Solar Water Heater Exchanger Removal, Glycol Removal
I have been an avid recycler in the Portland, Oregon, area for decades.  Portland is no slouch when it comes to environmental awareness and concerns.

After replacing my auto's antifreeze mixture, I had nearly two gallons (eight litres) that I didn't know what to do with.  So I stored it in the original anti-freeze gallon jugs and forgot about them.  After like five years, I had an accumulation of over eight gallons of ethylene glycol solution needing disposal.

I finally called the regional recycling authority about what to do with it.  Their answer was, for small quantities (less than 55 gallons), the city sewer system would appropriately deal with it.  Just flush it down the drain.

Of the various glycols, ethylene is not the first choice for solar heating systems due to its relatively fast breakdown rate.

More likely, your system uses either monopropylene glycol (1,2 propanediol, more commonly known as propylene glycol) or trimethylene glycol (1,3 propanediol) a bio-derived, thermally stable glycol. Unlike typical 1,3 propanediol, which is derived from oil or natural gas, the bio-derived “green” glycol is produced from corn sugar.  [reference]  Both of those are non-toxic and can also be flushed down the drain.