Need help identifying what I'm looking at

[Samir]

Hi,
So a few years ago I have bought a house with a concrete septic tank. The house was built about 12 years ago but was never lived in before. The septic tank has also never been used until we started using it. It seems to be working fine, but as of last few months we have started experiences occasional odor in the toilets that seem to be originating from the tank. So i decided to call a company to empty the tank.
However, the previous owners never clearly specified details on where exactly the septic tank was, so I dug up where they said it should be and although it looks like the septic tank cover, it seems too big. It seems to be made of concrete but its diameter is about 120 cm (3,5-4 feet). It appears to be hexagonal shape, with a handle on top, and is surrounded by rotting wood, which probably served as cast during construction. Also, it is covered in what seems to be thick, black plastic tarp, ribbed in shape, which I am hesitant to completely remove as am I not sure what its purpose is and whether I need to put it back after emptying the tank. So my questions are:
1. what exactly am I looking at?
2. what is the black tarp-like plastic covering the "cover"? Some improvisation?
3. Is it OK to remove completely the tarp and is it needed to put it back after tank emptying?
4. What is the small hole (about 2 meters - 6 feet away from the septic tank cover), which does not seem to lead to anywhere (I have dug it up before and can's see that it leads anywhere), but which is always the same and nothing seems to grow out of it? Some kind of vent? Snow melts over it very quickly.
5. How close can trees grow next to the septic tank? A small one is growing approximately 1m (3 feet) from the "cover". Should I cut it down?
Thanks for your help!

 

[Diehard]

If you started experiencing occasional septic odor in your house, the problem wouldn't be the septic tank.

All fixtures drain to that smelly septic tank. Which means, all fixtures must have some form of a trap on their drain pipes, that stays full with water, to act as a seal against those smells.

The toilet traps is built into toilets.
So if you were fairly sure the smell was coming from the toilet, either water level in the toilet bowl was very low for some reason or it's possible that it could be coming from a loose connection where the toilet sits on the drain pipe.

 

[Samir]

If you started experiencing occasional septic odor in your house, the problem wouldn't be the septic tank.

All fixtures drain to that smelly septic tank. Which means, all fixtures must have some form of a trap on their drain pipes, that stays full with water, to act as a seal against those smells.

The toilet traps is built into toilets.
So if you were fairly sure the smell was coming from the toilet, either water level in the toilet bowl was very low for some reason or it's possible that it could be coming from a loose connection where the toilet sits on the drain pipe.

Yes, I think you're right, the odor is probably coming due to a faulty trap, which I will look into.

 

[haloflood]

That is odd looking. For me, at least, I've never seen anything quite like that. I would guess the tank was poured-in-place instead of precast, which would probably explain the wood and the rough surface and no apparent inspection port. Also seems kind of small if it's 4 feet wide at most, but I'm comparing that to the old tanks in my area that are circular, about 6 feet in diameter, and precast.

Trees are not the best things to have near septic tanks, but they're not guaranteed to invade a tank. Probably depends on the type of tree and yearly soil moisture, as well, since the inside of a septic tank is a constant source of water and nutrients as long as someone is using it. When they do get into a tank, tree roots usually go through the gap between the lid and the tank, or around a pipe where it enters the tank. In those cases the lid can be removed, the roots cleared out, and the lid and inlet/outlet pipes resealed. I have seen a few times, though, where tree roots punched their way through the sidewall of a concrete tank, and a few tanks that were almost completely filled with roots. How long it takes for that to happen, I have no idea.

 

[fixitron]

As Diehard said, it is likely a problem with the seal between the floor and toilet. I estimate that around 70% of all toilets that I have encountered are loose on the floor, which often leads to a broken wax seal. I also see a lot of installs where a wax ring with a horn was used when a horn was not needed. When the toilet flange sits on top of the floor, there is often little room between the flange and the outlet of the toilet. A horn leaves little room for wax to seal.
This was built only 12 years ago?
I cannot imagine how one would pour a conventional style septic tank in place. It is difficult enough in a factory.
IF that is a septic tank, you would need a machine such as a backhoe to lift it to be able to pump it out.
My Rule#2- "The more you look, the more you see" and its corollary- "If you don't look, you don't see". You should lift that cover and inspect it and see if it is even connected to a leach field (or maybe a drywell?).