Don't dump your glycerin down the drain, it is not a Toxic compound but sewer plant operators have night mares of the biodiesel home brewer dumping it down the drain. It does not kill off the bacteria (bugs), but like children given too much candy it will make them unruly and not do their job well. Anaerobic type bugs love glycerin, but like a good parent with a bag of candy, it needs to be rationed. If you have a waste water plant that has anaerobic digesters you may work out a deal with them where they can regulate the flow into their anaerobic digesters or blend it with septage from septic tank haulers.
If you plan to dump it into your own anaerobic septic tank (you are on your own if have an aerobic septic tank), you better take some lessons from a plant operator who can show you (under a microscope unless you work for Microsoft and can see them with out one

) what bugs are good guys and which are bad guys, plus how to be a good parent to the bugs you are feeding the glycerin to so you can regulate the amount you are feeding
If this all seams too complicated, one option is feeding it to mulch piles but check with your state and local people to see if there are any regulations you need to follow.
If you are only making it for your self your compost heap may be all you need to send the stuff back to nature, but check your state and local regulations first.
If your very industrious, you may find some one who wants the glycerin for making soap
Steve
