Ethidium Bromide Waste
Usage in gels:
Monash suggest for liquid waste a number of treatments of which one includes the use of Bio-101 tea bags. The bag will absorb out 5 mg in 1L of solution overnight. Presumably this means that if you have waste as 30L this is also a 5 mg maximum absorption, though I am not sure if a minimum soaking time is to be considered. So 1 week should be sufficient and in practice we rarely fill a 30L container in 1 week, closer to 1 month
The maximum amount of EtBr in waste. The group uses a proprietary solution of known EtBr concentration (625µg/mL) that is added to the gels and the gel buffer and at a fixed amount:
If we assume that the EtBr in the gel (which is disposed of separately) is electrophoresed completely out of the gel and into the buffer to a theoretical maximum never really achieved, then 75µg of EtBr would be
in 1L of solution which equates to 2.24mg in 30L, i.e. the tea bag could be safely used at 1 per container with a generous conservative margin.
For the small tank, 1 drop in the gel = 25 µg and 2 drops in the buffer = 50 µg, total is 75 µg in 400 mL (50 mL gel + 350 mL buffer)
This is 187.5µg / L = 0.19µg / mL or 5.6 mg in 30 L. Since this would be the amount for only small gel waste and is based on all the EtBr being electrophoresed from the gel into the buffer, it should still suffice to use one bag.
However, I noted from the following link from Princeton Uni http://web.princeton.edu/sites/ehs/chemwaste/etbr.html that:
Thus our solution at 75 – 187.5 µg/L = 0.075 –0.187.5 µg per mL should be quite safe to just pour down the sink