Sometimes the area is very tight in wastewater treatment plants. As always, using the area efficiently is good engineering. In chemical treatment, mixing of chemicals are mandatory and they most commonly done in tanks with a mixer in it. But tanks occupy great area and sometimes the area is expensive or small.
Pipe flocculator is a commonly used technology to mix chemicals and wastewater where you can ill afford building large tanks to mix chemicals. Wastewater is pumped through the pipe and chemicals are dosed into the pipe that is long and elbowed. And thus, the mixing is done inside this pipe, saving a lot of space.