dirt separator before or after return circulator

All my primary boiler parts arrived and I am just about to put them together. As I am laying them out to maximize space efficiency, it occurs to me that the layout would benefit if I install the dirt separator before the return circulator back to the boiler. It also seems that if the dirt separator were placed immediately before, that the return circulator would benefit from return water that has just had any dirt removed from it. But I wonder if there might be a logistics reason why a dirt separator should be installed after a return circulator, just as an air separator should be placed before the supply circulator?

In case it matters, here is a link to the dirt separator that will be installed:
http://www.supplyhouse.com/Spirotherm-TDN100-FT-1-Spirotrap-Drain-Brass-Dirt-Separators-with-removable-head-Female-Thread-9714000-p

Thanks in advance for opinions, suggestions and advice.