It's actually a common misconception that it's safe to dispose of your old potions into the sewer system! Water treatment relies on a complex imp ecology which is easily disrupted by many common potion ingredients.
None of us (least of all, us here at the city government) want to deal with the Water Department's wrath. We all know how annoying it is to have our water shut off for maintenance. And, we all end up in the sewers now and again by accident. Let's keep our municipal sewer system a safe and effective resource for all by treating it right :)
Your old potions and unused ingredients should be disposed of by bringing them to Hazardous Waste. You can drop them off during our open collection days, which are currently 2AM to 4AM on the third Thursday after each full moon. Or, to schedule an appointment, just find a landline and dial 311 (note: due to staffing shortages, the call center menu tree and all City service representatives currently speak only Old High German).