Our septic system was reserved for toilet.

The bath lavatory washer and kitchen sink all drained into another
system that was used for flower beds, trees, worm beds and lawn.

Lots of care was used in what went down the drain, but it was a
system that worked for decades.

With that type of system, even heavy clay soil with trenches with 12
inches of gravel would support two people forever.

You need efficient dual flush toilets - use .8 gallon of water for
most flushes.