There are a few different reasons that your kitchen sink could be giving off a bad smell but lucky for us it is a pretty easy thing to cure.
The first thing you have to determine is where the smell is coming from. Is it actually coming from the drain in your sink or is it coming from under the sink. Place a stopper in the sink and fill it with a few inches of water. After the area has had time to air out a bit do you still smell the odor? If you do then the smell may be coming from under your sink.
Remove all the items from under your sink and inspect them for problems. Old sponges can grow bacteria and if a mouse happened to die under your sink that can cause a pretty bad odor. This actually happened to me when mice got into my vegetable oil bottle by chewing off the plastic top. Why they did that and why two of them did that within a week I don’t know but I did decide to move the oil to a high cabinet after that.
If you find that none of the items under your sink are causing the odor you should inspect your pipes for good connections. Sewer Gas smells very bad and if any of your waste lines have bad connections or are corroded you may have a problem that does not cause a large leak but lets gas escape.
Now if you still smell the odor and your sink is still full of water there are two more things that you can check before we go back to the drain.
The first would be your garbage disposal. Disposals have vent pipes that allow the water to drain easier when a large amount of water and product are being processed from your dishwasher. Normally these vents are attached to the top of your sink and if it is malfunctioning it could be allowing odors from built up material or sewer gas to exit out the vent or its lines.
The second is your dish washer. The water from your dish washer is processed through your garbage disposal and sink waste lines if there is a problem with any of the waste lines or even if dirty water is sitting in the dishwasher bacteria could be giving off a bad smell.
If any of these items need repair you can tighten lines or clean as necessary. Damaged parts and gaskets will need to be replaced.
Everything is ok the smell is coming from the Drain
Now that you have inspected your waste lines and the area under your sink we can go back to the sink drain.
If the odor is coming from your sink drain then you have three possibilities.
Your Garbage Disposal has buildup that needs to be cleaned.
Your waste line has bacteria buildup and will need cleaning. This is apparent when your sink drains slowly.
Your waste lines are installed incorrectly and the PTrap under the sink is not holding water to prevent sewer gas from exiting the sink into your kitchen.
All three of these things are pretty easy to cure the first thing you want to do is inspect your p-trap under your sink and make sure it is holding water. Place a bucket under the p-trap and remove the top union nut that holds the p-trap to the sink waste line down spout.
A small amount of water about a cup should come out of the p-trap. This water is what stops sewer gas from coming back into your kitchen. If you have water in the p-trap this is not the end of a sewer gas problem. If you ever hear bubbling or see water being forced up out of the kitchen drain then you have a waste line vent problem. Water from a higher place in your home or apartment building is being forced into the kitchen sink because it has no where else to go. In this case you really need a plumber to inspect how your plumbing is laid out and they may install a one way check valve to stop the backflow. This is common when plumbing is in the basement below or near the level of the main waste line that exits the building.
Waste Line Buildup
Now that you have eliminated the P-Trap as the problem you need to see if the waste lines have a large amount of bacteria buildup. With the P-Trap detached shine a light into the pipes and look for buildup. If there is a large amount of buildup it is likely a mixture of bacteria and food materials.
You can mechanically remove the buildup with a variety of tools including forcing a rag through the straight down pipe with a screw driver. This is very dirty work. If you have corrosion in chrome plated steel pipe you may want to just replace the lines for about $10 however expect this problem to eventually return maybe in weeks, months or years but it will return.
Food and Bacteria in your Garbage Disposal
Food and bacteria buildup in your garbage disposal is normally due to not using enough water while running the disposal but just like the rest of your waste lines over many years this material will build up no matter how well you follow proper use of your disposal.
Cleaning Bacteria and Waste Material
Your garbage disposal should have come with a users manual and it may explain that by dumping a large quanity of ice and water into the disposal you can clean the area fairly well.
In addition cleaning the bacteria from your waste lines and disposal should include the use of bleach. Standard household bleach that you use for washing clothes that does not have additives can be poured down your drain to kill bacteria and loosen up food matter.
The problem when using bleach is that pouring it down your sink is basically sending it right out into your sewer. The best way to add it to your garbage disposal is to pour one cup into the disposal and then allow it to sit then force ice through the disposal with a sink full of water. You want to step away from the sink when you activate the disposal and using safety glasses is not a bad idea.
A small amount of bleach can also be poured into your p-trap through your main sink drain but remember if you pour too much into the drain it will simply go into the sewer and be wasted.
Bleach should only sit in your drain for about 15-20 minutes no longer or it can damage your pipes over time. You would never want to let it sit overnight the same way you would not let drain cleaner sit in your pipes overnight or it will corrode the pipes and cause leaks.
I hope this howto helps you get the odor out of your sink and remember that proper use of your dishwasher, disposal and checking your p-trap if the water seems to be draining slower then normal is the best way to keep a healthy kitchen sink.
If you find that you need extended repairs especially if water is backing into your sink when it is used in other places in your home you should have a professional take a look at the design of your system. Make sure you get a quote on the price for an inspection only when you call and then you can choose to do the work your self or have them do it.