A team led by the National Environment Agency (NEA) is currently monitoring wastewater in 20 large foreign worker dormitories as part of the Government's gradual clearance of dormitories of Covid-19 to allow workers to return to work.
This is part of a pilot programme that aims to detect the spread of coronavirus through tests of wastewater, an approach that will impact how policies like individual testing and isolation are effected.
The presence of viral material in samples of wastewater from some dorms has led to more targeted swab tests on workers there, leading to more detection of cases and earlier isolation.
Conversely, the absence of viral material at other dorms has given the authorities added confidence that they remain free of the virus when no cases have been reported.
This detection method is based on the understanding that even asymptomatic individuals can shed virus in their stool, registering their viral load when an infection could otherwise have been missed.
Australia has also rolled out a programme of sewage testing to detect virus clusters.
In a statement last Friday, the NEA, together with national water agency PUB and Home Team Science and Technology Agency, said the programme is still in its early stages here, although there are plans to expand it.
The unusual method has been in development and put to gradual use since February, after the first case of the coronavirus was reported here on Jan 23.
Trial results have found a correlation between the concentration of coronavirus material in wastewater and the prevalence of Covid-19 cases in dormitories.
"The trending of Sars-CoV-2 (the virus that causes Covid-19) over time can determine if infection control measures taken have been effective," the agencies said.
But the method has its limitations. In particular, it seems less effective when there are few cases, a shortcoming that means more research is needed to understand the use of the method as an early warning system.
When Singapore had 160 Covid-19 cases nationwide on March 9, viral ribonucleic acid (RNA) was not detected in the wastewater samples that the agencies collected from the water reclamation plants where they had also been trialling the programme.
Viral RNA became detectable only later, in late March, correlating with the rise in cases in migrant worker dorms across the island.
"Several reports overseas have shown that wastewater testing at treatment plants could be useful for early detection of Covid-19 transmission in the community. However, at low-level transmission, wastewater surveillance at the treatment plant appears to be less sensitive than clinical surveillance of cases in Singapore," said the director of NEA's Environmental Health Institute, Associate Professor Ng Lee Ching.
She said this was because Singapore's clinical testing is intensive and picks up more cases.
According to statistics compilation website Worldometer, Singapore conducts about 98,514 tests per million of its population, ranking 22nd in the world.
NEA said viral material in wastewater "does not suggest the presence of viable or infectious virus".
It added: "Without a host, the virus will not be able to propagate over time in wastewater. As an added preventive measure, wastewater from locations with Covid-19 cases, such as hospitals, isolation facilities and dormitories, (is) disinfected with chlorine at the premises before discharge into the public sewers."
Clement Yong
