How to figure stocking in a mixed system

[TCLynx]

This is definitely a subject that doesn't have much study behind it yet.

 

At one point some one asked me how many fish they could keep in a 1000 gallon fish tank if they had a 4' by 8' 12 inch deep gravel bed plus a 4' by 32 foot raft bed?

My initial inclination would be to figure the amount of fish the gravel bed would support and then figure the amount of fish the raft would support based on the Friendlies Low density methods.  However, such a design is likely to overload the gravel bed with solids if all the water is going through the raft bed first.

 

I also tend to figure based on filtration and am not too worried about the nitrate levels and plant uptake of nutrients.  Seeing as the filtration must be up to the task or the entire system can crash and leave one with lots of dead fish to dispose of.  If the nitrates get too high one can simply add more plants/greedier plants, reduce feed for a bit, or water some soil plants and top up and if there are too little nutrients, one can simply pull out some plants for a while while.  This is easier with rafts/NFT as pulling and replacing plants doesn't stir up the system much.

 

My basic rule for stocking with the media based portion is 1 fish (grow out to 1 lb) per 7 gallons of grow bed (that would be the recommended stocking level.)

 

Then for the low density raft stocking (I don't know this one as well) but I think it is supposed to be around 3.5 square feet of raft space per pound of fish.  (This is because the low density raft system doesn't remove the solids and therefore the system has more nutrients in it per unit of feed given to a particular amount of fish.)

 

So for the above example,

4x8 gravel bed provides 32 cubic feet (7.5 gallons each) so the gravel bed could support 32 fish up to 1 lb each.

4x32 of raft is 128 square feet divide by 3.5 = 36.5 fish up to 1 lb so I add those together and get 68.5 fish supported by the filtration.

 

Again, this is just how I estimate an appropriate fish load for the filtration, keeping in mind that the gravel bed probably could become overwhelmed by the solids from such a fish load long term.

 

Also, the appropriate recommended numbers are from a "backyard" low maintenance point of view and stocking for commercial situations will be different and proper monitoring of dissolved oxygen to make sure the fish don't die overnight or during a power outage is important at higher commercial stocking levels.

 

[Harold Sukhbir]

TC i am recently looking at the question of mixing. I am assuming it's a closed system with one source for fish. Once we consider media beds as our principle means of filtration we are already confined to the rules of thumb( 1lb fish > 7 gal media) as you said. Go over this figure and filtration will not be sustainable over the long term.

To be true to standard solids filtration, using your example,realistically we'll need another 4x8 bed plus 3.5 extra cu.ft  totaling  32+3.5 = 35.5cu.ft. This would be low density backyard as you said.

The two questions seem to be filtration and nutrient availability.How to design a mixed system that will be sustainable. Of course we can have more, approximately double the media beds to accommodate the solids and reduce plant load in them to about one half to allow for the nutrient requirement of our raft. I have also thought of dividing the fish tank output, one to swirl/net filter then raft, the other from fish tank to bed then raft, although this would be more labor input. We can also use flow rate as a means of controlling fish waste from the fish tank on the line that is divided going to the raft to imitate a low density system for the 36.5 fish without added filtration.

Then there will be the question of aeration, in F&D aeration is more naturally occurring as compared to raft and  friendlies add 1.5 cfm/100 fish and this applies to low density systems as well. Commercially filtration demands are extremely high and I'm not sure if filtration through media will be appropriate.

[Kobus Jooste]

I think, in the long run, that we will have to find a way to put the media component on steriods if mixed systems are to break out beyond the fish - to - gravel bed ratio confines.  That said, I have managed to make my mixed system run into nutrient trouble without having messy gravel.  In fact, the fish tank is perfectly cristal clear.  40 large tilapia in an IBC tank looking happy, but plants showing signs of nitrogen stress / chlorosis with poor growth. I have to say at this point though, that I switched food right about at the time that things are not going that great and I have my reservations about the quality of the food (same manufacturer, different composition) that I am now using.

 

Every design will have its critical limiting factor, and in my system, it is the amount of gravel available.  I think having worms in the beds are crucial (I still do not have that) to get them to stay healthy as best we can, and then perhaps also looking at media filled structures other than the beds, such as Nate's towers or just media-filled pipe to increase the ability of the system to deal with solids.  We want to feed the system based on its entire plant volume, and we will just have to develop the means of ensuring that we deal with the solids innovatively.  This may happen through the use of deeper gravel, by having hybrid biological filtration, media filled small diameter structures or something else I have not thought of!

[TCLynx]

Well I have found that long run, a media bed system that is heavily stocked with fish (heavy stock of fish for me is actually about one fish per 5-10 gallons of fish tank and I have twice as much grow bed volume as I have fish tank) for me has had more than enough nutrients to support the plants in the gravel beds plus plants in the add on means of growing like NFT, towers, and rafts.

This might be related to the fact that my system has all the shells in it and so the pH doesn't drop below 7 and therefore my nutrient levels tend to run high as the plants have some trouble keeping up with the fish and bio-filter.

 

Anyway, if the solids are left in the system, and the system is allowed a bit of extra time to mature, I expect the balance can be found.  I have found there is a major difference in nutrient levels in relation to the feed.  Higher protein feed will cause far more ammonia/nitrite-nitrate for a system with less solids to handle.

 

Now if one wishes to run less gravel bed well, there are people out there testing out that idea.  Like Kobus and Chris Smith.  The question here is can smaller gravel beds keep up with the increased solids or will they require regular clean outs?

 

I know of some trials with aerobic digestion/mineralization of the solids within a system instead of removing the solids.  Those trials were cut short due to other issues but they seemed promising.  Basically a tank where the solids were diverted and chewed up with lots of bubbles and the clearish water allowed to flow back to the system.

[Harold Sukhbir]

Hi All,

TC said;  

 I know of some trials with aerobic digestion/mineralization of the solids within a system instead of removing the solids.  Those trials were cut short due to other issues but they seemed promising.  Basically a tank where the solids were diverted and chewed up with lots of bubbles and the clearish water allowed to flow back to the system.

 

Great Idea TC! A separate, remote station for mineralization, off route within the loop, massive DO introduction, and particulate free water transfer back in-loop. Certainly appealing for commercial application as we are definitely moving toward this.

[Kobus Jooste]

I have been paying some attention to this one since our fish food composition changed to higher protein levels, that throws my system design out of whack if you look at the scaling tool provided on the forum.  If you look at getting the first principles right, you need to scale your media beds according to fish food input otherwise you will have a mucky system.  Here then you have some options, but in the end, your combo of hybrid bio-filter and media bed / media tower surface area must suffice to get your nutrient management optimized to your fish tank volume.  After that, you can have a bash at addding as much media-less components as you can feed with your fish biomass. 

 

The problem is that, once you have the mineralization bed volume right, and you plant that to capacity, you may not have a lot left for media-less culture (using the calculator). As an example:

I have a system of around 530 gallons (2000 liter) fish tank, stocked to 20 kg per cubic meter of water (the max for the calculator).  My food, unfortunately, is 41% protein.  Thus, according to the calculator, I am in need of 17 square meters (183 square feet) of media bed @ 12 inch depth (30 cm) to deal with the food input.  Unfortunately for me, this is more than the total area of leafy greens required to deal with nutrients after mineralization, making the addition of media less components pointless if I plant up all the mineralization surface area.

 

Thus there are a number of options:

1) do not plant up all the media beds - kind of pointless if you had to pay to put all that gravel in.

2) make the beds deeper in order to cut on surface area needed.  I like this idea, as you can give yourself a lot more space by taking the media beds to just 15 or 18 inch depth.  I do not see a serious risk of anoxia if the beds are flood-and-drain.

3) have mineralization sites that are out of the way in terms of your planting space, such as under raised beds.  These will allow you to increase mineralization space without having a massive expanse of gravel.

 

I therefore propose the following:

Step1: Determine your desired fish tank volume, stoking density and feeding rate / feed composition.

Step 2: Determine how much media you need to deal with 100% on site mineralization.

Step 3: determine how much media bed surface area you want.  Make your beds a bit deeper, and see how that compares with the calculator volume at 12 inch depth. 

Step 4: Determine your mineralization bed shortfall, and add that in the form of media towers or bio-filter.

Step 5: Look at the calculator's output for leafy green bed potential for your system's original component ratio calculation, and use that as a rough guide for how much media-less components you may be able to support. 

[Harold Sukhbir]

Will the landscape change with added aeration? The rule of thumb applies for passive aeration. We have to weigh the cost and compare with additional aeration, an increase in oxygen to speed up mineralization.

[Kobus Jooste]

I think the increase in aeration will help, but the physical area that is needed to trap and retain the solids is the bigger issue.  Having more oxygen will help fish and bacteria by reducing competition for it, but to begin with, I think I will advise designs around sufficient redundancy in mineralization area until we know a bit more about hybrid systems.  What do you think?

Harold Sukhbir said:

Will the landscape change with added aeration? The rule of thumb applies for passive aeration. We have to weigh the cost and compare with additional aeration, an increase in oxygen to speed up mineralization.

[TCLynx]

Well I've found that (so far in my aquaponics journey of a few years) having twice as much gravel volume as fish tank volume seems able to support a good amount of fish without seeming to have much trouble with anoxia.  Now according to the Doctor's calculator and recommendations, he seems to think that the shallow bed depth (30 cm or 12 inches) is as deep as it should go and that the mineralization requirement should be based on square footage.  Now this is where I ran into the most problem with it.  Because according to the calculator and recommendation that the media beds be based on a square footage at 12 inches deep, well then my big system doesn't work.  It does not meet those recommendations. 

 

My 700 gallon fish tank with at least 140 pounds of fish (and that is now, I did have more in there.)  Is doing well with about 1400 gallons of gravel.  900 of those gallons of gravel is in beds 2 feet deep and another 300 gallons is in a bed that is about 18 inches deep and only a small amount in beds that are 12 inches deep.

 

According to the square footage rule, I don't have anywhere near enough grow bed to handle the mineralization of the solids in my system.

But, Going by the twice as much gravel as fish tank rule, my system is pretty good.  I must admit my flow rate is far higher than the calculator calls for (for my fish load it tells me to up the size of my fish tank and calls for less than a 300 gallon per hour flow rate!!!!)  Well my flow rate through the fish tank and to the grow beds is more than twice (probably more like 3 times) the volume of the fish tank per hour.

 

Anyway, I think deep beds are definitely the way to go if you want a combo system.  I can't back that with any hard science though.  Just simple flood and drain gravel systems with deep beds seem to be working for me and Outbackozzie

 

 

Now I agree that redundancy in mineralization area is a good thing I am a little worried for those systems with 1000 gallon fish tanks and big raft beds but only a few hundred gallons of gravel bed.

[Harold Sukhbir]

What if we find a way to increase the Bio-availability of nutrients in our AP systems by following TC's suggestion. We bypass solids into "digestion units"  supplemented with adequate oxygen and reintroduce the processed water into the system. This may solve the problem of over planting and solids buildup to an extent. The system would remain closed but at the time have latent nutrient in the loop.

[TCLynx]

ya just have to figure out how you will make it work mechanically.  Do you simply have the water flow through the digester or do you have to suck the solids out of a setting tank and place them in the digester?  How do you design the digester so that only clearish water escapes to the plant beds?  I don't know the physical design of the digester that BYAP used for their commercial set up.

Harold Sukhbir said:

What if we find a way to increase the Bio-availability of nutrients in our AP systems by following TC's suggestion. We bypass solids into "digestion units"  supplemented with adequate oxygen and reintroduce the processed water into the system. This may solve the problem of over planting and solids buildup to an extent. The system would remain closed but at the time have latent nutrient in the loop.

[Harold Sukhbir]

TC said; "or do you have to suck the solids out of a setting tank and place them in the digester?"

If we want to remove 50% of solids for processing, we can simply divide the flow from the pump with equal size pipe, if we need more or less, we can manipulate flow rates.The digester could possibly be a large swirl/net combination filter with constant aeration. With this, solids can remain for longer periods in the system to mineralize and not run the risk of the system becoming anoxic because of the presence of large amounts of oxygen. Although some of the solids will have to be eventually removed periodically, overall we can increase the bio-availability of nutrient for our plants. In clarifier's and net or swirl filters nutrients are underutilized and valuable nutrient wasted.

 

[TCLynx]

I don't know that they ever need to be removed?  I know of many gravel bed systems that find they never need to clean out the beds so provided the "digester" has enough capacity to cope with the solids, I expect that one will find the solids will seem to magically vanish.  Only time in a system where solids "have" to be removed is when there is not enough capacity to handle the mineralization and use the nutrients.  For example in a system where the fish load is too much for the plant/filtration side of the system, people might remove solids to use them in the dirt garden.  To me that seems a design flaw in the system but some people see it as providing fertilizer for the dirt garden and don't mind the extra steps involved.

Harold Sukhbir said:

TC said; "or do you have to suck the solids out of a setting tank and place them in the digester?"

If we want to remove 50% of solids for processing, we can simply divide the flow from the pump with equal size pipe, if we need more or less, we can manipulate flow rates.The digester could possibly be a large swirl/net combination filter with constant aeration. With this, solids can remain for longer periods in the system to mineralize and not run the risk of the system becoming anoxic because of the presence of large amounts of oxygen. Although some of the solids will have to be eventually removed periodically, overall we can increase the bio-availability of nutrient for our plants. In clarifier's and net or swirl filters nutrients are underutilized and valuable nutrient wasted.