Review of Coronavirus transmission in urban clusters: Survival in Water and Wastewater Systems

The ongoing Coronavirus Disease 2019 (COVID-19) pandemic has infected over 58 million people and claimed over 1.58 millions deaths globally (as of 11th December 2020) since its first outbreak in Wuhan, China in December 2019. Initially, the numbers of infected patients and death was largely contained in China with 98% of all confirmed infected cases. However, the increased rate of new infected cases outside of China like United States, Italy, and Spain raises questions on the virus characteristics and its routes of transmission. Although the main transmission modes of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are through direct contact and respiratory droplet/aerosol inhalation, current studies stipulate that SARS-CoV-2 RNA is found in sewerage, suggesting the potential transmission of SARS-COV-2 through wastewater systems. This paper seeks to review potential exposure routes of SARS-COV-2 in urban environments, the survival rate of coronaviruses that pose human health risks, and to provide relevant safety recommendations to reduce the impact of ongoing COVID-19 pandemic. There is an urgent need for wastewater effluent and water treatment supply epidemiology surveillance, especially in developing countries with subpar wastewater treatment systems and infrastructure to reduce human and ecological risks to protect populations from infectious diseases outbreak


INTRODUCTION
The novel zoonotic Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), an etiological agent of Coronavirus Disease 2019  has infected over 58 million people worldwide and continues to spread globally (WHO 2020b). This novel coronavirus (nCoV) was identified in Wuhan, China in late December 2019 and was tentatively named as 2019-nCoV (Coronaviridae Study Group of the International Committee on Taxonomy of Viruses 2020). SARS-CoV-2 is the newly re-emerging zoonotic coronavirus (CoV) that has resulted in a major outbreak after SARS-CoV in 2003  ). However, the fatality rate of SAR-CoV-2 is 2.3% which is lower than other coronavirus strains such as SARS-CoV and MERS-CoV with 9.5% and 34.4%, respectively (Petrosillo et al. 2020). The basic reproduction number (R0) represents virus transmissibility within a naïve population, indicating the average number of new infected cases produced by an infectious individual where R0 > 1 shows the number of new infections is likely to increase whereas R0 < 1 stipulates that virus transmission will cause less than one new infection case and the disease will eventually decline. To date, the estimate R0 of COVID-19 is 2-2.5 people which is higher than SARS (1.7-1.9 people) and MERS (less than 1 person) (Liu et al. 2020b;Petrosillo et al. 2020).
The present COVID-19 pandemic is one of the biggest global emergency crises that does not seem to be tapering off soon, with more new infected cases are reported across the globe (WHO 2020b). Current understanding of COVID-19 transmission is mainly based on previous SARS and MERS outbreak where it has been shown to transmit from person-to-person through direct personal contact and inhalation of respiratory droplets secreted from infected individuals (Shereen et al. 2020). Intriguingly, some studies have reported that SARS-CoV-2 was detected in the stool samples collected from COVID-19-positive patients, inferring that SARS-CoV-2 may potentially transmit through faecal-oral route (Wang et al. 2020;Woelfel et al. 2020). During 2003 SARS outbreak in Hong Kong, inadequate sewage system is believed to be a platform for SARS transmission where aerosolized faecal droplets containing coronavirus re-enters into apartment complex through sewage and drainage systems with strong upward air flows, inadequate traps and nonfunctional water seal (Lee 2003; Hung et al. 2006; Wigginton et al. 2015). Little studies have been done on the relationship between urban clusters formation and the spread of infectious microorganisms.
Therefore, enhanced environmental surveillance through wastewater system need to be addressed to reduce health burden, alert health officials to impose or withdraw control measures and inform local authorities of the potential outbreak in the near future.

METHODOLOGY AND DATA COLLECTION
This review article is aimed to discuss the possible exposure routes of the novel SARS-CoV-2 in urban environment and the survival rate of coronaviruses that poses human health risks. The literature analysis begins with materials collected from independent databases such as ScienceDirect, Wiley Only Library and The Lancet. In order to avoid duplication of articles, specific keyword combination were used together as advanced search such as 'coronavirus survivalibility', 'covid-19 survivalibility', 'coronavirus transmission', and 'covid-19 transmission'. The database search was set to English, and articles not related to either coronavirous and covid-19 survivalibility and transmission was excluded from this review. Based on the search results, a total of 1966 articles were related to the specific keyword combination (as demonstrated in Table 1). Further exclusion were filtered to review articles focusing on transmission of coronavirus and covid-19 in the urban environment setting, particularly for water and wastewater systems. Ultimately about 89 journal articles were reviewed in this paper. Data from other medium of resources were also collected from World Health Organization, the United Nations Development Programme, Centers for Disease Control and Prevention of the United States and Ministry of Health Malaysia, to help support statistical data for this review paper.  The main mode of SARS-CoV-2 transmission is via respiratory droplets secreted when an infected individual sneezes, coughs and talks, which means that this virus can be transmitted air-borne (Wang et al., 2021). Therefore, SARS-CoV-2 transmission is likely to happen when susceptible persons are in close proximity to COVID-19 infected individuals or by touching viruscontaminated inanimate objects or surfaces (Sahin 2020). Although SARS-CoV-2 is generally thought to be transmitted via respiratory routes, new evidence suggests that it may also infect gastrointestinal tract and its viral RNA was detected in the stool of SARS-CoV-2 infected hospitilized patients, indicating the possibility of this respiratory virus to spread through faecal-oral route . Previous research indicates that toilet water containing SARS coronavirus could generate aerosol droplets from toilet flushing, highlighting the potential spread of infectious particles in the indoor environment such as home and office as shown in Figure   The specific characteristics of viral particles can be vary depending on the sources but according to the World Health Organisation (WHO), droplets are defined as > 5 μm in diameter whereas airborne particles are < 5 μm in diameter (WHO 2020a). Respiratory droplets secrected from infected person are stable in the air for several amount of time before settling on the grounds or surfaces due to gravity (van Doremalen et al. 2020). The bioaerosols generated by the toilet flush are generally smaller than the threshold for respirable particle size (diameter 10 μm) with the greater portion of the mean particle sizes reported to be 5 μm or less in size based on an evaluation of commonly cited studies describing toilet bioaerosol particle sizes ( Previously, several studies have reported the ability of coronaviruses to survive on multiple surfaces at different temperatures (see Table 2). More recently, the stability of SARS-CoV-2 on different surfaces in different environmental settings was investigated ( . SARS-CoV-2 was is highly stable at low temperature (4°C) and susceptible to heat and common disinfectants such as 70% ethanol and household bleach (Chin et al., 2020). These findings suggest the indirect transmission of SARS-CoV-2 through contamined surface and airborne particles, and highlight the importance of frequent disinfection of touched surfaces or objects.  (Table  3). Other findings using transmissible gastroenteritis virus (TGEV) and mouse hepatitis virus (MHV) also showed that these surrogate coronaviruses are also stable for lengthy period of time at lower temperature of 4°C than higher temperature of 25°C (Table 3).  was collected after settling. Primary effluent is biological oxygen demand (BOD) and suspended solids of 110-220 mg/l. 5 Reagent-grade water (pH 6.0, turbidity 0.1 NTU)

Water and Wastewater Systems in Building as Coronavirus Transmission Route
To date, there is no report of SARS-CoV-2 transmission from sewage and human excretion.  (Regan, 2020). This air vent is one of the main parts of a building's plumbing system, and is also known as a vent stack that regulates the air pressure in the plumbing system through the roof of a building vertically (New York Engineers, 2020). The drain and soil pipe physically remove water and sewage (wastewater) from a building, while plumbing air vent pipes remove gas and odour of the waste from a building (New York Engineers, 2020). The air vent pipes are attached to the drainpipe line and releases pressure to help water flow smoothly through the drainpipes by providing fresh air into the plumbing system (New York Engineers, 2020).
Another possibility and risk of transmission from wastewater is when an infected person has used the toilet and flushes it without closing the toilet seat cover or lid, this action can generate concentration of virus-laden aerosols into the air ( Figure 2) (Wenhong, 2020). The flushing of toilet produces bioaerosol with a range of particle sizes, between 14−700 nm using a scanning mobility particle sizer and between 0.5−20 μm using an aerodynamic particle size spectrometer (Chattopadhyay & Taft, 2018). Based on a study by Lin and Marr (2017) that measured two commercial auto-flush mechanism toilets for the aerosolization of Ebola virus surrogate in wastewater, a total particle number of 1.7 to 2.6 million per flush and the total volume of aerosols generated in the range of 10−9 to 10−8 mL were reported. The study found that the size distribution across a particle size range (e.g., 1 μm to 20 μm, large droplets) and the distribution peaks over the particle size range of approximately 10 nm to 1,000 nm (i.e., 10-2 μm to 1 μm) (Lin & Marr, 2017). Figure 3: Representation of building plumbing system. Viral particles that are released into drainage could get trapped in the piping system. If the viral particles become aerosolised, the piping system could be the mode of transmission for virus spread within the closed building area.

GUIDELINE ON DISCHARGE PIPE
The wastewater discharge pipe should be kept minimum in distance, fewer bends and adequate gradients to prevent the transmission of foul air into the building, by using water trap at all sanitary appliances. The foul air may contain contamination that may cause nuisance or health hazard. The water traps come together as part of the integrated molded sanitary appliances such as the water closets (wc) or the gullies. To smaller fittings such as sinks and basins, they will be fitted with either a trap 'P', 'S' or 'Q' as ( Figure 4)  The U-trap is one of the common water traps used at sanitary appliances to avoid foul air into the buildings with certain depth and height of the seal (JKR, 2017). The depth of seal must be retained at no less than 25mm for a typical wash down water closet (WC) or for tubular trap at 75mm seal depth (Hall & Greeno, 2009 Figure 3, the vent-pipes are connected to the traps and directly to a discharge pipe.

URBAN WATER AND WASTEWATER TREATMENT
The human health risk of enteroviruses such as E.Col, Salmonella typhi, and Shigella spp. is greater than bacterial pathogens in urban surface waters (that can be the source of drinking water) and would require greater removal or treatment processes (Zhang & Wang, 2012). Water treatment can be defined by as the processes used to purify, disinfect and protect water against recontamination, and is highly dependent on around the clock energy supply (usually electricity) (UNESCO, 2019). This high energy dependency for clean water remains a challenge for developing countries, and as a result low-tech and nature-based filtration process is used that does not guarantee safe drinking water quality for its population (UNESCO, 2019). Safe drinking water quality persist in both developed and developing countries alike, and several waterrelated diseases such as cholera and schistosomiasis are widespread across many developing countries as majority of the domestic and urban wastewater is not treated before its release into the environment (UN-Water, 2018).
A study in Gothenburg analysed untreated sewage samples from Ryaverket wastewater treatment plant found seven (7) (Zhang et al., 2020). The overuse and abuse of antibiotics in preventing/treating bacterial diseases and promoting animal growth has promoted antibiotic resistance genes (ARGs) in bacteria, which poses tremendous risk to human population as these ARG bacteria are more difficult to control and kill (Bouki et al., 2013; Zhang et al., 2020) . Latest findings from Netherlands found the COVID-19 virus in wastewater at Amsterdam Schiphol Airport and the wastewater treatment plant in Kaatsheuvelservicing the town where the first reported COVID-19 patient in Netherlands lives (National Institute for Public Health and the Environment, 2020).
These studies illustrate the importance of monitoring and identifying typing strains in sewage and wastewater for viruses in order to provide an early warning for possible outbreaks (Hellmér et al., 2014;Osuolale & Okoh, 2017), and there is potential public health risk for infectious disease to infect a population through improper wastewater treatment systems. Generally, the wastewater or sewage treatment process is categorized in four (4) different phases: preliminary, primary, secondary and tertiary ( Figure 5) (CDC, 2015a; IWK, 2019). The preliminary sewage treatment process (or coagulation) includes screening, grit removal, pre-aeration and removal of rubbish, grit, oil and grease (IWK, 2019). The primary treatment includes sedimentation, floatation and removal of suspended solids and organic matter; while the secondary treatment is directed to filter biodegradable organic and suspended solids using biological unit processes, filters (sand, gravel and charcoal) and disinfection (CDC, 2015a; IWK, 2019). Finally, in the tertiary phase includes biological and chemical treatment to remove nutrients and pathogens, toxic substances including heavy metals and further removal of suspended solids and organic matter, through filtration and disinfection processes (chlorin or chloramine) (CDC, 2015a; IWK, 2019). In urban water and wastewater treatment, chlorine treatment or chlorination is the most common method to treat and disinfect drinking water for safe consumption (CDC, 2015b;EPA, 2002). Disinfection is the process to kill or inactivate most microorganism in wastewater such as bacteria, viruses or spores that causes illness to the human and animal population (CDC, 2015b). Chlorine is used either in compressed elemental gas, in solution (sodium hypochlorite -NaOCl, or in solid form (calcium hypochlorite -Ca(OCl)2 and added to water to kill harmful microorganism (CDC, 2015b). Therefore, for developed and most modern developing countries with high technology wastewater system that includes chlorination will be effective in provided safe water to its population. However, for least developed countries, the COVID-19 pandemic poses a real threat where untreated wastewater remains common practice, and without proper wastewater infrastructure, technical and institutional capacity and financing (UNESCO, 2019).

DISCUSSION
Multiple studies that have found coronaviruses survivability in water and wastewater (Casanova et al., 2009;Gundy et al., 2008;Wigginton et al., 2015b;Wolff et al., 2005) long enough for it to be a possible transmission method. Gastroenteritis (TGEV) and mouse hepatitis (MHV) viruses are able to survive up to 14 days in pasteurized settled sewage at 25° C, and up to 105 days at 4°C (Casanova et al., 2009). The high risk of potential rapid and mass infection is when contaminated water or wastewater with coronavirus becomes aerosolized (Casanova et al., 2009). Contaminated water and wastewater systems will defeat any quarantine or isolation measures as the virus remains infectious within then contaminated systems even after the infected individuals have been removed from the area (Casanova et al., 2009). It is very worrying as the COVID-19 pandemic SARS-CoV-2 virus might have the same survivability rate as the SARS-CoV virus since they belong to the same Coronaviridae family, and the SARS-CoV virus is able to survive up to 9 days in culture media at room temperature (Rabenau et al., 2005). Research also need to be done in testing the SARS-CoV-2 virus's survivability in water, as other human coronavirus HCoV 229E is predicted to survive up to 588 days at 4°C, and tested to survive up to 10 days in 23°C in tap filtered water (Gundy et al., 2008). SARS-CoV-2 virus transmission route is mainly through respiratory droplets, it is also important to review viruses survivability on fomites or inanimate surfaces. Previous research shows that the SARS-CoV virus is able to survive on metal, paper and plastic surfaces up to 5 days in room temperature (Duan et al., 2003;Rabenau et al., 2005). Similarly, looking at gastroenteritis (TGEV) and mouse hepatitis (MHV) viruses, it was found that they are able to survive on steel surfaces for more than 28 days at 4°C (Casanova et al., 2010). More importantly for health care providers, some studies have found that the HCoV 229E virus can survive up to 5 days on silicon rubber (Warnes et al., 2015) and up to 8 hours on surgical latex gloves at 21°C (Sizun et al., 2000). Equally important, Lai et al. (2005) found that the SARS-CoV GVU6109 virus is able to survive up to 2 days on disposable gowns in room temperature conditions. However, methods of disinfection and rendering the virus inactive is quite simple and low cost. The coronaviruses can easily be efficiently inactivated by disinfection procedures with 60-70% ethanol, 0.5% hydrogen peroxide or 0.1% sodium hypochlorite within one (1) minute (Kampf et al., 2020). Guidelines by WHO and CDC recommends avoiding physical contact, frequently washing hands with soap for at least 20 seconds or using alcohol-based hand sanitizers, wear a facemask, and avoid sharing personal household items such as dishes, eating utensils and towels (CDC, 2020; WHO, 2020a). Disinfecting and cleaning "high-touch" surfaces such as doorknobs, countertops, keyboards and phones daily by using household disinfectant and diluted chlorine solution (CDC, 2020). The common household disinfectant brand Dettol (with active ingredient Chloroxylenol) claims that their products are able to kill other coronavirus such as (SARS-CoV, MERs-CoV and HCoV) (99.9% inactivation) and therefore would be also be an effective disinfectant for the emerging coronavirus, SARS-CoV-2 (Dettol, 2019).

CONCLUSION
This review paper has highlighted the significant gap in the potential role of water and wastewater treatment spreading the COVID-19 pandemic virus, following previous research on different coronaviruses such as SARS-CoV, MERS-CoV, gastroenteritis (TGEV) and mouse hepatitis (MHV). New findings from Netherlands have proven that the novel SARS-CoV-2 virus is able to also survive in wastewater. There is an urgent need to conduct testing on wastewater effluent and water treatment supply to curb further outbreak in communities, especially in developing countries with subpar wastewater treatment systems and infrastructure to reduce human and ecological risks. Human coronaviruses can also remain infectious on fomites or inanimate surfaces up to 9 days and this data is especially important to health care providers so they do not become infected and infect others surrounding them as the virus can survive up to 2 days on their protective gowns and 8 hours on surgical latex gloves. Constant monitoring and testing of water and wastewater effluent is needed for epidemiology surveillance to protect populations from infectious diseases outbreak. Similarly, routine surface disinfectant using 60% alcohol-based solutions or ethanol will significantly reduce coronavirus infectivity and survivability, and the same should be expected for the novel SARS-CoV-2 virus.

DATA AVAILABILITY STATEMENT
All data, models, and code generated or used during the study appear in the submitted article.