Surveillance of SARS-CoV-2 RNA in wastewater treatment plants in Türkiye, Istanbul: a long-term study and statistical analysis

Wastewater-based epidemiology (WBE) is a powerful method that allows community surveillance to identify diseases/pandemic dynamics in a city, especially in metropolitan areas with high overpopulation. This study investigated the detection and quantification of SARS-CoV-2 RNA in wastewater and sewage sludge in two different wastewater treatment plants (WWTPs) in Istanbul, the 5th largest city in the world, during the COVID-19 pandemic. Statistical analysis was performed to examine the relationships between SARS-CoV-2 concentrations and COVID-19 case numbers, seasonal variations, and key WWTP parameters, including total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD), using principal component analysis (PCA). SARS-CoV-2 N genes were analyzed in influent, treated effluent and sludge samples collected between June 2021 and January 2022 by reverse transcription–polymerase chain reaction (RT-qPCR). Viral genes were detected in 23 out of 26 influent wastewater samples (88%) and in 4 out of 26 secondary treated wastewater samples. SARS-CoV-2 RNA concentrations in influent samples ranged from 0 to 11,469 ± 551.2 genomic copies (GC)/mL for all viral genes analyzed (N1: 109.7 ± 46.6 − 11,228.9 ± 543.05 GC/mL and N2: 864.5 ± 82 − 11,469.6 ± 551.2 GC/mL). In secondary wastewater samples, SARS-CoV-2 RNA concentrations ranged from 665.23 ± 48.6 to 2,833.17 GC/mL, while sludge samples contained concentrations ranging from 138.7 ± 20.1 to 2,027.9 ± 97.2 GC/mL. The findings of this work demonstrate the feasibility of RT-qPCR for quantifying SARS-CoV-2 in wastewater and suggest a weak to moderate correlation between SARS-CoV-2 gene concentrations and confirmed COVID-19 and death cases in the community. A correlation was also observed between the number of GC per TP and COD for both the N1 (Pearson correlation coefficient: 0.949 for N1/TP and 0.986 for N1/COD) and N2 genes (Pearson correlation coefficient: 0.960 for N2/TP and 0.993 for N2/COD) in two WWTPs. The study highlights the potential of WBE as a COVID-19 surveillance tool and as an early warning and control system for SARS-CoV-2 infections.