The purpose of this research included identifying the optimum cultivation conditions of five different strains of duckweed while evaluating the nutrient uptake of nitrogen (N) and phosphorus (P) in anaerobically digested dairy manure to promote biomass production.
The growth of duckweed was assessed on the cultivation parameters of temperature, pH, dissolved oxygen, light intensity, nutrient concentrations, and biomass production. Three strains, namely Landoltia punctata, Lemna gibba and Lemna minuta, were identified as the promising candidates for their high levels of nutrient uptake and biomass production. The temperature and light intensity were maintained in an environmental chamber at 25°C and 10,000 lux, respectively. The nutrient uptake through duckweed cultivation, characterized by the changes of total nitrogen (TN), total Kjeldahl nitrogen (TKN), and total phosphorus (TP), was assessed on the anaerobically digested dairy manure in three dilution ratios i.e., 1:13, 1:18, and 1:27 by volume.
In the dilution ratios 1:18 and 1:27 all duckweed strains grew successfully. However, in dilution ratio 1:13 all three duckweed species were inhibited by the high nutrient concentration. The batch system created an aerobic environment within the anaerobically digested dairy manure medium with a dissolved oxygen content of 2-6 mg/L. At the high light intensity of 10,000 (lux) a buffer was needed in order to keep the medium's pH constant to promote duckweed growth. This research compared the nutrient reduction of the microbial growth within the anaerobically digested dairy manure and a standard solution of 1.6 g/L of Hoagland E-medium to the nutrient reduction from the three strains of duckweed at the dilution ratios of 1:13, 1:18, and 1:27. Experimental results revealed that the average duckweed productivities were 1.50, 1.30 and 0.50 grams per square foot per day for Landoltia punctata, Lemna gibba, and Lemna minuta, respectively. At the dilution ratio of 1:27 the highest significant reductions came from Landoltia punctata at 86.0% for TN, 87.5% for TKN, and a TP of 89.5%. At the dilution ratio of 1:18 Lemna gibba got the next highest at 83.8% for TN, 85.6% for TKN, and a TP of 76.2%. Lemna minuta came in last with the highest nutrient reductions in dilution ratio 1:18 with 83.1% for TN, 84.7% for TKN, and a TP of 76.5%. A light intensity of 10,000 lux, pH of 6.5, a temperature of 25°C and a dilution ratio of 1:27 promoted active duckweed growth on anaerobically digested dairy manure.
We will continue the duckweed cultivation work to optimize manure nutrient uptake and to convert duckweed biomass into bioethanol.
Lide Chen, Assistant Professor/Waste Management Engineer, University of Idaho
Kevin Kruger (University of Idaho)
Kevin Kruger is a graduate student who conducted the duckweed cultivation tests.
This work is supported by the USDA NIFA and Idaho Agricultural Experiment Station.