Monitoring waves of the COVID-19 pandemic: Inferences from WWTPs of different sizes

Abstract

Wastewater based epidemiology was employed to track the spread of SARS-CoV-2 within the sewershed areas of 10 wastewater treatment plants (WWTPs) in Catalonia, Spain. A total of 185 WWTPs inflow samples were collected over the period consisting of both the first wave (mid-March to June) and the second wave (July to November). Concentrations of SARS-CoV-2 RNA (N1 and N2 assays) were quantified in these wastewaters as well as those of Human adenoviruses (HAdV) and JC polyomavirus (JCPyV), as indicators of human faecal contamination. SARS-CoV-2 N gene daily loads strongly correlated with the number of cases diagnosed one week after sampling i.e. wastewater levels were a good predictor of cases to be diagnosed in the immediate future. The conditions present at small WWTPs relative to larger WWTPs influence the ability to follow the pandemic. Small WWTPs (<24,000 inhabitants) had lower median loads of SARS-CoV-2 despite similar incidence of infection within the municipalities served by the different WWTP (but not lower loads of HAdV and JCPyV). The lowest incidence resulting in quantifiable SARS-CoV-2 concentration in wastewater differed between WWTP sizes, being 0.11 and 0.82 cases/1000 inhabitants for the large and small sized WWTP respectively.

Keywords: Human adenoviruses; JC polyomavirus; Pandemic surveillance; SARS-CoV-2; Small large WWTPs; Wastewater-based epidemiology.

Graphical abstract

Fig. 1

Prevalence of infection estimates and WW measured concentrations for SARS-CoV-2 (N1 assay) (a) WWTP-J in Barcelona, (b) WWTP-H and (c) WWTP-G. Values below LOQ are marked with an inverted triangle on the x-axis. WW values are expressed as daily loads. The sum of cases shows in shades of red were calculated using Eq. (2) (bright red), Eq. (3) (crimson-red), Eq. (4) (pastel red). Both y-axis are in log10 scale. The shading represents the enforced measures at the time: SL = strict lockdown; RM = reduce mobility.

Fig. 2

For WWTPs (WWTP- J, -G, -H): Linear correlations of the log converted values of viral concentration and daily viral loads with estimates for number of cases during the first wave (left half) and second wave (right half). (a–c and d–f follow Eqs. (2), (3)). Dotted line represents the 95% C.I. Difference in n between series is due to the daily flow at the WWTP inlet was not available on and daily GC loads could not be computed.

Fig. 3

Estimated number of active infections (lines, right y-axis) and WW loads of SARS-CoV-2 (N1 assay), orange bar plot left y-axis) and JCPyV (blue bar plot left y-axis) viruses for the second wave at all 10 WWTPs.

Fig. 4

Concentrations of SARS-CoV-2 (N1 and N2 assays), HAdV and JCPyV in different sized WWTP for the period July and November 2020. Midline shows the median, edges of the box plot show the 2nd and 3rd quartiles and the whiskers show the minimum and maximum value measured. Maximum incidences per 1000 inh. reported over the period July–November are shown next to the figure.

Fig. 5

Bubble Plot of the second wave showing the summed future 7d cases per 1000 inhabitants against the total daily N1 GC loads measured. Tick mark diameters are organised in three groups by size depending on the number of people being served by the WWTP. Red dots indicate measured concentrations below LOD and indicate only the maximum daily load possible. LWQI = lowest WW quantifiable incidence i.e. the lowest incidence of infection which resulted in a quantifiable concentration of N1 GC in WW.