Duckweed Performance in Cold Climates

Duckweed grows approximately 6+ months in Cache Valley Utah. This presents an issue when using duckweed for removing nutrients from the wastewater. I've observed duckweed as early as March and as late as the end of November; however, I've only noticed full duckweed coverage from about the first of May to November 19th (World Toilet Day). Fortunately, Wellsville currently has the capacity to store water (i.e. save for treatment in the warm season) during the winter, which they do. Also, their discharge permit allows for 432 kg-P/yr (72kg during the warm season and 360kg cold season). Wellsville discharges into the Little Bear River. This excellent report by JUB Engineering provides a summary of the treatment plant where we propose using duckweed nutrient removal. I've met a few engineers from JUB and would highly recommend them.

Figure: Duckweed (L. minor and Wolffia) at Wellsville Sewage Lagoons 22 March 2010--duckweed appeared immediately along shore as ice melted.

Video: Water Cress growing green in Cold Cache Valley Climate (2.5min)

Video: Water Cress growing green in Cold Cache Valley Climate (25sec)

The figures below show temperature trends for Cache Valley, including the cold temperatures in January when that the water cress survives. These charts were retrieved online from a database/network ran by Jeff Horsburgh of the Utah Water Research Laboratory (UWRL). Jeff continuously monitors environmental data along the Little Bear River and makes it available to the public online. This is a fantastic research tool made possible by data logging equipment from Campbell Scientific, Inc. (a local Cache Valley company).

Figure: Temperature Trend two weeks preceding video on 15 Jan 2011--Cache Valley Winter

Figure: Temperature Trend before video on 15 Jan 2011 Cache Valley Winter

Most research with duckweed is conducted in climates with a 9+month growing season. Laboratory tests are usually performed at 25'C. A difficult to find article, but frequently referenced, by Culley, D.D. Jr., et al., has this to say about winter and duckweed:

"Several species rarely flower and form no turions. During the winter season the fronds are greatly reduced but remain at the surface. Occasionaly, turion-like fronds will form, but the plants continue to slowly reproduce vegatatively. These plants are probably the best plants to utilize in a culture system as restocking is virtually assured. Lemna gibba, L. valdiviana, L. minor, L. trisulca and L. minuscula are five such plants that frequently show some growth in the cool season. In some cases, L. gibba (Culley 1978) also shows rapid growth under summer conditions, making it a candidate for continuous culture. Lemna minor, S. intermedia and S. biperforata also do not form turions and rarely flower. L. minor may be a candidate for continuous culture, but the latter two are more suited for culture under warm condition. L. trisulca is a delicate, submerged form and thus will be more difficult to culture and harvest.

"At present, mixed cultures appear preferable to monoculture to insure the best yield on a yearly basis. Maintaing mixed cultures can present management problems, due to competition for space, variation in growth rates, and harvesting techniques that favor removal of certain species. For example, Wolffia species are easily suspended in the water column when harvesting in a manner that disturbs the water. Over time, the larger and more buoyant species are removed, leaving an increasing biomass of Wolffia for expansion. L. gibba, a very buoyant plant, will rise above the more flattened fronds of, for example, Spirodela polyrrhiza. Unless the plants are carefully harvested to prevent crowding the culture will gradually be dominated by L. gibba."
-Culley

Full-scale operations:

Mr. Dudley D. Culley has researched full-scale duckweed systems; however, the majority of research and duckweed work is still limited to lab and small pilot-scale studies. I am aware of only a few full-scale operations with duckweed plants; and of those, I believe only one is still operable. Duckweed grows on many wastewater lagoons, but it's rare to find one periodically harvesting the duckweed for nutrient (or BOD, TSS, etc.) treatment and even rarer to find a system with a cold climate like Cache Valley, UT. Here is a small list of the full-scale operations I'm aware of:
Most Successful: Agriquatics Mirzapur System (Bangladesh): article, website, video
Closest to Utah: Boulder City, NV wastewater treatment Lemna Corporation project supervised by Don Donahue--11 acre facultative lagoon with duckweed for BOD reduction; abandoned after 10 years. Produced high amounts of duckweed biomass but due to inadequate solids handling became too burdensome too continue. Don mentioned in personal correspondence that the duckweed stopped growing around 10'C.
Problems: Biloxi, Miss. (terminated) and Paragould, Ark. (never successful due to algae/fungus issues) as discussed here.