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How to Dry 100 pounds of Duckweed

May 26, 2012

Last month, I received a request for a pound of dried duckweed by a biofuel company to be used in ethanol production tests. I figured that as duckweed is 94% water, I’d need close to 100 pounds of wet biomass. I drove up to my favorite pond in Illinois and harvested five 5 gallon bucketfuls. Once home, I unrolled a 30 x 30 ft piece of plastic on the front lawn and started spreading duckweed out.

“You look like you’re drying pot or something,” my teenage son observed as he watched me patiently thin out the clumps in order to insure even drying. I sat back and surveyed the dripping, green mass. Hmmm…. you don’t see a sight like this on someone’s front lawn every day. Good thing we are in the country!

It took nearly a week to dry it all, due to clouds, a couple of rain showers and cool weather. When rains came, I would roll the plastic and duckweed up like a cigar and stowed it under my camper. The half-dried duckweed would start heating up (decomposition)  so I only did this twice and then only long enough for the rain to pass by.

After six days, it was dry and brittle. I ended up with a garbage bag full and it weighed seven pounds. I picked out small twigs, tree seeds and leaves out of a pound of it. Then I rubbed handfuls through a colander to insure a uniform consistency. The resulting dried duckweed was dark green and smelled like hay. Very nice. Now to get the results of the ethanol experiment!

17 Comments leave one →
  1. March 18, 2015 2:09 pm

    When drying, green leaves do not make good use of the sun’s energy that falls on them.

    In smaller batches, a solar dryer, with a black solar collector can absorb more sunlight for a given area. Because of thermal siphon effect, warmed air will rise and vent out the top, taking advantage of the chimney effect, and in the process circulate in and around screened trays with duckweed, thus no electricity needed, but you like, you can add a small fan to increase air flow and prevent excessive temperatures.


    • March 18, 2015 10:05 pm

      Keith, super description of a thermo-siphon dryer. Back then, I only dried 100 pounds. With my current system, I dry 800 pounds routinely. The fans are a must in this situation.


  2. Anonymous permalink
    May 4, 2013 2:13 pm

    what kind of pellet press do you all use?


  3. September 29, 2012 4:52 pm

    When I said “spin cycle” I meant the one on the washing machine. Sorry about that.


    • September 29, 2012 6:00 pm

      To be honest, I’ve thrown both duckweed and filamentous algae in pillowcases and run them through the spin cycle of my washing machine. This was to remove all the excess water from the outside of the leaves/biomass for weighing out equal amounts for experiments. This method would give you a head start on drying massive quantities of duckweed, but the internal moisture would still need to be dried via solar or some other heat source. Preferably free.
      You must have a LOT of duckweed… What part of the country are you from? The amount of energy needed to dry duckweed from 94% moisture down to 10% moisture or so would require a lot of energy. Perhaps more than you’d get out of your final burnable product due to inefficiencies of energy exchanges. Perhaps drying it for pelletizing for animal feed would be a better way to go.
      For Btu comparison’s sake, I drive to a sawmill 3 miles from my home and pick up hickory and walnut tree planks and edgings for $5 a pickup load. I still have to dry it, but once I stack it with ventilation between the layers, I can walk away and forget it for the rest of the summer. Duckweed would require more manhours to turn and dry unless you are using it for animal feed- then it’s worth the added energy and time. I know I’ve talked about using duckweed as a heating source, but that’s if you have a good drying system with lots of free heat in the summer being wasted anyhow.
      Let me know what you end up doing with it. Occasionally I get requests from researchers who would like larger samples of dried duckweed.
      I like how you think. Keep it going!


      • September 30, 2012 7:54 am

        I was thinking of turning them into pellets for the pellet stove. I haven’t started my duckweed project yet, but the plan is to turn both my front and back yard into a pond and grow it there. I think getting the moisture out in the spin cycle, or bicycle would be a good head start, I would just need a good plan to dry out the rest of it. Duckweed pellets do burn according to what I have read. Thanks for the additional tips!


      • September 30, 2012 1:17 pm

        Hey, I didn’t quite read that through, I missed the compliment – thanks! I really appreciate having someone to brainstorm with – two heads are better than one, especially when the goal is the same.

        I had this crazy idea that the idea of people turning their lawns into duckweed ponds would solve an enormous amount of energy problems.

        But it takes a few crazy pilgrims to get the ball started, and as per usual, this is a boot strap operation.

        Keep blogging. I’ll peruse the rest of your entries over the next week. 🙂


        • October 9, 2012 10:57 pm

          If people would grow duckweed from waste or leached nutrients from their lawns, gardens, or compost piles, they’d be doing the environment a favor and they’d have lots of clean, green, and healthy duckweed for a variety of uses. Next spring, I’m putting in a couple of smaller, permanent ponds to do just that. As for using it for energy, maybe on such a small scale, it wouldn’t be worth it other than a nitrogen boost for a biomethane generation system. If you’ve got a few tons to harvest, then it might be worth your while to consider bioethanol as a product. Used fresh for animal feed or green compost, duckweed is great. When you want to extract the energy from it, it’s good to consider that fresh duckweed is 94% water. I’d love to see duckweed grown as a matter of course in EVERYONE’S garden as the “missing link” between terrestrial crops and aquatic crops such as fish production. Duckweed is a great plant and worthy of its day in the sun.


          • elysiafields permalink
            October 10, 2012 2:31 pm

            In order to generate enough ethanol from duckweed in order to warm my house in the cold Canadian northern winters, I’d need to grow 8,000 lbs. per day. Then huge facilities to process it. Pellets are realistic though. I can grow enough to make pellets for pellets stoves, which means free heat for me and I get to save some trees too.

            Drying it will take a bit of work, but mother earth is worth it no?


            • October 10, 2012 3:15 pm

              Mother Earth begs us to tread lightly, use what we need but do turn all “waste” back into the system for reuse. If I had to choose between using a tree to heat my home or duckweed pellets… First off, I’d make sure I’m “treading lightly” with my home being as tight and as well insulated as possible. Then insure I’m using passive solar and thermal heat retention materials in my home. Then I’d go with solar space heating and solar hot water. If I STILL needed a source of home heating, I’d then do the following:

              I’d next consider the Btu content of both heating materials- wood vs duckweed pellets, my time and equipment to process both, and all energy requirements. Based on my calculations, then I’d take my pick.. Perhaps for your situation, duckweed is a solid alternative to wood heat. Personally, if my wood was farmed sustainably and I had animals that could use duckweed as a food (or compost pile that needed a nitrogen fix) , I’d have no problem with using wood for home heating.. If the duckweed was sustainably-grown and I”ve got a low tech, semi-automated way to harvest, dry, and pelletize it, then duckweed is the way to go.

              It’s never a simple answer, is it? 🙂


              • elysiafields permalink
                October 10, 2012 4:27 pm

                In order to make duckweed pellets, you just shove the dried duckweed into the pellet maker. With other materials, you usually need to process them with another machine so that the pellet machine can handle them. I will be doing the usual winterizing around my home, but will likely look into wind turbines as opposed to solar panels, as their manufacture isn’t necessarily treading lightly on the earth. Wind turbines are becomming extremely efficient and the new design ensures birds stay away. I realize that wind turbines also are manufactured items, but you need fewer of those than solar panels. However there IS a new solar panel innovation that looks pretty exciting. It rotates!

                Seeing what I can do to accomplish the passive solar heating is definitely on the list of improvements I want to make to my property.

                I do have the time to cycle for a few hours a day to dry the duckweed – I work out for that amount of time anyway, so I can just make cycling my workout.

                If you don’t have the time I suppose that’s legit, but if you do have the time and just don’t want to spend the time, then you have to take responsibility for taking the easier option.

                Duckweed is way more environmentally sustainable than wood, hands down.


  4. September 29, 2012 4:50 pm

    I’m in the process of researching possible ways I can optimize biomass production from my small property. Re-doing the math, the duckweed I can produce now, unoptimized, would ALMOST provide enough fuel for winter.

    OK, this is primitive, but when you’re not rolling in dough, and if you’re super serious about finding another way, while at the same time, no other options are available, you can always purchase a five gallon cylinder used for drying salads, hook that up to your bike somehow, and spin dry 100 pounds of duckweed per day. While you’re doing that, there are these batteries you can buy, that get their charge from bike peddling. So every now and then, you can charge up 400w battery to use for back up.

    On the other hand, I wonder how well the spin cycle would work drying duckweed, and how much energy spinning it dry would consume, in terms of electric power…


  5. September 4, 2012 2:56 pm

    7 pounds of dry duckweed for every 100 pounds? Hmm… That means my smallish ponds would need to produce a lot more than 250 pounds per day to provide enough duckweed for my energy needs!


    • September 4, 2012 3:03 pm

      You are right. That is why I don’t push the bioenergy angle unless you can work directly with fresh, wet duckweed for energy conversion.


  6. September 4, 2012 2:54 pm

    I’m thinking I should find a way to use my sauna to do this.


  7. yassin permalink
    May 27, 2012 10:51 am

    Looking forward for the results


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