Hi All,
Ok, let me try to sum up the goals of the people who posted this thread to make sure I have understood everything going on until now.
1) people want sand in their main display tank for asthetic reasons
2) those that are proposing a deep sand bed want to maintain critters that may require several inches of sand that can turn into a "sink" for nutrients
3) in order to prevent the DSB "sink" from filling with wastes and leaching back into the system, people are trying to incorporate a wasting plenum to constantly or periodically draw the sludge water from the bottom of the sandbed. For the "lazy" people this will act as daily small water changes *wink*.
I'm hoping you guys get some good results but I like to play devil's advocate and try and poke holes in theories to see if people have answers or theories to cover my concerns.
Question 1)
Is there any chance of solids precipitating out of solution in your matrix of PVC under the sand bed? Possibly blocking the holes or eventually constricting the pipes themselves?
There are threads on RC where people end up having to do vinegar baths etc to clean up calcium deposits on pumps etc.
I think this would be more a concern with people doing less frequent and larger pulls as they would pull water into the pipes that has much different chemistry than the water would end up with after sitting in the pipes for days or weeks. Could drops in pH as the water is processed anaerobically in the tubes cause anything in solution to precipitate out?
2) particle migration of very small particles into the plenum space or even into the PVC pipe matrix
I understand the geometry involved in the layering scheme, but this assumes uniform diameter on the particles, and even packing.
It also assumes that the particles are not moving and no channeling occurs.
Even with the assumptions above, any particle smaller than the interstitial (sp) space is still going to move through the sand bed and accumulate in the plenum area.
When I lived in Tucson, I tore up some carpeting when replacing it with tile, and I was amazed at the amount of chalk fine dirt/dust that had made it under the carpet. What shocked me even more was the amount that had somehow even found its way under the foam padding under the carpet which was glued to the slab foundation. The foam was glued around the edges, and all throughout the floor space, the joint between pieces of foam had been taped, yet there was still almost a quarter inch of dust UNDER the foam.
What I am saying is you are probably going to end up with small particles accumulating in the plenum area. Maybe they will work their way around the edges, maybe they will be able to just work their way completely through the sandbed with the water you are pulling through during wasting, but smaller particles are going to come down into the plenum area. Once down there unless they are kept in suspension and pulled out during the wasting, they will end up settling. Once they settle, they are going to provide a nice surface area for biofilms or whatever so that they will not dislodge easily during the next draw. If this stuff accumulates to a depth to block the holes in your PVC matrix, you won't get even drawing, or may just end up with some PVC in the bottom of your sandbed.
Didn't ldrhawke originally have a way to backflush the wasting mechanism? Was there a conclusion in this thread on how that was going to be accomplished, or are you of the opinion that it won't be necessary so have simplified the system by going without a backflush mechanism?
3) compaction of your sandbed
By having smaller particle media above larger media, and drawing from the bottom of the tank, you are going to end up sucking small particles into the spaces between bigger particles, and basically compact your sandbed.
Since I don't expect this to happen evenly, this is going to be a source of uneven flow through the sand bed and channelling.
I also see this as being a possible problem with your stretching of the zones in the sandbed. If you compact the sandbed, and have layers where sand critters are not moving the sand and disturbing any compacted areas, this will end up stretching the anaerobic area and shrinking the area breaking down ammonia and nitrite/nitrate.
Basically if you compact the sandbed this will prevent the natural downward migration of water from the water column into the sandbed and will end up with the oxygen being consumed in the sandbed quickly and not replaced leading to the anaerobic area being predominant. (I believe this is why Dr. Ron states it is necessary to have such a diverse group of animals to basically stir the sand and help the migration of oxygen and other chemicals into and out of the sandbed)
Now if you are doing constant pulling of water into the sandbed, this may not be as much of an issue (maybe another reason to do more frequent smaller wastings than longer duration less frequent ones).
One thing that you may want to test to see if your bed is compacting is this:
If you are always pulling a consistent amount with the U tube idea (which I like) and it is gravity fed, keeping track of how long it takes for the U tube to fill with your wasting water and seeing if this increases over time will tell you if your flow is being constricted. Just something maybe to keep an eye out for as something easily measured outside that can indicate a problem developing inside.
If your times for wasting the same amount is consistent, that would be good, if they are increasing you are probably getting constricted flow in the sandbed because of compaction or filling of spaces by detritus (i.e. your sandbed is not able to process as much waste as you are pulling into the sandbed), if the times fluctuate where some days they are slower and then faster and this never stabilizes, that would indicate to me channeling is happening, and that as a channel develops the flush time decreases.
4) Even with wasting from the bottom of the sandbed, I still think you are only buying time before you have to rip out the sandbed because of detritus and phosphate accumulation.
The only way to prevent accumulation in a closed system is to balance intake and output, which I think is almost impossible for reefkeepers because there are too many variables.
How do you calculate the input of phosphates by feeding, dosing of chemicals, make up water, etc?
Assuming you can accurately account for the amount of phosphates (or any other thing you are trying to keep in check for that matter) that is being fed into the system, can you really accurately reflect the amount of phosphate being extracted from the system? Are the tests only for phosphate in solution? What about phosphate bound in the bacteria that you are going to be sucking out by the millions? Also, how do you account for any phosphate bound in the growth of inhabitants of the reef system?
If you know a way to accurately track inputs and outputs that would be a first step in developing a steady state system where you can have it go on forever. I don't see that being feasible for a reef system though.
5) What are the feelings of people on here about the need to periodically "cook" live rocks because they act as a sink for P and other nasties that eventually lead to algae outbreaks etc?
Is live rock refurbishing accepted as necessary reef maintenance?
If so, I think you would have to accept that sandbed maintenance is necessary to refurbish the substrate.
6) I didn't see anybody suggesting to incorporate vascular marine plants that would feed from the stuff accumulating in the sandbed as a means for extending the life of the sandbed.
Using macroalgae to strip P from the water column for export is good means of slowing any buildup in the substrate, and if you are going for a DSB in display tank, why not incorporate some manatee grass or whatever in the sandy area to try and suck it out of the sandbed the old fashioned way by growing plants you can harvest that absorb through roots for export.
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Personally I like the idea of wasting from the bottom of the tank, if this is the dirtiest location where wastes accumulate then it only makes sense to pull water out from this location for Water Changes etc.
It is similar in how people use skimmers to concentrate waste for removal from the water column, or concentrating waste by growing macro algae for manual export.
Is there any cheap reliable way to test for heavy metals in the plenum wasting you are going to do? I'd be interested in seeing if there is an accumulation of any heavy metals deep in the sandbed. If you don't introduce heavy metals into the system, I don't see how they could be accumulating though. The only inputs with any heavy metals would be your water supply, your salt mix, medications or dosing, and food.
If you limit the input of heavy metals, you won't have them accumulating. If you grow your own food for your reef from the wastes most people just throw away, you would slow down or prevent accumulation of heavy metals. Instead of draining the plenum waste to a drain after testing, why not drain it to a separate tank specifically for growing macro algae/pods that does not cycle water back to your main tank? That way you can harvest food algae like gracilaria or ulva for fishes that you may have in your reef etc.
It would also give you a test system to see if the waste from the plenum is really something that needs to be removed from the reef system. As you continue to feed the waste from the plenum to this separate test tank, which you shouldn't feed at all, you can monitor for the health of the tank inhabitants. Do you see this waste from the plenum eventually killing off the tank due to accumulating heavy metals? If the waste from the plenum actually results in a healthy system where there is plenty of macro processing the phosphate etc, and then everything feeding off the algae like pods etc and making a little food chain, you could then simply drain your plenum wasting to the sump or refugium and really set up a constant flow scheme, more like a large trickle filter where the goal then isn't to export from the bottom of the sand bed in the display tank, but to just keep anything from accumulating there.
Just my 2 cents, or $1.50 considering how long the above is.
The long term results will be very interesting, so take good notes along the way
