What should I do if the toilet return air hole is bubbling with water

Just order white cement to seal it (if you don't care about the color, just ordinary cement) and have the conditions to paint some white paint! It's better to use sealant, but you have to buy one with just a little, which is much more expensive!

When you want to seal it, you should first cut it deeper, which will make it stronger (stronger)!

In fact, it is true that the toilet and sewer pipe are not sealed, and the water is returned a little bit when flushing. However, there is no leakage in other places, and it does not hinder other things, so deal with it, and it will not affect the use for the time being!

Plug it with marble glue and wait for it to dry, or use plastic steel clay (chemical raw materials) on the market, which is very expensive.

There should be a pipe that is directly connected to the outside and connected to the toilet sewer, called the exhaust channel, otherwise it will exhaust into the toilet when the septic tank has high digestion pressure.