twentymak wrote:OK so basically I need a septic tank. It gets too complicated there. ...
No, a septic tank is when you balance all the wastewater in a large tank of several m3, the sludge accumulates at the bottom and the gray water leaves in a spreading.
In the case of reuse of shower water and washing machines, good grease filtration is required.
The principle of the grease trap is simple: a small tank with compartments with holes at the bottom. The floating swill accumulates at the top of the compartments, the degreased water passes through the holes at the base of the partitions of the compartments.
Imagine an aquarium in which you pour water loaded with soap and other micro-waste: you will see that this charged water remains on the surface.
It is basic as filtration but already it skims a lot.