Coronavirus has been found in sewage water in the Netherlands and is likely to have got there from the faeces of patients with Covid-19.
Research suggests that monitoring sewage water is a good way of detecting whether specific infections such as Covid-19 are present in the population.
The Dutch National Institute for Public Health and the Environment has previously found norovirus, antibiotic-resistant bacteria and measles in waste water using the same technique.
Only a small percentage of Covid-19 patients have the coronavirus in their gastrointestinal tract and will excrete it into the sewers. A small percentage have diarrhoea, and the virus that causes Covid-19 was detected in stool samples of some of these patients.
Dutch authorities insisted that all sewage workers would be safe from coronavirus infection if they took the proper hygiene precautions, such as wearing protective clothing and respirator masks.
The coronavirus was found in waste water at Amsterdam Schiphol Airport, Tilburg, and a sewage plant in Kaatsheuvel.
The virus was found in Schiphol's sewage water four days after the first person in the Netherlands had tested positive. The sewage plant treats water from Loon op Zand, where the first Covid-19 patient in the Netherlands lives.
Figures released on Wednesday revealed that a further 80 people had died from coronavirus in the Netherlands, taking the death toll to 356, and another 852 had been diagnosed with the virus. The increase of 852 patients was the highest recorded single-day increase in the country to date.
Jaap von Dissel, of the National Institute for Public Health and the Environment, said that the spread of coronavirus seemed to be slowing after the country had entered a partial lockdown. Mark Rutte, the prime minister, will decide next week whether to extend the measures.
"The exponential growth has probably been halted. It is sensitive to behaviour, but for the time being there is a positive trend," Mr Vvon Dissel said.