Thanks for your question, Dr. Iyer. The scaling-up of duckweed-based biofuels will depend largely on developing a cheap and efficient method of drying the plant material on-site. Duckweed has a water content of 92-95%, making transportation of wet biomass economically unfeasible. Sun drying is generally inefficient while heat drying requires a large input of energy. Mechanical squeezing of the plant tissue may effectively remove most water at low cost but has yet to be implemented on a large scale. Ideally, duckweed would be grown on open ponds, as opposed to within greenhouse enclosures, to minimize costs. Such an approach, however, demands careful consideration of many environmental factors, including climate and pests. To maximize throughput and yield, duckweed could be grown and harvested from multiple sources, dried on-site, and then transported to a central location for downstream processing.

Thanks for your inquiry, Dr. Clancy. Strains with high starch provide the most readily available source of fermentable sugars, maximizing biofuel yield. Plants contain two main reservoirs of fermentable sugars: cellulose and starch. Cellulose is packed into tough fibers that require harsh chemical conditions at high temperatures to break down into glucose sugar units. Starch, on the other hand, is easily extracted from plant material and directly hydrolyzed into glucose by simple enzymatic reactions. The resulting glucose is fermented by yeast into ethanol and then distilled. Subsequently, the ethanol can be blended with gasoline at variable percentages (E10, E15, E85) for use in vehicles. This overall conversion of starch into ethanol can be a highly efficient process; up to 95% yield has been observed.

However, a duckweed strain with high starch content is not the only important criterion. We need to find a strain that balances starch accumulation with a fast growth rate to maintain a steady, high yield under a particular set of growing conditions (e.g. pH, light intensity, temperature). These two metrics—high starch content and rapid growth rate—have the highest priority in our screening of duckweed strains for biofuel production.

Yes, there is. A published report (Xu et al. 2011) previously identified a strain of Spirodela polyrhiza with a high growth rate (12.4 g dry weight/m^2/d) and high starch content (31% dry weight, d.w.) grown on dilute swine wastewater. Using this strain, the researchers were able to obtain an ethanol yield of 6.42 × 10^3 liters/hectare, which is approximately 50% higher than that typically achieved using corn.

A preliminary screen conducted by our lab identified three S. polyrhiza strains and one S. intermedia strain that were promising. The S. polyrhiza strains had growth rates of 17.86-20.94 g d.w./m^2/d and starch contents of 18.5-27.6% d.w. The S. intermedia strain had a growth rate of 6.57 g d.w./m^2/d with a 42.6% starch content. Plants were grown under sterile laboratory conditions. We aim to select a strain that has both a good starch content and fast growth rate.

Thank you for your question, Dr. Koodali.

Thank you!

Thanks for your question, Dr. Harrison. The U.S. produces roughly 40 billion gallons of municipal wastewater per day (US Wastewater Treatment Factsheet, University of Michigan, http://css.snre.umich.edu/css_doc/CSS04-14.pdf). Assuming all of that wastewater is used for growing duckweed, 25,000 hectares of 0.6 m-deep ponds would be filled daily (9 million ha yearly). Recently, Xu and colleagues (2011) obtained an ethanol yield of 6420 liters/hectare/year by growing duckweed on wastewater. Using this yield for the sake of simplified calculations, I estimate that roughly 1.6 million liters of ethanol per day, or 60 billion liters per year, could be generated from US municipal wastewater, exceeding current US ethanol production. In addition, the 9 million hectares required for these duckweed-wastewater systems is less than the 14 million hectares of land devoted to cultivating corn for ethanol production (FAOStat, 2010; Sci Am, Feb 2013). Thus, based on these rough estimates, duckweed would indeed be a viable ethanol production platform for our current needs.

Hi, Dr. Yates. Yes, reducing or eliminating the need for fertilizer would be a game changer for the renewability and sustainability of biofuel feedstocks. That is why it is so imperative to integrate either agricultural or municipal wastewater as a replacement for conventional, industrially made fertilizers. Recycling the wastewater for crop irrigation or aquaculture systems are added benefits. To address your first question, we have obtained an ethanol yield of 2,407 liters/hectare/year at our local test site—assuming a 6-month growing period, 30% starch duckweed by dry weight, and a fermentation efficiency of 95%. As this was our first attempt, there is much room for improvement. Another group (Xu et al. 2011) calculated an ethanol yield of 6,420 liters/hectare/year.

For the second question, we do not know the amount of fertilizer necessary if production needs exceed wastewater ponds. A visiting chemical engineering student from South Africa (sponsored through our IGERT’s international collaboration!) is currently quantifying the duckweed biomass accumulation per unit of fertilizer. Another option is to build open ponds to accommodate the need and fill them with wastewater from existing facilities, since there is an abundance of municipal wastewater, as I mentioned in my response to Dr. Harrison’s question, as well as the untapped potential of agricultural wastewater. We have also been approached by farmers who want to use fallow ponds on their farms for duckweed production.

Thanks, Stephanie! The outtakes from the video are even better.

haha I can only imagine

Thanks for your encouraging words, Mihai!

Thanks, Wenqin!

Thanks, Patricio!

Hey Josh! Thanks!

Thanks for your question, Dr. Drayton! The best performing species from a preliminary screen in our lab have growth rates from ~18-21 g dry weight/m^2/d under sterile lab conditions. Another paper reports a different duckweed species to reach 30 g d.w./m^2/d in lab conditions. As you mentioned, duckweed in the wild is a different story. They often grow as multiple genera/species together. The smaller duckweeds fill in the spaces on the water’s surface that the larger duckweeds are not able to occupy. On open ponds, the duckweeds can also form thick (multi-layer) mats.

In contrast, algal systems are expensive and energy-intensive. The algae need to be churned frequently for maximal photosynthesis in closed bioreactors. In addition, algae are difficult to harvest due to their microscopic size.

Thanks, Ellen!

Thanks, Mike!

Haha. Thanks, Massimo!

Hi John! Thanks for your interest in the video! Please check your email inbox for more details.

Thank you!

Thanks, Christopher! We are really excited about the possibilities!

We could potentially use municipal and agricultural wastewater to support biomass growth. It would be a great way to treat the wastewater for downstream applications like crop irrigation and aquaculture. It’s a cheap fertilizer source!

Hi Geoffrey. Thanks for your question. One of the advantages of open pond duckweed cultivation is disease resistance. Chlamy cultures can be easily contaminated and are prone to attacks by bacteria, viruses, and other microalgae when grown on open ponds. The duckweed ponds that I have visited do not appear to have significant issues with disease or pests. There is very little literature that addresses diseases of duckweed. Another major advantage of a duckweed-based biofuels system is the ease of harvesting. Since duckweed floats, it is easier to harvest than algae which reside throughout the water column in open ponds. We can harvest the duckweed by simply skimming the pond surface. In contrast, harvesting algae typically involves flocculation. Flocculating agents are usually very expensive, especially for industrial-scale production, and so drive up the cost of algal biofuels. In addition, the algal sludge has limited use in downstream applications. Another difference between duckweed and Chlamy is the desired end product. As far as I know, algal biofuel research is primarily focused on lipid (oil) production for biodiesel, whereas our goal is starch production (however, oil production in duckweeds may also be feasible).

Go duckweed!

Thanks, Paul! Apple Keynote!