I have a small one storey 'Isan' house built by some local people from my wife's village. Originally the bathroom was two walls and a door in a corner inside the rectangular one room house but due to the house's small size I removed it, opened the back wall and built a big bathroom behind the original building.

The old septic tank - placed 50 cm behind the original house - is abandoned. It was the standard '3 tubes and a lit'. What was done was that the upper tube was removed, the lid put on, then soil was filled on top of the lid (50 cm) and the not reinforced concrete floor of the new bathroom was made on top of this fill.

The old tank was NOT emptied and NOT filled with soil. If the old tank produces methane it will have to escape sideways through compact soil or upward into my bathroom ...

Does anybody know if I am living on top of a bomb?

Like lighting a fart?

Natural processes break down the solids and liquids by bacterial action, the breakdown occurs in the absence of oxygen and the anaerobic conditions are referred to as "septic," giving the tank its name. Consequently, if the tank has been sealed, over time the liquid (effluent) will disappear and the sludge will dry out leaving a crud, remarkably there is very little smell during this process because it is not being agitated.

It is therefore doubtful combustion will take place.

I agree with Astana that I think the risk of combustion is extremley low but as the waste degrades it will compact and the integrity of the tank will also decrease over time leaving a void beneath the unreinforced floor.
It depends on the quality of the tank to begin with but at some time in the future, maybe many years from now, it's possible the floor could collapse.
If it was my property I think I would attempt to remove a small section of floor, pump out the waste from the tank and pump sand slowly back in trying to avoid any airpockets.
Shouldn't cost to much for a little piece of mind.

Good advice Roy but for better piece of mind you will need to plug the tank properly, sand alone cannot be trusted.

This is how I would do it Philo...

Step 1: Remove water and organic debris.

Pump any water, semisolid, or solid organic material from the septic tank. Organic solids, semi-solids, or liquid material should be disposed of safely.

Step 2:

Clear debris. Remove all hardware and foreign material or debris from the hole and remove debris from around the site.

Step 3:

Puncture the floor. It is important that water not accumulate inside the tank to form a perched water table. Removing the floor is preferred, but drilling or breaking the floor is usually adequate to allow drainage and prevent any accumulation of water after plugging or filling.

Step 4:

Plug/fill the structure. Plug the septic tank with local low-organic-matter subsoil (usually natural clay) material. Be sure this material contains no other potential contaminants and is moist enough to compact easily. The clay should be placed in layers of 6 inches to a foot (no more than 2 feet) and compacted to prevent settling. Some form of mechanical compacting should be used. Stop when the fill is within 3 feet of the surface.
.
Step 5:

Place grout plug. Level the lining at the desired depth, which usually is 3 feet below the surface, and complete the subsoil fill up to this same level. The structure is now ready for the plug of approved grout material which should be 6 to 24 inches thick. Sodium bentonite clay is recommended. Because of bentonite

Thanks for your informed answers. As the tank is only about 1 m in diameter and around 2 m deep and was only used by two people for some 3-5 months - AND MOST IMPORTANTLY - has no floor and is therefore draining to the soil, I would expect the liquid to be gone already. The house rests above the ground level araond 50 cm (so the tank top is about ground level) and the ponds that are in the vicinity still does not contain water (this rainy season) even if they are 2-3 m deep. I think I will open the bathroom floor, punctuate the lid, fill in sement/water and just let it be another experiece in the land of so so.

Thanks again.

Should do the job Philo just remember to get a concrete vibrator, you can get a pole one that would be small enough to fit inside the hole and give it a wiggle about

You are awful Roy but I like it.