A physicochemical model for rationalizing SARS-CoV-2 concentration in sewage. Case study: The city of Thessaloniki in Greece

Abstract

Detection of SARS-CoV-2 in sewage has been employed by several researchers as an alternative early warning indicator of virus spreading in communities, covering both symptomatic and asymptomatic cases. A factor that can seriously mislead the quantitative measurement of viral copies in sewage is the adsorption of virus fragments onto the highly porous solids suspended in wastewater, making them inaccessible. This depends not only on the available amount of suspended solids, but also on the amount of other dissolved chemicals which may influence the capacity of adsorption. On this account, the present work develops a mathematical framework, at various degrees of spatial complexity, of a physicochemical model that rationalizes the quantitative measurements of total virus fragments in sewage as regards the adsorption of virus onto suspended solids and the effect of dissolved chemicals on it. The city of Thessaloniki in Greece is employed as a convenient case study to determine the values of model variables. The present data indicate the ratio of the specific absorption (UV₂₅₄/DOC) over the dissolved oxygen (DO) as the parameter with the highest correlation with viral copies. This implies a strong effect on viral inaccessibility in sewage caused (i) by the presence of humic-like substances and (ii) by virus decay due to oxidation and metabolic activity of bacteria. The present results suggest days where many fold corrections in the measurement of viral copies should be applied. As a result, although the detected RNA load in June 2020 is similar to that in April 2020, virus shedding in the city is about 5 times lower in June than in April, in line with the very low SARS-CoV-2 incidence and hospital admissions for COVID-19 in Thessaloniki in June.

Keywords: Adsorption; Corona virus; Modeling; SARS-CoV-2; Virus concentration rationalization; Wastewater epidemiology.

Graphical abstract

Fig. 1

Schematic representation of possible interaction mechanisms of virus particles/fragments with a porous solid particle suspended in sewage. 1: mass transfer (convection and diffusion) from the surrounding bulk liquid, 2: adsorption, 3: desorption, 4: pore diffusion and 5: surface diffusion.

Fig. 2

Full resolution discrete topological model accounting for each pipe section and junction in the sewage piping network. In the schematic of the unit pipe (up-right corner) individual virus parts and solid particles as well as solid particles with adsorbed viral particles/fragments are shown to flow in sewage.

Fig. 3

Fully continuous limit topological model accounting for lumped regions in the sewage piping network.

Fig. 4

Combination of the discrete and continuous approaches is possible. For example the main flow paths from lumped small regions to the central flow can be treated explicitly in addition to the central pipe.

Fig. 5

SARS-CoV-2 viral copy numbers per mL based on the N protein and Orf1ab standard curves (a), and evolution of physical-chemical parameters throughout the study period: inlet flowrate and total suspended solids concentration (b), dissolved oxygen concentration and specific UV absorbance (c) and electrical conductivity (d).

Fig. 6

Evolution of relative shedding rate of viral RNA copies after June 3rd, 2020, as corrected by the present model. The overall low values after June 10th, is in line with the calm conditions observed in city hospitals during this period.