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I have seen a couple of videos on youtube where people are raising duckweed and freezing it into blocks. The fish really seem to like it. If it's frozen is it safe as a fish food? I don't want it to take over my grow beds.

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Gina - the ammonia will be down at 0.16 ppm and I've had nitrates hitting 80+ ppm before the set-up was refined a bit and they (the duckweed) do not go for it one bit. 

 

Benhehle - I have seen everything from reports claining a FCR of 1 for tilapia on duckweed through to people that swear that you should not use it.  Bottom line is all tilapia I have looked at (Nile, Aureus, Mozambique) are omnivores capabpe of eating a large proportion of diet in plant form.  I have also found "western" (read into this institutions that may be food industry sensitive) papers to be more sceptic and "Eastern" papers (read here production should be as cheap as possible to be economically viable plus we support farmers in rural applications) being more supportive. 

 

I'm sure a 50 - 60% substitution may work, and as it costs almost nothing to produce, the longer time to market will only influence your power bill.

 

Here is my system layout:  I have 9 beds in plug flow configuration, with water flowing in on one ind via a small solids seperator and out the other end directly into the fish tank.  Fish tanks has no duckweed in it, the swirl filter only poo and uneaten food thus the duckweed stays in the bed.  Not sure what happened at the Friendlies.

 

 

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At friendlies they put duckweed into a tank with a hole tank hapas net.  Duckweed got trapped between the next and the tank wall, died and rotted consumed all the dissolved oxygen and killed some fish in that same tank.  They didn't notice the black rotting slime between the net and tank wall and only noticed the problem when fish started dieing.

At least that is kinda the gist I got from the newsletter they posted about it once upon a time.

Hi Kobus,

Wow! Liking your Swirl a lot. Great design

Thanks, but in hindsight, I could have made the fines trap at the top a little bit more substantial.  It is a 5 liter bucket but I think it will be better if it is 15 - 20. 

Harold Sukhbir said:

Hi Kobus,

Wow! Liking your Swirl a lot. Great design

You may notice that often outdoor municiple sewage system are often green on top.  It is usually Duckweed.  Works well there too.   I do not grow it as a main source of food.   I have tried that and it simply was not practical.  I only use it to supplement,  especially fish I want food in front of all the time and I am going to be be gone for a while.   I have tried pelleting duck weed,   not practical.   In India,  They use duck weed to clean the human sewage, then gather it daily and feed it to their tilapia.    Good use of nutrients, however I would like to see another step between the sewage and the human food, but it works.   At least part of my aquaponics system will have a deep water component.   I grow the duck weed at the beginning of the deep water growbed,  if the duckweed starts coming close to the exit, well that duckweed is the first to go.   A word of caution for those that want to harvest wild duckweed.   It can be prone to all kinds of pathegens which makes it not worth the trouble.
Yea, parasites, fish, tadpoles, snails, insects and all sorts of other creatures can also come with the wild duckweed.

I found this in one of my wastewater textbooks - Natural Systems for Wastewater Treatment

I think you are going to have to work miracles with AP to get those protein values though Terri.  Waste water comes with phenominal ammonia levels, and in AP, you can perhpas squeeze by with 2 mg/L.  At the rate the duckweed grows in my test unit (ammonia levels at no higher than 0.25 mg/L now in winter) I cannot see how the plant can turn the low ammonia level into 40% protein in a few days.

 

That said, they are phenominal scavengers for resources (good and bad in terms of trace elements and heavy metals) and well worth trying.  With 380 g dry mass sample, I will know just what duckweed can accomplish in AP water.  Should have that type of sample by mid-summer.

I'm not a plant expert, but it seems to me that the protein content will fall within the the range quoted. What will be affected by a lower ammonia concentration is the rate at which the mother plant's fronds divide to produce new fronds (daughter plants).

"Duckweed fronds can double their mass in 2 days under ideal conditions of nutrient availability, sunlight, and temperature."

Terri, if you look at the info pertaining to duckweed protein content, then most of it is based on natural "pond full of dead litter"  or over 1000 mg/l waste water.  In my research system, I'm giving them between 0.1 and 0.25 mg/l, but their growth rate is not deminished by much - 3 to 5 day harvest intervals in summer. 

 

I do not have data yet, but I cannot see how the plant can sit in it's upper range of protein content if it is fed ammonia fumes.  The data source you cited was from waste water management (and also not quoting specific species, as "duckweed" is a group of 4 - 5 different genera), thus I assume they stuck to protein levels generated under those conditions in the table.  In nature, (as with algae - everyone always quotes the best case scenario there too) the reality is that the protein content can be as low as 6.8%, and the carbs as low as 14% (as opposed to 46% in a glut).

 

I'm not wanting to be negative, but am not expecting top notch protein from duckweed grown in the same water in which fish must survive.

Terri Mikkola said:

I'm not a plant expert, but it seems to me that the protein content will fall within the the range quoted. What will be affected by a lower ammonia concentration is the rate at which the mother plant's fronds divide to produce new fronds (daughter plants).

"Duckweed fronds can double their mass in 2 days under ideal conditions of nutrient availability, sunlight, and temperature."

Kobus, I don't look at your comments as being negative AP is a learning process and I'm here to learn. I have a fairly good understanding of the water quality aspect of AP, but plants and fish are another story. I usually kill house plants, which one reason why I like AP, I won't forget to water them!
I am planning a separate system for duckweed using kid pools. I don't know how it's all going to work yet but learning is half the fun.

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