Primary Wastewater Treatment is fundamentally concerned with the physical treatment of wastewater. This is mostly in reference to the sludge that settles out of solution. This can be measured by the use of Imhoff cones, as shown below.
Imhoff cone 
The important thing to remember about Imhoff cones is that it is simply glassware used in the lab to measure the settleable solids in wastewater. The "workhorse" treatment method for primary treatment is the primary settling basin (also called the primary sedimantation tank, as shown below.
Notice several things about the above graphic. Firstthe gravitation settling of sludge is sent of to another area for further treatment and disposal. Second, the treated effluent is sent of for further secondary (biological) treatment. And third, this particular design skims off the grease and other floating scum for further treatment and disposal. The diagram below shows another design:
Primary sedimentation tank
The "scrapers" move very slowly to collect the sludge that settles to the bottom. The final diagram given below is the Imhoff tank (not to be confused with the Imhoff cone mentioned above, which is simply glassware used in the lab). Pay special attention to the small opening at the bottom of the smaller chamber.
