I just installed a new garbage disposer for my parents a few days ago (when the old one broke years ago, they never replaced it, just removed it and redid the sink drain). The disposer works fine, and I ran everything according to the instructions, but there's a problem somewhere in the line that I can't find. As soon as I open the clean-out valve in the basement, the water will drain OK for a while, then it starts backing up again. I've snaked the line and used Draino with no results (no blockages found, either). I haven't climbed up on the roof yet to check for a blockage in the vent, but that's the next step, I guess. If there's no blockage, would an inline vent under the sink help? Also, I only have 11 inches to work with between the disposer and the drop line (I think that's the term) to the trap so I went with the straight line option rather than a second P-trap. Could adding a second trap possibly help? I've been searching for answers the past few days, and I'm getting desperate. I really don't want to do the dishes in the bathtub! The house is a 40+-year-old modular on a septic system and city water. Any help would be appreciated!

Where does it back up ftom, the cleanout or the disposal? Does everything else drain OK?

Within a minute or so of running water directly into the sink or into the side with the disposal, water backs up and equalizes into both sinks. It will VERY slowly drain if allowed an hour or two, or if I plunge until it releases. When I open the cleanout (estimating it's about 8 feet or so from the kitchen sinks), I get some water, but not that much. I haven't opened it yet when the sinks are full of water, so I might get a lot more if I did. Drainage was fine before installing the disposer, and my stepdad said they had similar problems with the old one. The vertical line runs about 18 inches to the pipe that leads up to the roof vent and to the basement. In the basement, there's a fairly sharp elbow then a slight slant running to the main sewage line. Leading out of the basement, the main line still appears to be descending some toward the septic tank, but I have no idea what it does outside or how many degrees it actually drops. My stepdad's theory is that there may not be enough drop in the line to handle a disposer or that the 1 1/2 inch pipes can't do it. I have to retract an earlier statement. I used a drain auger, not a snake, as far as possible and didn't hit any blockage, so I'll try snaking the line tomorrow afternoon. There does seem to be a lot of crud on the inside of the pipes, kind of like clogged arteries. Is there anything on the market to take care of that and possibly help the problem (assuming we can get this working in the first place)? With a 16-month-old who's into everything and wants to "help" Mommy, I'm trying to work on this late at night when he's sleeping. Thanks in advance. In hours of researching this problem online, I've hit on this website most often and have read many of your answers.

Clorox is amazing at breaking up clogs, but will hurt a copper drain pipe.

Clorox will also kill the bacteria in a septic system, so that idea is out. Thanks, though.

Here is something that won't kill the good bugs in your septic and its fun to watch. Mix a half cup of baking soda with a half cup of salt. Dump down the drain and then add a cup of vinegar, don't put your face over the drain. Tomorrow throw a bucket of really hot water down the drain.

Thanks for the advice, I remember the "volcanoes" from way back. Does anyone have some advice for the original questions concerning repeated backups & how to stop the problem from continuing? I finally found a clog just at the junction of the 1 1/2 inch drain line in the basement where it enters the main sewage line leading to the septic tank. There does seem to be good fall from the kitchen to the sewage line, better, actually, than from the bathrooms to the main line, but the big difference is 1 1/2 inch pipe vs. 4 inch. Any suggestions short of running all new and larger pipe from the kitchen to getting this to work? (Please see my original two posts in this thread). Thanks.

An inline vent will not change anything. A second trap is illegal and causes it to airlock. The pipe is sized correctly. Sizing small allows the water to scour the pipe therefore keeping it clean. Your problem is a clog, you would be best served to remove the clog. To use a disposer correctly you are to run water until the waste mass is removed through the pipes into the street. Folks don't do this so it clings to the pipe and gradually clogs it.