Duckweed's potential role in a hybrid aquaponic system

[Kobus Jooste]

We are still some way off from being able to grow our own complete food source for our fish, but many of us may want to substitute and grow some, if not all the food we use.  I have been looking at the applications of hybrid design for some time, and think there is something important to say about the potential use of duckweed species in a hybrid system. 

 

We spend a lot of time trying to get our mineralization ratios correct in order to prevent any serious Ammonia build-up in our systems, but at the same time, would like as much of the Nitrates that the process generate to end up as plant growth.  Many people would therefore be concerned about having a mass of duckweed sucking nutrients out of their system.  I have found something quite interesting about having duckweed in an aquaponic system.  This system is dedicated entirely to duckweed at the moment, and the trouble that it has given me in terms of algae growth made me realize something quite interesting.  Duckweed will likely "starve" before they go after Nitrates present in a system.  I have a set-up with 20 small koi in it, 7 square meters of duckweed and a small 150 liter biofilter as back-up should the duckweed pack up.  Well the efforts of the bio-filter is going unused, while the Ammonia levels remain below 1 mg/l. 

 

 

 

 

As can be seen from the graph (1.5 months) Ammonia levels have stayed between 0.15 and 0.3 mg/L, while Nitrates have steadily climbed to 20 mg/L.  The slight spike in the Ammonia is likely from the good growth I'm getting for the fish, and switching a UV-sterilizer on in order to knock back the micro-algae that love the remaining nitrogen in the system.  For me, this is an indication that duckweed beds can be introduced into a system without a huge impact on the nutrients available to the other plant crops.  Having slightly elevated Ammonia levels (still below danger levels) will give you an extra food supplement for your fish without hampering the growth of your food plants.

 

 

9 Beds of duckweed growing well off 20 small koi.  Only slightly elevated stocking densities may be needed to give you this much growth without comprimising your regular crops. 

[Sylvia Bernstein]

cool stuff, Kobus.  What do you think the duckweed is consuming, if anything?

[Kobus Jooste]

The duckweed will first and foremost go after Ammonia as its nitrogen source, and then it is capable of scavenging for all the macro-and micro nutrients present in the system.  It's final chemical composition depends a lot on what is in the water stream, but in my system, they literally whither away without Ammonia even if there are some Nitrates available - they do much much better on Ammonia.

Sylvia Bernstein said:

cool stuff, Kobus.  What do you think the duckweed is consuming, if anything?

[Harold Sukhbir]

Simply amazing Kobus! No wonder it's being used in waste water treatment. It would be interesting to find out their nutrient consumption per weight as a comparison to our veggies.

[Kate Mink]

What do you do with your duckweed?

[Kobus Jooste]

Right now, it is a research project for a hybrid system where the aquaculture unit biological filtration is used as a base for fertilizer.  I would like to use duckweed in the future as part of a food source, but for now, it is fertilizer

Kate Mink said:

What do you do with your duckweed?

[Greener]

Very interesting Kobus.  I was just debating integrating my duckweed tank with the system, and if I did where in the flow I should put it.  Sounds like it would be best fed straight from the FT, not after the gravel beds/biofilters.  That works better for me too.

 

I am hesitant to integrate the duckweed tank because I fear it spreading and taking over the system (especially if my catfish end up not liking it).  I assume it could cause problems with the pumps, in the lines, or by getting constantly dumped into the gravel beds.  I want to keep as many solids out of the GBs as I can.  

 

Is it difficult to keep duckweed from spreading in an integrated system?  I will have covers on all open-water tanks (except the duckweed tank of course), this should help.

 

[Kobus Jooste]

I'm going to be cautious about what I say, as I recall a post in a newsletter from a prominent US AP business basically advocating the total abandonment of any attempt to work with duckweed.  It is dark over here now, but I can add pictures of how I set up the overflow from the duckweed beds back to the fish tanks tomorrow - basically a standpipe protected by a larger outer guard pipe that is drilled at the bottom, to ensure that water is not sucked off the surface of the duckweed bed.  No duckweed has ever escaped this set-up.  I also harvest regularly (4 - 5 day rotation), so nothing dies and goes into plumbing. I have a basic inlet and the return flow describe earlier.  Duckweed seem to do much better on ammonia, and I think the fact that my system showed rapid duckweed growth with ever-increasing nitrate loads underlines this fact for the species I was using at least.  Growth may be weak if they are fed water from a post-filtration source.  If your system is in the open, or if you use one of the very fine duckweed species (Lemna gibba is what I use) you could have accidental spreading or small colonies escaping an overflow - there I am not sure about risk at all.  I have had them in a closed system for more than 6 months now and no issues as yet.

Greener said:

Very interesting Kobus.  I was just debating integrating my duckweed tank with the system, and if I did where in the flow I should put it.  Sounds like it would be best fed straight from the FT, not after the gravel beds/biofilters.  That works better for me too.

 

I am hesitant to integrate the duckweed tank because I fear it spreading and taking over the system (especially if my catfish end up not liking it).  I assume it could cause problems with the pumps, in the lines, or by getting constantly dumped into the gravel beds.  I want to keep as many solids out of the GBs as I can.  

 

Is it difficult to keep duckweed from spreading in an integrated system?  I will have covers on all open-water tanks (except the duckweed tank of course), this should help.

 

[Greener]

A solids-lifting overflow or SLO is the name, as I was just reminded yesterday.  :) 

 

Great to hear that this works well for containment.  I may give it a shot, with the duckweed between my FT and my sump.  Since my pumps will be at the bottom of my sump, I should have a good amount of buffer to control duckweed spreading through the pumps to the GBs.

 

I will take a look at lemna gibba and other types of duckweed.  Sticking with the biggest is probably best anyway.  

 

I have been assuming duckweed is legal in AZ but I should look that up too.  Anyone from AZ know for sure?

 

Thanks again!

 

[AlbertF]

i made a tube weir for my duckweed tank

[Kobus Jooste]

Thanks for the inputs - here are two pics of what I did.  The base of the Lemna beds are raised above the water level of the FT, thus I use bottom drainage out of the last bed and into the fish tank, which as you can see, is not showing any signs of duckweed problems.  The standpipe is drilled at the dark line on the inside, while the guard pipe is drilled at the bottom.  This set-up works for me.  Incidently, the last two grow beds, as can be seen in the drain pic, shows typical signs of "scrubbing zone" growth described in waste water treatment literature - plant growth is a little slower and weaker as the duckweed scavenge for nutrients.  These are the only beds with signs of algae.

[Greener]

nice, thanks for the pics.