Table 1.
Summary of identified factors that may impact SARS-CoV-2 RNA concentrations in wastewater or its interpretation.
| Factor | Expected impact on SARS-CoV-2 RNA concentrations or relevance | References |
|---|---|---|
| i) Shedding-related factors | ||
| Shedding peak | Early infections have a predominant influence on RNA loads. | Wu et al., 2022 |
| COVID-19 recovery | Reduction in RNA loads, but contribution to residual noise in RNA concentrations. |
McMahan et al., 2021; Wu et al., 2022 |
| Shedding load distribution | Shedding load distribution may possibly be used to estimate total infections. | Huisman et al., 2021 (preprint) |
| Shedding rate | Shedding rates can be calculated for a specific community, but may vary depending on underlying population. | Schmitz et al., 2021 |
| Incontinence aids | High-risk populations may not contribute to RNA load when incontinence aids are used. | Acosta et al., 2021 |
| ii) Population size | ||
| Population size | Positive relationship between population size and RNA concentrations. | Wilder et al., 2021 |
| Population size | Positive relationship between population size and RNA detection rates. | Wu et al., 2021 |
| iii) In-sewer factors | ||
| Solid particles | Distribution of viral RNA between solid phase and aqueous phase. Possible reduction in aqueous phase due to solid particles. | Jørgensen et al., 2020 (preprint); Fores et al., 2021; Graham et al., 2021; Weidhaas et al., 2021; Westhaus et al., 2021 |
| Organic matter | Reduced RNA concentrations in aqueous phase for increasing organic load, dependent on physicochemical properties (e.g., humic-like substances). |
Hong et al., 2021; Petala et al., 2021 |
| Dissolved oxygen | Increased RNA concentrations in aqueous phase with increasing dissolved oxygen. | Petala et al., 2021 |
| Flow rate | Increased or reduced concentrations. |
Wilder et al., 2021; Vallejo et al., 2022 |
| Travel time | Reduced RNA concentrations. | Weidhaas et al., 2021 |
| Wastewater pH | Possible reduced concentrations at low pH with high concentrations of volatile fatty acids. | Hong et al., 2021 |
| Chlorination | Reduced RNA concentrations, depending on fulfilling of chlorine demand. | Zhang et al., 2020; Hemalatha et al., 2021 |
| Wastewater temperature | Possible reduced RNA concentrations. |
Ahmed et al., 2020a; Bardi and Oliaee, 2021 |
| Ambient temperature | Possible reduced concentrations, situation specific. | Arora et al., 2020; Hart and Halden, 2020; Hong et al., 2021; Vallejo et al., 2022 |
| iv) Sampling strategy | ||
| Sampling frequency | At least two or three non-consecutive samples a week to minimize noise in trends. | Feng et al., 2021; Graham et al., 2021; Huisman et al., 2021 (preprint) |
| Sampling mode | 24-h composite flow-proportional sampling of influent wastewater to capture RNA loads adequately. |
Ort et al., 2010 (not included in review) |
The expected impact on SARS-CoV-2 RNA concentrations is given for a relative increase in quantitative factors. For categorical factors, the expected effect or relevance is briefly mentioned.