The unsung hero of your sanitary drain system is no doubt the P-Trap. The p-trap is an inexpensive device found on every drain except the toilet in your home or, business that serves as a barrier between your nose, and the foul gases, and, diseases that lurk in the sewer system of your town, or, city.
The P-trap is a “U” or “J” shaped bend that holds water in that shaped section providing a liquid seal between you and those sewer gases. When water goes down the drain the excess water is water over the dam so to speak allowing the drain water to enter the sewer system without the seal ever breaking.
In the past less effective methods were used to provide the seal. Those methods included bell traps, s-traps, drum traps, and a few others. These all had limitations in the sense that they either failed to provide good reliable drainage, were difficult to clean, or, were subject to the trap siphoning due to not being vented properly because of the design. In 2003 an outbreak of SARS in China was traced to ineffective traps.
A common mistake by DIY’ers is to not install the drain pipes under the sink properly, or, to remodel a kitchen installing a deeper sink and garbage disposer without paying attention to the height of the drain in the wall. This results in the trap being installed in a manner that alters the depth of the water seal on the trap. This results in poor drainage, odors, and possible early failure of the disposer. The depth of the water in the trap is a set depth and must not be altered..
Today’s p-trap properly installed and vented is very reliable at draining and provides excellent sealing. There are rules at to how it is installed and vented. Generally the wall arm coming out of the trap is horizontal with a pitch of 1/4″ per foot minimum downward into the wall. Once inside the wall it can either turn down into a vertical drain or continue on a pitched horizontal run. However in any case it must have a vent before the drain turns downward. There are also limitations of the total developed length of the horizontal run before having a vent. Consult your local code so your installation will meet the code in your area. Failure to properly vent the p-trap can cause siphoning of the water out of the trap allowing sewer gases to enter your home.
When your sink becomes clogged it is a good idea if you can remove the trap and inspect it and the pipes from the sink drain to the wall to see if the blockage is in any of those pipes. Use a bucket under the trap to catch any water remaining in the sink and drain pipes to keep the mess under control. If the blockage is not in those pipes but into the wall you would probably be best served by calling a plumber to clean the drain line.