I am an environmental consultant and I would not worry too much about it. The pathogens of concern in domestic waste water don't live long on the ground. Keep your kids and friends away from the spot for a week or so, and pickup any offending solids with a shovel and burry them, not much else needed.
I like that term "offending solids". I am a bit les refined.
If there is any crap left laying around, bury it or throw it back in the tank.
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just a bit of the layman's trivia.
open field dumping of septic waste is still allowed in certain areas of Indiana. As long as the area meets a listed criteria, honey dippers can simply empty his truck on the open ground.
EEEEWWWWWWwwww!!!!!!!!!!!!!!!!!
All I can say is you simply have to hope that your favorite hiking trail is not one one of the acceptable areas.
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more unneeded knowledge:
in a local city, the sludge from wastewater treatment plant is left to ripen in a local contained area. It is then used as soil when the city plants trees and such in the area.