Harvesting Duckweed By Skimming

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Title: Harvesting Duckweed By Skimming
Author: Smith, Ryan Andrew
Advisors: Jiayang Cheng, Committee Co-Chair
Joel Ducoste, Committee Member
John Classen, Committee Co-Chair
Abstract: Duckweed is a floating aquatic plant that has been used in a variety of water treatment applications. The plant must be harvested for the chemical removal from the wastewater to be complete. This project investigated the suitability of a harvesting system that would collect the plants by skimming them from the water surface to maintain the crop at a desirable density. The observation was made that clumps of plants could form, as opposed to a single layer of individual plants, which affected the skimming of the duckweed mat. Experiments were performed to find the necessary depth and slope of the skimmer for clumps to move in at very low flows. The skimmer crest was rounded and the root length of the plants ranged between 0 and 6 cm. A depth of approximately 1.5 cm was found to be the minimum depth where clumps moved into the skimmer consistently. The skimmer slope could be 3o with clumps still flowing down the skimmer base. A concept and full-scale prototype were developed. The prototype used a skimming funnel that floated beneath the duckweed mat. A pumping mechanism was connected to the funnel that pumped to the bank. A vortex formed over the funnel orifice during pumping, which moved plants into the funnel and exit pipe. Flow data was collected at various funnel depths and author-defined vortex strengths were noted. Using the data, empirical equations were found that could be used to calculate the needed pumping rate to achieve a desired vortex strength at a given funnel depth. Experiments were conducted to evaluate the circulation of the duckweed mat and thus the ability of a skimmer-based harvester to harvest from all areas of the water surface. Tracers were placed on the duckweed mat and natural wind forces and flow forces from the skimmer were applied. The tracers were then monitored for approximately 2 weeks. Some movement occurred, but there was no consistent rotation in the plant mat. It was determined that duckweed could be harvested well using skimmers, yet the lack of significant circulation of the plants under wind force and flow forces from the skimmer was a limitation of the prototype.
Date: 2004-03-06
Degree: MS
Discipline: Biological and Agricultural Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/1830

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