Questions by Scleractinian
I have no problem trying to answer your questions, as long as you are a "civil" engineer....a little humor back :strooper:
Could you elaborate a bit on the hydraulic problems with using eggcrate over the grid, as suggested?
Keep one basic thing in mind, which I know you understand. Fluids always flow to the lowest pressure area. Or to put it another way, fluids will find the path of least resistance and easiest way out of or into a container. This basic law of Fluid Dynamics is being ignored in trying to justify and make an egg crate plenum work.
When a fluid transfer system is being designed for low flow rates and low pressure. It becomes very difficult to control where and how fluid moves. The slightest change in resistance to flow will cause major maldistribution.
You have several ways to over come this problem, one is to increase the flow rates and/or increase the back pressure to obtain even distribution.
A grate plenum system will develop very little back pressure, unless you have very high flow rates. We don't have this in CPW so I have designed it to work for reduced flow rates.
Tank substrate is never perfectly uniform, and even becomes less uniform as detritus starts to fill voids, currents move the bottom around, and it ages. With low flow rates and low pressure the slightest variation in the bed density or depth will cause the fluid flow to that area or the slightest increase in backpressure will divert the flow around the area. With low flow rate systems flow can virtually stop into areas of very small increases of resistance.
As an example: The substrate bed in a tank is never perfectly flat. Density or porosity is not perfectly uniform. Also many tanks are set up with the substrate lower in the front of the tank and get higher in the back. If you had and egg crate plenum on the bottom you with very low resistance to flow, and if you removed a gallon of fluid from the plenum, what area on the surface of the substrate do you think the plenum would be replenished from.
Obviously nearly all the fluid would be pulled into the plenum from the front that has the shallowest depth and the lowest pressure drop. Little or no flow would come from the back.
Also when you pull the fluid out of the plenum, the closest lowest pressure drop zone, to that point of discharge, will be the area the fluid is pulled from.
An example. Exaggerated for effect. If you had your egg crate plenum 30 feet long and you where removing fluid from one end. How much fluid to you think would move down into the plenum chamber from the far end of the tank...Zero or non-measureable to be more accurate.
If you're trying to avoid short circuiting and channeling, it seems that drawing a small volume from a more discrete plenum space would actually help to accomplish this...
Ido not I understand your question? But drawing a small volume assures of short circuiting. You are not trying to avoid short circuiting as much as you are control it. Short circuiting will always happen. You are just trying to make it happen more uniformly and over a larger area.
What is your meaning of discrete plenum space?
Visual Maldistribution a couple of Examples:
Picture a 20 foot long piece of 1" pipe, capped at one end and connect a water hosed to the other end. The same size 1" hole drilled every 6 inches. To make it easier to visualize flow is out not in. (Fluid flowing in or out not much difference in the principle I want to demonstrate. ) Open the valve wide open hold the pipe horizontal with the holes up,. What do you think you will see. Most of the water will come out of the holes closets to the hose with the end. Make the 1" hole 1/32".....because you have water pressure and are restricting the flow, the water will shoot up a stream close to the same height uniformly from each of the holes.
1. If the pipe is large enough in diameter so it had little pressure drop from friction, water would shoot out of the all holes, and all the streams would rise to about the same height.
2. If the pipe was a small diameter you may have water shooting out at decreasing height as you moved away from the hose end.
3. Now take and add a very short riser pipe, 1/4" in height, to some of the holes. With high flow rates, which causes a pressure drop across the holes, the water will still shoot out relatively the same height.
Now slowly close the water valve and reduce the flow. As you reduce the flow you will see As you reduce flow, the holes slight 1/4" risers, will stop flowing any water at all and all the water would come out of the holes without the riser. That is caused by only a 1/4" in water column back pressure.
What does this have to do with a plenum on the bottom of the tank.....everything. The only way you can equalize the flow into or out of the plenum is to either increase the flow rate or reduce the outlet area, both of which are causing an artificial pressure drop. What you are doing is causing a controlled short circuiting. The more small holes simply control where you want the short circuiting to happen.
If you now covered this plenum piping I describe, or used no piping, and had the flow into our out of the egg crate plenum, which had little or no pressure drop at low flow. You would have accomplished nothing. You would be right back to the original maldistribution of the 20' long pipe with 1/4" risers.
What if you placed a sheet of acrylic, perf'd with uniformly distributed orifices and sealed to the tank sides, over the eggcrate...?
This is probably the next closest approach that could be made to work. A sheet with a lot of small holes. The problem would be plugging. So to try and avoid plugging you would cover the holes with filter material. If you used a thick spun filter material, similar to the Poly Filter a lot of us use to remove phosphates, it would actually add a plenum space above the holes. For the most you would now have nearly the same maldistribution conditions your went to all the trouble to get around with a plastic sheet drilled with holes.
What would you think of a continuous (non-pulsed, dropwise or via peristaltic pump) withdrawal of 'liquor' vs. your your present method
This is back to the low flow condition which will cause maldistribution.
Are you hoping to move solids through the bed... at relatively very low velocities and only intermittently?
NO....the idea is to allow the bacteria in the bed to eat the solids and only see waste in solution.
Seems slower, uniform flows might make for best hydraulic uniformity, but would perhaps not be compatible w/ the chemistry desired...?
Back to the maldistribution problem. The chemistry aspect is a whole topic by itself. The best rate of removal of the concentrating waste on the bottom of the DSB using CPW has yet to be established. What I am doing removing a gallon a day in a 60 gallon system works. It is optimum, probably not.
Several things affect this. Everyone's tank size and bio loading is different. Also, there is a lot of analysis to do in the chemistry of what is happening at the bottom of a DSB. The Theil data indicates that the build up of nitrates and phosphates may be accelerating with time and concentration of the anoxic sulfite soup that is building up. It appears that phosphates may be going back into solution which may be because of the chemistry and pH reduction. If that is the case, this could possibly be used to our advantage and allow us to remove more phosphate build up then we would get without the drop in pH. But, it also appears the reverse is true with Nitrates. They are also increasing which may indicate the sulfide reduction is taking over the bed and slowing the nitrate cycle down. So we may want to waste more frequently to stop that. This is a topic I am discussing with Dr. Randy or chemist whizard.
On the other hand, we may find wasting more once a week is more beneficial that wasting smaller amounts daily. This will only be know with more time and more study. I don't believe anyone reviewing the CPW concept and Theil's data can scientifically or logically reach the conclusion that a DSB will work better without using it.
Also, have you considered testing your 'liquor' for PO4, Nx, etc? Might be interesting to relate what's coming out w/ your observed RedOx in another part of the system.
You have not read or looked at the referenced sites at the beginning of this thread. Theil did an excellent study and collected great data years ago, the impact and significance of of this data has been ignored. It was for a DSB with a plenum, but I contend you would see very similar data from a non-plenum DSB. There is no sound logic to believe otherwise. A lot of study of this has been and is being done. It is the basis of believing CPW will benefit our tanks and improve water quality.
I just stumbled across this thread, and had been considering installing a small (2"x2") plenum connected to a 1/4" poly line to be sampled occasionally by withdrawl via a syringe (primarily 'cause I'm crazy and wanted to see what was going on in the sandbed) in a tank currently being assembled. I might just scale that plenum up a bit.
I am not sure that only doing CPW in a small area of a tank will demonstrate or prove anything.
All of the fluid flow principles, I am trying to explain, I have been using for 20 years in processing waste. There is little to no difference in how they apply and how they benefit fluid flow equalization. The same physical and engineering principles apply at the bottom of a reef tank or maintaining an optimum growth environment for microbes in our patented organic waste treatment process.
Plugging......I have been using the CPW plenum system I describe for over 6 months in my tank. The velocities are low enough to avoid rapid plugging. The flow rate of my system has not changed all, which would the indication of partial plugging. IF.... plugging did start to occur, I would simply back flush and open up the holes. All I need to do is open and close a couple of valves for a minute to accomplish this the way my system is piped. Just like is done with all sand filters in water treatment plants.
I have no intention of getting into any more ****ing contest trying to convince anyone, that uses this forum only to get attention and be noticed, that believes that water flows up hill and uses babble to prove it. I will discuss any topic or gray area on CPW with a "civil" engineer, or non-insulting lay person.
Babbling insulting idiots will be ignored.
I think if you review my past posts, you will see doing it with a sense of humor has always been my manner. :mixed:
... it just may be a year or two before I turn that valve. Maybe I'll turn it when I start thinking my DSB needs a flush ...
