The presence of the corona virus in sewage water could be an important indicator of a second wave, said Minister Hugo de Jonge (Public Health, CDA) in July when corona measures were relaxed. The Netherlands is now fully engaged in that second wave and almost everyone has been taken by surprise. The use of sewer data is complicated, sewer researchers, water companies and RIVM acknowledge. But the RIVM hopes that in two weeks, GGDs will be able to properly interpret the data about the sewage water in their own area – so that they can detect an increase in the number of infections at an early stage.
Employees of water companies now use tablespoons to shovel bottles full of sewage water at 317 water purification plants every week. In April, only 29 water treatment plants had so-called ‘sample cabinets’. Sewer monitoring is also carried out in other countries, such as the United Kingdom and the United States.
One of the problems encountered by RIVM was a lack of clarity about the number of households that discharged at one sewer measuring point. If the virus is found in the sewage system, you need to know how many residents the sewage water comes from in order to be able to say something about the number of infections. “The Central Bureau of Statistics has had to find out which postcodes discharge on certain purifications. That was quite a job ”, says Ana Maria De Roda Husman, microbiologist and head of the Environment department at RIVM.
Also read: The corona pandemic is leaving its mark in the sewers
The amount of precipitation was a second problem. Rain dilutes the sewage and thus reduces the amount of virus per liter of water. This must be taken into account when interpreting the sewage water data. The intention is that the corrected data will be readable on the government dashboard from Tuesday. Then everyone can see how many coronavirus particles per 100,000 inhabitants end up in the sewer in their municipality. De Roda Husman: “That makes it possible to compare congregations with each other.”
Under and over world
Can a possible third wave be detected more quickly by these measurements? In the second wave, the rise in the sewer seemed to be less ahead of test data than in the first. “Tests are now ten times as many, so the number of positive tests has also increased earlier,” explains Gertjan Medema, microbiologist at the water company KWR. Medema previously demonstrated the relationship between the amount of virus particles in the sewage system and the number of infections.
Ewout Fanoy, infectious disease control doctor at the GGD Rotterdam-Rijnmond, also confirms that what he and his colleagues already know from the test streets is now mainly visible in the sewer system. “Sewer monitoring confirms the current picture and is a piece of the puzzle that helps in determining whether the epidemic is going up, stabilizing or falling.”
He thinks that sewer monitoring can prove its value as a predictor especially in times when few people are infected with Covid-19. “Then you can use it as an early warning to quickly draw attention to a neighborhood.”
Medema saw this effect during both waves. He refers to a graph about Utrecht (in Rotterdam they only started measuring after the first wave). “There you see that at the start of the first and second waves the number of virus particles in the sewer increases earlier than the number of positive tests and hospital admissions.”
That makes sense, explains De Roda Husman of the RIVM: “The virus enters your stool sooner than you get complaints and you are only tested when you have complaints.”
He passed on the increase that Medema saw to the GGDs of the municipalities concerned. But because the technology is still so new, they had many questions. “If the trend goes up, a municipality is alert. But I also get questions such as: what will the trend look like next week. ” Much is still unknown about how the figures should be interpreted. And the figures can still fluctuate a lot. “We need to measure more frequently so that trends can be identified more quickly.”
It is also important to bring the ‘upper and underworld together’, says Miranda de Graaf, virologist at Erasmus MC. In Rotterdam, she receives the sewer information per district. “We compare that with what we see in people.” Erasmus MC does this together with KWR, general practitioners of Rijnmond Healthy and the GGD Rotterdam-Rijnmond.
Jars with poo
Katendrecht has one GP practice. De Graaf is investigating whether the virus of patients who report there with complaints can also be found in the sewer. The Covid-19 patients receive a questionnaire and are allowed to hand in a jar of poo. In the lab, researchers look at the genetic characteristics of the virus particles and the number of virus particles in the stool. “Not all infected people have virus particles in their stool, which is about 40 percent of patients.”
Due to these differences, the number of virus particles in the sewer can fluctuate greatly. “In a large city, the differences between people disappear in the average, in one neighborhood this happens less,” explains Medema of KWR.
Based on the genetic characteristics of the virus particles, the researchers try to trace the source of a seat of the fire. “And if we miss something, suppose we see something in the sewer that we cannot find among the patients, then you know that you need to test more.” But we are not there yet. De Graaf is now working on combining all this data.
Besides Katendrecht, she also looks at Ommoord, Rozenburg and a large part of Rotterdam South. By taking areas of different sizes, she examines how far she can zoom in on what is happening.
Everyone goes to the bathroom
Ewout Fanoy, infectious disease control doctor at the GGD Rotterdam-Rijnmond, takes the sewage monitoring of the RIVM and KWR into his discussions with the municipality about the situation in the city. “The figures about the sewer are still very high, so we think: that is not a good sign.”
Fanoy would prefer to be able to read the sewer even more specifically. “For example, that we can measure at one nursing home.” There they can take action early if virus particles are detected in the sewer. “But we can’t get that done yet.” As with the GPs, it is therefore necessary to look above ground to see what is going on in a care home – extra work for the employees. “And they are already under a lot of pressure.” Above-ground monitoring is needed at the outset to find out what the data from the sewer means. He hopes this can be set up in December, when the pressure on the workforce eases.
Fanoy also sees another advantage of monitoring the sewer. “At the moment many people from neighborhoods where less educated people live are tested positive.” There can be several reasons for this. One of the questions that GGDs have is whether everyone knows how to find the test streets equally well. “But everyone goes to the toilet, so maybe the sewer can provide more insight into this.”
A version of this article also appeared in nrc.next on October 30, 2020
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