May 14, 2012

Thesis published:  Duckweed Uptake of Phosphorus and Five Pharmaceuticals:  Microcosm and Wastewater Lagoon Studies

(plus new job started at WesTech Engineering, Inc.)


  1. Hello
    My name is Ali, I am working on application of duckweed in Siberia, and I am very admire your work. you mentioned in your thesis (chapter three, page 46) that you used air pump for adding dissolved oxygen. How much dissolved oxygen you added periodically and why?
    Thank you in advanced

    1. Ali, thank you for the comment. I wish you well with your project. I'd be interested in learning more about it. A quick answer to your question is the dissolved oxygen increased from 4.45, 3.8, and 2.5 mg/L up to 7.9, 6.7, and 5.9 mg/L in cells #1-3 and the measurements were made when the duckweed was in the dark. Dissolved oxygen was added via a fish bubbler/stone located right before cell #1 which pumped air at 2.5-3.33 scfh. These measurements were made before starting the airlift pump. I also added a fungicide at the same time that air started getting bubbled into the reactor. The main purpose for doing all this was to get rid of fungi growing in the reactor.

      Now for the long answer:

      I had to go back and check my notebooks since it has been a few years since growing the duckweed. I added the recirculation system after things like chlorosis, fungi, and algae started to make it difficult to grow duckweed in the lab with synthetic nutrient solutions. One of my committee members was Dr. Bruce Bugbee (professor of plant physiology at Utah State University) and he recommended maintaining dissolved oxygen by increasing the flow rate across the root zone in order to prevent disease. He has an article about this on his website:

      Bugbee, B. (2003). “Research: hydroponics and plant nutrition - principles of nutrient management in recirculating hydroponic culture.” Crop Physiology Laboratory, (July 25, 2016).

      Here is the excerpt from the article about flow rate and dissolved oxygen:

      "Designing Hydroponic Systems: The Importance of Flow Rate

      "Most hydroponic systems have inadequate flow rates, which results in reduced oxygen levels at root surfaces. This stresses roots and can increase the incidence of disease. Oxygen is soluble only as a micronutrient, yet its uptake rate is much faster than any other nutrient element.

      "The nutrient film technique was designed to improve aeration of the nutrient solution because of the thin film of solution, but the slow flow rates in NFT cause channeling of the solution and reduced flow to areas with dense roots. The root surfaces in these areas become anaerobic, which diminishes root respiration, reduces nutrient uptake, increases N losses via denitrification, and makes roots susceptible to infection. The problems with the nutrient film technique have been discussed by several authors. Bugbee and Salisbury (1989) discuss the importance of flow rate and adequate root-zone oxygen levels."

      At first, I recirculated water at approximately 100 liters per day (one bed volume turnover) using a peristaltic pump. I added the aquarium fish bubbler to add dissolved oxygen after finding fungus growing on the duckweed. Before I added the bubbler, the dissolved oxygen concentrations were 4.45, 3.8, and 2.5 mg/L in cells 1-3, respectively (note: nutrient solution entered in cell #1 and exited in cell #3). Two days after adding the fish bubbler, the dissolved oxygen increased to 6.2, 5.6, and 4 mg/L in cells 1-3 and I made the following note in my notebook, "Improved, but still under par." Apparently, the target dissolved oxygen concentration was higher but I don't remember what it was—maybe 100% saturation. Two weeks later, the dissolved oxygen concentration was 7.9, 6.7, and 5.9 mg/L in cells 1-3. I measured these dissolved oxygen concentrations just before the grow lights turned on when the reactors were in the dark.

      The air lift pump was added several months later. I think I finally figured out that I was pumping water and adding air using two different pieces of equipment, so I decided to simplify the process by pumping the water with air. The aquarium bubbler was a small 2-watt unit that produced 2.5-3.33 scfh air. The air lift pump was submerged 8.25 inches and discharged at a height of 12 inches (4.25 inches above the water surface). The air lift pump had a flow rate of 750 mL/min and I think it only operated when the grow lights were turned on.