Two methods to clear caustic soda from a drain (in a environmentally sound way) are:
(1) Hot water
(2) Strong vinegar (nasty chip vinegar). The safe but strong acidic properties of vinegar will partly neutralize the strong alkaline properties of caustic soda, producing a soapy solution.
PLEASE DON'T ASSUME THE RESULTING SOLUTION FROM THE MIX OF CAUSTIC SODA AND VINEGAR IS SAFE! IT MIGHT STILL BE A STRONG ALKALINE!
I hope you find this post useful.
John
