The water you use doesn't go away. Whatever goes down the drain ends up at the treatment plant. While the plant effectively removes settled and organic materials, it is not designed to remove certain chemicals and metals. When toxic chemicals are dumped or rinsed down household drains, they pass through the system largely untreated and end up in the creek where they may threaten sensitive aquatic life.
In addition, anything that goes down a storm drain goes directly to creeks and streams, eventually passing untreated into the Illinois River.