Constant backups in a bathroom sink are often the result of a clog in the trap. Beneath the sink is a bend in the drainpipe, which is often referred to as a P-trap or J-trap due to its shape.
Basics of Trap Function
The bend in the trap is there specifically to hold a bit of water, so it is never meant to be fully empty and dry. The held water serves an important purpose — it creates a seal that prevents sewer gasses from coming up the line and backing up into your home.
The trap also provides another handy function. Items that shouldn't go down the drain are often caught in the trap. So, for example, if you drop an earring down the sink, you may be able to retrieve it because its weight may cause it to remain inside the trap. Of course, this same design component that traps something like an earring can also lead to the entrapment of other less savory items that can cause a clog in the line.
Common Clog Causes
Most clogs in the bathroom originate with soap scum. Soap scum gets into the drain and coats the inside of the trap. This sticky scum then catches everything that flows past, such as hair, skin cells, toothpaste residue, and even bits of paper. Anything else that ends up down the drain that shouldn't, like floss, just compounds the problem.
To make matters worse, most of these trapped materials are organic matter. Organic matter can become a host to a microbial mat, which will then further clog up the drain. Often you can see the growth of this mat as an orange or pinkish residue that forms around the outside of the drain in the bottom of your sink.
Clearing and Prevention
Clearing the clog is relatively easy. The simplest and least damaging way is to place a bucket under the trap to catch any water. Then, use a pipe wrench to loosen the joints on either side of the trap and remove it. Empty it out in the bucket and wipe it clean before replacing it. Just make sure to check the joints to make sure they are tight and not leaking before removing the bucket.
As for prevention, use a screen or hair catcher in the drain to cut down on debris. It's also a good idea to pour boiling water down the drain once a week or so. This washes out the soap scum and destroys any microbial mat that is forming.
Contact a plumber in your area if you are experiencing repeated clogs in the trap beneath your bathroom sink.