I would like to install a water purification unit under my double kitchen sinks. I currently have a dishwasher and a garbage disposal connected. What is the proper code when doing this. More specifically, may I connect the dishwasher drain to the garbage disposal instead of the overflow port on top of the sink. I want to use the overflow port hole for the purifying units facet. I live in Escondido, California. My sink is cast iron and porcelain. If I can not hook up the dishwasher directly to the garbage disposal I would like to drill a new hole for its facet. What type of drill bit do you suggest I use?
You can hook up the dishwasher to the garbage disposal - that is a normal installation. There should be a location on the disposal - typically a plastic part that gets knocked out to connect the dishwasher hose. You will probably need an adapter for the disposal - this often comes with the disposal but is probably long gone. If you post the model of the disposal, we can provide more info.
"Drilling" sounds like you may be referring to a traditional air gap in the top of the sink. Dishwashers can be connected that way, but they can also be set up with a high hose loop. Basically the drain hose from the dishwasher loops up to just under the counter, effectively forming an air gap (the only way sewage could get backwards into the dishwasher would be if the sink filled up to the same height as the top of the hose loop). The other end can go into the disposal. If you do this then the old air gap can be removed and the hole used for water purification or soap dispenser or whatever you want. See one of my other dishwasher answers for a diagram.
Code in most places says that the dishwasher must be connected to an air gap. Some allow for a loop in the hose and then direct to the drain or disposal.
The drain goes to the air gap FIRST and then to the disposal. It sound like you have that set up in place. Air gap holes are typically 1 1/4". Check your instructions of your water system to get the size opening required for it.
You would use a 1 1/4-inch diameter hole saw to cut a new hole. The material that you are drilling through would dictate what kind of saw you would use. A regular hole saw would work for a wood or laminate counter top, but you would need a diamond encrusted hole saw with water injection ( or just keep feeding it water with a squirt bottle to keep it cool ) for granite or similar type counters. I have never drilled through cast iron so i can not comment on that.