Are you sure it's not the water that smells like sewage? It could be bacteria growth in the drain if you only smell this in the bathroom or if other places smell is the same it could be in the water heater or water plant itself especially if your area was affected by the latest storm.

Try to get some of that water in the cup, go inside the room away from the bathroom, smell it and see if it's not your water.

AAV

If there is an AAV (Air Admittance Valve) it could be that.

Before I got deeply involved in the mechanical parts of the drain, I would clean it. First I would remove any accumulated hair. I use a shorter version of this tool.

"UltraTech 36 in. Lighted Mechanical Pick-Up Tool"

It can be found by searching Home Depot on the string above.

I insert it with the jaws open, into the drain deep enough to insure it is in the hair. Then rotate if several times. Close the jaws and slowly pull it up, while rotating it until the hair comes clear of the drain. Usually this bring all the hair from the drain in a nice neat ball. You may need to do it a couple of time.

Once the hair is out of the drain I would clean the drain with some thing like Drano.

IF you still have the smell after running a lot of water down the drain I would look for vent problems in the system.

@GN Builders L.L.C The water has no foul smell or taste. That water is being used to brush teeth at night, and even after four or five minutes of run time the water tastes neutral. It definitely smells like a gas, and it only happens when the water is left on for a while.

@Joseph Corlett, LLC There is no AAV system, but could this be the drain waste vent associated with the sink in question simply stopped working?

@krnuttle Okay, I ordered that part

If there is no AAV and the vent stopped working, you have a clogged, or if you had renovation done it could be an improperly installed vent.

Have a plumber check your vent stacks, they regulate air pressure in your pipes and if the vent is clogged, etc it cannot remove gas and odors from your house.

Make sure you get the shorter one it is easier to use. You may find it in the tool department. Mine was actual suppose to be for get screw and bolt that drop down into the engine compartment. Mine is about 12" long

@GN Builders L.L.C You may have just helped me figure out another problem as well. All of the bathrooms and tubs/showers in this home have slow drains. I even had some new pipe installed and that did not really resolve the issue. Now I am reading that the vent system, when clogged, will cause drains to run slow. I went outside and it looks like these 50-year-old vents have no caps on them! In this condition, isn't it a given that some organic debris will have accumulated down those vents?

Assuming the vents can be snaked out, can I easily obtain third-party caps to help keep the vents protected going forward?

The best thing to do is to have a plumber take a look at everything and see if he can pinpoint the issue onsite.

Because even if the distance from the trap to the vent exceeds the max distance allowed per code there is a possibility that the trap seal gets vacuumed out from the trap and sewer gas will enter your home through that trap...

Good luck!

@GN Builders L.L.C I will definitely be calling the plumber, but it looks like the connection between the sink trap and the vent is happening inside the sidewall of the building, and it is not possible to visually inspect or clear that junction from the trap. Maybe that kind of design was allowed 50 years ago when the home was built? I gather that in modern homes the trap vent connection is visible in the back of the trap under the sink.

Aside from asking the plumber to inspect and clean out the vents from the roof, should I be asking him to open up the traps and inspect the connection to the vent from inside the opened trap? What remediation can be done, other than cleaning, if the connection is inside the sidewall of the building?

Should I be buying a cap for the vents on the roof?

You can tie some chickenwire over the tops of the vents, but I've never seen caps made for the job.

As @Joseph Corlett, LLC said, I also never seen caps made for vents or any need for them. See what the plumber says.

@GN Builders L.L.C Why would no one cap a vent when falling debris from trees could so easily go down the tube?

A vent cap could create more issues than you bargain for, especially in the wintertime. Leave it alone they were designed to work the natural way without any obstructions.

Good luck

We put a borescope down our drain vents, and about 11 feet down the 15-foot drain vent you run into a pile of organic debris. The debris exactly corresponds to the Redwood and Pine trees that rain down on every other part of my yard. So four feet of organic debris crammed into those vent pipes seems like enough to possibly be hampering air flow.

What is the right way to extract this material? A plumber's snake/auger is designed to push a clog through a pipe or sometimes pull it back out. My problem is different. I have 100 dry pine needles and redwood tree leaves, and I need to somehow pull them out, not push them through into the sewer, where they could provide other problems.

There are "grabit" tools like this one, but they are far too short and far too small on the tip.

https://www.amazon.com/General-Tools-70399-Lighted-Mechanical/dp/B002RHP7TS

I need a spring-activated grabber head that would widen to about 1.5 inches and grab and secure many tiny pieces of organic material as we pull it back out. What kind of tool can I use for that?

I had a issue with sewer gas smell suddenly. What had changed? I replaced the faucet and plunger in bathroom sink. So I googled. I found out that if the overflow in bathroom sink gets clogged in somehow can cause a suction that brings up sewer gas smell in the sink. So I tested to see if overflow was clogged by pouring water in just the over flow, it wasn't draining and got sewer gas smell.

I guess we used too much plumbers putty installing the new plunger and it got into the overflowoverflow .. My husband and I are amateuramateursamateurs, and had a heck of a time installing it. After I cleared the clog no more sewer gas smell.

We moved into this house about 10 years ago. The overflow on the sink was my first big challenge. When we inspected the house I noted that drain in that sink ran slow. I assumed it was hair in the drain. It was not.

I took the trap off and it was clean. I did everything I could think of but nothing worked.

One day I accidentally got the water level near the overflow hole and realized it was plugged. Ran a wire into the hole and worked the plug out. The drain has ran perfectly ever since.

I had the exactly the problem with a foul sm com from the bathroom sink, and this was after having a new bathroom suite fitted,tried loads of different fixes,airAir admitanceadmittance valve, new traps, bleach, drain cleaner, nothing helped until a plumber that iI know took a look and spotted the mistake straight away.

Turned out that the waste pipe from the sink that was running across under the floorboards joined the bathwaste via a tee then onward to the soil pipe where it dropped away ,because all the waste were the same diameter the water in the trap of the basin was being sucked out when the bath was being emptied because there wasnt enough room in the last plpe to the soil to allow for air flow,

The last pipe was being flooded with water creating a vacuum within the basin waste which would draw just enough water from the trap to break the 'seal' and allow the sewer smell to pass through it

The fix was to have the bath and sink waste joining into a larger diameter pipeso that there is air within the pipe on emptying the bath

This fixed the problem straight away noticed the difference

Hope this helps you

Tony

@Tony Lewis I could barely follow that, but the sense that I have is that the pipes from the sink were the same diameter as the sewer pipe that they emptied into. Is that the idea?

Let's get specific. A typical sewer pipe is a 4-inch diameter and a typical sink pipe might be 2 inches. What were your diameters?

I have an ongoing problem with my bathtub not emptying out fast enough. That is a 2-inch pipe going into a 4-inch sewer. I had always wondered if it would be worth trying a 4-inch bathtub drain pipe going into a 4-inch sewer pipe, but your post makes me think twice about that.

Specifics ok, waste system before the fix were 1,5 inch pipes from Bath and basin each one about 2 meters in length to a joining T ,Pipe drops down through bathroom floor to kitchen ceiling still 1.5 to 90°bend then across ceiling for about 3 meters into a 4 inch soil pipe.           After being fixed ,the 1.5 bath and basin no longer join via a T,they join into a 3 inch pipe independent of each other, now a 3 inch pipe runs along kitchen ceiling to the 4 inch soil.This now allows a sufficient amount of air within the pipe to let the water flow freely ,this stopped the water being sucked from the trap caused by a vacuum effect as the smaller pipe filled completely with water

@Tony Lewis That describes the "fixed" system. What were the dimensions of each part of that run before the fix?

Pipes from basin and bath and everything up to but not including siol pipe were 1.5 inches diameter