The cold water in my powder room stinks of mildew when we first turn it on. After it runs 20 seconds, it's fine. Nobody can figure it out. It is the powder room on the first floor. The main water line access is directly below in the basement. Outside water smells fine as well as other taps.
Background info that could be helpful: we have copper piping, the faucet was replaced last year by a friend (not a plumber), we had a leak in that main water line access closet and a licensed plumber fixed it (about a year ago). We had a bathroom reno this winter and the new shower is on the wall directly behind the faucet. I noticed the problem start in March. The odor is NOT coming from the drain. Your hands actually smell from the water!
This is driving me crazy as I have small children who wash their hands in there! Any help appreciated!

Hello suziqy, hard to say what's going on but I would start with replacing the supply lines to the faucet, if their plastic Ive found a few that get a lot of build up. If that doesn't work then I would consider chlorinating and flushing out the system, find a co. that specializes in water purification systems. They can chlorinate the system and flush it out for you safely.
Good luck-----Zeke

It may be interior of the faucet. Some manufacturers now make faucets that look like metal, but are plastic. Maybe the plastic guts inside the faucet smell.

Thanks, guys. You both bring up very good points. My suspicion has been that it is something directly related to the faucet. I think I'll buy a new faucet and supply lines and get a licensed plumber to install, then see if I need to go further. This one was a Moen and not a very good one, I think...

Sounds like you have bacteria in the cold water system. Moen is a top of the line faucet. Replacing it is overkill in my book. If this were my call I would shut vthe water off at the stops and disconnect the cold water supply from the angle stop and fill a glass of water from the angle stop. If it smelled I would follow Zekes advice and chlorinate the system by a professional.
If the water didn't smell then the problem's local and you could replace the cold water supply, open up the faucet, pull and replace the cartridge and fill the open valve body with bleach and let it set for a few hours or overnight. I would then flush out the valve, replace the new cartridge, reassemble the faucet and raiser and see if the odor's gone. If you simply replace the faucet and supplies and the infestation is further down the line then you'll be left with the same problem. Good luck, Tom

Speedball... two questions. If it was in the system, wouldn't the other cold water taps be affected? Also, doesn't copper piping prevent bacteria growth in pipes? Thanks!

99% of the time the problem is with the supply lines. Moen valves are made of brass and lines are copper this prevents mildew.

speedball...two questions. if it was in the system, wouldn't the other cold water taps be affected? Also, doesn't copper piping prevent bacteria growth in pipes? thanks!
By "system" I meant anywhere in the cold water lines. Sorry if I mislead you.
If the supplies were plastic then I would replace them. Kylar wascorrect about the supplies and mistaken about copper and brass preventing bacteria from growing. After I changed out the supplies I would leave the water off open up the valve, pull the cartridge and soak both the cartridge and valve body in bleach. Don't forget the aerator. Good luck, Tom