And thou shalt have a shovel amongst thy weapons, and it shall be, when thou sittest down abroad, thou shalt dig therewith, and shalt turn back and cover that which is come from thee. Deuteronomy 23:13
There you have it, the basic sewage disposal system. Dig a hole and bury it, like a cat. I used to have an Irish cat but it had a poo and buried itself. Anyway, onto the origins of septic tanks. Prior to us “digging a hole” our very early ancestors used a form of waste disposal consisting of a biological treatment via land application (we shat on the grass). This was fine for the moving tribes and nomads, there was loads of space and the chances of you treading in someone else’s were slim and even if you did the chances of you being bothered were slimmer but as soon as basic societies were formed we needed another way. A more civilised way.