So my kid is taking a Chemical Engineering course called Unit Operations and the Professor asked...

[FBaggins]

4. Wrong answer

The company you used to sell it to has gone out of business... but that likely means that one of their competitors now has more business from picking up customers from the one that went under. Sell to them.

Alternatively, the stuff sells to the public at $25/L. Open up a subsidiary and sell it direct for 25% less than the competition.

IOW - It's only "waste" when it has no market value. Losing a customer doesn't mean that a marketable chemical becomes "waste".

You of all people should recognize this from debates re: the disposal of nuclear materials that other reactor designs could use as fuel.

I explained to my son that this is a very real problem. Most of the world's waste hydrochloric acid waste is deep welled.

Not exactly. It's injected intentionally as part of the oil/gas production process (clearing out base materials clogging wells)... but IIRC, the largest portion of the market is PVC manufacturing.