Never pour used antifreeze down the drain or in the street. Both ethylene glycol (which other antifreeze coolant manufacturers use) and propylene glycol (which we at Timberline BRands LLC used exclusively) are toxic – even propylene glycol which is less toxic than ehtylene glycol to the point that only massive amounts ingested will cause issues in humans. Pure ethylene glycol is toxic to aquatic and land based life forms. Fortunately, it will eventually break down into water and carbon dioxide.
Used antifreeze also picks up heavy metals like lead during use in the engine. Used antifreeze contains more than just ethylene / propylene glycols and water. Contaminants found in used antifreeze, such as lead and benzene, can cause serious health and environmental problems. Antifreeze manufacturers also add a variety of chemicals to antifreeze to inhibit rust and corrosion. These chemicals can endanger the environment. Large quantities of glycol can suffocate aquatic life and disrupt sewage treatment processes. (Much of this information taken from http://www.eetcorp.com/antifreeze/antifreeze-faq.htm#q12).
These should not be released to the environment. Properly dispose of used antifreeze at an appropriate collection center. The EPA has some good information on their website include antifreeze recycling and Consolidated Screening Checklist for Automotive Repair Facilities Guidebook.