Analysing virus levels in sewage flowing in pipes under the city of Malaga has been a useful tool for local officials fighting Covid-19 since the start of the pandemic in the spring, as faeces flushed away can carry the virus even before symptoms show.
Now the municipal water company, Emasa, has been giving more details of the innovation it introduced in its sampling for the second wave; the testing of the city's sewage is now divided into 16 zones, which allows scientists to get a more precise idea of the spread of the virus in the city.
Tests are made every Monday at the same points, usually just before feeder pipes from each zone join with the main pipes, so as to avoid mixing neighbourhoods. The samples are sent to a laboratory in Alicante and the results fed back to Malaga city hall the next day.
Officials point out that although they always take the samples at the same time, results can vary if it has rained heavily or if there is a lot of detergent in the water. A zero reading doesn't necessarily mean there is no virus, as the testing is not that sensitive. Nor can a test distinguish between high or low concentrations of the virus; testing is most useful as an early warning.
A comparison of the first district tests from the beginning of October, at the start of the second wave, with data from the end of November shows the spread.
Back in October there were positive readings in the northern and central inner suburbs and the city centre, as well as El Palo to the east and Misericordia to the west.
By 23 November, the only parts of the system where the virus couldn't be detected were outlying, more rural areas to the west of the municipality and on industrial estates.