drain vent smells

[dinghy]

OK, the drain pipe under my sink has its own air vent- i assume this is because it could not be easily connected to the one on the roof?- and it smells , i believe, of sewer gas from my septic. It is only ever under one sink, but i believe that sink is the straitest line to the septic tank. Whole place was only built 6 months ago but never had a problem until recently and it will only last a day or two. There must be a trick to fix this, but all i can find is filters for a roof vent pipe. Though that might help the smell i think i noticed once outside. I do have a garbage disposal that releases bio-something-or-other that breaks down everything...would running that more often help the problem under the bathroom sink half way across the house?

[LazyPup]

Hello Dinghy and welcome to the homerepairforum...

First of all let me say that I am in complete agreement with you concerning the presence of sewer gas in the structure. Not only is the smell of sewer gas offensive it also presents a number of health and safety concerns and every effort should be made to locate and correct the source of your problem ASAP.

Due to the nature of the primary gasses that make up sewer gas locating the source of sewer gas in the home can be rather difficult. Sewer gas primarily made up of Hydrogen sulfide, Methane, Ammonia and Carbon Dioxide which is developed by the decompostion of organic matter. It is primarily generated in the septic tank but it can also be generated in lessor amounts by the decomposition of any organic materiial that may be adhering to the inner walls of Drain, Waste & Vent pipes. You stated that this is new construction within the last 6 months so it is unlikely that there is any appreciable buildups on the pipe walls.

In your post you stated that you only notice the smell occassionally. This may be due in part to the physical weight of the gasses. When compared to the physical weight of atmospheric air Hydrogen Sulfide is slightly heavier (!:19 to air), Carbon Dioxide is a bit heavier (1:53 to air) while Ammonia is lighter than air (0.59 to air) and Methane is the lightest (0.55 to air). All plumbing systems are vented to atmospheric air, therefore the sewer gasses are mixed with ordinary air in varying concentrations. There is also no specific ratio for the production of the gasses therefore the movement of vent air within the system is constantly varying. If the greatest concentration of sewer gas is one of the lighter than air gasses the sewer gas rises and vents to atmosphere however if the greater concentration is one of the heavier than air gasses the air and gasses in the sewer settle towards the septic tank. The motion of these gasses is also influenced by the motion of water flowing through the pipes so the concentration of any one specific gas at a single point will constantly vary by the motion of air, sewer gasses and water within the system and to some degree the motion is influenced by changes in the normal atmospheric pressure, thus you may smell the gas one day then not smell it for a day or two. In addition, hydrogen sulfide has a very unique property. While the rotten egg smell of hydrogen sulfide is very offensive it is also quickly ignored by the human olfactory nerves. This means that you may initially smell it very strongly when you enter a room but within a few minutes your nose will ignore it, thus it may still be present and go totally unnoticed.

At higher concentrations sewer gasses can cause asphyxiation or can be explosive. While it is extremely rare that sewer gasses could accumulate to dangerous levels in the home, none the less, the potential is there thus the plumbing codes require that all openings to the Drain, Waste and Vent system must must be properly vented or have a trap with a minimal water level of 2".

The codes do take some rather differing views on how vents must be terminated.

Under the Uniform Plumbing Code all vents must be terminated through the roof to atmosphere. (The UPC will permit a maximum of one air admittance valve per structure with the expressed consent of the local inspection authority.)

Under the International Residential Code every structure must have one "main Vent" which must run undiminished in size from the building main drain through the roof. Once the main vent has been established additional auxillary vents may be terminated through the roof, in the attic space, though the wall under specific conditions or you may use an unlimited number of Air Admittance Valves.

You stated that you have a vent under your sink. Being new construction I would assume that your DWV system was inspected therefore if you do have a vent under the sink it should also have an Air Admittance Valve(AAV) attached to the vent opening. An AAV appears like a small plastic canister with a little opening in the side that is screwed into a thread adapter on a vertical riser. (See the attached illustration). Inside an AAV there is a spring loaded diaphram that will let air into the pipe but will automatically close to prevent air or sewer gasses from escaping. Normally we can expect an AAV to have a service life of 5 to 8 yrs but as is the case with all mechanical devices they can fail at any time. If you determine that you have an AAV and suspect that it may be malfunctioning the solution is to replace it. Fortunately they are relatively inexpensive (about $5) and can be easily installed by just screwing into the thread mount at the base.

If you have a digital camera you might take a picture of the pipe in question and post it so we can help determine exactly what it is.

NOTE: From the profile you are new to the forum and I understand that the program will not let you post a picture until you have made a minimum of 5 posts. If that proves to be the case you may email the photo to me directly and I will be glad to post it for you.

LazyPup@yahoo.com

[dinghy]

There is a "Studor mini-vent" on top of the drain and it's labeled sewer vent. These are actually under all sinks in the house, but there are vents through the roof as well. So, i've ordered some biological somethings from septic remedy that promise to eliminate all the gasses. Does this stuff work or should i try replacing the air vent?