Muriatic acid is commonly used as a pool chemical. Any acid is dangerous to handle, toxic to the environment, and absolutely must be disposed of properly. Before you dump this substance down the drain, read on.
Research your options for disposal of this chemical. You will discover that pouring it down the drain is a bad idea. Do not try to neutralize the acid with baking soda or the like. The resulting chemical reaction releases heat and gases, both of which can become dangerous quickly.
Call your local recycling center. They are set up to handle hazardous household waste or can point you in the direction of someone who can. You may also call your household trash-removal service to see who handles hazardous waste in your town.
Carry the chemical to the hazardous-waste facility in the original container with the lid properly sealed for disposal.