Michael Newby wrote: I know it's not really the bubbles adding that much dissolved O2 but the overturning/agitation of the water exposing more surface area.
Actually...bubbles DO increase the dissolved oxygen in the water. And the smaller the bubbles are, the more surface area they have. It's the amount of air/water interface that you have that is important, because oxygen will move from where it is plentiful to where it isn't. One diffuser stone that puts out lots of tiny bubbles can easily have the effective area of a lot of riffles.
Now if you make your riffle area porous, like pumping and dripping the water over pumice stones, then that will also increase the surface area of the water exposed to air. I make my compost tea in 5 gallon buckets and use the smallest aquarium pump on the market with a 1" diffuser stone. It puts out enough air to make the bucket go from anaerobic (stinky) to aerobic in just an hour or so. Let your nose be your guide. If your process is foul smelling, then it probably isn't getting enough air through it, and you need to think of some trick to increase the amount of air -- bigger pump, more diffusers for smaller bubbles, spongy media with lots of surface area, etc.
John Elliott wrote:... think of some trick to increase the amount of air -- bigger pump, more diffusers for smaller bubbles, spongy media with lots of surface area, etc.
That's really what I was thinking of with the riffles - another trick to increase the surface area of the water exposed. From doing my diligent internet research, it would seem that for large brewers (rain barrels and bigger) the consensus for aeration is pumping the water somehow, not just relying on the bubbles to turn the water over. I personally am leaning towards an airlift pump with a vortex, but I like playing with different ideas just because.
I guess the thing I'm really wondering is can you reach maximum O2 in the water stream with just an airlift and a vortex, or would adding another step be warranted to squeeze a little more in. I really wish I could afford one of those O2 meters but it was hard enough buying the heavy duty air pump (Ecoplus comm. 7, it's a beast).
I'm thinking should be able to brew 250-300 gallons at once in my water trailer with the air pump I got and using all the other tricks for aerating the brew. I couldn't get all the parts I wanted at the little local hardware store to make a proper airlift, so for right now I just made a quick and dirty brewer with a 55 gal plastic drum with the pump blowing through a 16 port manifold held down with a rock - man that baby was going! Maybe a little too vigorous but it looked an smelled sweet after about 30 hrs. I'll try to get pictures of the next batch going.