Our kitchen sink has a disposal on the right side and a 'normal' drain on the left. It hasn't always been normal though. When it was installed, the drain pipe from the S trap to the wall ran 'uphill.' I probably shouldn't state the obvious, but that's a problem for a drain because gravity is supposed to pull the water away—down the drain.
The sink drained, but it was always very slow. I put a level on the pipe going to the wall and the bubble was higher near the wall (so it was sloping the wrong way).The problem, in a nutshell, was that the pipe extension directly under the sink was too long, so it pushed the trap down further than it should have.
PVC pipe can be connected using several methods. Joints can be cemented together with epoxy. Some joints are threaded together almost like two garden hoses. Others use something like a compression joint: turning the nut compresses a gasket or ring tightly around the pipe to prevent leaks.
The extension under my sink connected to the trap using the compression-type joint, so this was a simple fix. I simply removed the trap and the extension, sawed about 1 1/2 inches off the pipe and re-assembled the entire drain. This allowed the drain water to flow downhill. Now the sink drains more quickly and thoroughly.
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