It is common for homes, businesses, and other sites with running water to encounter clogged or slow-running drains. Water takes longer than usual to drain, and the sink, shower or bathtub may be left with an unpleasant greasy residue when the water does finally drain away.
Blocked drain or sewer problem
A single slow-draining drain could be an isolated problem, but if the building has a few slow drains then there may be an issue with the sewage system. If so, check it out with your council or local sewage department.
Drain cleaner
Drains can become clogged with soap scum, grease, hair and other debris in the water. Use a store-bought drain cleaner or a DIY-recipe to clean out the drain.
For a home remedy, start by pouring half a cup of salt down the drain. Follow this with half a cup of baking soda, then half a cup of vinegar. After twenty minutes, pour very hot water into the drain for three minutes.
Repeat if required. Once the drain is clean, use a screen or filter at the mouth of the drain to catch debris, and avoid pouring greasy, thick and sticky solutions down any drain.
Manual clean
Sometimes it is necessary to clean out the drain manually by using a long, heavy length of wire. Insert the wire into the drain and move it around to loosen the clog. Pour hot or boiling water to clear out the clogged area.
Source: https://www.tele-management.ca/2013/07/home-plumbing-how-to-fix-slow-drains/
