I've been adding STPP for upwards of 9 years, no issues thus far with my aerobic septic ... can't say if there may be different effects on a "traditional" system.

I have a traditional septic and have been using it for almost 10 years with no issues. In fact I had the septic people come out to pump it as it’s been awhile and they said it wasn’t full and looked good. Before I started using stpp I had the septic pumped yearly. I’m now having it done every three to five now.

We used to have stpp in detergents with septic tanks, right? then they took out the stpp. Now adding it back by hand shouldn't be an issue. I have a septic tank, old style, on the farm. It was put in God knows when. Ive lived here 28 years and never had it pumped. 5 years ago we had to have it opened and it looked good. No need to disturb the system. We had tree roots in the incoming line. I'd been using stpp for several years at that time. My tank is a huge cement tank with a heavy cement lid. there is know access unless you get a digger and unearth the thing.

Thank you all. I was pretty young and definitely not a homeowner doing my own laundry when phosphates were removed from detergents.