Aspects of the present disclosure provide various apparatus and methods. In some embodiments, an apparatus is provided for mixing a gas with a liquid. The apparatus may include a pipe having two ends. The pipe may provide a main fluid path and may have an interior surface having a first groove. The apparatus may also include a helical vane disposed inside the pipe. The vane may have a first projecting tongue that engages the first groove. The apparatus may also include a gas injection port on the pipe adapted to inject gas into the fluid path upstream of the helical vane. In some embodiments, the helical vane may be a 3D printed component.