I live on a private well and have had a tank for a while. Every few months I'd have to chlorinate the tank (with hydrogen peroxide) to get rid of the rotten egg smell.

Two local plumbing companies advised that the smell was caused by sulfate-reducing bacteria, confirmed by numerous online sources.

I could get rid of it by draining a gallon out of the tank, opening the temp & pressure relief valve, pouring hydrogen peroxide into the tank, and letting this solution flush through the pipes and sit for a few hours.

About a month ago I noticed the smell again and was planning to chlorinate the system. Unfortunately shortly after, the tank was leaking, so I replaced it and "upgraded" to a tankless system.

Now the problem is, after a month, the water still smells. Obviously there is no more anode rod or sitting water, so there must be bacteria still in my pipes. It's mainly coming from one faucet in the kitchen.

How can I flush that pipe, or kill the bacteria in some other way?

Odor in the cold water that goes away after water flows... the most likely source of the hydrogen sulfide is Sulfate-Reducing Bacteria 1

  • 1
    "As far as i know" does not sound like concrete evidence. Have your water tested, test a sample direct from the well and one from a fixture within the house. i would guess that it is the water itself that smells and it smells that way from the well before it enters your home. Sulfur in well water is common. Sulfur smells like rotten eggs. perhaps a water treatment system is what is needed.
    – Alaska Man
    Oct 29, 2017 at 9:48
  • Tankless water heaters are "supposed" to be installed with special isolation valves which allow disconnection of the heater both from the incoming cold supply and the outgoing hot. (The standard installation of a tank only has one cut-off in the incoming cold, but some might have two cut-off valves.) But in addition the widely used valves for tankless have a second set of valves (incorporated in the same valve bodies) which open up the heater for flushing and descaling. You could use the descaling settings to flush with hydrogen peroxide, but I question whether this is necessary. Oct 29, 2017 at 12:44
  • These are the type of valve recommended for a tankless WH: homedepot.com/p/… Do you have valves like this? Oct 29, 2017 at 13:04
  • If you have these special valves, you could use them to flush your hot water distribution lines with peroxide. You would do this by setting the cold water valve on the tankless heater in the flush configuration and leaving the hot water valve in the normal operating position. Then you would pump or draw the peroxide into the system there and it would go through the heater and into the hot water distribution system. You could pump it or use a shop vac to pull it through each faucet. You could use the vac to get a siphon started which could be used to fill the entire hot water distribution sys. Oct 29, 2017 at 14:04
  • @Alaskaman I did have my water tested. I do have a functioning water treatment system. "I would guess that it is the water itself that smells"- what was that you said about lack of concrete evidence? "Sulfur smells like rotten eggs"- no it doesn't. Hydrogen sulfide smells like rotten eggs. In my case, hydrogen sulfide released by sulfate-reducing bacteria, as confirmed by two local plumbers. Now how about an upvote for a good question?
    – ferret
    Oct 29, 2017 at 14:32

1 Answer 1


Well, the standard method of chlorinating well systems to kill bacteria in the pipes is to use chlorine (peroxide is an oxidizer, but it's not "chlorinating" in my vocabulary) in the well, and pump it throughout the system, wait several hours, and then flush it out.

Presumably you could use peroxide if you have a sufficient amount, but it's likely to be more expensive than using chlorine bleach (plain, unscented as I would hope would be obvious.)

Interestingly, "overkill" generally is less effective at killing things that a proper level of chorine. My particular well with 200 feet of 6" well full of water above the pump takes about 3 quarts of household bleach. Process is to open the well head, run a hose to the well head and turn it on, and add bleach with the hose running so it gets circulated. Once the chlorine is well-mixed in the well, run water from each tap until you smell the chlorine, then shut it off. Wait several hours, pump most of the chlorinated water out with the hose to somewhere that isn't your septic system or a body of water (run it onto the ground), then flush the pipes in the house once there's no more smell of chlorine from the hose.

I'll go look for references and edit.



This publication suggests a smaller dose and longer contact time:


  • The guy I use for well testing uses a gallon and cycles it like you suggested, on homes that are not occupied he lets it sit until the next day then while doing the flow test that pumps all the excess out of the system, at the end of the test he takes samples to the lab.+
    – Ed Beal
    Nov 29, 2017 at 14:17

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