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I'm removing a flush toilet, with the plan to put a composting toilet in its place. The old toilet was short but elevated on a small platform. The new toilet is taller, so I'm removing the platform.

The drain pipe is black plastic. I assume I can't disassemble it at a joint, but must instead cut it with a hacksaw. Is that correct?

What's the proper way to cap the pipe? Maybe some day I'll want to put a flush toilet in again, so it needs to be suitable for doing that.

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You didn't mention if the drain pipe is accessible from under the floor. Assuming it does not pass directly through a concrete slab, the only safe way to terminate and seal the pipe from backwash and septic gas is to cut it off and glue the proper sized cap on with primer and PVC cement. If and when you need to reinstall the drain, simply cut off the pipe again below the cap and use a coupling to continue the pipe run back to a new closet flange. Do not use a screw cap. They are for temp use or pressure testing only. The hardware can rust, loosen and leak.

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There's a lot of information I didn't include! This is in a motorhome. The blackwater tank is immediately below the plywood subfloor, so the drain pipe is very short. If I cut it below the floor level, there may not be enough to glue the coupling to. There are no other sources of blackwater, so the tank will just sit empty. I tried to make the question general, and to make it more about conventional construction, and then map the answers back on to the motorhome. I found a properly sized cap which I will probably just let sit loose on the top of the pipe, under the bucket. –  Jay Bazuzi Jan 9 '11 at 1:40
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I'm not sure exactly what kind of pipe that is, but they make runner plugs that you put inside the pipe, and then tighten a wing nut to expand it to seal. After you cut it off, try that. If that doesn't work, maybe just put a regular cap over it with the correct cement for that kind of pipe.

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As far as I am aware, a test plug is not meant as a permanent way to plug a pipe. They are meant for temporary use only. –  James Van Huis Jan 7 '11 at 5:27
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