A badly done garbage disposal plumbing job leaves you with the risk of clogging your drainage system and causing all kinds of plumbing problems. If you have never installed a garbage disposal unit before, you would be advised to either call a professional or research thoroughly how to undertake the project.
Planning to install a waste disposal unit requires careful consideration. Be sure to understand how your current set up works. If you have a double sink set up in your kitchen you will need to remove the drain pipes from the sink you choose to adopt the garbage disposal unit. Double sinks are double drained and the feed pipe which goes from one sink to the waste pipe will have to disconnected and removed. Underneath the sink, be sure to turn off the stop valves for both hot and cold water. They should have isolator valves coming from each of the faucets.
Once the isolator valves have shut the water off, undo the small tail piece from the sink which will have the garbage disposal installed. That is the part directly under the drainage hole. Loosen the tail piece and then trace along to the next point of connection. This could either be a longer tail piece or a outlet T. In some cases it could be a Y-fitting, so if it is, undo the tail piece that connects the two sinks and remove it.
Attach the garbage disposal unit with any parts that were supplied with it. After that you will need to make sure the exit pipe is facing the other sink, so line it up correctly before you completely tighten it. If it is running directly across at a level, measure a straight piece of pipe. If it goes into the wall behind the other sink at a angle, turn the garbage disposal to face that in a straight line, then tighten it. You will need to cut a new tail piece to fit between the garbage disposal and the opposing sink. Make sure you measure it exactly and cut the piece long enough that you can screw on the other tail pieces to attach both ends. You will also need to be precise about the P-trap that runs from the drainage in the correct way. Remember that at this point you will need two adaptors if you are using a Y-fitting.
Both the garbage disposal and the sink need to drain into the main drainage system, which is usually a pipe out to the exterior wall. Secure all the fittings with plumbers putty or Teflon tape. Once you have secured them and they are tightened and lined up, you can proceed to turn the isolator valves back in to the ‘on’ position. The water will then start to flow back through the system and you can turn on the waste disposal and make sure everything runs ok.