Jaques Finlay, Ben Janke, Poornima Natarajan, Dan Larkin, Jack Distel
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Background
Because duckweed rapidly absorbs nutrients from the water column, studies show duckweed biomass is 0.2% phosphorus, harvesting offers a rare opportunity for true nutrient export - a management feat typically only achieved through dredging. The City of Bloomington trialed strategies for duckweed harvest to determine if it was a feasible activity. These trials demonstrate a cost-effective, scalable method to physically remove phosphorus and restore favorable pond states.
Harvest Trials
The Bloomington trials evaluated two active removal strategies to determine operational feasibility. Both methods utilized floating booms to concentrate the biomass into a thick mat near the shore.
- Siphon-Pump Method - attempted to pull a water-and-plant slurry through a trash pump and into a mesh separator on shore. The separator worked great; however, the high density of the coralled duckweed frequently clogged the pump.
- Extraction Method - Once coralled, the material was lifted directly from the water surface. In this case, we used a vacuum truck - not recommended, but it worked as proof of concept.
These experiments demonstrated that while mechanical siphoning requires further engineering, manual or mechanical extraction is relatively easy, scalable maintenance solution.
Photos of the Siphon-Pump Method
Photos of the Extraction Method
Results
The siphon approach was abandoned due to issues with the pump. All full pond trials were completed using the extraction method. Three ponds were fully harvested in 2022.
Photos of the pond before harvest showing a murky lake.
Photos of the pond after harvest showing a clear lake
The harvests were able to clear a 1-acre pond in about five hours, from set up to take down, to complete each pond.
A key success was the "touch it once" rule, ensuring efficiency by moving duckweed directly from the water into a disposal receptacle.
The use of 100-foot floating booms was a standout lesson; they were highly effective at concentrating duckweed. A small boat was used to maneuver the booms which prevented people from tromping through the pond.
We learned that collection points must be at least one foot deep to prevent the mass from hitting bottom and to allow the booms to function without rolling over the material. Our vac truck hose could not extend that far and forced us to "rake" the duckweed into the suction end.
Passive vs Active Removal
Passive removal functions as a "set up and leave it" system, utilizing infrastructure, like continuous surface skimmers or pumps, to capture small amounts of biomass throughout the growing season.
Active removal is an event-based harvest utilizing a crew who use equipment to go in, get the duckweed, and get out. The City only experimented with active removals.
Note, the siphon-pump method was designed based on a passive system. Its issue of clogging would not likely occur in a passive scenario; the proportion of water to duckweed would be much higher.
Passive can achieve continuous, low-maintenance nutrient export. But, the device must be able to last in the field without constantly requiring maintenance. If successful, passive removal could provide a strategy for continuous phosphorus removal, making constant duckweed growth an advantageous tool for maintaining pond functionality.
Cost-Benefit and the Future
The Bloomington Trials:
Staff cost: 3 staff x $60/he x 3 hours = $540
Conveyor cost: $100/hr x 3 hours = $300
Total cost per 1,000 lbs duckweed: $900 (rounded)
Cost per pound of phosphorus removed: $420
While initial trials using a vac-truck were expensive, they proved the concept's mechanical viability. Transitioning to optimized equipment, like conveyor belts, can significantly reduce costs.
Photo of duckweed after harvest.
Photo of two men; one holding harvested duckweed
The piles of duckweed post harvest. Due to the water that the vac-truck sucked up in addition to the duckweed, it was difficult to calculate total weight removed. We estimate that it was at least 1,000 pounds of wet mass per pond. It was likely more.
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