How to safely sterilize urea-containing growth medium?
Autoclaving is the best way for sterilization. If your growth medium is large, how much chemical disinfectant do you think you would need to bring up to its working concentration?

Add some acid to your waste medium (so that it's only weakly acidic), not as disinfectant, but to suppress ammonia formation. Urea will still break down during autoclave, but ammonium ion (pKa~9) will generally stay in solution.