Aquatic Weed Removal

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Plowing the pond
Team Name Nemo
Clients & Support
Team Members
  • Bryan Barrett
  • Nicole Fletcher
  • Autumn Pratt
  • Stephen Walters
  • Sarah Willis
  • Duration Fall 2016 - Spring 2017
    Faculty Advisers Dr. Dev Shrestha
    Shop Mentor James Founds
    Client Paul Warnick and Arboretum Associates

    Team Nemo is building a remote controlled aquatic weed harvesting boat. For the past few years the ponds in the University of Idaho Arboretum have been carpeted with duckweed and Azolla. The weed harvesting boat has been commissioned to address the overgrowth. It will provide caretakers the ability to cut back growth immediately, and by removing biomass it will reduce the nutrient overload which is causing the problem. Similar commercially available machines are much larger and are driven by an on-board operator. Our boat will better suit the scale and needs of the arboretum.

    Problem Definition

    Past Attempt: Proskim
    Azolla Mat on Pond.
    Azolla Mat on Pond

    Problem Statement

    The excessive aquatic weed growth in the arboretum ponds is not aesthetically pleasing. We are looking for a long term, ecologically friendly, low-maintenance, and cost effective solution.


    Two small ponds were dug in the UI arboretum several decades ago. They were lined with bentonite and filled with natural runoff from rain and the arboretum irrigation system. The irrigation system runs on reclaimed water, which is presumably nutrient rich. The ponds have low circulation volume (a pump at the bottom of the arboretum recirculates water back to the head of the upper pond through a 2" pipe.) These conditions encourage nutrient buildup and weed growth.

    Previously Attempted Solutions

    The Arboretum Horticulturist (Paul Warnick) has made several attempts to solve the weed problem, but none have achieved desired results.

    Attempt Type Explanation/Cause of Failure
    1 Aquatic Herbicide Herbicide Weed decomposition suffocated fish
    2 Proskim Skimmer Mechanical Removal Clogs on feathers/snail shells
    3 Aluminum Sulfate Nutrient Re-balance (tie up Phosphate) Unusable due to high pond pH
    4 Grass Carp Biological Control Carp do not seem to be keeping up with weed growth
    5 Aeration Nutrient Re-balance (tie up Phosphate) Seemed to help, but not sufficient

    The following specifications are important to our client in order for an attempt to be considered successful.

    Design Specifications

    Specification Specific Requirements Target Values
    1 Labor Low operator hours 8 man-hours per week or less
    2 Maneuverability Low weight, movable by 1-2 people No component more than 100 lb
    3 Maneuverability Reasonable size Under 10 ft long
    4 Cost Within budget Under $15,000
    5 Safety Safe for operator, other pond life Negligible risk with proper operation

    Project Learning

    Weed Types

    Sarah and Nicole preparing to collect water samples.

    Duckweed, water meal, Azolla, and filamentous algae are all common water weeds which prefer quiet nutrient rich water. All but filamentous algae are free floating plants, and in other situations can be valuable as fertilizer or animal feed.

    Azolla is able to fix nitrogen into a usable form directly from the atmosphere through symbiotic relationship with cyanobacteria. This means it contributes even more to the nutrient buildup in the pond as it decomposes. Because it always has a ready supply of nitrogen, phosphorus tends to be the limiting nutrient. Azolla has a competitive advantage when Nitrate:Phosphate ratios are low.

    Azolla specimen.
    Azolla Specimen

    Pond Chemistry

    Ponds with low circulation tend towards eutrophication, or nutrient overload. This is especially true when they are subjected to run-off from fertilized lawns or fields, a definite condition in the Arboretum. Eutrophication encourages weed overgrowth, so removal or locking of nutrients in the system is a necessary part of pond maintenance. This can be done through removal of biomass (for instance weeding or harvesting fish), through draining and refilling, or through any biological or chemical process which causes nutrients to either escape or bond into an unusable form. Phosphorus is especially hard to remove because it has no gas phase.

    Because Azolla has a competitive advantage when the nitrate:phosphate ratio is low, increasing the nitrogen levels may help discourage that particular infestation. If the ratio of dissolved inorganic nitrate to dissolved inorganic phosphate (DIN:DIP) is less than 10:1, Azolla likely has a competitive advantage. (Source: Dr. Wilhelm, UI College of Natural Resources) To find the DIN:DIP ratio, it is necessary to measure the concentration of nitrate and ammonia (components of DIN) and phosphate (DIP).

    Azolla Density

    The density of wet weeds is needed in order to design volume-to-weight capacity ratios for the boat and to estimate area cleared per trip. Most of the weed mass is Azolla, so we are using its density as a working estimate. We could not find any sources which listed Azolla density, so we measured a wet sample ourselves and found the density at approximately 17-20 pounds per cubic foot. An interesting note: Azolla is highly water repellent (like duck feathers) so even during collection it is unlikely to gain much waterlogged weight.

    Material and Hardware

    In designing the boat and choosing from available parts, we have researched and considered a lot of information on materials and standards. Often the best place to start is reading about existing machines which fulfill similar functions or operate in similar conditions as the boat we are designing. Looking up paddle-boats, for example, provided good information about how different types of metals (aluminum vs. stainless vs. galvanized steel) hold up in different types of water. Similar sources also brought up how to avoid galvanic corrosion when matching fasteners, a problem which would not have likely even occurred to us as amateurs. Moving into the more concrete parts of design - choosing from available hardware - also implies a lot of learning. For instance, considering options for the conveyor brought up everything from sizing with ANSI sprocket specifications to types of positive drive conveyor belting. The overarching lesson here is that a cost-effective design uses parts and sizes in common use.


    Design Process

    Design Decision Process

    The problem was at first very open ended: we were simply requested to reduce the weed problem by any means possible. We began by brainstorming a variety of ideas, which we presented to our client.

    Type Possibility
    1 Mechanical Remote Controlled Water Plow
    2 Remote Controlled Conveyor Collection
    3 Manned Version of Plow or Collection
    4 Chemical Nitrate Addition (Remove Azolla Advantage)
    5 Enviromental Buffer Zone
    6 Partial Pre-rain Drain (Water Renewal)
    7 Added Aeration
    8 Biological Azolla Weevil

    Final Design

    The Arboretum Associates were most interested in a mechanical solution. The remote control aspect appealed to them, and they liked both the plow and conveyor ideas. We decided to construct a boat which could support either a plow or conveyor attachment, so both could be tested. Our theory is the collection method will be more useful for low-density growth, while the plow will be more effective for a thickly overgrown pond. During construction our initial detailed design had to be modified. For instance, we had planned to use PVC pipes to provide buoyancy, but found that the inflatable pontoons from a fishing boat were a much lighter and better solution.

    Photo Overview Photo Overview
    16 NEMO Pontoon.jpeg
    • Weight: 12 lbs (2 x 6 lb each)
    • Inflatable
    • Weather-resistant
    • Rated for 350 lb load

    16 NEMO BoatInShop.jpeg
    • Weight: 36 lb
    • Corrosion-resistant aluminum (6061 Aluminum chosen for welding workability)
    • Provides connections and support for pontoons, motor, and attachments
    • Center space is 42" x 42", holds collection basket during conveyor attachment use
    • Allowing the basket to be partially submerged means the collected weeds support their own weight.
    16 NEMO PlowBoatInShop.jpeg
    • Weight: 10 lb
    • Corrosion-resistant aluminum, stainless steel fasteners
    • Removable netting (using netting rather than a solid surface reduces unnecessary drag)
    • Adjustable angle hinges
    • Cotter-pin connections to frame
    • 6' full expansion span

    16 NEMO ModularBelt.jpeg
    • Weight: 52 lb
    • Corrosion-resistant aluminum, stainless steel fasteners
    • Modular polyethylene positive-drive belt and sprockets (from Forbo) - eliminates chain and other rust-prone parts
    • Adjustable conveyor speed
    • 12V 100W conveyor motor also connects to battery in rear of boat
    • Cotter-pin connections to frame
    • 4' Center to center axle distance
    16 NEMO MotorSarah.jpeg
    Motor and Battery
    • Motor weight: 32 lb
    • Battery weight: 45 lb
    • Remote control trolling motor (Cayman Haswing, 12V, 55lb)
    • The remote controls speed and direction of the motor. However, the controls need to be integrated with the conveyor controls, and the range needs to be extended.
    • 12V deep cycle marine battery

    16 NEMO OnshoreConveyor.jpeg
    Onshore Conveyor
    • Onshore conveyor possibility to help with plowing collection (suggested purchase)
    • Supplier: Bastian Solutions
    • Portable Parts Conveyor - 7' Long x 12" Wide
    • Supplier

    Implementation and Testing

    Our full-system test with the plow went well. We drove it around unmanned for a while, but found that it also worked very well as a manned craft. (We were still using the remote control, just at a much shorter distance.) This adds a nice option, especially until we have a more reliable remote control connection. A couple improvements to be made became evident during the test:

    • When the plow is being used without a rider on the boat, it will need added weight in front to keep the boat level.
    • A wider plow would be no trouble for the motor to push, and would make work more efficient.

    16 NEMO PlowTestRC.jpeg
    16 NEMO PlowTestAutumn.jpeg

    Testing the boat with the plow attachment - with and without rider.

    The test was made on May 1, so the weed growth was still low.

    However, the plow performed well on the weeds which were present.

    Future Work

    Our motor currently has a remote control range of 30-50 ft. A longer range remote system needs to be constructed which integrates a kill switch and the controls for the on-board conveyor belt with the controls for the motor. We decided not to pursue this in the time constraints we had because we thought it should be done thoroughly and robustly. Attempting to adapt the motor without enough time to do it properly risked breaking it.

    The points we discovered during testing would be simple to solve and good to consider - adding a removable weight to the plow is a trivial problem, and making a wider plow would be simple.

    If a future team takes up the project, we think it would also be wise to monitor the nutrient levels in the pond, and begin to build up a better understanding of the affecting factors.

    Team Members

    Team "NEMO"
    Member Biography Discipline
    Bryan Barrett
    16 NEMO BryanBio.jpeg
    I’m from Coeur d’Alene, ID. I love nature and am interested in the eco-hydrological side of Environmental Engineering. Biological Engineering
    Nicole Fletcher
    16 NEMO NicoleBio.jpeg
    I was born and raised in Seattle, Washington. I am currently working on my final year of school and hope to be working with biohazardous waste and damaged ecosystems doing bioremediation and restorations when I graduate. Biological Engineering
    Autumn Pratt
    16 NEMO AutumnBio.jpeg
    I'm from Moscow and am in my final year here at UI. I plan to go to graduate school next year, hopefully doing research in robotics. Biologically inspired engineering and soft robotics are particularly interesting to me. In free time I like writing, music, and almost anything outside. Mechanical Engineering
    Stephen Walters
    16 NEMO StephenBio.jpeg
    I am from Meridian, Idaho and enjoy the outdoors, dirt-biking, and working with heavy machinery. Mechanical Engineering
    Sarah Willis
    16 NEMO SarahBio.jpeg
    I’m from Kooskia, Idaho. I particularly enjoy 3D solid-modeling and Mechanics of Materials. Interests outside of school include spending time with friends and family and listening to music. Mechanical Engineering

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