The Urban Water Cycle as a Planning Tool to Monitor SARS-CoV-2: A Review of the Literature

: COVID-19 is a terrible virus that has impacted human health and the economy on a global scale. The detection and control of the pandemic have become necessities that require appropriate monitoring strategies. One of these strategies involves measuring and quantifying the virus in water at different stages of the Urban Water Cycle (UWC). This article presents a comprehensive literature review of the analyses and quantiﬁcations of SARS-CoV-2 in multiple UWC components from 2020 to June 2021. More than 140 studies worldwide with a focus on industrialized nations were identiﬁed, mainly in the USA, Australia, and Asia and the European Union. Wastewater treatment plants were the focus of most of these studies, followed by city sewerage systems and hospital efﬂuents. The fewest studies examined the presence of this virus in bodies of water. Most of the studies were conducted for epidemiological purposes. However, a few focused on viral load and its removal using various treatment strategies or modelling and developing strategies to control the disease. Others compared methodologies for determining if SARS-CoV-2 was present or included risk assessments. This is the ﬁrst study to emphasize the importance of the various individual components of the UWC and their potential impacts on viral transmission from the source to the public.


Introduction
Coronavirus disease 2019 (COVID-19) is responsible for a disastrous pandemic that, as of June on 2021, has resulted more than 3 million deaths and more than 180 million infected people worldwide. SARS-CoV-2 that causes COVID-19 is characterized by its efficient transmission via liquid droplets (saliva and nose), aerosols and surfaces that have been touched by symptomatic or asymptomatic patients [1][2][3]. The virus can enter the body through the eyes, nose or throat. A rapid growth in infections has been observed throughout the world, with epicenters in Asia, Europe and North America. The pandemic is generating abrupt and radical changes in global dynamics in terms of economic, social, environmental and human health issues. The ongoing rise in infections, deaths and inadequate human immune system responses highlights the importance of a careful evaluation of SARS-CoV-2. In particular, evaluations should focus on the short-and long-term impacts on public health, different viral transmission routes, and potential strategies for the prevention and control of the virus [4][5][6][7][8][9].
According to the literature, the main transmission routes for other viruses are either through direct contact or through microscopic droplets or aerosols generated from sneezes Table 1. Reported evidence of SARS-CoV-2 measurements in faecal samples. [26] China 73 The viral RNA test results remained positive in faecal matter forlonger than in pharyngeal swab samples. [28] China 305

Authors Country Number of Patients Comments
Based on a comparison between two series of patients, there was a higher positivity rate for the group with severe symptoms vs. those with mild symptoms (94.6% vs. 82.5%) [29] Singapore 18 Using PCR, the virus was detected in the faecal matter of four out of eight patients.
[20] China 84 Faecal samples from a higher proportion of patients with diarrhea (69%) were positive for virus RNA than from patients without diarrhea (17%).
[ 30] China 42 The presence of SARS-CoV-2 RNA in the faeces of COVID-19 patients was not associated with gastrointestinal symptoms or disease severity. Faecal samples from 67% of patients remained positive for viral RNA after pharyngeal swabs were negative.
[31] USA 1 Analysis of faecal matter obtained on day 7 of the disease yielded positive results. [22] China 10 Rectal smears from eight children consistently tested positive even after their nasopharyngeal tests were negative, increasing the possibility of faecal-oral transmission.

Urban Water Cycle
According to Peña-Guzmán et al. [32], the Urban Water Cycle (UWC) is the spatiotemporal interaction between water and hydrological processes, as well as the supply, treatment, distribution, consumption, collection, and reuse that is carried out in urban or partially urban areas. Based on the interconnections and multiple processes that exist within this cycle, many authors [11,[33][34][35][36][37][38], have examined how SARS-CoV-2 can be monitoring into urban waters, mainly wastewater and affluents.
As shown in Figure 1, traces of virus can enter the urban water cycle mainly through the use of water by people infected with COVID-19. The traces in wastewater are associated with the discharge of fluids or faeces from the infected population. Depending on the city 's sanitation infrastructure, wastewaters are discharged directly into the receiving water bodies (e.g., surface waters) or sent to WWTPs. These WWTPs may or may not remove the virus, depending on the treatment technology that is applied.

Wastewater and SARS-CoV-2
The introduction of SARS-CoV-2 into wastewater through human waste sources is a global health concern during the current pandemic. Further, our limited understanding of potential virus transmission through wastewater and the viability, persistence and inactivation of the virus using current treatment processes lead us to question the current water quality and wastewater management strategies [39]. This suggests the need for precautions and the strict control of faeces of infected patients with the coronavirus (mainly in hospitals). At the same time, there has been increased management, measurement and monitoring of wastewater quality related to viral presence [40,41]. This is because in wastewater could result in high concentrations of viral RNA in the receiving water bodies if the wastewater is not adequately treated [42]. Hence, to help contribute to the monitoring of the virus across the globe, academic and governmental communities (mainly in developed countries) have initiated strategies that seek to report and quantify the presence of the virus in wastewater and surface water sources. Multiple approaches have been implemented, such as risk assessments of contact with contaminated water, quantification of genetic chains, determination of SARS-CoV-2 genetic chain, detection methodologies, evaluation of treatment efficiencies and epidemiological assessment and surveillance. These approaches are being used to obtain additional information on the virus, to better understand its presence and control its transmission through wastewater [43][44][45][46][47][48]. According to [49], viral RNA can be detected in wastewater, even when only one person in a population of 10,000 is infected with SARS-CoV-2. This emphasizes the high levels of potential for viral transmission through wastewater and the importance of the high sensitivity of current measurement methods. A total of 142 studies were found in scientific articles in 38 different countries (Table 2). This table shows the country where the study was conducted, the component of the UWC where the measurements of SARS-CoV-2 are carried out, and the specific objective of the study. The search for studies was carried out between January 2020 and June 2021 in scientific article databases.       [110] Italy X

Wastewater of five WWTPs
Quantification of RNA SARS-CoV-2 in wastewaters for epidemiological applications.

Wastewater of two WWTPs
Evaluation of various methods for detection of SARS-CoV-2 in wastewaters. [112] Italy X X Wastewater of two WWTPs and four pumping locations of a sewer system Quantification of RNA SARS-CoV-2 in wastewaters for epidemiological applications.

Wastewater of eight WWTPs
Alternative methods for measurement of SARS-CoV-2.
[114] Japan X X Wastewater of one WWTP and a river Quantification of RNA SARS-CoV-2 in rivers receiving wastewater discharges.

Wastewater Treatment Plants (WWTPs)
As indicated in Table 2, most studies have reported the presence of the virus in WWTPs, either in wastewater entering the plant, in effluents or at different stages of the treatment processes. These studies account for 84% of the reported cases. Additionally, more than 50% of the WWTP component measurements were carried out in more than two WWTPs that were located in different cities in the same country. One of the main objectives for this type of monitoring is to use WWTPs as an epidemiological surveillance system. According to [183], WWTPs capture the viral loads of 104 to 106 individuals in a single sample, which facilitates spatial analyses and accelerates epidemiological investigations. Hence, the obtained biological measurements (quantity and occurrence) of SARS-CoV-2 from WWTPs could reflect the community's health and act as an indirect population-level diagnostic tool. Indeed, the study of WTPs by [152] allowed them to identify growth trends in viral loads according to study area and analysis time, which allows government entities to exercise specific epidemiological control measures. On the other hand, Kumar et al. [34], evaluated the temporal variation of COVID-19 occurrence in India. This information could be used for controlling the growth of the virus in wastewater. Results from Ahmed et al. [43], Trottier et al. [85], Randazzo et al. [131], Medema et al. [117], Hasan et al. [144] and Wurtzer et al. [84], among others, led to the conclusion that these analyses could generate early warnings about the presence of the virus that include individuals with mild and no symptoms.
In addition, the wastewater component of the UWC makes it possible to understand the behavior of SARS-CoV-2 and the capacity of the WWTPs to eliminate this virus. Wastewater effluent discharges generally flow into receiving water bodies that are then used as drinking water supply sources for cities located downstream within the watershed. In some cases, these waters are reused for other purposes [184], creating a public health problem [185,186]. The removal of SARS-CoV-2 from wastewaters was evaluated by Westhaus et al. [88]. They observed that while three conventional activated sludge treatment plants did not efficiently remove SARS-CoV-2, ozonation treatment improved the removal performance. Besides, Balboa et al. [187], Randazzo et al. [131] and Rimoldi et al. [108] found that the removal efficiency was 89% after the secondary treatment and 100% after the tertiary treatment. This suggests that each WWTP's efficiency at removing the virus mainly depends on the type of treatment process it applies. Thus, evaluating each treatment process of WWTP would allow a better understanding of viral elimination in wastewater.
Studies on sludge and the wastewater that results from WWTPs (screening, primary and secondary sedimentation) make it possible to evaluate the risks associated with the handling of these media and the consequent health impacts due to the virus's ability to survive from hours to days in wastewater [66,149,188]. For example, Zaneti et al. [61,62] used quantitative analyses with various scenarios to determine the likelihood of health risks for WWTP workers and concluded that there is a need to create protection protocols and develop training and preparation measures for municipal WWTP personnel. Balboa et al. [187] observed that the secondary treatment sludge did not contain SARS-CoV chains. However, they found high viral loads in the primary treatment sludge. The enveloped virus's high affinity could explain this for biosolids which leads to viral retention in sludge. Hence, the higher solid content in the primary sludge retains more viral particles compared to the secondary sludge. Research on sludge allows for confirming or refuting studies to be carried out on wastewater from WWTPs (as the influent). Finally, the poor or lack of wastewater treatment facilities in underdeveloped or developing countries poses a greater risk to public health.
Researchers studied the production of microbial aerosols and their health impacts on plant operators during WWTP processes [38,189,190]. Recent publications have proved that COVID-19 is highly stable in aerosols (viruses live for several hours) and on surfaces (viruses live for several days) [1]. Hence, the microbial aerosol exposure of workers during WWTP processes needs to be addressed to create safe work environments. Balboa et al. (2020) and other studies did not find virus chains in the secondary effluents (<11%), reducing the risk of aerosol production during the aeration process. However, the preliminary results indicate a need to expand these types of studies, both in treatment systems and in different components of the UWC [46,190]. A survey developed by Dada and Gyawali [191] where online data on WWTP characteristics in New Zealand were collected (without measurements, therefore not included in Table 2) provides further evidence on aerosolized viral exposure. The researchers in this study characterized exposure to SARS-CoV-2 via inhalation and determined it to be low. However, Gholipor et al. [104], observed a high risk in WWTP workers due to the exposure to bioaerosols. The measurement of RNA chains at every possible step in WWTPs should be considered in order to control the dissemination of viruses in the WWTP environment.

Sewer Systems
Sewage that leaks into surface water might enable virus transmission through airborne spray and enter drinking water systems. Therefore, sewage networks represent the second most crucial component of the UWC and are featured in the greatest number of studies on SARS-Cov-2 (24%). The most effective use of the results obtained from sewage network samples is a potential epidemiological monitoring tool. This approach is called wastewater epidemiology (WBE), which allows for the development of early warning monitoring systems [183,192]. Gonzales et al. [152], Curtis at al. [156], Kuryntseva et al. [123], Fangaro et al. [59], Betancourt et al. [161] and Colosi et al. [155], among others, have proposed epidemiological models and analyses of growth rates according to the study area and temporalities. These proposed models and methodologies have made it possible to understand the continuity, behaviour and growth of the virus in a given population [56,193,194]. Additionally, this kind of study provides reliable information on the behaviour of the virus in asymptomatic patients and allows researchers to determine the number of undiagnosed infections in a population [192]. These studies can also help evaluate the impacts of the sanitation measures that are recommended by public health authorities [163]. It is important to mention that WBE studies in developing countries have shown great potential for epidemiological surveillance and control tools. For example, Iglesias et al. [50] determined, with high reliability, the changes in the prevalence of COVID-19 in a marginal community in Argentina, even with low coverage of sewage systems. This study illustrated that in developing countries where COVID-19 tests are limited, this web/larger-scale approach is a useful decision-making tool for public health authorities.
Other authors have proposed monitoring programs to understand and identify the behaviour of the virus in sewage systems. For example, a study by Petala et al. [92] proposed a mathematical model that looked for possible effects of SARS-CoV-2 RNA based on commonly measured parameters, such as dissolved oxygen and total suspended solids, to explain the behaviour of the virus in the pipes of a sewage system. It is important to expand the studies by including parameters such as temperature and pH, among others, which may impact the survival time of the virus in wastewater [195]. Previous studies of other SARS-like viruses showed that at 4 • C, the virus has a longer survival time compared to at 20 • C [196]. In the case of the SARS-CoV-2 viral genome, survival was detected at higher ambient temperatures (above 40 • C) in wastewater. Further research is needed to understand the effect of environmental parameters on the persistence of the new SARS-CoV-2.
Furthermore, authors such as Scott et al. [173], Crowe et al. [172] and Gibas et al. [167] conducted measurements on educational sectors (university and colleges) and Wong et al. [127] evaluated the trend of RAN SARS-CoV-2 in wastewater from a residential building to evaluate the temporal epidemiological behavior and identify and prioritize control strategies. This opens the door for the sectorized application and prioritization of different sectors, since it allows to evaluate the feasibility of the epidemiological strategies elaborated by local authorities or the sanitary measures adopted by the populations.

Surface Waters/Groundwater
Similar to WWTPs and sewage systems, SARS-CoV-2 monitoring in surface waters also indicates that the virus is transmitted from WWTPs to natural water sources. Often, untreated wastewater is discharged into the surface water (river, lakes), affecting groundwater sources. This is especially relevant in low-income countries and regions, including rural and peri-urban communities where untreated surface and groundwater sources are often directly used for drinking water. Surface water monitoring in low-income/developing countries requires more attention to control the potential risk of community spread of COVID-19. To date, surface waters have mainly been measured for viral loads, focusing on the possible use of these measurements as an epidemiological tool. According to Guerrero-Latorre et al. [82], the viral loads that were measured suggest that the number of people infected in the city of Quito are likely higher than that reported in the official data. This indicates the need to expand epidemiological data. Additionally, the lack of wastewater treatment in the city led to higher source water viral loads compared to other studies in which WWTPs were utilized. Rimoldi et al. [108] observed the same situation in Italian surface watersheds, where they found high viral loads in three rivers due to wastewater that was not treated or inefficiently treated or from combined sewage overflows in those rivers. The same types of evaluations were performed by Haramoto et al. [114] in rivers in Japan and by Zhao et al. [75] in rivers and lakes in China and found no positive values for viral load were observed in these water bodies, which can likely be explained by the presence of WWTPs in these study areas.

Wastewater from Hospitals
Wastewater from hospitals presents serious environmental and public health risks due to the presence of high concentrations of medical waste. In addition, the presence of SARS-CoV-2 in hospital wastewater poses additional risks of COVID-19 transmission [75,106]. Studies have explored the efficiency of the different technologies to treat this wastewater. Zhang et al. [76] found that viral RNA was removed after a preliminary disinfection treatment with sodium hypochlorite. However, after disinfection, SARS-CoV-2 RNA was found in the septic tank effluent, likely due to the release of viruses embedded in faecal particles. The high organic content and solid compounds in faeces decrease the efficiency of the treatment, which therefore requires an increase in hypochlorite doses in the septic tank to achieve complete viral elimination. Similarly, Arora et al. [97] showed the efficiency of sodium hypochlorite as a virus treatment solution in hospitals. These results demonstrate the need for optimized disinfection treatment systems that are effective at disinfecting wastewater from hospitals. Appropriate disinfection treatments and/or alternatives should be considered in prevention protocols to control COVID-19 transmission. Additionally, in developing countries where there are no WWTPs, it is necessary to implement treatment measures that reduce viral loads in sewage systems or water sources that receive sewage discharge.
Also, authors such as Hong et al. [125], Xu et al. [94] and Arora et al. [97] reported the use hospital effluents as pilot studies for the quantification of viral loads; based on the number of patients who have been admitted with CO-VID-19, models that reduce the variability and uncertainty of the relationships between the loads and the number of infected are elaborated. These models can then be applied to larger spatial scales.

Benefits and Outcomes of Monitoring the Different UWC Components
Based the literature review carried out, Table 3 presents benefits and outcomes associated with the monitoring of each UWC component.

Spatial Analysis
The country with the highest number of reported studies that examine SARS-CoV-2 in the UWC is the USA with a total of 39 (28%), followed by India with 10 (7.1%), Spain with nine (6.4%), Canada with eight (5.7%) Italy and Brazil with six (4.2% individual and 8.4% by two countries), Australia, France and Germany with five (3.5% individually, and 10.5% by three countries), the Netherlands with four (2.8%). Countries such as China, Japan and England with three studies each and other countries with one or two studies represent a total of 24.8%. It is important to emphasize that some studies involved multicountry measurements showing the potential of collaboration networks that facilitate the monitoring of the pandemic worldwide. Figure 2 presents the global distribution of the reported studies on SARS-CoV-2 measured in the UWC.

Wastewater treatment plant
High research opportunities, due to the variability of existing treatments and the combinations that can be generated.
Allows monitoring of solids generated in primary and secondary treatments, where highly reliable results are obtained. Relevant for epidemiological control studies, since the wastewater contributing areas are known facilitating SARS-CoV-2 evaluations in conditions where it is not possible to monitor various locations of the sewer network. Allow to observe the dynamics of viral loads (growth and decay), which is useful for epidemiological analysis purposes.
In combined sewer networks the dilution rate can be very high, which could generate variability in the measurements.

Hospital efluent
High research opportunities, due to the variability of existing hospital water treatments. Optimal control location for quantification and establishing relationships between viral loads and number of infected by COVID-19.
Does not allow a wide spatial scale to be considered for epidemiological surveillance strategies. Does not allow to identify areas with asymptomatic infected persons

Sewer network
Opportunity to evaluate specific areas, such as educational centers, residential, commercial and industrial areas, among others, which allows the development of very specific epidemiological surveillance strategies. Allows to evaluate the behavior and the spatio-temporal dynamics in large wastewater drainage areas. High opportunity to expand research to improve knowledge about the behavior of the pandemic, and on the environmental alterations of SARS-CoV-2 at different spatial and temporal scales.
In combined sewer systems, the dilution rate can be very high, which generates variability in the measurements.

Surface Water
Allows identifying the dynamics of viral loads, which could be used as an epidemiological tool. Many watersheds are bordering between cities, provinces (states) and countries, which increases the opportunities to conduct epidemiological evaluations at large scales.
In waterbodies receiving large number of wastewater discharges, it is very difficult to identify the contributing areas of the viral loads, since it's hard to disaggregate the measured concentrations of SARS -CoV-2-According to Figure 2, high concentrations of studies were conducted mainly in North America, Europe, Oceania, and Asia. In Latin America, studies have been carried out in Brazil, Chile, Argentina, Ecuador and Mexico. In Africa only one study was reported in South Africa. This distribution of studies indicates that most epidemiological monitoring in wastewater or natural waters have been conducted in highly industrialized countries. It is important to mention that the absence of investigations and measurements in wastewater, mainly in Africa, Central America and parts of Latin America, is due to relatively low sanitation coverage and limited capacity to diagnose infection [120]. It is also worth noting that according to the United Nations in 2017, 80% of the wastewater worldwide (>95% in some developing countries) is discharged into receiving water bodies without prior treatment or with little preliminary treatment, which poses a significant challenge for these countries in terms of water pollution and monitoring of public health [197,198].
The low coverage of sanitation and wastewater treatment, the shortage of certified laboratories and the low financial investments in public health in developing countries [199] indicate that monitoring SARS-CoV-2 for epidemiological purposes is a huge challenge. It is thus urgent to provide these countries with the required infrastructure and human resources as well as with the scientific capacities to implement monitoring of SARS-CoV-2 for epidemiological surveillance purposes.

Conclusions
The SARS-CoV-2 virus has generated huge numbers of infected people and unfortunately, has resulted in more than 1.65 million deaths worldwide. Most measurements and quantification efforts focus mainly on individuals who are currently infected. However, new studies based on wastewater epidemiology are increasingly being reported. An urban water cycle is a useful tool that allows for the expansion of integral management of urban water resources. Additionally, wastewater epidemiology can be used to identify interconnections between water sources and discharged pollutants during the various processes and at individual components of the system. These characteristics make this monitoring approach for COVID-19 in UWCs a vital tool for epidemiological monitoring and control studies and is quickly gaining popularity worldwide. This approach comes with a high benefit-to-cost ratio and can provide temporal and spatial estimates of the number of infected individuals in a given population [34,117,200].
Patients infected with COVID-19 can discharge SARS-CoV-2 in their faeces, which allows us to use wastewater and surface waters to monitor viral loads. The impact of SARS-CoV-2 in wastewater and surface water on the environment and on public health compels thorough research to assess and manage the associated risks [201]. It is also necessary to expand our understanding of the behaviour of the virus in wastewater. We need to be able to estimate the impact that characteristics and conditions of the environment and water (e.g., pH, temperature, light exposure, the concentration of solids, dissolved oxygen and organic matter) have on the survival of the virus [33,202]. These studies help to determine the conditions that favor environmental transmission. They are also incredibly important since large amounts of untreated wastewater are discharged into surface waters, which are then used in agriculture, fishing and recreational activities [185,203].
The diverse techniques used for the detection and quantification of SARS-CoV-2 levels have shown high efficiency when used as epidemiological monitoring tools. However, it is important to mention that differences in measurements can be observed in their quantification, due to the characteristics of each test. Public health authorities must take into account these differences and their possible impact for epidemiological decision-making [204,205]. Therefore, it is essential to continue the improvement of methods and procedures for the detection of SARS-CoV-2 in the different components of the UWC. In addition, it is essential to develop new methodologies that extend the knowledge and facilitate the massification of the application of epidemiological studies [188]. Developing conventional and robust techniques allows for the extrapolation and large-scale application of epidemiological studies using wastewater, facilitating their wide application and reducing technical and conceptual errors, as well as economic costs.
The objective of the measurements is to quantify the concentrations of SARS-CoV-2 in UWC waters as a public health tool. New research opportunities must be generated as a result of such monitoring. It is essential to identify the behavior of SARS-CoV-2 in the different types of wastewaters (residential, commercial, industrial, institutional, etc.), and evaluate the relationships with the physico-chemical characteristics of these waters, since factors such as pH and temperature may influence the results. Also, it is necessary to compare the concentrations of SARS-CoV-2 at the different steps of WWTPs treatment processes.
It is essential to identify adequate treatment systems that can be used to reduce virus loads in community and hospital wastewater. Apart from secondary treatment (>90% removal), studies show the need for tertiary systems or disinfection treatments to remove viruses efficiently. Lesiemple et al. [193] and Bhatt et al. [206] are useful resources as they reviewed different treatment processes and provide recommendations for wastewater treatment. It is paramount that we study the presence of SARS-CoV-2 in surface water, groundwater and drinking water, mainly in sectors where water treatment is unreliable (including rural areas with inadequate sanitation and developing countries). These studies should be mainly carried out in heavily urbanized watersheds where untreated wastewaters are discharged into surface waters (which also affects groundwater) and drinking water treatment for human consumption is not available [207].
It is necessary to create collaborative networks between highly industrialized countries and developing countries for the application of surveillance strategies of SARS-CoV-2 in UWC waters for epidemiological purposes. The previous experiences and knowledge acquired by the scientific community of the countries that have already used high benefitcost protocols for SARS-CoV-2 surveillance in waters must be transferred as soon as possible to the authorities of developing countries.
The monitoring of SARS-CoV-2 in the various components of the UWC has been growing rapidly. Countries such as the United States, Holland, Italy, Brazil, Spain, Australia and India have already benefitted from the implementation of surveillance protocols during this pandemic. Indeed, authors such as Kopperi et al. [208] have already proposed a standard methodological approach for the study and epidemiological surveillance of SARS-CoV-2 in wastewater, that could be applied worldwide.
At this time of global and local re-opening, where new strains are increasingly being identified, the use of epidemiological control with wastewater becomes a powerful tool for decision-making and public health planning. Additionally, since mass vaccination processes are progressing in many countries, epidemiological monitoring in wastewater will allow the identification of areas where vaccination must be intensified.

Conflicts of Interest:
The authors declare no conflict of interest.