Here in Quebec, where the Min. of Ag. once had a mobile ethylene dioxide
fumigation trailer c/w hot caustic tanks, the approved method of disposing
of the used caustic is as follows:-

once the sodium hydroxide solution has cooled, add 5 gallons of
concentrated "acide chlohydrique" to 45 gallons of caustic.  This brings the
ph of 14 to ph 9.
Once neutralised the solution can be poured onto the soil at an acceptable
site.

I have not translated the term "acide chlohydrique" so as not to make an
error. Whether this acid is dangerous, l do not know. As to a suitable site
for disposing of the resulting liquid - we always presumed that it could be
the end of the garden or a corner of a field. Thousands of hives have, in
the past, been treated in hot caustic. The treated equipment has to be aired
for quite some time and the components are always less rigid.  Myself, I am
not sure if it's really worth the effort. Now some beekeepers have built
large wax dipping tanks to sterilise their equipment, very similar to the
New Zealand system and that "appears" to be working okay.
Peter

> The major problem is danger to YOURSELF and of course the finding of a
> suitable place to dispose of the used NaOH.  If you are NOT a chemist,
sodium
> hydroxide is dangerous to handle, and HOT "lye" is EXTREMELY DANGEROUS to
you.
> I would be strongly opposed to an amateur using NaOH today, and legal
disposal of
> the used NaOH would be almost impossible.