Hi,
My girlfriends family live in spain, and are having problems with smells around their septic tank, I have looked at the system, and it doesn’t seem right. It should be noted the smell comes after a shower or bath, and it only comes outside around the septic tank, not in the house.
There is a septic tank, which has an overflow for water which goes into another tank and then in turn into a soak away. The water overflow comes out of the top of the septic tank, which seems right.
There is then another 2 pipes coming from the septic tank. One is a ventilation pipe that leads up and around the house (supposed to be for exit gases) the other is another pipe (I’m told it was installed to balance pressure), again this is an open pipe, but it goes down the hill.
The first problem seems to be, the proposed ventilation pipe (leading up and around house) is connected to the bottom of the septic tank, and the other pipe from below is connected into the top of the septic tank. This seems wrong, surely the vent pipe should come out of the top of the septic tank allowing gases to rise away?
Secondly this second pipe installed to ‘equalise pressure’, and allow gas to pass through the first seems wrong, especially how it is installed. Firstly is it even required? Secondly if so shouldn’t it be again an upward vent pipe, installed somewhere else?
I have read about AAV’s, to release pressure when a bath is un plugged, these seem logical, and I’m wondering if these are needed to stop vacuum build up and possible gurgling of toilets/sinks. Is this required, or will a vent direct from the septic be enough? There seems to be no ‘Main soil stack’ so putting a vent/AAV in the house could be hard
My initial thought to sort this problem is to install the change the vent pipe (up and around house) so it comes out of the top of the septic tank.
Do I then need to install a second vent or AAV around the house? Does this need to be above the water outlets.

The inlet pipe is the highest and there should be a few inches of space between the bottom of its T and the top of the floating crust. This to create as little turbulence as possible when the water and solids flow in. This also provides space for the various gases, mainly carbon dioxide and methane to escape above the roof of the home, these gases are the result of the bacterial action within the septic tank.
The water in the tank is just level with the bottom of the outlet pipe, and the bottom of the outlet T is 40% below the surface of the water/floating crust and the bottom of the tank.

The septic system is very simple, it just relies on the bacteria in our toilet to continue the process started in our gut. Out toilet is 70% water to start with and after the bacteria have finished most of it has been converted to various gases the rest is water and a very small amount of compost falls to the bottom of the tank. This needs to be removed when the septic tank is half full. If the solids fill the tank above the half way mark, then there is every chance that some of the suspended partly processed solids will find their way into the drain field and block it.

A septic usually smells when the lid or lids are loose and rain water flows into the top of the septic tank making the top of the floating crust wet.