Where I used to live near Lake Michigan, rain gardens became trendy, even though we were all on municipal sewers, mine was in line after my rain barrels, a sort of dip in the yard where certain vegetation was planted.
I also ran all old tank water thru that planted area to my pond, of course in the dead of winter, i had to adjust water changes to the reality of the situation.
Now where I live, 50 yards from the Pacific, the entire island is on septic tanks, and I run all old fish water thru a fruit and veggie garden.
As far as cooking oils and fats, I sop them all up and use the saturated paper to start the BBQ (I've always hated the taste petroleum charcoal starter fluid imparted), using the paper with olive oil soaked paper in the charcoal starter chimney is a great alternative.
As an aside, using something like Rid-X as a probiotic in the tanks, might in the end also be benificial to the septic system, especially those certain times of year when terrestrial plants are dormant.
The Use of Probiotics in Aquaculture (Read!)