One month ago I experienced bubbling in my downstairs toilet when someone was taking a shower upstairs. The toilet bubbling concerned me and I noticed a little water on the floor the next day, nothing much, as if it had just overflowed very slightly.
I called a plumber who came out and inspected the drain line and found that the line was blocked with dried sludge. He mentioned it was probably because when the septic was last emptied in the summer, about 10 months ago, that the drain line was not sprayed out and flushed. He did that and everything was resolved, so he said.
About one week ago, I experienced some bubbling in the downstairs toilet again and tried to remedy the problem with double the recommended dose of clog remover (Drano) down the nearest sink. I put my laundry in the washer and timed it for 4 AM so it would be done in the morning. The next morning I went to use the restroom and opened the bathroom door into about an inch or two of water. Some blue stuff came up the toilet with water and had flooded the downstairs bathroom, there was nothing really gross except smelly water and some toilet paper. There was standing water in the washer as well.
The same plumber came out and inspected the drain line again and found a blockage of toilet paper. We were both concerned about some kind of damage to the pipe and so he flushed the drain line again and showed me the whole drain line to the first toilet, with a camera snake. It showed no damage he said and I couldn't see anything obvious. He showed me how to unclog the septic inlet if I heard bubbling again.
Today, I noticed my toilet downstairs was flushing poorly so I went and checked out the septic inlet, and sure enough there was a bit of toilet paper in there.
This has not been a problem until recently and I have lived in this house for about 10 months. What could be causing this problem to suddenly appear? No new people have come to live with me and I have been making sure to flush waste first and then toilet paper second, as recommended by the plumber to see if that helps the situation.
In addition to this, I also bought a bulk package of Drano from Costco and used it on all of the drains.
At this point, the only change I have made that could affect the system is that I installed an above ground sprinker run (4 sprinkers) to my lawn, which is above the drain field.
Is it possible that this could be causing the water level to rise periodically inside the septic tank to backflow under heavier use (shower + toilet or washing machine)?
What can I do to further diagnose the issue and is there anything that comes to the mind of the experienced plumber here?