This article was originally published from « Antenne Pays Basque »
Why do we find so many plastic sticks from cotton wool buds (and tampon applicators) on our shores?
Because they have either been thrown directly into the natural environment (although the proportion of those is small) or more likely because some people throw them into the toilet bowl rather than disposing of them in the bathroom bin.
So why are they not filtered out by the gratings at the sewage treatment plants?
A small amount of cotton wool buds goes through the mesh if they are pointed towards the grating, but in most cases the answer lies somewhere else.
There are two types of sewerage systems which get the water to the wastewater
treatment plants:
The first is called ‘combined’, and it collects used domestic water (in green on the diagram) and stormwater runoff (in blue on the diagram) in the same sewers. This all-in-one system revolutionised urban hygiene in the 19th century and helped avoid numerous floods. Most of the larger cities are still running on combined sewerage systems. The other type of sewerage network has separated sewers. Used household water is channelled into sanitary sewers, with stormwater runoff going into separate sewers to be taken to the sewage treatment plants.
Under normal precipitation conditions, the two sewerage systems work fine, and the sewage treatment plants discharge purified water into the natural environment in line with the applicable regulations.
When exceptionally heavy rainfall occurs, where there are separate sewers, stormwater is directed into retention basins (to be treated at a later point in time), and the sewage plant carries on treating the used domestic water whose output remains stable.
In the case of combined sewers on the other hand, the network overflows and a large part of the water is discharged untreated into the natural environment without going through a sewage plant as they cannot deal with such volumes. Cotton wool buds, sanitary towels, tampon applicators, tampons, and all other objects which were thrown into the toilet are then directly disposed into our waterways. They flow into the oceans and seas and eventually get washed up on the beach… or they may pierce or obstruct the stomachs of marine birds!