Application of Passive Samplers for SARS-CoV-2 Wastewater Surveillance

Abstract

SARS-CoV-2 wastewater surveillance is a promising tool for monitoring the spread of infection during pandemic outbreaks. 24-hour composite sampling of wastewater using autosamplers is the preferred means for wastewater surveillance sample collection. Autosamplers however require a significant capital cost and furthermore some sampling locations are not amenable to autosampler deployment because of a lack of space and lack of access to electricity. Grab sampling is an alternative to auto sampling for wastewater surveillance, however it may be less effective compared to 24-hour composite sampling due to the possibility to miss the collection of shed disease targets during critical shedding events. Torpedo-style passive samplers packed with medical gauze and tampon-style passive samplers are alternatives to grab sampling when deployment of autosamplers is not possible. Torpedo-style and tampon-style passive samplers are characterized as being easy to deploy and collect and have shown promise for disease surveillance using wastewater. Although passive samplers have shown the ability to detect SARS-CoV-2, they have not demonstrated the ability to quantify the viral load in the wastewater due to the fact that the collection of the liquid phase of the sampler is not consistent across the deployment period of a passive sampler. As SARS-CoV-2 disease targets have been shown to largely partition to the solids phase of wastewaters, it is hypothesized that mass fraction quantitation may enable passive samplers to quantify wastewater signals comparably to autosamplers. In this study, wastewater samples were collected from the same location over a period of three months from a sewer access point at the University of Ottawa using conventional 24-hour auto sampling. Two types of torpedostyle passive samplers and a tampon-style passive sampler were tested to assess whether passive sampler measurements of SARS-CoV-2 N1 and N2 gene targets can be used in the place of autosampler quantitated values. When comparing the wastewater characteristics of centrifuged pellets collected by various passive samplers and a conventional autosampler, the results of this study showed that the torpedo-style passive sampler packed with two pieces of gauze (P2) collected significantly lower water content compared to the autosampler, and P2 collected significantly greater total solids and volatile solids compared to the autosampler. When measuring SARS-CoV-2 N1 and N2 signals, the results indicate that N1 and N2 gene region copy numbers from all of the samplers were not significantly distinct. However, the P2 sampler, a torpedo-style passive sampler packed with four pieces of gauze (P4), and the tampon-style passive sampler (T) captured a greater quantity of pepper mild iii mottle virus (PMMoV) gene targets compared to the autosampler; where PMMoV is the most commonly measured fecal biomarker for wastewater surveillance of SARS-CoV-2. The greater quantity of PMMoV gene targets compared to the autosampler was likely due to proportionally higher total solids and volatile solids in the centrifuged pellet material captured. When N1 and N2 measurements were normalized against sample volume, pellet mass or PMMoV gene copy numbers, P2, P4, and T showed no significant differences compared to the autosampler. In contrast, differences were observed between passive samplers and the autosampler when PMMoV measurements were normalized against the matrix volumes or pellet mass. High statistical percentage differences were observed between all passive samplers and the autosampler. Overall, passive samplers are reliable, cost-effective devices for sampling disease targets in wastewater if results are expressed as copies/g or copies/copies PMMoV. These devices are feasible substitutes for autosamplers when detection and quantification of SARS-CoV-2 in wastewater are required. P2 passive samplers using units of measurement of copies/g are recommended for SARS-CoV-2 surveillance in the wastewater.