Creating Floc in the classroom

I'm the youth ed manager at Denver Water in Colorado. We do a water treatment demo in the classroom, and currently we use the first part of the Winkler DO test to create floc. I'm looking for a better demo that creates white floc and doesn't turn the water brown. I've tried using alum and mixing it with alkaline iodide azide reagent. It creates wonderful, white floc. What I don't know is if it's safe in a classroom setting. Can you help?


