Rain water has always been a problem for the municipal sewage systems in Spain.

The first rains after a dry spell are highly polluted by the solids they pick up on the streets implying that the flow reaching the treatment plant is excessive and carries a greater load than the plant is designed to handle. If the usual option of spillway is used, this highly polluted water is dumped directly into the main waterway and building storm tanks at the treatment plant to temporarily hold the water until it can be treated is not always viable when there are pump stations before this point that do not have the capacity to pump such elevated flows.

At the job that DRACE medioambiente carried out in Santander and other towns around the Bay, it was impossible to concentrate in one storm tank all the flow from the watershed due to lack of space in the treatment plant that we were building and because of the investment that would be required to prepare the numerous pumps that would have to be installed. Thus, the solution chosen was to build numerous smaller storm tanks along the sewage lines. This solution has been used recently by other cities with the same problem, like Madrid.