats—AMR—in wastewater, waterways, and groundwater has lagged behind other AMR initiatives. The inclusion of wastewater and of professionals most knowledgeable about the wastewater pathway in AMR work is an area that requires significant attention and innovation.