There is a lot of kitchen folklore about what is fine to wash down the sink. Here is what actually happens in the pipe.
Myth: coffee grounds clean your pipes
Fact: coffee grounds do not dissolve. They settle in the trap and the bends of the pipe, and stick to any grease, forming a thick sludge. They are one of the most common things we find in blocked kitchen drains.
Do instead: put grounds in the food waste caddy, compost or bin.
Myth: eggshells sharpen waste disposal blades
Fact: waste disposal units do not have blades to sharpen. Eggshells break into small, gritty pieces that settle in pipes and combine with grease, and the membrane can wrap around other debris.
Do instead: compost them or put them in food waste.
Myth: rice and pasta are fine because they are soft
Fact: rice and pasta keep absorbing water and swelling after they go down the sink. Combined with grease, they can form a stubborn, starchy plug.
Do instead: scrape plates into the bin or food waste before washing up.
Myth: hot water and washing-up liquid make fat safe
Fact: washing-up liquid breaks fat up for a short time, but it cools and sets further down the pipe or in the gully outside. Over time this builds up into blockages, and contributes to fatbergs in the sewers.
Do instead: let fat cool, then scrape it into the bin. Wipe greasy pans with kitchen roll before washing. See what to do with cooking fat after Christmas dinner.
Myth: a little bit does not matter
Fact: blockages build up from many small amounts. A sink that drains fine today can be narrowed by months of grounds, grease and food.
Myth: biological products fix blockages
Fact: enzyme products can help with mild build-up over time, but they will not clear a blocked sink. See do chemical drain unblockers work?
What can go down the kitchen sink
Water, washing-up liquid and small amounts of food residue from rinsed plates. Use a sink strainer to catch the rest.
If your sink is already slow, see why does my sink keep blocking?, how to clean an outside drain gully or blocked sinks.