DSB Heresy

[Scleractinian]

The only conflicting views are from people that simply do not want to accept the data below and feel they have some way to improve upon what I have described it takes to build a CPW system to work effectively.

Just a comment here that we will probably address more in the chemistry forum. Note that some of these test kits involve redox chemistry. A little nitrite shows up as a lot of nitrate in some kits, for example. Since the plenum is a reducing situation where there may be redox species like nitrite and sulfite and other things, I tend to worry about the primary data that was generated by Thiel (not a chemist), and is being used to test these systems as well.

I'm very interested in your opinions of the potential merits of the 'process' Randy... I may be oversimplifying, but it seems that, if we can understand what's actually going on and manage it that the 'process' might function as a kind of infinitely deep sandbed...

My questions about automating effluent withdrawls are inspired by the periodic dosing (batch treatment) used in wastewater treatment systems. Do you think that it might be possible to treat a sandbed w/ a plenum and effluent drain as kind of a 'batch reactor' using this 'process', or that a VERY (dropwise) slow, consistent, flow might function like a denitrator coil, but be effective on PO4 as well?

Is anyone familiar with the design and function of small intermittent sand filters (ISF) used for on-site wastewater treatment?

BTW, it's (usually) clay if particle size is less than (about) 2 microns.

 

[ddoering]

Hi Shoestring,

First let me thank you for actually responding to my questions in regards to DSB and the substrate depth and size.

I had read a lot on DSB and old tank syndrome, and with all the different opinions and theories my head was spinning.

If the main issue with a DSB is phosphate buildup and release as the bed goes anoxic and bacteria dies, and not massive die offs, maybe I will just go with a DSB.

I want to have a lot of grazers and other critters in the tank, which is going to require algae to feed them, so if the problem is not the tank getting nuked and large die-offs, a DSB that lasts a few years before I have to transfer animals and then clean it out doesn't sound too bad. By then I would probably want a bigger tank anyway

I also value your opinion that coarser would be better, but you seem to be taking this approach for a different reason than ldrhawke was proposing.

You think coarser would be better because it would allow you to siphon out the solid wastes, and so they would not need to be processed by the bacteria because they are removed.

This makes sense, and is basically a modified under gravel filter that you are dumping the waste water instead of trying to re-use it after biological filtration. But if you are not depending on the sandbed for biological filtration, does it really matter what depth the sandbed is? In other words, wouldn't a coarse bed of 2 inches be better than a coarse bed of 5 inches if you are just trying to siphon out waste before it is broken down? Less bed to go through, less chance of clogging in the bed etc?

From ldrhawke's description of how this is supposed to work, you want water flow to suck the waste down into the sandbed so that the bacteria can go to work on it, and that the waste water siphoned from the bottom will have all the wastes in higher concentrations and soluble so you can export them.

This is why I don't understand why bed depth would make a difference. I see it how you are describing, which is basically an under gravel filter and vacuuming the detritus from the sandbed, but doing it from the bottom instead of from the top.

If you are actually relying on the biological processing in the bed, and just trying to prevent the deep anoxic area where sulphate reduction or whatever takes place, resulting in buildup of hydrogen sulfide and the anoxic layer moving towards the surface, then I would expect a shallower bed fo finer grain sand might also work.

It would be worth experimenting with, since I already have 10 bags or so of southdown sand that I bought in preparing for setting up a tank.

It will have to wait until I actually move out of this apartment so I can setup my first tank. That is why I am on these boards, trying to learn from other peoples' mistakes and experience

Cheers,
Doug

 

[ddoering]

ldrhawke,

Frankly I find your responses throughout this thread have been very antagonistic. You seem to lash out at anybody who asks a question or does not get down on their knees and bow to your greater wisdom for proposing this filtering method.

I tried to ignore your arrogance, and cocksure attitude to my first post where I was asking questions, and where you just blindly assumed that I had not read the thread. (It's not as if I hadn't had to get past it in some of your previous responses)

When I tried to explain why in detail I had asked the questions to demonstrate that I had indeed read the thread, I get more flippant remarks from you attacking me.

First I didn't read the thread, and my questions could be answered if I only would review the previous posts.

Next my questions are "good questions" but you brush them off as if they were an insult to ask you since I can answer them myself, or because there is no answer to those questions yet.
(You can't have it both ways, they can't be answered by reading the thread, and yet have no answers.) Also, just so you know my personality type, if I knew the answers I wouldn't ask you a question, I would just state that you were full of BS, and would prove it.

Lets clear some things up:

1) You are the person who started this thread, proposing this methodology and getting snooty naming it CPW.

2) You are the one proposing this will fix what is wrong with DSB and prevent crashing.

3) You are the one who stated in the beginning that you don't need long term results to know this will work.

4) You are the one who flamed somebody who was skeptical when stating they wanted more long term data to show the success of this method before they would embrace it as the next holy grail of reefkeeping.

5) You are the one who makes claims to being an expert in this area, with your company doing waste management etc. You even went so far as to put down somebody else who's knowledge in fluid dynamics would far exceed mine. So when I ask a question, it is showing that I am acknowleding you have knowledge and experience that I lack, and I am questioning your method to get a better understanding of how it works. Any idea that can not stand some questioning, and give responses to explain principles underlying the theory should not be presented as you have done, basically stating that what you say is FACT.

I give the idea merit, but nothing is so perfect it can't be improved upon. And if you do not see the benefit of having people with different viewpoints asking questions or making suggestions, why are you posting to this board?

I think your questions in your last post are useful, and would provide a good basis for a methodology for testing different grain sizes in the bed to see if adjusting this improves on the idea you proposed. Oh, and as you are so fond of basically blowing off answers and not giving data, you can find out the distribution of particle size of southdown sand somewhere here on RC. I believe it was a post in Dr. Ron's forum.

As you can clearly read, my hobby experience is just starting, and I don't even have a tank set up yet. This is why I come to these boards, and read, and ask questions. Some of my questions may seem stupid, some may re-ask something you feel you already answered, some may simply be me playing devil's advocate.

As for my motivation asking about smaller grain sand, it is simply that I have already bought some southdown and would like to use it instead of having to pay an overpriced amount at a LFS for specific sand.


Overall, I think the idea has merit, and can probably use some fine tuning/experimenting to see what comes of it to benefit the hobby.

Your attitude can definately use some adjustment. Quit being so defensive and antagonistic when people ask questions etc. If you can't stand people calling your baby ugly, don't enter it in any beauty pageants.

Doug

P.S. I think the naming of this methodology as CPW is pretentious BS, but that is just my opinion.

 

[Shoestring Reefer]

I had read a lot on DSB and old tank syndrome, and with all the different opinions and theories my head was spinning.

I know the feeling.

If the main issue with a DSB is phosphate buildup and release as the bed goes anoxic and bacteria dies, and not massive die offs, maybe I will just go with a DSB.

I want to have a lot of grazers and other critters in the tank, which is going to require algae to feed them, so if the problem is not the tank getting nuked and large die-offs, a DSB that lasts a few years before I have to transfer animals and then clean it out doesn't sound too bad. By then I would probably want a bigger tank anyway

Well, they usually seem to last a few years. There's a DSB poll, if you're interested in some very un-scientific statistics. Just remember to vacuum the poop. I have a DSB, but I'm going to have jawfish, so my DSB really won't function as a DSB, anyways. I'll be hopefully getting a house in a year or two (cross fingers) so it will be moved, anyway. I'll probably want a bigger tank, too.

I also value your opinion that coarser would be better, but you seem to be taking this approach for a different reason than ldrhawke was proposing.

You think coarser would be better because it would allow you to siphon out the solid wastes, and so they would not need to be processed by the bacteria because they are removed.

Yes, I imagine it functioning differently that ldrhawke proposes. The way I picture it, there is a net flow down through the bed, so not much is going to come up from the bed-certainly not much from the bottom of a deep bed. If that's the case, there won't be much biological filtration for the tank-the filtered water will leave the system. But, if there won't be much biological filtration for the tank, why bother in the first place? I suspect the top-most part of the bed will help with water column filtration, but I wouldn't imagine the bottom of a 5" bed with downward flushing 3 times a day will contribute much.

This makes sense, and is basically a modified under gravel filter that you are dumping the waste water instead of trying to re-use it after biological filtration. But if you are not depending on the sandbed for biological filtration, does it really matter what depth the sandbed is? In other words, wouldn't a coarse bed of 2 inches be better than a coarse bed of 5 inches if you are just trying to siphon out waste before it is broken down? Less bed to go through, less chance of clogging in the bed etc?

From ldrhawke's description of how this is supposed to work, you want water flow to suck the waste down into the sandbed so that the bacteria can go to work on it, and that the waste water siphoned from the bottom will have all the wastes in higher concentrations and soluble so you can export them.

This is why I don't understand why bed depth would make a difference. I see it how you are describing, which is basically an under gravel filter and vacuuming the detritus from the sandbed, but doing it from the bottom instead of from the top.

If you are actually relying on the biological processing in the bed, and just trying to prevent the deep anoxic area where sulphate reduction or whatever takes place, resulting in buildup of hydrogen sulfide and the anoxic layer moving towards the surface, then I would expect a shallower bed fo finer grain sand might also work.

Well, I'm not sure. I think that what you say has merit, but I'm just stuck in the DSB mentality, I guess. Deeper just "feels right" to me.
BUT- I'm not setting one up. If you are, just go with your gut. It appears that you will have the second tank set up like this, so you could be a pioneer. Go for what you think is right, and keep us posted.

It would be worth experimenting with, since I already have 10 bags or so of southdown sand that I bought in preparing for setting up a tank.

I only bought 3 or 4. The last bag was torn and half-empty (or, half full?) of sand, so I had to scoop it up off the floor. It's a shame that DSBs turned out to not be as great as we all hoped; South Down/Old Castle was a pretty nice find.

It will have to wait until I actually move out of this apartment so I can setup my first tank. That is why I am on these boards, trying to learn from other peoples' mistakes and experience

Cheers,
Doug

I know the feeling; moving is a pain, and having a tank doesn't make it easier.

 

[Shoestring Reefer]

I give the idea merit, but nothing is so perfect it can't be improved upon.

I agree, and I really enjoy the process. I'd say it's the engineer in me, except that pretty much everyone seems to enjoy it. That's why I've stuck around.

Try to not let anyone here ruin your day. Trust me, I get pretty ticked, too; but then again, there are a lot of great people on here with decades of experience who will spend incredable amounts of time helping us newbes out.

 

[H20ENG]

Quote by scleractinian:
"Do you think that it might be possible to treat a sandbed w/ a plenum and effluent drain as kind of a 'batch reactor' using this 'process'"
This is the way I think this "method" SHOULD be used if we want to retain the beds ability to denitrify. It is very possible to remove too much water from the bottom and reduce the number of denitrifiers by flushing the bed with oxygenated water. IMO, the flow rate can only be adjusted by testing the effluents DO content, or very gradual changes with consistent nitrate testing of the main tank.
It almost sounds as if ldrhake no longer wishes to use his bed for denitrification, only for nitrification. Cant get a straight answer.... Maybe he can clarify.
Keep up the great commentary, guys! Funny how some of these things get turned into soap operas.
Chris

 

[ddoering]

Hey Shoestring,

Once I get things setup I will definately be posting to show progress and get feedback, as well as seeing if anybody has ideas on improving things.

It is also good to get an idea of things that you might want to test for, so you can do so from the beginning and keep as accurate of data as you want so others can compare their experience to them. (Part of the reason I was asking about what is building up in the bed... I want to know if it is something
I can test for and should when I get my setup cycled and going).

I try to give people the benefit of the doubt on here, and really do appreciate the help from people who have taken time to answer questions or provide insight on new ideas. From researching and reading on here, I have already probably saved countless fish/corals and money that would have been lost, and I haven't even cycled a tank yet.

Cheers,
Doug

 

[Randy Holmes-Farley]

I'm not sure what criteria would be useful to show that draining water off of a plenum was a better procedure than a DSB alone, with or without a plenum.

What exactly are you guys proposing to use to say that it is a success?

 

[ldrhawke]

I'm not sure what criteria would be useful to show that draining water off of a plenum was a better procedure than a DSB alone, with or without a plenum.

What exactly are you guys proposing to use to say that it is a success?

Good question....

I know you are not a big believer in what an ORP meter shows, because there are so many variables and different readings for every tank.

I do believe the changes in ORP read out is indication of good and bad changes in tank chemistry.

The coral and fish appear much happier in a tank with an ORP reading of 350 to 400 compared to one at 200. I also know this is subjective, but my observation and a number of other reef keepers observation agree. Nearly every commercial aquarium uses ozone, and an ORP controller, to raise ORP levels. Many still believe you cannot keep a tank at its best without it. But, that is another topic.

Example: When I feed my tank ORP drop 30 or 40 points and then comes back up over the next few hours.

I had been using a declining ORP reading as an indiction if the tank is getting out of balance and water quality is dropping. When making a 15% water change the ORP would move back up and the response to change would happen quicker, than with a low ORP.

To me an ORP reading is like a looking at a barometer for the weather. Doesn't necessarily tell you everything but is a good indicator of change or rate of change.

Back to your point...."what will determine CPW is a success".

As I have posted, my tank experience has been, even with making 15% weekly water changes, to maintain an ORP setting of 350Mv, my ozone feed would cycle all day. When I was wasting only a half quart a day with CPW, ozone would still cycle all day.

When I increased my CPW rate to 1 gallon a day, my ORP stopped cycling. Not only stopped cycling but continued to climb to just under 400mv. It has virtually eliminated the feeding of ozone, which I have set at 350mv on the controller.

I know we can question the value of ORP and what this all means, but to me it is an indication that I am getting positive results.

Before my SPS would not open up and appeared to be struggling. My frog spawn was shrinking. They are now both keep their polyps open all day, appear to be growing, and in general the whole looks much better.

Why do I think CPW is the cause of this? Based on logic and data, waste concentrates in a DSB. CPW removes some of that concentrated waste and reduces the amount that migrates back to the top of the substrate by keeping the flow toward the bottom of the substrate and releasing build up.

How do I know this.....

1.The nitrate and phosphate in the waste is always much higher than what is in the water column.

2. If, instead of dumping the wasting fluid down the drain, I allow it back into the tank, my ORP meter drops over 100 points in a few minutes and takes almost a day to recover.

3. I am wasting only about 1/2 of what I was removing with water changes and getting improved ORP readings without ozone feed.

3. If I drop a few pods or newly hatched brine shrimp into what is being wasted they immediately die.

4. I know I have one more way of measuring and reacting to water quality; other than measuring Ca,Alk,Ph,Ni, Ammonia,etc.etc,etc. in trying to maintain life in a bottle.

It is logical if you remove the waste from where it appears to be concentrating, instead of simply pulling it from the water column, that you will have to remove less.

Chemical and biological chemistry and the what, how, and when is nice to know and understand. It may help you improve results. As more reef tank keepers apply CPW an start to develop more history and data, I am sure we will see further improvements.

The most important thing to me is always results.

As in a lab, sometimes you analysis what you have, even before you know exactly why you have it. Often the physical order of making a reaction is as or more important than the chemicals used. Experimenting with the variables to improve the results is what normally follows.

I think I learned this when I was ten years old watching a movie, in the Museum of Science and Industry in Chicago, about the men and how they discovered aluminum and vulcanized rubber in kitchen labs.

 

[Shoestring Reefer]

My 0.02

My 0.02

I'm not sure what criteria would be useful to show that draining water off of a plenum was a better procedure than a DSB alone, with or without a plenum.

What exactly are you guys proposing to use to say that it is a success?

Good question IMO, Randy. That's not really been discussed, and is a pretty important topic.

Well, long-term success is out, because that can be achieved with either DSB or DSB with plenum, or with BB. Plus, you would need to wait 10 years (assuming it didn't fail before then.)

If my head is on straight, ORP testing of the bed is out, because DSBs are bound to have low orp. Correct me if that's wrong.

Maby ORP testing of the water column, but many people don't even monitor orp; having an OPR of 320 in one system and 360 in another doesn't necessarily one system is a success, the other a failure. Unfortunately, it's one of the few things we can observe.

You can't test the water for phosphate, because macro-algae just suck that up. Nitrite and ammonia should be undetectable unless something really wrong, too.

Maby nitrate in the water column?

Maby a side-by-side comparison of two similar tanks (maby even sharing the same water, to eliminate water quality contributions to bed degeneration) to see which is the first to release phosphate (by waiting for the blooms, and then testing the bed material, possibly sampling at various depths.) Maby, bed testing could be done in the meantime. Obviously, more than my little phosphate test kit would be required, but if a laboratory is what it takes, that's what it takes.

I don't think a test of the waste water really can indicate success, because that water is thrown away. Sort of like testing RO waste water to see if your RO is working properly. It can indicate the performance of the system for tweaking purposes, and maby changes in trended data would indicate potential or impending failure, but I'm not sure.

I guess the problem, as I see it, would be making a controlled experiment, and keeping in mind that success/failure is based on long-term water column quality (because that's what we're after in this hobby) and not bed quality (because the bed is just a tool, not the goal.)

 

[Shoestring Reefer]

From what I've seen, an ORP meter is about $100.00. If, in addition to that, coarse (ie, not South Down/Old Castle) substrait is required (at $0.50 to $1.00/lb), you're talking about $200 to set up a bed in a 55-gallon tank, not to mention the plumbing.

That puts it out of reach of many beginners, especially considering that 1) a lot of us are in apartments, and will be moving, anyways. and 2) A DSb costs about $20 to set up.

It would be nice if a non-orp way of testing the water could be found, just from a financial standpoint. That would get more nubes trying it sooner, building a bigger pool of reefers with experience.

 

[H20ENG]

$0.02 more....
One thing to consider, in defense of ldrhawke, is the other methods that were so widely adopted (Plenum, DSB, Wet/Dry, etc.) WITHOUT the testing data we all want to see here. These methods have shown their good and bad after many installations, and this is just another method that may prove to work or not after more use and tweaking by reefers.
I propose that we still dont really have a handle on what really goes on in a tank, though we are getting pretty close.
To say this is the end-all to filtration would be wrong. Even ldrhawke has said that it will require more testing, (though he seems to have most of the answers.....).
At least, its good discussion to further the knowledge of the hobby.

 

[6 Line]

I would just like to concur with "ddoering"

You have stated on here how "YOUR DESIGN" is such an improvement on the DSB and Plennum designs, to which many people have had great success and many people have had failures.
I for one think that you have just jumped on the back of a system to which there are some technical issues relating to there demise, and started BLOWING YOUR OWN TRUMPET on how fantastic and wonderful you really think you are, because you added a drain off system.
Your system is only a modern day example of what Edward Frankland designed in 1868 after the invention of the Septic Tank by 'Louis Moureas' in 1860 (was not named a Septic Tank until 1895), Edward F added fine sand settlement chambers to allow the efluent from the affore mentioned Septic Tank.

So this brings me to another point you made on how good your company is to give you your so called credibility on this board, You have only benefitted from what the Greeks started in 300B.C.E then passed to the Romans etc. I think they call this Evolution and Knowledge Transitional Developement.

If you give advise, you need to accept criticism or questioning to allow others to Develop there own knowledge, alas you see fit to great peoples questions with contempt and dismiss them as simply not worthy of sensible explinations.

I am with others on this board who take things as they come and share what I have learned with others. I find people like yourself very infuriating due to there own self opinionated thoughts ."Look at me I'm brilliant"

I hate to burst your bubble, but lets wait for your system to see 10 years with no problems to start dictating to others how magnificent you are!!!!

Regards
Jay.

P.S. Us English do like a joke, but we like to be in decent company to appreciatte it.
:bum:

 

[Shoestring Reefer]

$0.02 more....
I propose that we still dont really have a handle on what really goes on in a tank, though we are getting pretty close.

...

At least, its good discussion to further the knowledge of the hobby.

That sums it up quite nicely. It's a shame we have to wait several years to see if a method panns out or not, though.

 

[ldrhawke]

Quote by scleractinian:
"Do you think that it might be possible to treat a sandbed w/ a plenum and effluent drain as kind of a 'batch reactor' using this 'process'"
This is the way I think this "method" SHOULD be used if we want to retain the beds ability to denitrify. It is very possible to remove too much water from the bottom and reduce the number of denitrifiers by flushing the bed with oxygenated water. IMO, the flow rate can only be adjusted by testing the effluents DO content, or very gradual changes with consistent nitrate testing of the main tank.
It almost sounds as if ldrhake no longer wishes to use his bed for denitrification, only for nitrification. Cant get a straight answer.... Maybe he can clarify.
Keep up the great commentary, guys! Funny how some of these things get turned into soap operas.
Chris

I thought I have been giving straight answers from the beginning.

I initially hoped and believed a DSB/P system could be set up to complete the nitrate cycle. I no longer believe that based on present experience and knowing about what time and volume is required for de-nitrification. There is not enough volume in a tanks DSP to efficiently complete the nitrification cycles.

Everyone seems to feel just because a nitrification cycles takes place in a DSB that is all there is to it. They totally disregard loading and transfer required to process all the waste in the water column above.

Can you set up separate additional systems to complete they nitrification cycles...certainly. I have said numerous times that can easily be done.

This is not the question you should be asking. These are the questions to ask.

1. Is the cost for de-nitrification equipment and operational oversight worth it compared to simply dumping 1/60th your system volume into the drain?

2. If i did install all the equipment to complete the total nitrification cycle have I totally treated the waste water, or are the other type of waste such as phosphate, silicate, and other organics not fully stabilized that I will be recycling back into the tank?

3. What sort of additional treatment or filters will I have to add on to remove phosphates, silicates, and organics after de-nitrification?

4. Is it easier, cheaper, and more logical to simply look waste a gallon ( or 1/60th) from the plenum?

5. Don't I have to waste even more water with normal water changes anyway?

6. Don't I need to replenish the minor elements used up and put them back into the water, no matter what I do? Isn't is simpler and easier to do it with fresh salt mix than separate costly additives?

I now look at the substrate bed as a large biological dash pot that can absorbs shock loads.

I look at it as a good source of food for the coral.

I will make up for a DSB's biological processing inefficiency in simplest, easiest, and most cost effective manner.....dump 1/60 of my volume every day into the drain.

If you read my past posts I have said this same thing time and time again. Maybe listing it in this manner will make it easier to understand.

 

[Randy Holmes-Farley]

The reason that I asked about what might demonstrate that plenum draining is a benefit is that there are a host of proposed technologies, from ozone to eco-aqualizers. Knowing that there is a clear benefit from using them is far from trivial.

 

[Scleractinian]

Originally posted by
I thought I have been giving straight answers from the beginning.

...

If you read my past posts I have said this same thing time and time again. Maybe listing it in this manner will make it easier to understand.

ldrhawke-- Ummm... I should have been more clear in my post (since I'm VERY sure that you must have READ it), but my last questions above were intended for Randy, and for anyone familiar w/ small scale ISF technology.

The tone of your responses is what leads others to post in a way that you seem to find disrespectful. This thread is not yours. WADR, you seem to feel that those who question, critique, propose potential improvements to, etc. your concept are personally attacking you. I might suggest that this is not the case, that you lighten up a bit, and that you realize and accept that RC is a FORUM, intended for the polite exchange of ideas.

If you'd prefer that people not be skeptical of your idea, or that they achieve a level of enlightenment approaching yours before they post questions, then perhaps you should host your own discussion elsewhere...

I appreciate the fact that you posted the idea here, and resepect your efforts in trying to help people understand it better, but...

Remember, I am an idiot, not an engineer or CEO, and am probably a Greenie and an EPA type, who apparently can't read.

 

[Scleractinian]

I'm not sure what criteria would be useful to show that draining water off of a plenum was a better procedure than a DSB alone, with or without a plenum.

What exactly are you guys proposing to use to say that it is a success?

The barebottom crowd seems to be suggesting that conventional (ala Shimek) DSBs have some undefined crisis at around year 4.

Got 5 years?

Randy, if PO4 accumulation and release is supposed to be the problem w/ aging sandbeds, is there a practical way for us to compare PO4 inputs (food and water) and exports (skimmer, water changes, and especially CPW (TM) effluent, etc.?

 

[Randy Holmes-Farley]

Randy, if PO4 accumulation and release is supposed to be the problem w/ aging sandbeds, is there a practical way for us to compare PO4 inputs (food and water) and exports (skimmer, water changes, and especially CPW (TM) effluent, etc

I'd think you would be comparing two large numbers (input and output), looking to see if the small difference between them got bigger or smaller. Given the difficulties of measuring phosphorus in things like solid macroalgae, skimmate, coral skeletons, and fish growth, I don't think it would be doable.

 

[Randy Holmes-Farley]

Do you think that it might be possible to treat a sandbed w/ a plenum and effluent drain as kind of a 'batch reactor' using this 'process', or that a VERY (dropwise) slow, consistent, flow might function like a denitrator coil, but be effective on PO4 as well?

Yes, that seems possible. I'm not sure how you'd decide what spped or frequency of draining was optimal. Maybe an ORP electrode stuck into the plenum, drain when it hits a certain low ORP threshold, and stop when it hits a higher threshold.

That would keep the plenum from getting too anoxic (if, indeed, that typically happens) and also prevent it from becoming too oxic, shutting down anoxic processes until the oxygen is depleted again.