This is just for people who can't get enough of activated sludge. Since I'm not an expert on it I thought I'd direct you to couple a places to find more info.
Where can I find activated sludge? You can always find it at the wastewater treatment plant. Take a guided tour of one. Go to either the Albany, Guilderland, or Glens Falls wasterwater treatment plants.
Are there more models for activated sludge? You bet. Take a peek at this SimWorks program. I haven't had a chance to download it yet. Or check out this book review. Derin Orhon and Nazik Artan wrote a 600 page book all about activated sludge. Unfortunately I don't have the $95 to get it.