The drum filter is now operational*. Observations:
1. The sprayer is very loud, to me at least; it kind of spoils the serenity of the pond plus I don't want to annoy the neighbors. I'm thinking about fabricating a lid or cover, maybe using foil-covered 2"-thick wall foam.
2. It cycles more often than expected, whatever that means. It's probably the consequence of having never run anything other than a settling tank, so there's all sorts of crap - figuratively and literally - to be filtered out. I assume over the coming weeks it'll calm down and the cycle time will slow.
3. It's surprisingly "moody"; sometimes it'll cycle within 5 minutes and the next time it might be 30 minutes. It might be due to the ring on the drum not being perfectly square (back to my rant about "good enough" plastic drum manufacturing). Because the ring isn't perfectly square, there's a small gap that travels around with the rotation. Depending where the gap ends up, above or below water, it speeds up or slows down how long it takes for the filter to load up. The lower the water flow the more that matters.
4. I never really thought through the consequences of running a drum filter. Right now, ever 10 minutes or so it produces about a gallon of icky water that has to go "somewhere." If the yard was finished it would run to plants but until the yard is sorted it's getting dumped into the storm drain, which in my case, runs out to the street. Not sure how much the neighbors will appreciate stinky green water flowing by their driveways - maybe the sound of the sprayer will distract them.
5. The previous system was nearly passive with a settling tank and moving-bed bio. About the worst that could happen was for the air pump to quit. Now, I wake up at night wondering if: the float switch has stuck, the chain broke, the solenoid valve failed, the timer module shorted out, a wire broke, or a million other things that can cause a small disaster. Because my setup is pump-fed for now, I probably need to add overflow ports so that if the filter doesn't successfully trigger the sprayer, the incoming water can bypass the drum so it doesn't overflow. The point is, if you're going to use a drum filter, fabricated or bought, know that you're buying into a system with a much higher chance of failure and the need to keep a close eye on the unit.
*The drum filter is currently being used with the old pond, where the bottom drain feeds the pump which pushes water to the drum filter. The new pond will gravity-feed the filter.