Fountains of sewage

Just after Christmas, we noticed something strange going on with our basement bathroom. It features a "pumping toilet" since it lies below the level of our septic tank.

The pump started pumping occasionally for no apparent reason.

At first we suspected a slight leak in the toilet. To test it, we turned the water off to the toilet. It still pumped, but with less frequency. Then we noticed that it seemed to pump mostly when we had guests using our upstairs guest bathroom.

And one day, we went downstairs, and found that a fountain of sewage had erupted from the shower drain in the basement bathroom. It was right after some guests had been in. We cleaned it up, and it happened again the next time we had guests in.

Hmmmmn...

So we called a plumber, who generously said they couldn't help us, but re-directed us to a Canmore company that inspected and put cameras down drain lines to diagnose problems like these. They came by today. They popped the covers on the septic tanks, and found the first tank REALLY full, and the second tank just half full with water gently dripping into it. In fact, the first tank is SO full that the level is ABOVE the level of the drain line in the basement. They put a camera down that main drain line running to the septic tank and found it clear of debris but half-full of water. 

Hmmmmn...

The current diagnosis is that there's a blockage somewhere between the two septic tanks, backing up the water into the house. It will allow some water to flow slowly between the tanks, but when guests are in and we all shower at once (say, in the morning before skiing), the water backs up from Tank 2 to Tank 1 into the drain line of the house, flows into the vent line of the basement bathroom, fountains out the shower and into the toilet where it is pumped back up.

So we're scheduled to have the septic tanks pumped out and cleaned on Monday -- a semi-annual thing the previous owner has been doing for 20+ years. We'll try to clear any blockage, and check why the level is so high (and we might try to get a high level alarm installed).

But it doesn't explain how the water in the septic tank is higher than the level of the line draining into it -- a line that has some water in it, but is not full.

Hmmmmn...