Who hasn’t wondered while flushing the toilet: Where does it all go? Find out this Saturday, Sept. 15, when ALCOSAN hosts its annual open house. The ALCOSAN facility, located on the North Side by the McKees Rocks Bridge, takes in the county’s waste water — from street run-off after rains to the toilet you flushed this morning. If it ends up in the sewer system, it goes to ALCOSAN. Yup, that includes the dog’s tennis ball that rolled into the sewer grate.

The family-oriented event is billed as “the region’s largest watershed and environmental festival,” and includes the new “Wet Weather Village” educational set-up; presentations on microbiology and watershed life; hands-on activities; and games and food. Also, ALCOSAN’s fun-loving mascot, Frankie the Fish, will be on hand.

There are also guided tours of the treatment plant, where visitors can learn what happens to Spot’s tennis ball — and all the poop and gross stuff — once it gets to ALCOSAN. For starters, it all goes into the biggest “barrel” you’ve ever seen, before a series of mechanical and bio-chemical processes turn it all back into clean water. We take it for granted, but it's cool to actually see how it works.

The event is 9 a.m.-4 p.m., and free. For more info and driving directions, see ALCOSAN's website.