In this work, conducted over a year in Spain’s Valencian Region, we aimed to study the multifactorial public health risks associated with sewage reclamation by analysing a range of contaminants including human enteric and respiratory viruses, antibiotic-resistant bacteria (ARB), antibiotic resistance genes (ARGs) and microplastics (MPs). The findings underscore the need for a One Health strategy to successfully address these interconnected risks effectively and to successfully achieve the United Nations Sustainable Development Goals.
The One Health Paradigm
The One Health approach emphasizes the interconnectedness of human, animal and environmental health. This perspective is of great interest in sewage reclamation, where contaminants can cross boundaries between ecosystems, affecting not just human populations but also animals and the broader environment. Here, we integrated this holistic view by examining contaminants that pose risks across these domains.
Key Findings and Implications
Persistent Viral Contaminants:
In our study, we investigated the presence of human enteric viruses, including human norovirus (HuNoV) GI and GII, human astroviruses (HAstV), hepatitis A virus (HAV), hepatitis E virus (HEV), and rotavirus (RV), along with respiratory viruses such as SARS-CoV-2, influenza A virus (IAV), and respiratory syncytial virus (RSV), in influent, effluent and biosolid samples from six wastewater treatment plants (WWTPs). We found that influent wastewater samples contained high concentrations of these viruses, making their complete removal through standard treatment processes challenging.
Post-treatment, we observed reductions in virus concentrations, but these were often below 2 Logs for viruses like HuNoV GI, HuNoV GII, HAstV, and RV—significantly less than the ≥6 Log reduction required by European regulations (EU Regulation 2020/741). Although HEV showed a reduction greater than 4 Logs, the persistence of other viruses in effluent samples suggests ongoing public health risks, especially for crops irrigated with reclaimed water. It is important to note that while RT-qPCR detected these viral genomes, they may not represent infectious particles. However, other studies have identified infectious enteric viruses in reclaimed water using capsid-integrity assays or cell culture, indicating potential risks.
We also examined somatic coliphages and Escherichia coli as indicators of faecal contamination. Reductions of 1.83 Log PFU for coliphages and 1.65 Log CFU for E. coli were observed, which did not meet EU standards, revealing the limitations of the WWTPs in reducing microbial risks.
We also identified novel viral indicators of faecal contamination, including Pepper mild mottle virus (PMMoV) and crAssphage, in effluent samples at concentrations consistent with previous research. Notably, this was the first study to report PMMoV levels in biosolids, raising concerns about the potential spread of this plant pathogen through agricultural use.
Regarding respiratory viruses, while RSV and IAV were effectively removed, SARS-CoV-2 was consistently detected in effluent and biosolids, highlighting the need for further studies on non-respiratory transmission routes.
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Antibiotic Resistance Genes and Bacteria:
In our study, we observed very high levels of Extended Spectrum Beta-Lactamase-producing E. coli (ESBL-E. coli) in influent samples, averaging 6.63 Log CFU/100 mL. The water reclamation processes applied by the WWTPs resulted in only modest reductions of 1.43 Log, leaving effluent levels at 4.30 Log CFU/100 mL—3 Logs higher than reported in other studies. This significant persistence highlights the role of effluent water in the potential dissemination of antibiotic-resistant bacteria (ARB) into the food chain, especially when used for irrigation. Similarly, high levels of ESBL-E. coli in biosolids suggest the need for further treatment before their application in agriculture.
In addition to resistant bacteria, our study also identified 11 different antibiotic resistance genes (ARGs) across effluent and biosolid samples. Sulphonamide resistance genes (sul1 and sul2) were the most prevalent, with higher concentrations in biosolids than in effluents, underlining the risk of biosolids as carriers of ARGs. Notably, blaCTX-M, an ARG associated with beta-lactamase resistance, was detected at levels 4 Logs higher than viable ESBL-E. coli, likely due to the persistence of DNA in the environment and its protection by extracellular materials.
Interestingly, certain ARGs like cmlA_2, which confers resistance to phenicols, were absent in effluent samples but present in biosolids, suggesting differences in environmental conditions or microbial populations. The presence of these ARGs, alongside high ESBL-E. coli levels, raises concerns about the spread of resistance through both irrigation and soil amendment, emphasizing the need for improved reclamation processes. From a One Health perspective, the dissemination of ARGs in aquatic environments poses risks not only to human health but also to animal health.
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Microplastics as Emerging Contaminants:
Our study highlights the extensive presence of microplastics (MPs) in wastewater, which poses significant environmental and public health risks. WWTPs play a crucial role in reducing MP concentrations, and we observed a decrease in MP levels from influent to effluent samples. However, the efficiency of MP removal varied with particle size, with larger MPs being removed more effectively (77-100% reduction), while smaller MPs (1-300 µm) showed lower and more variable removal rates (4-59%), consistent with previous studies.
The types of polymers detected, such as polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS) and polyamide (PA), were consistent with those reported in other studies. WWTPs were more effective at removing higher-density polymers like PA and PET, likely due to the density separation step during treatment. Despite this, over 90% of MPs detected in our samples were between 1 and 300 µm in size, with fragments being the most common shape.
The release of MPs into the environment through water effluents and biosolids is concerning because MPs can act as carriers for harmful pollutants, bacteria and viruses. These “plastispheres” can facilitate the spread of pathogens, including antibiotic-resistant bacteria, and increase the risk of viral transmission through electrostatic adhesion. Research has shown that MPs can support bacterial communities, including pathogens, and may even promote infections, such as IAV, by resisting traditional disinfection methods like UV treatment.
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Conclusions and Recommendations
Overall, the study’s findings emphasize the need for a comprehensive approach to wastewater management that integrates the One Health perspective. The presence of viral pathogens, antibiotic-resistant bacteria, ARGs and microplastics in reclaimed water and biosolids demonstrates the limitations of current wastewater treatment technologies. This necessitates the development of more effective treatment methods. Emerging technologies, such as membrane bioreactors and ultrafiltration, offer promising solutions by enhancing sludge separation and effluent quality. Additionally, integrating microalgae-based processes may provide a sustainable and effective method for eliminating MPs and other harmful contaminants from reclaimed water.
Furthermore, the importance of continuous monitoring and risk assessment in water reclamation processes cannot be overstated. Integrating environmental, human and animal health data is essential for comprehending the full scope of risks associated with sewage reclamation and for developing effective mitigation strategies.
In conclusion, as water reuse becomes increasingly vital in addressing global water scarcity, adopting a One Health approach is crucial for ensuring that reclaimed water is safe for agricultural use and does not pose undue risks to public health and the environment. This work also emphasizes the importance of collaboration among policymakers, researchers, and public health officials in developing and implementing solutions that protect both human health and the environment.