Finally figured out my recurring bathroom blockage issue

I went through the exact same thing with a 4-inch PVC line running under my gravel driveway. For two years, I was obsessed with saving water, thinking my old septic system couldn't handle the load. I even switched to that thin, single-ply Scott tissue and used the 'eco' mode on my Bosch dishwasher every single time. Like you, I found out through a camera inspection that a heavy truck must have compressed a section of the pipe, creating a four-foot belly. My plumber explained that by using so little water, I wasn't creating enough 'scouring action.' The solids would just hit that standing water in the dip, lose momentum, and sink. Over a few months, it would turn into a thick paste that eventually choked the line. I was essentially starving my pipes of the volume they needed to actually function.

Once we dug it up and reset the grade with a proper bed of crushed stone, I stopped being so stingy with the water. Now, I make sure to run a full tub of hot water once a month and let it drain all at once to help push everthing through. It's much cheaper to pay a slightly higher water bill than to have a guy out with a snake every six months.

My recurring clog turned out to be the exact opposite of a belly in the line. I had a similar six-month cycle, but my issue was a 'soft blockage' caused by a low-flow TOTO toilet and a high-efficiency showerhead. I wasn't dealing with a dip in the pipe, but rather 'mound buildup' because my pipes were acutally graded too steeply.

When the plumber checked it out, he explained that the water was rushing away so fast that it left the solids and heavy soap scum behind to dry on the bottom of the pipe. Over a few months, that layer would get thick enugh to snag everything else. I ended up having to swap out my showerhead for a high-flow model and I started doing a weekly 'full-flush' routine where I fill the bathtubs and release them at once. Since I started using more water volume to actually scour the line, I haven't had a single backup in over two years. It’s wild how trying to be conservative with water can actually backfire on your home's infrastructure.

I had that exact belly under my patio. Instead of digging it all up right away, I started filling the kitchen sink and both bathroom sinks to the brim every Sunday morning. I'd pull the plugs all at the same time to create a massive wall of water. It’s been eighteen months since my last backup.

I also switched from that thick liquid detergent to a powder, wich stopped that gummy sludge from building up in the standing water. Digging is the permanent fix, but the 'tsunami method' saved me a few grand for now.

Update: I tried that trick of filling all the sinks and dumping them at once. It defintely helped clear out the standing sludge, but I'm still seeing slow drainage from my low-flow toilet. Has anyone successfully adjusted thier flush volume to help with a belly, or is replacing the fixture the only real fix?