Are Deep Sand Beds, DSBs, dangerous to use in a marine aquarium?

[simonh]

that is fine, then think of them as a lagoon substrate. the problem with that is that we now have to explain why we are not doing regular major substrate disturbing events. those pesky tropical storms. we are emulating nature right, or are we?

That's the problem in sand bed discussions they seem to gravitate to extremes. It's either a deep sediment, or it's a shallow lagoon been kicked up by storms etc. It's actually 4-6" of sand in the bottom of a reef tank. I've never said anything about emulating nature or shown P flow within nature. I'm more interested in P flow with 4-6" of sand in the bottom of a reef tank connected to a body of water directly above it.

These discussions tend to describe a scenario where you have a perfect system. until you put 4-6" of sand in the bottom. Suddenly the addition of some sand has created a black hole with extraordinary powers that pull everything nasty into them. Until one day "something" bad happens. And the black hole is the culprit as it became full of "something".

ok, but still this is an increase in material. an increase in material is an increase in mass of the system. a sign of eutrophication. i guess this extra material could be trace elements, but that would lead to a toxic sludge, also something not wanted in our systems. any way you look at it, this material is not something we want in our systems indefinitely.

It could be mainly inert. We don't know. TOXIC sludge sounds better though!

when do you decide to take the trash out or flush the toilet? when does the trash can stop being useful for keeping trash? you can keep putting stuff in it when it is full, but it just falls to the floor. i think you are missing the role bacteria have in this and the sand itself. take the very top layer of a substrate. it is completely clean, and so is the water. now lets a touch of P to the water. to reach equilibrium the sand binds the P to reach equilibrium. now comes the bacteria. they take the P off of the sand, they fall into the substrate. now there is an open site for the P if the water and the sand are not in equilibrium. this poor bacteria with the P either lives, dies, or gets eaten. either way the P is now organically bound. the P is still sunk in the substrate and has not changed the equilibrium with the water column. this just keeps going on and on. the bacteria take the P, then do whatever. leaving a new place for P to be bound from the water column. of course some of this organically bound P will become inorganically bound by bacterial action, but as they are deeper in the substrate the harder it is for the P to reach an equilibrium with the water column because of all of the material in the way creating a barrier.

My trash can is a little different. Their is no "ecosystem" or chemical reactions reaching equilibrium states at work just me chucking lumps of plastic in each day until it is full. The analogy might sounds clever but really has no place in the discussion. By your description this P seems to be cleaved from calcium phosphate then just dumped into the sand.. more P binds, it's cleaved build up in bacteria live/dead. Your reactions always seem to proceed in one direction! NEGATIVE!

there is nothing wrong with that, and a substrate can be used to great affect in this manner for a while. a skimmer is not going to get it all, and in fact if the calcium carbonate bacterial tug of war did not exist our systems would not be alive. skimmers are fantastic exporters of organic P. i think they need to be bigger though. i am also a big fan of settling tanks, think empty live sumps. a place to see and easily remove accumulating detritus. conical setting tanks being better because of the nice valve on the bottom.

You might as well pull the live rock too. The process you are describing for the P been bound to sand then bacteria taking it and building up endlessly is going to be happening on your live rock too as that is CaCO3.

that they contain. CO2 and carbonate are just other sources of elemental C for bacteria.

I'd like to see some references to marine bacteria typical in our tanks breaking down carbonate or bicarbonate to supply their carbon needs. Lots of elements contain carbon but it doesn't mean it is available to organisms. The reason organic carbon dosing seems to work is that there must be a limited availability of easily available carbon for use by bacteria in our tanks. The forms of organic carbon dosed are easily broken down for uptake by the organisms. If they were able to use carbonate theres a huge supply already in the water column!

 

[power boat jim]

simonh, after reading all of your replies that, from I gather, basically state you dont believe in or were aware of the phosphate cycle. I would be curious to know what exactly you think happens to the ever increasing input of phosphate in a reef tank environment and why a sand bed cannot and does not become a p sink with limited capacity for n and p.

If I am incorrect, please say so but that is the tone I have gathered by reading your replies to the principals layed out by reefin dude each time he tried to explain the p cycle in many different ways.

 

[simonh]

I am aware of the P cycle in the ocean. I've read many books on reef aquariums and reef ecology. I am aware that P can bind to calcium carbonate, although that binding capacity in an aquarium is quite a limited and that it there are also equilibrium relationships at work and the CaCO3 is not an endless sink. I have a pretty good grasp of chemistry altough I'm no chemist.

I'm not sure showing a picture of the phosphate cycle in nature is that relavent in a discussion of DSBs. As I said earlier I don't have the rabbit in my tank, the weathering and soil errosion or the deep sediments or geological cycle.

I do have a tub of flake food in my hand. I have a protein skimmer that has been shown to export phosphate. I drip limewater that has been hypothesised to precipitate phosphate and may then be skimmed out. I use iron oxide hydroxide PO4 remover that has a strong binding capacity for PO4. I msy even grow some calurpa in a refugium that I harvest and that removes PO4. I have a tank full of animals that also use some of the PO4 in their growth. I change some water sometimes etc....

Throwing a phosphorous cycle of what happens in the sea isn't going to convince me that all the PO4 I am adding into my tank is going into the DEEP sediment as shown in that diagram. It's a world away from my little microcosm and whilst it may be nice to try and map each item in the cycle e.g. my skimmer is foam on the beach... erm my PO4 remover is erm... It doesn't really work! As I have pointed out what happens in a tank with 4-6" of sand connected to the bulk water column above is my interest for the purpose of this discussion. Not what happens in billions of gallons of water with different components from thousands of miles of deep sea where nutrients may become sunk away from the reef or near to land where rock errosion and rivers may bring P into the system. It's just irrelevent.

Some PO4 may be bound to my CaCO3 sand, particularly when the level in the bulk water are high, when lowered I would expect some of that P to come back into equilibrium. Some may be traped away in the CaCO3 if the matrix has been growing. There may be some detritus in my sandbed that is in various states of been processed and the P may be currently passing through some infauna in the sandbed. Some will be released back into the water column in spawning or when perturberd. Some makes biomass. When less detritus is available some of the population will die due to lack of food, some existing organisms may eat them to keep living. What's left maybe inert, it may be toxic we don't really know. That's my view.

Nitrogen is somewhat easier as it has a gas phase and given denitrification somewhere in the system is quite easy to get rid of from our aquariums.

My purpose of getting involved in this discussion was to try and understand what Geoff thinks is going on but I'm not a whole lot clearer. I don't think we will ever agree as to what we think is happening to P in a reef tank. The more this thread goes on the more I think there is some P conspiracy theory at work.. now my calcium reactor is working for agent P

 

[Reefin' Dude]

That's the problem in sand bed discussions they seem to gravitate to extremes. It's either a deep sediment, or it's a shallow lagoon been kicked up by storms etc. It's actually 4-6" of sand in the bottom of a reef tank. I've never said anything about emulating nature or shown P flow within nature. I'm more interested in P flow with 4-6" of sand in the bottom of a reef tank connected to a body of water directly above it.

These discussions tend to describe a scenario where you have a perfect system. until you put 4-6" of sand in the bottom. Suddenly the addition of some sand has created a black hole with extraordinary powers that pull everything nasty into them. Until one day "something" bad happens. And the black hole is the culprit as it became full of "something".

the P flow is the same in our systems as it is in nature. sorry, i did not mean shallow as in substrate, but depth of water. there is no way anybody can consider the substrates we keep in our systems as being deep compared to a marine substrate. the differences in depth of water deal with the occurrence of major disturbing events of the substrate. the deeper the depth the less likely there will be regular major disturbances of the substrate from wave action. this was the "extreme" point i was trying to make. one either disturbs the substrate like a shallow sea, or one does not like the deep sea. do we treat our substrates like the shallow seas, or like the deep sea. the access to sunlight though creates a major wrinkle for treating our substrates like the are the deep sea.

It could be mainly inert. We don't know. TOXIC sludge sounds better though!

it doesn't matter whether it is inert or toxic. the material in the substrate is still increasing. a sign that the system is becoming more eutrophic. if there are any toxic material in the food, then it is going to end up on the bottom of the system. whether it is a BB or a substrates system. what one does with this collection of material is up to the aquarist. they can either siphon it out or hide it in the substrate. nobody is saying you can not keep a substrate. we are just trying to explain what is going on in them.

My trash can is a little different. Their is no "ecosystem" or chemical reactions reaching equilibrium states at work just me chucking lumps of plastic in each day until it is full. The analogy might sounds clever but really has no place in the discussion. By your description this P seems to be cleaved from calcium phosphate then just dumped into the sand.. more P binds, it's cleaved build up in bacteria live/dead. Your reactions always seem to proceed in one direction! NEGATIVE!

you are right, we do not have an ecosystem(phosphate sink) in our houses. why not? why not keep enough rats, roaches, and other smaller organisms to eat our trash? why not collect all of the waste material from our toilets and spread it out all over the house and let microbes do something with it, not sure what they would do with it besides make it smaller, they are not going to make it disappear. matter can not be created nor destroyed. of course this is all going on while you go out to the store and get more food to bring into the house. this does not work. if it did, we would be doing it. we have running water and trash service to remove our wastes. that is what we must do in our little glass houses. we are the running water and the trash service.

the important part is that in our houses we can see when exports are not equal to imports. in our system it is not as easy. the left overs can hide in a lot of different areas.

You might as well pull the live rock too. The process you are describing for the P been bound to sand then bacteria taking it and building up endlessly is going to be happening on your live rock too as that is CaCO3.

you are right, it can. the difference is that LR is surrounded by flow. if the flow is good enough, then the flow will remove the bacterial flock from the LR. those puffs of detritus that occur when blowing off the LR. this is the same puffs that happen when you disturb a substrate. the processes are the same. if the puffs are not occurring on the LR, then absolutely the LR will behave the same as a substrate and will become a P sink. it will not be relying on the give and take of the bacteria and the calcium carbonate to keep it "clean".

"cooking", purging, or curing LR utilize this process. it also happens in a substrate, the difference being that we do not disturb the substrate to get all of this bacterial flock/puff out of it on a regular basis. if we did, then the P sink would be refreshed ready to do its job again.

I'd like to see some references to marine bacteria typical in our tanks breaking down carbonate or bicarbonate to supply their carbon needs. Lots of elements contain carbon but it doesn't mean it is available to organisms. The reason organic carbon dosing seems to work is that there must be a limited availability of easily available carbon for use by bacteria in our tanks. The forms of organic carbon dosed are easily broken down for uptake by the organisms. If they were able to use carbonate theres a huge supply already in the water column!

only elemental carbon contains carbon. other elements are other elements, but other compounds can contain carbon absolutely. life on earth depends on bacterias ability to utilize carbon in its various forms. all of the elemental processes start with bacterias ability to turn the element into a solution or form that other organisms can utilize. whether that is absorption or through ingestion.

Ammonium regeneration and carbon utilization by marine bacteria grown on mixed substrates

Consumption of dissolved organic carbon by marine bacteria and demand for inorganic nutrients

Active heterotrophic bacterial assemblages involved in inorganic carbon fixation in the
Pacific Northwest coastal margin

sorry, there are others. we can mix and match autotrophs and heterotrophs in google with bacterial carbon utilization and get all kinds of fun results.

G~

 

[Fredfish]

the P flow is the same in our systems as it is in nature.

It is?

Seems to me there are some major differences. Far more differences than similarities.

 

[simonh]

I'll skip the discussion of P as I don't think we are ever going to reach agreement on what we think happens in our glass boxes. It's the weekend so I'll just enjoy a :beer:

only elemental carbon contains carbon. other elements are other elements, but other compounds can contain carbon absolutely. life on earth depends on bacterias ability to utilize carbon in its various forms. all of the elemental processes start with bacterias ability to turn the element into a solution or form that other organisms can utilize. whether that is absorption or through ingestion.
sorry, there are others. we can mix and match autotrophs and heterotrophs in google with bacterial carbon utilization and get all kinds of fun results.

I am aware of the difference between the element C and the carbonate ion which is made up of C atom surrounded by 3 O atoms.

You made an assertion that calcium reactors were needed because the bacteria in the sand were using the carbonate to acquire their C. I am aware that some compounds can be broken down by bacteria/algae to require a nutrient they need. I am not aware of carbonate been used in this way by bacteria as you described.

If you were using a calcium reactor and the bacteria were using the C from carbonate ion we would expect to see calcium rise over time relative to alkalinity. Not something I've seen. The same reactions would take place if using limewater or any other method of calcium/alkalinity supplementation.

 

[Fredfish]

Hmm, image did not display in the last post. Lets try this again. Here, in a picture, is what Simon has been trying to communicate:

After you remove all the things not present in our tanks what does that leave us? The water. The sand. Even the precipitated solids are a question mark. That is not enough to say that the P cycle is the same in our tanks as in nature. In fact, it is probably enough to reasonably conclude, without setting up a whole series of experiments, that it is not.

The P does not magically disappear, so we need/want to know what is happening with it. There is nothing wrong with looking to nature for processes that may apply in our tanks, but to make the link, you need to establish that the proper conditions for those processes also exist in our aquariums. You cannot simply assume.

To establish those links repeatable experiments need to be set up to prove that the process in question takes place. Then and only then can you state with certainty that deep sand beds are [insert your favourite hypothesis here]...

Until those experiments happen, the best you can write is "This is what I think is happening."

That's enough for one post.

 

[power boat jim]

The natural cycle is not that much different then the one that takes place in the tank. You just need to look at it in simple terms of a process and not in the literal sense of a rabbit being a major source of the oceans phosphates .True, there are just not as many specific sources for a tank as there is in a natural system but so what, but both systems have input. Both systems will precipitate out phosphates, one into the sub strait the other possibly into the rock bottom of the ocean. In either case its part of the bottom of the system . In the end we can remove the sub strait and all thats bound to it by using a siphon or a just scooping out the sand. The earths process takes a bit longer but its all a result of scale and the result is the same, phosphates are removed through plate tectonics and stored until they are recycled back to the system, again both systems can employ physical removal. We dont recycle phosphate but we continually add more through feeding, once again same result .

So the two system are more then comparable and using one for a basis on how the other should work seems more then viable to me.

 

[Fredfish]

The natural cycle is not that much different then the one that takes place in the tank.

As evidenced by what? Saying something does not make it so. It does make it opinion.

Both systems will precipitate out phosphates, one into the sub strait the other possibly into the rock bottom of the ocean.

Same question as above and more. Into what form? What compounds? Soluble or insoluble? Under what conditions? under what conditions does the sink no longer accept more P? Under what conditions does it release P?

 

[power boat jim]

I dont know but the fact these processes happen in both places using the same elements/nutrients, makes it hard to deny there must be strong similarities, Thats it thats my point. The questions you ask I cannot answer but I think you already know that. Im not going to try to find out either because its not going to change anything for me. If you need beyond a doubt evidence that the two process are not exactly the same I wish you well in finding out. Also I see no evidence brought forward to disprove that there is a significant difference between p deposition in the ocean and in a sand bed in a reef tank.

 

[Fredfish]

I dont know but the fact these processes happen in both places using the same elements/nutrients, makes it hard to deny there must be strong similarities, Thats it thats my point. The questions you ask I cannot answer but I think you already know that. Im not going to try to find out either because its not going to change anything for me. If you need beyond a doubt evidence that the two process are not exactly the same I wish you well in finding out. Also I see no evidence brought forward to disprove that there is a significant difference between p deposition in the ocean and in a sand bed in a reef tank.

Thanks for the honest answer. On the internets, I try not to go on assumptions. It always gets me in trouble.

I'm not trying to change your mind, but I do think it is important for others reading to understand how you and I get to what we believe, particularly for issues like this where there is a lot that is unknown.

Here are some things I do know:

1. Back when deep sand beds were newer, there seemed to be two groups of people: those that had nutrient issues with higher feeding regimens, and those that did not. People Like Borneman, Toonan and Schimek got curious and went about investigating why. I remember Toonen setting up a series of tanks with different depth and grades of sand to see what differences might come up. Both Toonan and Schimek did animal counts (involves staring through a microscope at slides inscribed with little square and trying to count the number of animals per square, fun stuff :eek1: ).

It was experimentation like this and results where sandbeds stayed clean and N and P stayed low in the water column that got us to the recommendations posted earlier by Paolo. Sand bed depth, grain size and mix and most importantly species diversity and density were critical to getting a functioning live sand bed.

there were a number of people who ran tanks for multiple years under these conditions without fouling or clumping.

2. We do not know in detail what processes are at work in a sand bed, but we do know that you can get a range of results. If you do not get the right mix of depth sand type and species diversity and density your sandbed will become a nutrient sink, but it does not have to be that way. There are people who have posted to this thread that have been very successful long term.

 

[power boat jim]

Thanks for the honest answer. On the internets, I try not to go on assumptions. It always gets me in trouble.

.

You are welcome, Im not smart enough to lie my way through a process Im not familiar with other then on a basic level and it wouldnt help me nor anyone else if I tried.

The only experience I have with a sand bed is when I tried 10 years ago and it was a disaster. My only point was to make was if a cycle is present in nature,then there should be some similarities to that cycle even scaled down to our tanks. I dont think you could ever tell someone that a sand bed comes with an expiration date no matter how many scientific studies you run and how detailed of information you get on the N and P cycle in both environments.

My sand bed lasted a year others are going well after five. The process that made my sand bed go septic are certainly at work in the 5 year old bed but they just have not gotten to the point where it has become a sink full of anaerobic detritus. The big question is why. This is where the arguments begin. Some people want a numbers and research that say at this P level the bed no longer works and if your water parameters maintain this number the sand bed will last forever with no maintenance required. Neither statement could ever be proven one way or another due to the unlimited amount of variables we as hobbyists bring to the table. So all that said I hope you understand all I was just trying to make a general statement about similarities that can exist in the P cycle in nature vs our tanks. :beer:

 

[nmbeg]

I do not pretend to be a biologist or chemist, nor to have any insights into any of this. I have two comments/questions:

1) the global phosphate cycle, while important to learn, is NOT going on in our tanks. The differences between the macro and micro level are too much to just assume that the global cycle occurs on a smaller scale in our tanks.

2) the phosphate sink theory seems to rely heavily on gravity bringing nutrients downward. Why isn't it possible that bacteria that incorporate the phospahate flow back into the water column? Or the plankton? Or the pods as they get eaten by fish, and pooped back into the water column, etc. I have no knowledge of this, just asking a question.

3) the "everything poops" theory seems to rely on solid poop. Do we know what snail poop, worm poop, crab poop, etc look like? Are they solid or liquid, flowing back into the water column? Again, I have no knowledge, just asking a question.

One final comment: the "you must flush your toilet" or "take out the garbage" comparison fails because proponents of DSBs are trying to create a recycling ecosystem--the same is not true of our houses. On a small scale, some people attempt similar limited cycles in our houses--houses with septic systems and wells recycle their liquid wastes. Compost bins recycle unused nutrients that we bring into our houses.

Those systems are incomplete, but fun. Same thing with a DSB--it may never be complete or even "work," but it's fun.

 

[nmbeg]

Sorry, thought of a couple more comments:

1) someone mentioned earlier about focusing on the organisms we want in our tanks, and not trying to support an entire ecosystem to help support the other organisms. For me, a "live" sand bed IS one of the organisms I want to keep. For me, it's more than just a filter like my skimmer, it's a cool, living thing. I get just as much enjoyment looking at a worm trail or a nitrogen bubble as my wife gets from watching the yellow tang. (I am sure there are some people reading this that get just as much excitement from watching their skimmer--but we will leave that for another thread)

2) the entire discussion on this thread has been about P. Can we then conclude that a DSB DOES work effectively on the N cycle? Indefinitely? Safely? If I run phosphate reducer with a DSB, is that a safe and complete system (recoginizing of course that there are other ways to have a complete and safe system; and that there are other components like WC and skimmer, etc).

3) I apologize in advance if this belongs in the noob forum: I am still trying to figure out the correct way to plumb a rabbit into my system. Should I use a deep substrate bed in his cage, or shallow? Bare bottom? Are there certain species of rabbit better at de-phosphatization than others? Thanks in advance for your help.

 

[power boat jim]

Sorry, thought of a couple more comments:

1) someone mentioned earlier about focusing on the organisms we want in our tanks, and not trying to support an entire ecosystem to help support the other organisms. For me, a "live" sand bed IS one of the organisms I want to keep. For me, it's more than just a filter like my skimmer, it's a cool, living thing. I get just as much enjoyment looking at a worm trail or a nitrogen bubble as my wife gets from watching the yellow tang. (I am sure there are some people reading this that get just as much excitement from watching their skimmer--but we will leave that for another thread)

2) the entire discussion on this thread has been about P. Can we then conclude that a DSB DOES work effectively on the N cycle? Indefinitely? Safely? If I run phosphate reducer with a DSB, is that a safe and complete system (recoginizing of course that there are other ways to have a complete and safe system; and that there are other components like WC and skimmer, etc).

3) I apologize in advance if this belongs in the noob forum: I am still trying to figure out the correct way to plumb a rabbit into my system. Should I use a deep substrate bed in his cage, or shallow? Bare bottom? Are there certain species of rabbit better at de-phosphatization than others? Thanks in advance for your help.

The answer to question 2 is in theory a dsb was supposed to be the be all end all for nitrate reduction. In practice its not due to the wide variety of variables each one of us adds to the equation. There are no hard fast answers to your questions because no one can ever quantify how much detritus a cubic inch of sand can break down into a harmless mass over a given amount of time using every possible scenario found in each one of our reef tanks.I would venture to say there are certain general maintenance regimes that keep beds running longer then others,so using logic and the laws of physics a dsb is a finite resource that cant be expected to take an infinite amount of crud for an indefinite amount of time and continue to function without a little help. How much help it needs is where the debates usually begin.

As for question 3 I would give you an answer about the rabbit but I think we would have to split too many Hares to come up with a way to do it.

 

[sh0ck]

There are many possibilities and experiments also.

If you for example build DSB from silica based sands. Get loads of sponges etc to battle silicate leach, there will be no chemical P binding to your sand bed and you should have pretty good overview about actual P input/output situation in your system to tweak it.
Additional NO3 dosing can help battle P, iron powder is quite cheap and we can continue about many different approaches to DSB.

But yes if u just pile up standard aragonite sand and expect no maintenance filtration system, u will hit the wall sooner or later.

 

[Fredfish]

...
2) the phosphate sink theory seems to rely heavily on gravity bringing nutrients downward. Why isn't it possible that bacteria that incorporate the phospahate flow back into the water column? Or the plankton? Or the pods as they get eaten by fish, and pooped back into the water column, etc. I have no knowledge of this, just asking a question.

3) the "everything poops" theory seems to rely on solid poop. Do we know what snail poop, worm poop, crab poop, etc look like? Are they solid or liquid, flowing back into the water column? Again, I have no knowledge, just asking a question. ...

It is entirely reasonable to ask what mechanisms there might be to bring nutrients like P back up into the water column. Bacteria can be mobile. I don't know if they are in the sand bed.

I do know that there are critters that move though the sand bed quite easily. There are worms that have their feeding apparatus pointing down and their arse end and breathing apparatus pointing up in order to take advantage of food in anaerobic zones. The 'crap' does not always just move down.

As for a noob posting in an advanced thread/forum, ya can't learn to swim if ya don't jump in the deep end. :fun2:

Edit: Oh yea, poop. You'd be surprised how well studied critter poop is. They all excrete some form of solid.

 

[Fredfish]

The answer to question 2 is in theory a dsb was supposed to be the be all end all for nitrate reduction...

Every new thing that comes along is the be all and end all until its not. We never learn do we.

 

[billsreef]

It is entirely reasonable to ask what mechanisms there might be to bring nutrients like P back up into the water column. Bacteria can be mobile. I don't know if they are in the sand bed.

I do know that there are critters that move though the sand bed quite easily. There are worms that have their feeding apparatus pointing down and their arse end and breathing apparatus pointing up in order to take advantage of food in anaerobic zones. The 'crap' does not always just move down.

Bioturbation is the fancy term for what you just described

Lot's of different processes going on in our tanks and sand beds. Tangs are the ocean version of rabbits, plenty of plants...after all algae comes under the heading of plants, not to mention seagrasses that are indeed true vascular plants related to terrestrial lilies.

IMO our tanks probable compare more closely to a lagoon than the open ocean reefs. If you have some good strong currents and tidal flushing to clean clean out the gunk, you get nice some nice reefs in those lagoons. Lack of strong currents and lack of tidal flushing, and the sediment starts building up. Do your maintenance (tidal flushing) and grow some nice corals or try the no maintenance routine and have a nice marsh full of detrital sediment build up

 

[vitz]

grain size of the sand used in a dsb is as equally important, if not more so, a consideration for dsb performance/'issues'.

while very deep (8"+) dsb's can certainly be run w/little to no negative impact on a system, today w/ oolitic 'sands', one can get the same benefits from creating anoxic and anearobic filtration zones w/a depth of sand of only a few inches(2-4)-thereby minimizing the potential for 'dirt trapping' w/in the dsb.