Viruses are quietly moving through wastewater treatment plants all across the world, surviving processes intended to keep water safe and clean. New research reveals that these hidden microbes may have a significantly greater role in public health, water safety, and antibiotic resistance than regulators have previously identified. 

In a study published in Biocontaminant, researchers used advanced genetic techniques to track viruses in large wastewater treatment plants in China and Singapore. By studying samples ranging from raw sewage to final treated water, the researchers discovered that many viruses survive the treatment process and interact closely with disease-causing bacteria. 

"Wastewater treatment plants are designed to remove pollutants and known pathogens, but viruses have largely been overlooked," stated corresponding author Shu Hong Gao from Harbin Institute of Technology. "Our findings demonstrate that viruses are not only passive passengers. They actively modify microbial activities, potentially influencing both therapy efficacy and health risks." 

Using metagenomic sequencing, a technique for reading genetic material directly from environmental samples, the scientists identified 99 virus families in wastewater and sludge. Peduoviridae and Casjensviridae were constantly present throughout the treatment process, from incoming sewage to processed effluent. Their ability to live implies that they could serve as biological indicators of how well treatment methods are working. 

For decades, scientists have primarily assessed wastewater safety by monitoring bacteria such as Escherichia coli. However, the researchers found that E. coli levels did not indicate viral activity. Instead, opportunistic infections such as Pseudomonas aeruginosa and Aeromonas caviae behaved similarly to dominant viruses. 

"This challenges the idea that one or two standard bacteria can represent overall biological risk," according to Gao. "Our findings suggest that alternative indicators linked to viral populations may provide a more accurate picture of treatment effectiveness." 

The researchers also investigated what these viruses do within treatment plants. Many had supplementary metabolic genes, which alter how bacteria digest food and substances. Some of these genes may help bacteria break down contaminants more efficiently, thereby aiding in the cleaning of water. 

However, there is a potential drawback. Certain viral genes appeared to provide antibiotic-resistant bacteria a competitive advantage, raising fears that treatment plants may unintentionally contribute to the spread of resistance. 

Gao observed that these viral functions have both positive and negative effects. "They may support pollutant removal, but they can also increase the risk of resistance spreading among pathogens." 

A machine learning study found that most viruses attacked bacteria in the Pseudomonadota group, which includes many drug-resistant types often present in wastewater. Importantly, several viral functionalities persisted even after disinfection, implying that existing treatment methods may not adequately address virus-related hazards. 

The researchers believe that expanding wastewater surveillance to include viral monitoring could improve public health protection, particularly as treated water is widely reused. 

"Understanding virus host networks gives us new tools to manage biological risks," according to Gao. "With better monitoring and targeted process optimisation, wastewater treatment can be made safer and more resilient in a world facing growing public health challenges."