It is a good assumption. But if the field gets saturated enough the water will overflow the toilet.
I'm not at all sure that this is related to your problem, and it does appear to be elevation related, but you should not have any siphoning effect going on. That tells me that the basement toilet may not be vented properly. A proper vent should break any sort of siphon action.Does this make sense to you?
Our basement toilet won't flush during times of heavy rain. This happens about 1 or 2 days a year, usually in winter. We've learned to live with it. I have two fields - a new one installed about 4 years ago and a diverter valve - this issue happens no matter which field I'm on. As soon as it dries out, the system works fine the other 363 days a year. And this cycle has happened for the last 3 years. 3 years of 363 straight days of a normally working system tells me that my fields are fine - perhaps the situation is not ideal - yet it's just the nature of what I have.
So the ground becomes extremely saturated, the fields cannot purge off water, and the tank fills with water, as does the main line coming into my basement slab.
There is a syphoning effect happening with the basement toilet. The water in it is actually "connected" to the water in the drain, since that drain is full up to the toilet. Since those waters are touching, as the drain tries to purge, it pulls the toilet water down with it, interupting it's flush cycle. When the drain line finally dries, there's an air break and thus the toilet works normally.
Yet we continue to use water upstairs, and it does not overflow the basement toilet. I believe this is because the weight/pressure of the water upstairs (with more gravity behind it) is actually pushing/forcing the water toward the lowest drain - which is actually the septic fields, not the basement toilet or shower. I specuate that if we had no basement toilet/shower, we'd never even know we were having an issue.
So - based on everything above, is it reasonable to think that even when the fields are super-saturated, since they are lower than the whole house and every bathroom, natural water pressure from the weight of water in the house drain pipes could actually force water through the tank and squeeze it out the drain fields?
Out of curiousity, why do you think my two cement septic tank lids would leak, being buried 3 feet down? Do you mean rainwater might leak into them just because they aren't an airtight fit? Common sense tells me that's possible, but with dirt packed all around them, and being in an area with great runoff, I can't imagine very much water could leak in.
Or are you saying during times when my system is overloaded - sewage might be squeezed OUT?
If I put risers on and I experienced one of these overload events - I feel like the riser tubes would fill up too, then they would leak sewage now onto my lawn. Their tops would in fact be higher than my basement drain, so maybe they'd never fill up though.
As for the 50-flush party I had - yeah, that's not techincally much water use for a fully working system. What I was implying was that just a day earlier, there was standing water in my backyard, the washout was full and basement toilet unflushable. That next day, all worked perfect, as if the system was at full capacity. So even though they were saturated just a day before, my fields must have really been able to quickly purge off water and allow the system to rebalance in pretty short order.
If you think your Studer vent is not working all you have to do is unscrew it & see if there is a difference in the flush.Venting has been mentioned a bunch of time - there is a studer valve for the toilet - it's in the wall behind. Before we finished off that wall, a plumber was in for other work and I had him check it - he raised it so it was up in an open ceiling void where it could get more air.
The basement toilet always "burps" once at all times when you flush it...even when we're not having issues. Like one big pre-flush bubble comes out. Our original home inspector actually noted it as a possible venting issue.
Still - that plumber checked it and the venting was "right". Apparently not.
When we have the rain events and "slow" basement toilet (or when it stops for a day) - the water is sucked right out of the toilet. So however that is happening, something is pulling it from the other side.