We have a semi-outdoor kitchen / storage area with cement flooring and a few 30cm x 30cm floor covering doors with sewage access. These sewage access floor lids (sorry I don’t know if theres a technical term for these ????‍♂️) are also made of solid cement.
 

is there anything we can do to completely eliminate the smell? 

Not great options to eliminate the smell from escaping gases. You can either relocate the access/drains to outside the kitchen, or perhaps try to vent them outside but they will need to vent somewhere. In the interim you can try these various bacteria/enzyme preparations on a regular basis to help keep things in check. 

@creative1000 a picture would help a lot.

2 hours ago, blackcab said:

@creative1000 a picture would help a lot.

It sounds like a grease pit cleanout. We have one outside behind the house and eventually when we add on a kitchen, I have to decide whether to keep it there or try and provide an access port that doesn't end up on the kitchen floor

Here are some photos. 

Most likely the rectangular ones are grease traps and the round one is the septic tank.

I just heard about a hardening foam that can completely cover/seal it, and be easily cut away if access is required. 

22 hours ago, sometimewoodworker said:

Most likely the rectangular ones are grease traps and the round one is the septic tank.

The rectangular ones are the same as we have in the back yard for the septic overflow. The OP need to look inside to see what pipes are there.

On 3/17/2021 at 9:20 AM, tonray said:

In the interim you can try these various bacteria/enzyme preparations on a regular basis to help keep things in check. 

Can you give some names for these. Our new septic smalls a bit more than it should.

23 hours ago, GreasyFingers said:

Can you give some names for these. Our new septic smalls a bit more than it should.

Can get at Mr. DIY

 

The tanks should have vents separate from the access covers. You need to figure out where the vents are, and you need to make sure they are clear.

If the vents are blocked, it will be difficult if not impossible to stop the smell. It has to go somewhere....