It's been about a decade since I had a real, good orchid collection, and I'm just getting back to it. But I thought I'd share how I used humidity trays. I layered them with lave pebbles, and allowed the water level to be just lower than the top of the pebble layer. Whenever I added water I also put in just a couple of drops of bleach - not enough to smell, but enough to pretty well kill off any algae. I used old toaster oven racks over the top of the trays to set the plants on so that they wouldn't contact the water. It worked pretty well for me.

Someone mentioned that humidity trays may actually not do anything unless in an enclosed space. That may be so, but my experience was that they created a micro-climate around the plants which raised the humidity enough to keep them happy. YMMV - and remember, after so long away I'm sure that my methods are about as vibrant as dinosaurs.

I have a drip tray under mine that collects the water/fertilizer after watering. It seems to evaporate pretty quickly. For a while I tried gravel to increase humidity, but what a mess!