Solutions to stop biogas

Few weeks ago, when I moved an one-year-old refrigerator to clean up 
kitchen, some oil was on the floor. I suspected some refrigerant leaked
because that happened to my dehumidifier before.
Now, the refrigerator is repaired, but the causes result in the corrosion of copper tubes must be identified. 
I guessed one cause is biogas (沼氣), which came from sewers.
Biogas contains mainly methane (甲烷) and some hydrogen sulfide (H2S). Hydrogen sulfide not only causes rotten egg odor but also erodes metals such as lead, iron, and copper. Heat-exchanging pipes in a refrigerator are made from copper. 
Copper reacts with hydrogen sulfide and forms copper sulfide (CuS). In addition, biogas is heavier than air. Thus, biogas gathers on the ground and corrode those tube.



One solution is to replace with one-way water outlet as shown in pictures. It is a clever design and costs about 2 to 3 dollars. That uses a weight as a leverage to counterbalance a valve. When there is no water, the valve remains closed. When the water flows into the outlet and outweighs the metal, the valve opens. Thus, no air will come from sewers most of the time.


Another solution is to DIY a N-sharp tube. You will find similar design, called a trap, like the drainpipes of lavatories. All you need are four 90-degree tubes and some PVC glue. The water resided in the tube builds air pressure against air from sewers underneath.
I, finally, replaced two outlets with modified PVC tubes in the kitchen and three in bathrooms with the one-way-valve outlets.