Thanks MikeS:
I thought maybe you guys went to Hawaii and just stayed there! :lol:
You know, this is starting to sound like a discussion. Kind of scary, huh?
You, Mojo and many others here know w-a-a-y-y more than I do about this, and I appreciate your help and response.
Advection > horizontal movement of a mass of air. ( Webster's ) I assume that they mean water flow just below the surface that is transferred from flow that is just above the surface. Ok, I think I've got that part.
So, advection makes flow in the substrate, "close" to the surface, which is variable based on amount and direction of flow above the surface, along with the particle size. Your 3-5mm size is going to allow a "deeper" advection gradient, but deeper than what, and how much deeper?
Well, we don't know "how deep is deep", or "how close is close". I'll take a wild guess here and say maybe 1/4" depth of advection flow at Goeman's 2-4mm grain size with fairly high horizontal flow, and maybe 1/2" in a particilar area where the flow is directed at the substrate.
If we compare your 4mm avg. to Goeman's 3mm avg. then using the square function you get 80% deeper, or using the cubic function you get 140% deeper. So you might get down to 5/8" and even 1 1/4" in some areas, of "advection flow".
Now diffusion is going to take over. I'll admit that I don't understand diffusion very well. Maybe you could point me towards some good articles on that subject. I have not seen much of anything. There are a lot of widely varying opinions about it's depth for a particular grain size, and this is exactly where you're at with with your new tank and setup.
On wasting, I think I'd prefer to get away from calling it "continuous", and start calling it high frequency. As in HFPW. Oh my god, a new anacronym!:shock: What I envision is exactly along the lines of what LDRHawke was doing some time back, with a little( or a lot ) more experimentation on wasting frequency and volume, as well as substrate size and depth.
I asked LDRHawke to chime in with some of his previous results, but he has not responded yet. His original scheme was one pint, 3 times a day. On my 20 gal. of actual water, with a footprint of 265 sq. in., that would be 9/64" of "forced" advection "downflow" that would occur at a 100 gph flow rate for 4.5 seconds once every eight hours.
That would be about 7/16" per day which may or may not be a bit high. This is where wasting frequency and volume along with grain size and depth come in. It seems to me that this would extend both the aerobic and anoxic zones to some degree, and reduce the anearobic zone as well through wasting.
I don't know the rate at which diffusion occurs, or how it goes up and down at the same time either for that matter. Downflow that is equal to or greater than the upward diffusion rate may or may not be an advantage. this is something that needs to be found out.
It doesn't seem though, that the diffusion rates that we currently have in our substrates are somehow already perfect, either by accident or even by "mother nature", so HFPW gives us some control in this area. I or we need to find out how much control is beneficial here. At least with a properly designed "wasting plenum" in place we can make adjutments as we learn more about what depths the chemical and biological processes are taking place.
Once we do , frequency and volume can be adjusted to accomodate various grain sizes and depth.
On detritus removal, it is my current preference to use high flow( I'm at 20 X volume ) smaller grain size and skimming to keep detritus out of the substrate to begin with, at least most of the animal poop kind. Wasting at an appropriate frequency will force advection through the upper layer, and allow the use of smaller grain size at the surface( the aerobic zone ) for the purpose of rejecting detritus. Larger substrate could be utilized below that to aid in diffusion, if deemed appropriate.
Thanks again Wave98