Aquatic Weed Removal

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


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.

Background

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 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.

 Design Specifications 

Project Learning
Weed Types



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.



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
Design Process



- Remote Controlled "Water Plow" with Shoreline Conveyor Belt: Small boat with a mesh plow can be used to "herd" weeds towards the shoreline, where a conveyor belt facilitates scooping them out. (This is a scaled down version of a system we saw used by Weedo).

- Remote Controlled Collection: Small boat collects weeds and brings them to shore. This is also an idea which has been scaled down to fit a small pond. The biggest challenge with scaling this is maintaining sufficient buoyancy, since buoyancy (like volume) decreases with the cube of length. To address this a water-permeable mesh basket could be designed so that the captured weeds support their own weight.

- Manned Version of Plow or Collection: Either concept could be translated to a slightly larger manned version if our client would prefer simplicity. It did appear that this would make construction more expensive.

- Buffer Zone: A swath of grass or other long-rooted plants around the pond could control nutrient and sediment inflow. This is a common and low maintenance water quality solution.

- Partial Pre-rain Drain: Partial draining of the pond system before expected rains could help renew the water. When rain falls on a full pond, much of the overflow will be the fresh rain. If the pond is lowered to receive the expected rainfall, old water is flushed and fresh water is retained.

- Nitrate Addition: Adding nitrates could reduce the Azolla problem, since Azolla has a competitive advantage in low nitrate to phosphate ratio environments. However, it would not address the long-term nutrient overload. Before proceeding, we would need confirmation that the arboretum ponds have a legitimately low nitrate to phosphate ratio.

- Aeration: Adding more aerators to the pond would increase the dissolved oxygen at the pond bottom. This would encourage the growth of aerobic bacteria which break down organic matter and improve water quality. It would also reduce pond odors, which would be valuable in the arboretum.

Chosen Solutions

Remote Control Convertible

Note: breakdown of components and estimated costs is included in "Detailed Design Review" - see attached documents.

The idea of a remote controlled boat appealed to our client as a fun solution and a good way to test concepts at a small and low-cost scale. He wanted to try both the plow and the collection ideas. We decided to construct a boat which can support both options. The plow will be detachable and can be replaced with a conveyor belt and basket for on-board collection. 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.



We initially struggled with how to create a customized hull, but realized that a pontoon design would be both simpler and more suited to the slow-moving, barge-like function of the boat. We took inspiration from DIY PVC pontoon boats online.

Components can be broken down to:

1) Base and frame: ''This is the foundation of our boat. It should be lightweight, strong, and provide sufficient buoyancy for itself, the motor box, and either the plow or conveyor (these will be detachable). It does not need to support the full weight of collected Azolla, since the basket design will allow the Azolla to support its own weight. We decided to use 8" PVC to provide the buoyancy, as it is durable and provides a steady base for our boat. We put closed-cell foam inside the PVC so that any cracks will not cause the boat to immediately sink. For the frame we chose 6061 Aluminum L beams, because they are lightweight, corrosion resistant, and workable for welding. We initially chose dimensions of the boat as about 5'x4', (for a basket size 4'x4') based on a balance of capacity and maneuverability. This was refined through calculations of the buoyancy and weight over a range of sizes - we did not want the PVC to go much under half submerged, but we also wanted to keep the land weight of the base and frame low. Showing the results of our calculations to the Arboretum Associates, we settled on a compromise of basket size 3.5'x3.5'. This means the land weight of the component is about 130 lb, and it has a carrying capacity of 250 lb before sinking - we estimate using about 120-140 lb of that capacity, which would mean the PVC would be 49-53% submerged.''

2) Plow: Plow dimensions were chosen as 6' at full expansion (sides of plow will be on hinges so that the covered area can be reduced as needed for maneuvering or speed.) The plow frame is being constructed out of aluminum, again because the material is lightweight and corrosion resistant. A plastic mesh (similar to screen-door mesh) is being used to create the plow surface. This will allow some of the water to pass through but retain the Azolla, hopefully reducing unnecessary resistance.

3) Motor Selection: ''Motor sizing depends on the dynamics of the boat's movement in water. RC boat motors are not really appropriate for our situation, because they are meant to run quickly for a short period of time, and are mostly sized for smaller boats. The slow moving nature of our boat suggests using a trolling motor. There are RC trolling motors available, but they are only meant for controlling the motor from the helm of the boat, and so do not have the range we need. Our team has two obvious options: a) Purchase and RC trolling motor and extend the range. b) Convert a trolling motor to RC. We are looking into which option will be simpler, as none of us is very experienced with electronics.''

4) On-boat Conveyor: The conveyor frame is being constructed out of the same type of aluminum L pieces as the boat frame. The only notable thing about the conveyor frame is that it must include a device to tension the belt. For this purpose we are creating a sliding end piece which can be moved outwards by tightening a bolt. The belt itself we are ordering from a conveyor belt supplier. We decided that a positive drive belt would be best for our purposes, since it eliminates the need for a chain or other rust-prone parts. (Positive drive is a conveyor belt style where the belt itself interacts with the sprockets - something like a timing belt.) Since it is difficult to make a well-grounded calculation of the power needed for the conveyor, we are waiting until it is put together so we can measure the torque required to turn it. This will allow us to better size the motor.

5) Basket: We have found some fishing-type baskets for sale online which will likely suit our purposes. The basket needs to be strong and lightweight. Water should be able to pass though the walls, but not Azolla or duckweed. A major challenge is creating an simple way for the operator to remove the basket from the boat when full, since it could easily weigh 500 lb. We are considering a system in which the basket would be released from the bottom of the boat when unlatched. Then the boat could be driven forward, out off the way, and the full basket could be winched out of the water.

6) On-shore Conveyor: We plan to purchase this component from an outside seller, since it is something which is already commercially available. The on-shore conveyor must be durable, non-damaging to the pond bank, and light enough for 1-2 people to set up. We have found a viable option, but are waiting until our boat is validated to make the investment. 

Current Work

''Our project is currently in the heavy construction phase. We have the PVC pieces ready and are cutting and welding the frames for the various components. Major design points still in discussion are:''

A) sizing motors

B) adding RC capability.



Document Archive

 * [[Media:16_NEMO_PreliminaryDesignReview.pdf|Preliminary Design Review (Fall 2016)]]


 * [[Media:16_NEMO_DetailedDesignReview.pdf|Detailed Design Review (Spring 2017)]]


 * [[Media:16_NEMO_PhosphorusTest.jpeg|Preliminary Phosphate Testing]]


 * [[Media:16_NEMO_CombinedSchedules.jpeg|Combined Team Schedule (Fall 2016)]]