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Frida Isaksson
Frida Isaksson View original picture , opens in new tab/window

The use of normalization to better understand trends in the concentrations of SARS-CoV-2 in sewage

Published: 12 April 2022

Frida Isaksson has now finished her research project as part of her degree in Natural Resource Engineering at Luleå University of Technology (LTU). Her study aimed to increase knowledge on factors that affect the concentrations of non-infectious SARS-CoV-2 RNA fragments in sewage and to show the impact of using alternative normalization methods on interpreting viral concentrations in sewage samples.

Many factors affect the concentration of RNA from SARS-CoV-2 in sewage, including how many people are ill, how much virus is excreted in the faeces and how the viral excretion rate is related to, for example, the course of the disease and the type of variant. The fate of the virus is also affected by the environment in the sewage system, how many people use the sewage system and how much groundwater and precipitation flow into the sewage system and dilutes the samples. The number of people currently using the system is especially important when you have a large proportion of commuters or many tourists during parts of the year. The level of dilution of sewage by groundwater and rainfall can be extensive in Sweden (in average about 100%), but this varies depending on the location, time of year and also whether it has recently rained or not. The purpose of this thesis was to investigate the use of different normalization methods to take into account dilution effects and the number of people contributing to the sewage system on analyzing the concentrations of RNA from SARS-CoV-2. Data were evaluated statistically in relation to the number of clinically determined cases in Luleå and Råneå.

"The results showed a high correlation between the number of clinically determined cases and normalized values of SARS-CoV-2 in sewage, using either nitrogen, phosphorus or pepper mottle virus concentrations to predict the number of people contributing to the sewage system on the day of sampling", says Lian Lundy, guest professor in Urban Water Engineering at Luleå University of Technology.

The results also showed that these normalization methods worked better when comparing the two different treatment plants in contrast to sewage data, which were only normalized to account for flow variations measured at the treatment plants. In Råneå, with a small treatment plant, a peak in SARS-CoV-2 RNA concentrations was identified in the sewage 5-8 days before clinical test data showed the same increase (depending on the normalization method). For the larger treatment plant in Luleå, the corresponding lag time was 0-2 days.

During the sampling period of January-March 2021, the flows into the treatment plant were fairly stable (precipitation fell as snow and groundwater levels were slightly lower than usual). Further, the recommendation to work from home where possible meant that the number of commuters was lower than normal, with few business or holiday trips impacting on population numbers. Due to these factors, no clear differences could be seen between the normalization methods based on excreted amounts of nitrogen, phosphorus or pepper virus in urine and faeces. More studies during other seasons are required to see if there are further differences, and if different normalization methods should be used depending on the season.

Frida Isaksson has now completed her studies at LTU and started working as a water engineer at the consulting company AFRY. Lian Lundy, LTU, supervised the degree project and Annelie Hedström, LTU, was the examiner. Anna Székely and Nahla Mohamed from SLU in Uppsala have also contributed to this project with particular expertise in the detection of RNA in sewage. LTU has financed the project through the competence centrum Drizzle.

Annelie Hedström

Annelie Hedström, Professor

Phone: +46 (0)920 492309
Organisation: Urban Water Engineering, Architecture and Water, Department of Civil, Environmental and Natural Resources Engineering
Lian Lundy

Lian Lundy, Visiting Professor

Organisation: Urban Water Engineering, Architecture and Water, Department of Civil, Environmental and Natural Resources Engineering