Most powdered detergents contain high amounts of salts which aren’t particularly good for anything downstream of your dishwasher.
I’m unsure of a salt-free dish detergent, but I can gleefully recommend Oasis Biocompatible Laundry Detergent for everyone, but especially anyone with a septic or greywater laundry system. It cleans great and breaks down naturally into plant food.
“Septic safe” is a pretty loose definition, and usually means “this will not break anything” not “this will naturally break down into something that is not harmful to the ecosystem outside of your septic tank”, but it’s usually a good start