This morning I received a phone call from Joe Devinny, a professor of environmental engineering at the University of Southern California. I quoted Devinny as an expert source in my story on sewer gases at a downtown condominium complex.

I had asked him to explain the difference, if any, between the city’s fix to excessive sewer gases backing up into the condominium’s pipes and what the condominium’s homeowners association is requesting.

In technical terms, the city installed a “flapper value” on its main sewer line and the HOA wanted a “trap” or more precisely a “p-trap.”

Yesterday, Devinny told me there wasn’t much difference. This morning, he said he had thought about his comments some more and wanted to clarify. I invited him to write a couple paragraphs for this blog:

While a p-trap and a flapper valve have the same purpose, they operate on different principles. A p-trap is just a low spot in the line, which remains filled with water, preventing the flow of gases back up the pipe (there is one under your kitchen sink). A flapper valve is a hinged plate that is pushed open when water is passing, and which is closed by gravity or a spring when water is not passing.

In a large residential development, there may be flow in the drain much of the time. This could hold the flapper valve open for a significant amount of time, allowing gases to pass back up the pipe (which is not full of water). A p-trap would not have this problem, as it blocks gases from passing at all times. Now that the flapper valve has been installed, we will soon learn whether it is effective.

Since we’re on the subject, I’ll also post this e-mail I received later last night with comments from Michael Stenstrom, a professor in UCLA’s civil and environmental engineering department. I had asked UCLA’s department for assistance as well.

A “P trap” is a simple device that always provides a water seal. If you look under your sinks at home you can see the trap. It has a section that looks like a “U.” The U retains water so that sewer gases cannot come up into the room. If you have ever been away from your home or checked into a guest house that has been unoccupied for some time, you might be able to detect an odor in the bath room or around sinks. This occurs because all the water evaporated out of the trap. This is the same trap that fills with debris and creates a blockage. When you use a liquid plumber, which is usually NaOH (lye), it dissolved some of the organic debris, like hair, unclogging the P trap.

Toilets also use this same principle. If you look at low-flush toilets from the side, you can usually see the outline of the internal pipe, which is also in the shape of a P trap.

A flapper valve is anyone of a number of one-way or “check” valves. If the flow moves in the opposite direction, the flap or gate or even a sphere moves backwards and stops the flow.

P traps are used in every house hold for all sinks, showers and toilets have them build in. That’s code and it cuts down on odors. Flapper valves are used in more frequent in larger piping systems, and require maintenance. P traps have no moving parts.


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