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I'm using urea-containing growth medium for experiments with bacteria (1-2 l/day). After the experiment, the growth medium has to be sterilized and disposed. I did this so far by autoclaving, but our technician complained about the ammonia smell that develops (urea breaks down when heated, releasing ammonia).

I'm pretty sure the amount released into the air is not dangerous (I've emptied the autoclave several times), especially if you don't purposefully breathe in the first gust of air that comes out after opening, but I think it's useful to look into alternatives. Also I'm not sure if the autoclave might get damaged on the long run.

Does anyone have experience with this? Do you see a way to sterilize larger amounts of growth medium reliably without heating?

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  • $\begingroup$ You can use chemical methods to sterilize the media - I have used cleaners which develop chlorine dioxide for this purpose. This is smelly as well as you will always smell some of the chlorine dioxide when emptying the vessels. $\endgroup$ – Chris Mar 5 '15 at 11:39
  • $\begingroup$ Is the bacteria potentially pathogenic? What bacteria is it basically? $\endgroup$ – WYSIWYG Mar 5 '15 at 12:25
  • $\begingroup$ It's Sporosarcina pasteurii, not pathogenic and not a GMO. I just wrote an email to our safety officer to inquire if sterilizing is absolutely necessary, but I guess I will have to. Also, I might use GMOs in future, and then it's a must. $\endgroup$ – Dominik Mar 5 '15 at 12:29
  • $\begingroup$ Bacilli can form spores; I am not sure if this one can. Sometimes I use concentrated acid (H₂SO₄) to disinfect stuff, especially flasks that are long unwashed (with media still in it; they contain a variety of organisms). You can neutralize the acid after the treatment. It is a little dangerous though. Common disinfectants should work for you. $\endgroup$ – WYSIWYG Mar 5 '15 at 13:08
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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.

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You should check with your institutions guidelines as different facilities have different regulations but generally it is acceptable to:

In a fume hood and in a ventilated (unsealed) container:

1.add liquid bleach to a final concentration of 20% (add 1 part bleach to 3 parts culture)

2.Let sit overnight

3.Dispose down drain with copious amounts of water

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