Can aqueous acetonitrile or isopropanol in HPLC waste (at any concentration) legally be poured down the drain for disposal? Also, I've read a small bit on distillation techniques for recycling acetonitrile from HPLC waste. Are these techniques that pay for themselves only if producing large amounts of waste?

DO NOT POUR DOWN DRAIN!

Collect the waste and find a waste disposal service.

Don't pour down the acetonitrile, whats wrong with you? Your like a crazy person!

Dear David,
I can only agree with the other two, don't you ever pour down the acetonitrile.
Acetonitrile is poisonous and endangers not only humans but everything live in the water. Please check the safety data sheets...
As for the recycling techniques: I don't think that you can gain back HPLC quality, so for small amounts it really would be loss of time...And there again I agree: find a waste disposal service.

Don't forget, other than ethical issues, by pouring ACN down a drain, you will be breaking the law regarding discharge. When (not if) caught, the penalties can be quite steep.

Let me preface this by stating all of our HPLC waste is sent to a state approved waste disposal facility as should be the case everywhere. That said, most municipal waste treatment plants can handle low concentrations of acetonitrile and acetonitrile can be biodegraded. Don't confuse your emotions with facts.

I wish to improve peak separation between two closely eluting peaks by using recycling chromatography approach. I am using C-18 column (reversed-phase column). How do I do this without having to resort to a second column which is the traditional approach for recycling, but instead using the same column for recycling. Is this workable as an approach?

It is possible, if your pump is equipped to do so and if you have a low volume in your pump compared to the peak volume.
However, you will be better of doing the recycling with two columns, and switching valves. The technique is called the "walking column". It appears that you are familar with it.

Re: dumping acetonitrile,

the picture is a little complicated:
it's toxic, volatile, and not quickly biodegradable.
That means it isn't a great candidate for removal
by typical municipal wastewater treatment, but
perhaps small amounts might be broken down. The
bottom line is that the authories on this are
at the wastewater treatment plant and regulatory
agencies, not necessarily on the chrom forum.
A lot of times, you can dump really odd things.
For example, I can legally dump (with prior
notification) radionuclides like 14C, but I can't
legally dump ethanol. I could not have predicted
that, even with an sanitation background.
It will vary from place to place, so what
I can do is probably very different from what
you can do.