[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Sand
Date: Wed, 7 Feb 2001 09:32:37 -0500 (EST)
From: Ivo Busko
> ...I would definitely feel more confortable with _no_
> H2S and black stuff !
Considering the described extent, that's understandable.
> OK, here is a description of the substrate: lower layer
> is 1 1/2" deep, silica (pool filter) sand mixed with crushed
> vermiculite (originally a 11 qt bag) and 40 oz Substrate
> Gold. Mid layer is 1" silica sand. Top layer is 1/2" 2-5 mm
> river gravel. The entire thing sits on top of a UGF plate
> covered with a thin layer of foam to prevent the fine mix
> to seep thru the plate. The original idea was to provide
> low-flow reverse circulation. It didn't work in practice
> because at low-flow conditions channels form and divert
> all the water thru a few spots. I keep it running anyways
> to avoid a dead spot situation under the plate.
Had it been me in this situation, I would have done a couple of things
differently. One is pretty obvious (I think you know which, now) and the
other a little more subtle.
The subtle one is the depth of the topping layers. I use a blended mix of
clay litter, iron sulfate, Osmocote and sand for my bottom layer to achieve
about the same intentions as your mix, so it's not only finely- grained but
nutrient rich at the start. It's a modified version of Quackenbush's
original model, but the point is its composition and the need to keep it in
place. Dan suggested topping the mix off with an inch- and- a- half of fine
gravel as a minimum.
In following the list and UseNet over the years, I've seen quite a few posts
from those attempting to duplicate Dan's success with this. Of course,
hardly anyone who posts does so to report _success_, asking instead as to
why theirs failed. The most common theme to all the failed attempts was the
depth of the topping layer. Most of them had topped the mix with either a
far too shallow layer or with a large- grained material, and in almost every
instance of this the main concern was anaerobic activity in a deeper mix.
Dan kinda liked a good- looking tank all right, but didn't exactly relish
having to do a whole lot of work keeping it trimmed and manicured. He
tended toward plants that weren't so "explosive" in growth patterns and it
offered him a couple of advantages, particularly with his mixed- layer
approach. Not having to trim as often was a good thing, but not having to
_replant_ as often was even better. Each time you have to remove a plant's
root structure from a mixed substrate you risk pulling the richer layer up
to the water column.
_Because_ he tended toward those "low maintenance" types of plants as the
bulk of the plantings, his gravel depth of 1.5 inches worked well as a
sealant. But a lot of _us_ *do* like the faster- growing species, especially
those that can provide us with certain nutrient- limiting capabilities (ask
Chuck Gadd why he's growing so fond of Ambulia lately).
At any rate, most of us end up replanting a little more often. I've found
that as a practical guide, it takes at least a couple of inches of something
as tight as sand to do a proper seal job for us mere mortals.
So it would seem to me that "mixing" would be compounded by what is probably
the focus for your problems - the RUGF setup.
> With time the substrate collapsed by about 1/2". Due to
> compaction, and not seepage thru the plate (I can see
> under the tank). I can't tell if the compaction was due to
> the presence of vermiculite or not. Both layers seem to
> have gotten thiner, and the gravel layer partially got mixed
> with the rest, but it's hard to tell...
Exactly - it is hard to tell. But in the absence of definite confirmation,
is it not reasonable to assume that it's a combination of _all_ of these?
Vermiculite can change volume depending on its water content. Soak it long
enough and mix it well enough and you get a pretty "mushy" mix. So it
wouldn't be hard to see it eventually blending into the sand a little more
thoroughly over time. And as you pointed out, the gravel will settle down
into the sand as well.
All in all, though, I'd have to say that it's probably a _very good_ thing
that your RUGF *didn't* establish a generalized, more even flow rate across
the whole bottom, though. I can see how it might make sense as an attempt to
provide a substrate flow while keeping costs within reason. But it would
seem pretty difficult to duplicate the very low flow _rate_ of a convective
current with any kind of pump that would be required for the filter plate.
And the convective current setup has a clear advantage when it comes to
setting up the flows themselves. It's been a while since I've read, for
example, George's articles on cables, but I seem to recall that it's a
combination of both proper wattage _and_ spacing that's important. The key
seems to be establishing "localized" effects to even out upflow and downflow
patterns in clusters of "micro- currents". It's the sum of the micros that
give the effect of the macro - like using a differential equation to find
the area under a curve, or the old saw about never being able to get
anywhere because you must first travel an infinite collection of halfway
points.
> Could it be that the reverse water circulation contributed to
> compaction ? It sounds more likely that, if anything, it would
> counteract any compaction tendency.
As you've already noticed, the pressure from the plate will be relieved
through the path of least resistance. Sometimes it'll be a noticeable
"spring" welling up from within the bed (at the extreme), sometimes a
localized fluidization of the substrate with the effect that you were
originally seeking across the entire bottom. Again, this would be a primary
result of the flow rate caused by having to pump the water through the plate
_mechanically_.
Water _can_ be seeped through a bed of sand. There's too many sand filters
attached to swimming pools and well pumps for me to believe it's a lost
cause. Let's look at the differences between the two, then.
A sand filter is just a container of sand in its simplest form, where water
is flowed to the top surface and allowed to drain down through the sand.
This has the effect of straining the water for any impurities large enough
to be trapped within the interstitial spacing between the sand grains. Same
thing as a micron filter, but much easier to clean :-)
It's those spaces that are the key, and to be of any service must initially
remain open for flow. So the filter sand is normally screened for a fine,
but consistent grain size. This allows the sand to compact without closing
off, as there's nothing in the way of "silt" to quickly fill in the choke
spaces. And since the water sits atop the sand, its weight helps compact the
sand and keep it from flowing, and means that the _water_ is confined by the
_sand_.
In a RUGF setup, the only thing keeping the substrate from flowing is its
own weight, and this is offset even further by the amount of water it
displaces (buoyancy, specific gravity, some other buzzwords...). Any
additional upward current would decrease the effective weight of the grain
further as it fights the friction of the flow itself. Now the sand is
contained by the water, and can move out of the way when circumstances
allow.
And you may have increased this tendency further by supplying your own form
of "silt" to close off the sand. Remember the crushed vermiculite?
> Anyways, as the substrate got more and more compact,
> there was a slowing in the growth rate of most plants, the
> gradual disapepearance of pearling, and the appearance
> of the anaerobic zones, that now are pervasive. Plants still
> look healty and do grow, but at a much lower pace than
> before...
By now, you've probably developed quite a network of "live" and "dead" zones
of varying consistency. But I don't mean "life- or- death" when I say that,
only as a way of comparing, say, nutrient densities or flow rates.
The sand in the immediate area of each of your miniature "artesian wells" is
probably fresher than the areas in between. Probably _too_ fresh - think
fluidized bed filters, here. And the areas in between, the "dead" and
motionless areas, are where the organic wastes and nutrients are collecting
as things settle in. I would imagine, too, that this type of "gravity"
settling might be a little more effective than the compacting caused by
gentle settling over time - it would allow things to pack in more tightly in
a much more immediate time frame as everything moves around until it gets
"locked" into placed. The flow would also force the smaller sediment to be
able to find a space to fit as well, like pieces in a jigsaw puzzle.
I do have an idea for reducing the flow through the plate significantly,
possibly retrieving the situation over time. Since this reply is getting
rather lengthy, though, I'll send it through as a follow- up message.
-Y-
David A. Youngker
nestor10 at mindspring_com