The rapid global transmission of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) led to a pandemic commonly known as Coronavirus Disease 2019 (COVID-19). To date, this pandemic has affected approximately 686 million people and claimed more than 6.8 million lives worldwide. Even though this virus is primarily transmitted through respiratory droplets or direct contact with an infected person, SARS-CoV-2 has also been detected in feces. It is, therefore, possible that SARS-CoV-2 may be excreted in human feces and other bodily secretions, such as saliva and urine, and be transported subsequently to wastewater treatment plants.

Study: Survival of SARS-CoV-2 in wastewater. Image Credit: Avigator Fortuner / Shutterstock

Background

The recent identification of SARS-CoV-2 in feces has raised the concern that it could be an additional route of viral transmission. A similar occurrence was recorded in 2003, during the first SARS pandemic. During this period, SARS cases increased due to aerosolized fecal matter, which resulted from faulty sewage pipelines within an apartment building in Hong Kong.

Data have been scarce regarding the isolation of viable SARS-CoV-2 from wastewater. It is imperative to understand how this virus dwells in wastewater as the majority of developing countries with low vaccine coverage still struggle to provide clean drinking water and safe sanitization.

Previous studies that evaluated the survival of SARS revealed that enveloped viruses remain viable for a small period compared to non-enveloped viruses. Unfortunately, due to the risks linked to culturing viable viruses and the need for biosafety level 3 laboratories, few studies have been conducted on determining live SARS-CoV-2 in wastewater and other environmental water matrices. However, a prior study revealed that infectious SARS-CoV-2 survived for 2.3 days in a river environment and 1.1 days in seawater at 20°C. 

Another study pointed out that SARS-CoV-2 survived for 1.7 days in wastewater at room temperature. The viability of SARS-CoV-2 was found to be greater in filtered river water and wastewater compared to unfiltered river water and wastewater samples.

About the Study

A recent study in the journal Science of the Total Environment investigated the survival of SARS-CoV-2 in filtered and unfiltered raw wastewater at 20°C (room temperature). This study also assessed the viability of SARS-CoV-2 in secondary effluent at the same temperature.

Wastewater samples were collected on December 13, 2021, from a wastewater treatment facility in Arizona, USA, which receives an inflow of around 22 million gallons per day. The physiochemical properties of the sample were assessed. Raw filtered wastewater sample was prepared by passing the crude sample through a 0.2 μm filter membrane.

Study Findings

A gradual decline in the infectivity of SARS-CoV-2 was observed in the three wastewater matrices. Consistent with previous studies, this study revealed a quick decrease in infectivity in unfiltered and filtered raw wastewater compared to secondary effluent. This finding indicates rapid SARS-CoV-2 inactivation in complex matrices compared to simpler matrices.

A similar inactivation rate of SARS-CoV-2 was observed in filtered and unfiltered raw wastewater during the first four hours. This inactivation could be triggered by enzymes present in wastewater. SARS-CoV-2 is an enveloped virus containing a lipid bilayer membrane that is susceptible to enzymes (e.g., lipases and proteases), which leads to inactivation.

The inactivation rate was higher in unfiltered raw wastewater after 24 hours than in filtered wastewater. This could be due to competition between indigenous microbial populations, such as bacteria, metazoans, protozoans, and SARS-CoV-2 for survival. Some of the indigenous microbes produce proteolytic enzymes in unfiltered wastewater that affect the survival of SARS-CoV-2.

Previous studies have shown that viruses adhere to the organic matter and suspended solids present in wastewater, which protects the virus against predation, photodegradation, disinfection, and other processes that affect its viability. The amount of time required for the inactivation of 90% of SARS-CoV-2 (T90) was estimated to be 10.4, 10.8, and 18.3 hours for unfiltered raw, filtered raw, and secondary effluent, respectively. 

Typically, it is challenging to isolate viable SARS-CoV-2 from raw wastewater using standard filtration-elution procedures due to the high decay rates of the virus in this matric.

Conclusions

The authors claim this study to be the first to assess the survival of SARS-CoV-2 in secondary treated effluent. This study revealed the survival of SARS-CoV-2 in unfiltered/filtered raw sewage and secondary effluent at room temperature. Since the wastewater residence times in sewage systems are typically less than 24 hours, it is highly unlikely that aerosols in wastewater treatment facilities would contain elevated concentrations of SARS-CoV-2 at room temperature. In the future, survival rates of SARS-CoV-2 in wastewater must also be evaluated at other temperatures.

Journal reference:

• Sherchan, S. et al. (2023) Survival of SARS-CoV-2 in wastewater. Science of the Total Environment. 882. https://doi.org/10.1016/j.scitotenv.2023.163049, https://www.sciencedirect.com/science/article/pii/S0048969723016674#bb8785