Author(s): Antonio Lastra De La Rubia; Jaime Botello; Alejandro Pinilla
Linked Author(s): Antonio Lastra de la Rubia
Keywords: Wastewater-based epidemiology; SARS-CoV-2; COVID-19; Public health
Abstract: During the first quarter of 2020, due to the evolution of the COVID-19 pandemic, several studies on the validation process for monitoring and detecting traces of SARS-CoV-2 in wastewater started in multiple countries. Canal de Isabel II, as the company responsible for water infrastructure management in Madrid region, started to monitor the spread of SARS-CoV-2 in wastewater in March 2020, in the middle of a nationwide lockdown. VIGÍA project was released to the health authorities in early July, just before the second wave started. The monitoring system has been implemented over the entire Madrid region, with an approximate population of 7 million inhabitants by means of 289 sampling points, the biggest wastewater surveillance system in Europe. VIGíA is based on weekly sampling, data validation, statistical analysis, and representation. The results are shared daily with the Health Department of Madrid for consultancy and decision-making purposes. Madrid urban drainage system is mainly combined (mixed sewage, stormwater, and industrial effluents). It is unclear how detectable pieces of SARS-CoV-2 virus from infected people interact with above effluents. However, to detect unusual dilution that could potentially affect virus detectability, physicochemical parameters are also monitored for outlier detection. COD appears to be most relevant in detecting unusual wastewater composition, given that it is more sensitive to high dilutions due to rainfall episodes and higher than usual pollutant presence from industrial effluents. Chloride levels and electrical conductivity are also monitored as additional criteria to detect unusual composition although they are more stable. Samples with out-of-range values are resampled to verify or rule out qPCR results within 2 to 3 days. It has been found that a weekly sampling strategy offers adequate quantification with fixed sampling hours for every point to reduce the effect of daily variations. SARS-CoV-2 global signal in wastewater shows good agreement with new reported cases and new hospital admissions, with wastewater concentration ahead of both series. The commonly accepted explanation for this lag is that SARS-CoV-2 can be found in faeces of patients before the symptom onset. Results range between 3 to 11 days of anticipation among the studied sewersheds. This study has demonstrated wastewater-Based Epidemiology capabilities as an early warning tool for the current COVID-19 pandemic in Madrid region. Sampling points selection criteria were essential to this aim: a maximum of 25.000 equivalent inhabitants, a 3,5 km distance to population centres and a 2,5 km distance to the last discharging point are preferred to obtain optimal results. Future work will include a permanent epidemiological surveillance system in which a subset of 87 out of the current 289 sampling points will be monitored and in the event of virus detection, more detailed surveillance points will be activated in that sewershed to pinpoint infection hotspots.