can I replace this Triac with Relay?

www.way2project.in/projects/2011/04/night-alert-circuit/

-Can I remove this Triac and replace mechanical Relay with one general purposes transistor as a relay driver?
-Can I inject output of D1 into the base of Relay Driver transistor to drive 12V relay? I think I should have to change output resistor R11 with 10K (approx) to bias relay driver transistor. Am I done?

R11 will probably need to be > 120Ω but its value (and the transistor type) will depend on what current the relay coil draws.
D1 won't be needed in series with the transistor base but it could be used (a faster diode would be better) back-biased across the relay coil to suppress voltage spikes when the relay switches off.
Take all necessary precautions if you will be switching mains voltage with the relay, and use one with mains-rated contacts if so.