Saltcorner
By Bob Goemans
By Bob Goemans
Hi Bob
I've read a fair amount of your material and other people's views on the biomechanisms of what is happening within a plenum sand bed. It seems that not very much is known about actual mass transfer rates within the sand bed and I guess non-intrusive measurements for this are difficult and expensive to make. I'm a Ph.D. chemical engineer familiar with fluid mechanics, adsorption mechanisms and bio and pharmacokinetics - are there any major review literature papers that you can steer me towards?
Secondly - I am an amateur marine hobbyist with 2 years experience. I have some issues with my tank which I have been discussing with others and had the following reply from Scott at Wetwebmedia. (The bulk of the email is my text, Scott's comments are denoted ). He suggested to email you and that you might be interested in my intent to use convective currents and temperature to drive the biological mechanisms in a plenum system. Below is a copy of the email thread. Do you think the heater cable will be effective?
Thirdly, the presence of dissolved oxygen in the plenum does seem very counterintuitive (as detailed in your article in Jan 03 FAMA). Surely there has been an argument made by the bed having defects and there being a route for bulk liquid carrying liquid into the plenum but then I suppose this would not then explain why this area has a higher concentration of NO3 and other compounds than the rest of the aquarium water. How certain are the experimental measurements for the dissolved oxygen(DO)?
Regards
Geoff Purdom
Here's the web thread-
"I have some high Nitrate (80mg/l) and phosphate (3mg/l) problems in a 24 month 80 gallon set up. The setup gets a 5-10% water change on a weekly basis and is stable with a medium/high load of eight fish, two shrimp and a blue lobster. I tried to introduce an anemone, but it never fully open and unfortunately starved. I also have a bubble algae problem.
The set up is in two tanks, a 55 gallon which cascades its water to a 30 gallon tank which then returns to a wet/dry sump in the basement with a protein skimmer, mechanical filter, carbon, heater, UV then back to the 55 gallon tank.
My questions are:
1. Will I realistically be able to get phosphate down to a reasonable level for anemones and reduced algae? What should I aim for and will aluminum oxide be the most economic route. The phosphate comes presumably from accumulation from food.
<Well, ideally, you want to shoot for 0.05mg/l on phosphate, and less than 10 mg/l nitrate, if you can. I think it is certainly possible to lower phosphate in your system. I think there is certainly some phosphate coming from food, but there is also probably some phosphate in your source water. If you aren't using one already, you should look into an RO/DI unit, which can help you start off with more pure water. Another thought is that the mechanical filter which you are using needs to be cleaned (or have the media contained within it changed) more often (maybe twice a week). High nitrate and phosphate are definitely signs of nutrient accumulation. Attacking both together is possible. Your goal here should be to maximize nutrient export mechanisms. Your water change schedule is good; you may even want to try 2 smaller (5%) changes per week, to dilute the nutrients before they have a chance to accumulate. Also, try to really adjust your skimmer until it yields at least a couple of cups of dark yucky skimmate a week. It may take a fair amount of tweaking, but it is possible to get good results if you work with it. I'd employ extra chemical filtration media, such as Poly-Filter (my personal favorite). This stuff really works! Make sure that you are using a high-grade, phosphate-free carbon, and change it regularly. Another though would be to employ macroalgae, such as Chaetomorpha, Gracilaria, or Halimeda, and harvest them regularly. They will utilize many of the same nutrients that your nuisance algae are and they will ultimately beat out the lower algae. Harvesting them essentially removes nutrients from the system directly...yep- it works>
2. I have a tee'd off connection from the wet/dry to a 80 gallon rubberware container which will act as a plenum or it could be a refugium, and new sump. The water will be pumped back to the wet/dry on a recycle loop. I am planning on using 160 pounds of aragonite, coarse coral mix as a 4-5 inch bed over a 1 inch plenum space.
<Well, a plenum is a very useful tool if constructed correctly. It does require specific void space, particle size, etc, so do read up carefully if you plan on pursuing a plenum setup. Don't deviate from the established methods for constructing plenums - the techniques and guidelines were developed after a great deal of research. Check out Bob Goemans' web site (www.saltcorner.com) for much more on plenums. Bob is the authority on this system in the U.S. and can really give some great input! If you opt for a remote deep sand bed without a plenum, try a fine, oolitic aragonite like Carib Sea's Aragamax Sugar-Fine sand, and shoot for 4 inches or more.>
I have read that the plenum relies upon slight convective currents to effect as large an anoxic denitrifying layer as possible. Also since the sump/plenum is in my basement I am concerned about temperature control. I figure that I will put 30feet of 150W heating cable under the aragonite to deal with heating needs and achieve some convective mixing through the aragonite bed. Do you think this will work - will it be beneficial?
<To be quite honest with you, I have not seen this done before. It is an interesting idea, but I'm not sure whether or not it is worth the effort. I'd really encourage you to contact Bob Goemans on this one!>
The reason I started thinking of the cable heater is that the plenum will be in my basement which is about 40degF at the moment and only 50 -55 in summer. I live in New England. I figured that the rubberware tank itself (even although it will be insulated) would need some degree of heating and then subsequent small power head or air lift to get some recirculation.
<Absolutely...I agree.>
However this would NOT help the maintenance of temperature within the aragonite bed which would depend upon conduction for its heat from the main liquid bulk. Next I figure that the microbiological rates are probably favored by maintaining a reasonably high temperature 80degF. Therefore I have to find someway to keep the substrate at 80deg.
<Very logical approach.>
Cable heating is used here and there for encouraging plant growth in non-marine aquariums and this suggested to me what might work as a possible solution together with enhancing some flow through the sand bed. All the reading I have done to date - as you state - suggest some but only a slow vertical flow of water through the bed is recommended for the plenum to maximize its anoxic layer. However no where seems to suggest what is too fast or too slow.
<Well, as you would probably agree-the faster and more efficiently that nutrients are turned into energy, the better your water quality will be. So, it's important to create a system to foster these processes. I do see the logic in your method...very interesting!>
Even with my proposed setup I will not know what vertical linear flow rates I will be achieving anyway. My intent was to concentrate the heating cable into three longitudinal strips so that there will be cold:hot:cold:hot:cold:hot:cold strips the length of the tank (hope this is reasonable explanation). I will be using 110VAC roof de-icing cable. (I have ground fault trip electric and will be very cautious). The cable is waterproof and fully plastic coated therefore should be okay for continuous immersion.
<I have no experience with this device...do proceed with caution if not using an aquarium-specific heater cable system.>
Initially I intend on leaving the cable on 24 hours but could put this on a timer if the mass transfer does not seem to show that the plenum is doing its stuff of removing nitrate. However I will wait 12 months to be sure everything has had a chance to mature and monitor concentrations along the way.
<Sometimes standing back and letting nature do its job really is the best procedure.>
I must admit that I was torn between setting up a refugium in place of the plenum and instead using plants and light. Do you think maybe I should do half and half and plant Caulerpa with a plant spectrum 24W florescent tube on one half?
<I am very anti-Caulerpa for a variety of reasons - I'd use different macroalgae for this purpose...but do utilize some macroalgae. Perhaps you may want to employ a DSB in your main system, and add the refugium in line? Just a thought.>
However - if I do, then I will not know if the main idea ever worked.
<Ahh- the joys of experimentation!>
Lastly, my skimmer is a CPR BAKPak, which is a small footprint venturi in-sump skimmer but only rated to 50 gallons. I have not had a chance to read through all the skimmer stuff you and others have posted. However I do need to finish and get this plenum up first.
<Well, a skimmer is so vital to your system that you may want to push the skimmer up to number one on your list. The BakPak is a great skimmer - but I think it's operating at the edge of it's range in this system, so an upgrade is highly advisable.>
Lastly, I did not add in the previous email that before the plenum I will be putting in a 30 gallon trash can with flow distributor filled with polystyrene peanuts to act as a trickle filter to also increase my ammonium denitrification. The water will drain through the trash can trickle filter by gravity into the plenum. Currently my ammonium is 0.2-0.3. I figure for minimal expense I could get more aerobic degradation. I built this piece ahead of deciding on the plenum, which will presumably also achieve the same end effect of further NH3 & NO3 oxidation in addition to the anoxic denitrification. I have read elsewhere that this is not recommended as enhancing the NH3 oxidation will increase my NO3 problem.
<That's my thinking.>
This seems logical but at worst case if all the NH3 is shifted over to NO3 - I am only looking at a small increase in NO3 to my current concentration. Thanks for your comments and any further thoughts are appreciated.
<Well, you certainly have some great ideas and a nice approach! I think that simple, biological-based systems are always best. You will be surprised at how a simple idea, such as a deep sand bed (with or without a plenum), a great skimmer, good husbandry, and live rock can do the trick. Check out Anthony Calfo's Book of Coral Propagation for some more information on the concepts that you touched on. Also, You may want to pick up Live Sand Secrets by Bob Goemans for a simple review of plenum dynamics. Most of all - Have fun! Good luck! Regards, Scott >
Hi Geoff,
Thanks for a very interesting letter! Let me begin by saying that Sam Gamble can provide you with the 'scientific' literature requested. Contact him at keysmariculture@aol.com.
As for the heating cable, it's an issue Sam and I discussed many years ago when a reader questioned its usefulness. As Sam then noted, and which is discussed in our CD book 'The New Wave,' the more productive temperature range of the microbes existing in the sandbed is in the 70ยบ F range. Keep in mind the microbes themselves exist on the sand particles in the bed, they do not exist in the plenum area (void space). With that said, two problems arise from heater cables located in the sandbed. The first is that the area nearest to the cable will be the hottest, thereby effecting the well being of any microbes/microbial processes nearby. Also, the quality of the cable itself comes into play, as some are not too well regulated and/or do not dispense heat evenly. However, even more important is the fact that the generated heat rises, and causes rising convection currents that work against the downward flowing diffusion process. It's those downward processes in a plenum bed that are invaluable when it comes to microbial processes being in a 'balanced' state. Again, if you would like more in depth reasoning, contact Sam for the related 'science speak' side of the issue.
When it comes to the oxygen content of the plenum, there's been much conjecture about its presence, lack of presence, or how it would get there. As mentioned in the FAMA article, be assured it does accumulate in the plenum at a greater amount then present in the bed above (Sam has personally tested it.). And that's the reason plenum systems are so much more efficient than sandbeds directly on the aquarium bottom. It's that small amount of oxygen that keeps the majority of the above sand zone in an anoxic state/condition (0.5 - 2.0 mg/l). Something that deep beds directly on the aquarium bottom "do not' accomplish.
As for oxygen taking a direct path to the plenum through an opening of some kind, that is not as logical as it sounds. If that were to occur there could well be an access of oxygen in the plenum as it would then tend to be in balance with the oxygen in the bulk water. That would then halt denitrification in the above bed and result in a system with high levels of nitrate because only nitrification was mostly occurring! And, because the levels of nitrate and other compounds in the plenum space vary, it appears the plenum is a moderator/reservoir where bulk water excesses are temporary stored until the bacteria in the very efficient anoxic zones can fully process them. Keep in mind there are often temporary or ongoing excesses in our aquariums and ever increasing levels of nitrate should cause every aquarist to realize that his or her microbial processes are not in balance. If they were, then bulk water nitrate levels would be extremely low.
And I must add that I truly believe efficient utilization of nutrients by bacteria is much more important than that of export. When one concentrates of microbial balance, less time, cost, and effort need be put into export!
There are some areas in your letter that I would like to add some additional comments. I agree with Scott that you need to 'greatly' reduce the phosphate level, and that most phosphate comes from the food fed and the water supply. His suggestions for correcting should be heeded. Yet, when it comes to the phosphate level, I would prefer something closer to 0.015 mg/l and have found that when that level is attained, there is very little or no further algae growth. To accomplish that in a previous aquarium I first needed a test kit that would accurately attain that level. I used the Merck phosphate test kit 1.14445.0001 to accomplish that goal. And I've found that 'iron' based phosphate removing media is far superior to the aluminum-based products. Also, I should add that I never found water changes to be an effective way to control high levels of nitrate. Once the change is accomplished, nitrate from deep inside rock or the sandbed will flow back out into the bulk water and the level in the bulk water will quickly be what it was a day or two before the change was made. Yet, water changes are beneficial for most systems for many other reasons, as is quality skimming, and the use of the Poly-Filter.
Will you ever get the nitrate level low, and low would be >10 mg/l, is a good question. Increasing the area for nitrification with additional trickle areas will probably do away with any on-going nitrite, yet increase the nitrate level. If it were my system, I would think about creating a very large plenum area and/or possibly reducing the existing bioload. Mother Nature can only do so much! And, keep in mind, there is some denitrification equipment on the market that might be helpful if reducing the bioload is not amiable.
Hope this helps,
Bob
