Connecting Schottky to Postive Supply
This circuit is not good.

The very important thing to understand is that it is using an opto-isolator to give complete electrical isolation between the mains on the left and the logic on the right. There must be no connection between the two.

So what's wrong?




The return path is connected to earth. This should never be done. It should be connected to the 230 V AC neutral.
SD2 has no termination. This is a warning that something is odd about the circuit. It might be that SD1 and SD2 are both in the one 3-pin package and that we don't need SD2. It's not clear from the schematic.
The 220k resistor needs to be rated for 230 V AC which is \$ 230\sqrt{2} = 325\; V \; peak \$. A 400 V resistor would be considered wise. If you can't find one then use two series-connected 120k, 1 W resistors.
The 10R resistor is doing nothing.



  Does connecting the Schottky diode SD2 to 24V supply is advisable? 


No. There is no 24 V supply in your schematic anyway.


  If so, is there any advantage over connecting to 3.3V supply?


Never do that.


  The Zener diode ZD is added as an extra protection. Is this required just in case of a worst scenario?


The Zener diode limits the voltage on the 100R + 4N35 opto-isolator to 5.1 V. It is required.



It seems to me that you are not yet qualified to do this project.