That drain is most likely 1.5", what does your Saniflush say about drain diameter? Some only require 3/4". If yours requires 1.5" or less you should be OK but it never hurts to ask your local building/plumbing inspector, you may need a permit and I know you must be by code

Hi guys...

The saniflush pump allows for a 3/4" to 1" discharge pipe to connect to an 1.5" Pipe, but you cannot cut into your sink drain before the PTRAP (would ptrap the pump waste) or even after the PTRAP as this could interfere with the venting of the sink.

Perhaps you could cut into the drain pipe prior to it going into the floor and run that in under the sink and connect onto that as shown in the installation manual for these (see instructions or website www.saniflo.com ), but as Ballengerb1 stated, probably a good idea to check with your local plumbing inspector on this as some towns/cities want this to come off a MAIN DRAIN LINE (with cleanout), so you really are best to check, OK?

Back to you...

Mark