I would suggest what we call ...

I would suggest what we call a septic tank.  I have attached a file.  The tees are important we call them baffles.  They will allow the FOG (fats, oils, grease) to form a scum on the water.  Settleable solids will form a sludge on the bottom of the tanks.  Measure your flow a provide enough detention time for two or three days.  The effluent can then be sent through a rock or sand filter.  This process will reduce the COD and BOD.  The water then should be good for irrigation or to flush toilets.  Much more treatment would be needed for potable use.