I had this commercially produced "first flush" diverter that I got in a pinch because I was so busy with everything else. It has failed, and now I'm back to figuring out what to do. This thing was a plastic ball suspended by a stainless spring inside a PVC Schedule 40 tee...as the rain ran over it, it would fill with water and eventually drop over the bottom outlet of the tee, plug it up, and then divert the cleaner water through another tee located above it into the cistern. My stand pipe and the ball exploded this past winter when it froze due to not draining the way it was supposed to. Back to the drawing board. My goal is to divert the first 15 gallons from each rain event to waste, and collect the rest. This should clean the roof of leaves, bird crap, atmospheric fallout, pollen, etc. My idea is to have a 3" PVC drop pipe from the gutter to a 15-gallon HDPE drum. The drum will be fitted with a faucet at the bottom that I can crack open to slowly drain the barrel between rains. At the top of the drum, there will be a tee. At the bottom of the tee, I'm going to construct a cage out of plastic, metal, or whatever I find that will work, and place a PVC blow-up ball from the dollar store, like maybe 8" - 12" in diameter. This should fill up the drum with the first flush of 15 gallons (along with bird crap, leaves, etc, etc,) float the ball up against the bottom outlet of the tee, and direct the rest of the rain event into the overflow to the cistern. Final cost: $8 for used coke syrup 15-gallon drum $6 for plastic faucet and bulkhead fitting from tractor supply $1 for ball about $30 for PVC pipe and fittings. this might be a complete failure; the old "ball displacing the water trick" is the only thing I remember from fluid mechanics...any ideas or does anyone have a set up for filtering or having a fixed-volume diversion for roof water harvesting that didn't cost $200?