Mixing system for the introduction of chemical substances in a fluid to be treated, comprising: - a container comprising an external wall , an internal wall , a bottom and a cover , - at least two concentric mixing chambers, a first central chamber defined within said internal wall and a second chamber defined outside said internal wall , said first chamber and second chamber being in communication with each other, - an inlet duct for a fluid to be treated, configured for the introduction of said fluid from the outside into said first chamber , - an outlet duct for said treated fluid, configured for the outflow of said fluid from said second chamber to the outside of said container , said inlet duct , first chamber , second chamber and outlet duct being configured to define a labyrinthine mixing path , - stirring blades positioned inside said first chamber , operated by drive means through a rotation shaft , - at least one injection passage for the introduction of a chemical additive in one or more points of said labyrinthine mixing path , said inlet duct being positioned below said outlet duct with respect to a configuration of use of said mixing system , - said stirring blades comprising at least one first group of blades , - said first group of blades being positioned below said inlet duct with respect to a configuration of use of the mixing system , characterized in that said at least one injection passage comprises: - a first injection pipe directly connected to said inlet duct ; - a second injection pipe directly connected to said first chamber ; - a third injection pipe directly connected to said second chamber . - a container comprising an external wall , an internal wall , a bottom and a cover , - at least two concentric mixing chambers, a first central chamber defined within said internal wall and a second chamber defined outside said internal wall , said first chamber and second chamber being in communication with each other, - an inlet duct for a fluid to be treated, configured for the introduction of said fluid from the outside into said first chamber , - an outlet duct for said treated fluid, configured for the outflow of said fluid from said second chamber to the outside of said container , said inlet duct , first chamber , second chamber and outlet duct being configured to define a labyrinthine mixing path , - stirring blades positioned inside said first chamber , operated by drive means through a rotation shaft , - at least one injection passage for the introduction of a chemical additive in one or more points of said labyrinthine mixing path , said inlet duct being positioned below said outlet duct with respect to a configuration of use of said mixing system , - said stirring blades comprising at least one first group of blades , - said first group of blades being positioned below said inlet duct with respect to a configuration of use of the mixing system , characterized in that said at least one injection passage comprises: - a first injection pipe directly connected to said inlet duct ; - a second injection pipe directly connected to said first chamber ; - a third injection pipe directly connected to said second chamber . <EOS>