COVID-19 is a new illness but belongs to the large family of coronaviruses (CoV), which were first characterized in the 1960s by the Dr. June Almeida. Most of these microorganisms circulate among animals as pigs, bats, cats or camels, but sometimes they jump to humans (known as spillover event).
They normally cause mild to moderate upper-respiratory illness. However, recent types of coronaviruses have produced more severe diseases: Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV). The outbreak that we are currently suffering is SARS-CoV-2, causing coronavirus disease (COVID-19).
SARS-CoV-2 is transmitted human-to-human through droplets or direct contact but it has also been detected in fecal samples. Therefore, it is important to consider that symptomatic (and maybe also asymptomatic) affected people could spread the viruses through their excreta to water environments, including wastewater, surface water, drinking water or marine water.
May coronavirus be present in different water environments?
As declared by the World Health Organization (WHO) “there is no current evidence from surrogate human coronaviruses that they are present in surface or groundwater sources or transmitted through contaminated drinking-water”. However, persistence of coronavirus in drinking water is possible, especially if a low concentration of residual disinfectant is present in the distribution system. In case microbial biofilms colonize distribution systems, coronavirus could enter homes (but no cases have been known).
With respect to sewage water, there is no evidence yet about the survival of COVID-19 in this environment. Nevertheless, previous studies have demonstrated the presence of SARS-CoV in hospital and sewage wastewater originated from the fecal discharge of infected patients. In fact, during SARS outbreak in 2003, a leakage in a sewage pipe produced the aerosolization of water droplets, which contained coronavirus.
Other studies highlight the persistence of traces coronaviruses in wastewater treatment plants, specifically in primary and secondary sludge effluents (prior to disinfection).
Which is the survival of coronavirus in water?
The survival of coronavirus in water depends on several factors:
Temperature is one of the most affecting parameters, as high temperatures contribute to denaturation of proteins and activity of extracellular enzymes. For example, at 20ºC, SARS-CoV persisted for 2 days in hospital, domestic sewage and tap water. On the contrary, if the temperature decreases to 4ºC, the persistence is higher than 14 days.
Light exposure also affects the survival due to solar or UV inactivation, as it can destroy nucleic acid and disrupt their RNA.
The presence of organic matter or suspended solids is also important, as coronavirus may be adsorbed on them. A study with other virus (Murine hepatitis) demonstrated an adsorption of the virus on solid fraction of wastewater up to 26%.
Antagonist microorganisms increase the extent of inactivation of coronavirus.
As COVID-19 virus is an enveloped virus, with a fragile lipid host cell membrane, it is very susceptible to oxidants, such as chlorine.
Which are the methods to inactivate coronavirus in water?
Chlorine has been demonstrated to be an efficient and economic compound for bacteria and microorganism removal in water. The current water disinfection methods (in drinking water, wastewater and water from swimming pool treatments) are effective against non-enveloped viruses and bacteria. Since non-enveloped virus and bacteria are known to be more resistant to disinfection treatments, these methods can also be effective against coronavirus.
With respect to the optimal addition of chlorine to water, it is important to consider that when chlorine is added to water, three types of reactions can occur. This affects the availability of chlorine for disinfection purposes. First, substances like manganese, iron or hydron sulfide in water react with chlorine irreversibly. Secondly, chlorine may react reversibly with organic matter and ammonia, forming weak disinfectants (chloramines). Thirdly, chlorine may react with water giving efficient disinfectants as free chlorine or free residual chlorine. It also depends on the pH of the medium.
Ultraviolet irradiation is also expected to be effective in protecting water wastewater from coronavirus for the reasons mentioned before.
Which are the recommendations of the WHO?
Regarding drinking water, in centralized disinfection “there should be a residual concentration of free chlorine of ≥0.5 mg/L after at least 30 minutes of contact time at pH < 8.0. A chlorine residual should be maintained throughout the distribution system”.
In those cases where centralized water treatment and safe piped water supplies are not available, other household water treatment technologies have been proposed, including “boiling or using high-performing ultrafiltration or nanomembrane filters, solar irradiation and, in non-turbid waters, UV irradiation and appropriately dosed free chlorine”.
In WWTPs, “wastewater carried in sewerage systems should be treated in well-designed and well-managed centralized wastewater treatment works. Each stage of treatment (as well as retention time and dilution) results in a further reduction of the potential risk. A waste stabilization pond (that is, an oxidation pond or lagoon) is generally considered to be a practical and simple wastewater treatment technology that is particularly well suited to destroying pathogens, as relatively long retention times (that is, 20 days or longer) combined with sunlight, elevated pH levels, biological activity and other factors serve to accelerate pathogen destruction. A final disinfection step may be considered if existing wastewater treatment plants are not optimized to remove viruses”.
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