This is where the magic happens. Look for:
: The bottom of the first chamber is designed to accumulate sludge (solid waste), and the top of the liquid in both chambers has a layer of scum (oil, grease, and floating solids). 2 chamber septic tank dwg
You do not combine them. A grease trap (for kitchens) is a separate DWG located upstream of the septic tank. The septic tank DWG should specify an influent BOD (biochemical oxygen demand) of <200 mg/L after the trap. This is where the magic happens