A static mixer element and a method for mixing first and second (or more) fluids using a static mixer element. The static mixer element includes a directional flow axis which points in an intended downstream direction opposite to an intended upstream direction. The static mixer element also includes interdigitated static mixer blades. The blades each have a concave side which faces generally in the intended upstream direction at an acute angle with respect to the intended upstream direction. The static mixer element is placed in a pipe with the directional flow axis of the static mixer element pointing downstream. The first and second fluids are introduced into the pipe upstream from the static mixer element. Optionally, other static mixer elements can be added downstream, wherein even-numbered elements are rotated ninety-degrees about the directional flow axis from odd-numbered static mixer elements.