I found this out almost by accident in the FL sand-pit where we live. I’ve been gradually building a garden by digging a pit, throwing in a layer of charcoal, and then dumping in all the kitchen scraps until it was full. Then our drains started going glug glug glug, and the pattern of plant growth in the septic drainfield changed markedly– looks like a partial drainfield failure, but we can’t afford to replace it yet :/ So we are nursing the septic system along by using our charcoal pits for greywater processing, too. We haul the greasy dishwater out in buckets and dump it in with the kitchen scraps. Next thing you know, urine was going there too, and boiled chicken skins, and dead fish carcasses I hauled home from the beach. When each pit is full, I cap it with another layer of charcoal (I’ve started adding animal bones to the burn barrel), throw on some mulch and topsoil, and plant something. We’ve had some nice lettuces and sunflowers this year, and the cherry tomatoes are just coming in, sweet and delicious. Pretty good for white quartz sand! Not quite brave enough to start throwing poop in there– I’m nervous about not being able to get far enough from the well on our lot– but I’m kind of obsessed with not letting usable nutrients leave the property in trash bags now.