Among the many avenues that viruses can use to infect humans, drinking water may pose only a tiny risk for spreading certain viruses like the novel coronavirus. But in cases where there is unauthorized wastewater disposal or other events of inadvertent mixing of wastewater with water sources, the possibility of transmission through drinking water remains unknown.
Using a surrogate of the coronavirus that only infects bacteria, researchers have shown that existing water purification plants can easily reduce vast quantities of the virus, thereby protecting household water from such contagions. In particular, the researchers showed that the water purification step called coagulation could alone get rid of 99.999% of the virus, markedly decontaminating water for consumption.
Viruses can be categorized in two structural types: those that have an outer fortress, called an envelope, and those that do not. This envelope, consisting of a lipid bilayer and attached proteins, has multiple functions including aiding the virus in entering host cells. Several infamous viruses have a protective envelope including coronaviruses and the Ebola virus. Studies have found both enveloped and non-enveloped viruses in wastewater; however, most research has solely focused on the survival of non-enveloped viruses after wastewater and water treatment.
It is well known that wastewater mixes with drinking water supplies; in many countries, including the United States, wastewater is purified and used as drinking water. If enveloped viruses persist in wastewater, there could be a minuscule chance that these viruses make it into drinking water supplies.
At treatment facilities, raw water generally undergoes a three-step purification process: coagulation, filtration, and disinfection. In the coagulation step, certain metallic salts are added to initiate particles suspended in water to join together into millimeter-sized clumps. These clumps then settle down as sediment and are easily separated from the water. The researchers tested to see if enveloped viruses also assembled into bundles during coagulation.
For their experiments, they added a surrogate of the coronavirus that specifically infects bacteria to clean water. Next, they separately tested the action of a coagulant commonly used in water treatment plants. After coagulation, they studied small samples of the virus-infused water under an electron microscope and found that the virus strain assembled on the coagulants, forming clusters. They then checked the presence of infectious viruses in the water after removing the clumps and found there was a 100,000 reduction.
The researchers noted that although they used the coronavirus surrogate for their study, the results are readily generalizable to other viruses that have similar surface characteristics — notably, a lipid bilayer envelope and similar spike proteins. In the real world, wastewater contains a slew of viruses, unlike the team’s experiments that included just a single strain of virus. In their next set of experiments, they plan to investigate if coagulation is still as effective at decontamination in these scenarios.
The work suggests that surface water treatment plants might already be well equipped to meet virus regulations for drinking water and coagulation is just the first step in the water purification pipeline. Additional purification steps will only attenuate enveloped viruses further, alleviating associated health risks even more.