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Wastewater-based epidemiology (WBE) is successful in the detection of the spread of SARS-CoV-2. This review examines the methods used and results of recent studies on the quantification of SARS-CoV-2 in wastewater. WBE becomes essential, especially with virus transmission path uncertainty, limitations on the number of clinical tests that could be conducted, and a relatively long period for infected people to show symptoms. Wastewater surveillance was used to show the effect of lockdown on the virus spread. A WBE framework tailored for SARS-CoV-2 that incorporates lessons learnt from the reviewed studies was developed. Results of the review helped outline challenges facing the detection of SARS-CoV-2 in wastewater samples. A comparison between the various studies with regards to sample concentration and virus quantification was conducted. Five different primers sets were used for qPCR quantification; however, due to limited data availability, there is no consensus on the most sensitive primer. Correlating the slope of the relationship between the number of gene copies vs. the cumulative number of infections normalized to the total population served with the average new cases, suggests that qPCR results could help estimating the number of new infections. The correlation is improved when a lag period was introduced to account for asymptomatic infections. Based on lessons learnt from recent studies, it is recommended that future applications should consider the following: 1) ensuring occupational safety in managing sewage collection and processing, 2) evaluating the effectiveness of greywater disinfection, 3) measuring viral RNA decay due to biological and chemical activities during collection and treatment, 4) assessing the effectiveness of digital PCR, and 5) conducting large scale international studies that follow standardized protocols.

The current study investigated the mobility of four silver nanoparticles (AgNPs) stabilized using different capping agents and represent the common stabilization mechanisms as well as surface charging scenarios in reactive and nonreactive porous media. The AgNPs were (1) uncoated H2-AgNPs (electrostatically stabilized) and (2) citrate coated AgNPs (Citrate-AgNPs) (electrostatically stabilized), (3) polyvinylpyrrolidone coated AgNPs (PVP-AgNPs) (sterically stabilized), and (4) branched polyethyleneimine coated AgNPs (BPEI-AgNPs) (electrosterically stabilized). The porous media were (1) quartz sand (QS), (2) ferrihydrite-coated sand (FcS), and (3) kaolin-coated sand (KcS). The H2-AgNPs and Citrate-AgNPs were readily mobile in QS but significantly retained in FcS and KcS with more deposition achieved in the KcS media. The deposition of the H2-AgNPs and Citrate-AgNPs followed the order of KcS > FcS > QS. The PVP-AgNPs breakthrough occurred more rapid as compared to the H2-AgNPs and Citrate-AgNPs but the deposition of PVP-AgNPs followed the same order of the electrostatically stabilized AgNPs (KcS > FcS > QS). The BPEI-AgNPs were readily mobile regardless of the porous media reactivity. Physicochemical interactions were the dominant filtration mechanism in the majority of the investigated cases but straining played the major role in the deposition of the electrostatically stabilized H2-AgNPs and Citrate-AgNPs in the KcS media. The results highlight the importance of both the stabilization mechanism and capping agent chemistry as key factors governing the transport of AgNPs in the environment.

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