Divide it into several

If you want to make a septic tank yourself, then make a hermetic "bunker" in which the drains will be replaced. It is recommended to divide it into several cameras, which are connected by overflowing holes. The upper chamber collects large particles, the second and third — liquids. At the output you can make filter wells through which water will go into the soil. This well should be made above the level of groundwater by about a meter or even more.