Preliminary Literature Review

Title: Literature Review for Duckweed Systems: P-removal, Growth, and Harvest
Created: 29 April 2010
Author: Jon Farrell

Background:

Nutrient removal from wastewater prevents eutrophication from occurring downstream where the wastewater is discharged into water bodies such as rivers and reservoirs. One nutrient removal system that has been researched extensively over the past 40 years (Culley) utilizes duckweed plants (Lemnaceae) which uptake nutrients like N, P, K, Ca, and Mg into its biomass as it grows. Duckweed systems rely on three basic principles: nutrient uptake, harvesting, and solids management.

Duckweed plants typically contain more phosphorus in its tissue than other floating plants, which makes them suitable for phosphorus removal (Alaerts, Reddy). Duckweed systems usually treat sewage lagoons that receive weak municipal wastewater containing 1-4mg-P/L; however, duckweed is also used to treat swine lagoon waste containing 62.5-135mg-P/L (Chaiprapat).

Harvesting is an essential component of duckweed nutrient removal systems because it physically removes the phosphorus from the system via the biomass. Without harvesting, the plant tissue would die, settle to the bottom of the lagoon, decompose and then release the phosphorus and other nutrients back into the water column. This harvested biomass can be used as compost (Donahue), fodder rich in protein (Culley), or to generate fuel like methane (Clark).

Duckweed:

Duckweed grows naturally in almost every region with a growing season of at least five months. Most studies involving duckweed take place in climates with 9-10 month growing seasons; however, several also take place in regions with only 5-7 month growing seasons (Culley). Duckweed is a monocot, it floats on water, and has one of the fastest growth rates of any of the macrophytes. Duckweed is the common name for the Lemnaceae family of plants, with species like Lemna minor, Lemna Gibba, Spirodela Polyrhizza, and Wolffia (genus name). Duckweed studies range from full-scale operations with ponds covering 200m2 (Edwards) to 11 acres (Donahue); to pilot scale operations with only a few m2 (Reddy, Zimmo), to lab scale tests in jars with only 0.004 m2 surface area (Chaiprapat).

Phosphorus Removal:

Many studies have pointed out a direct correlation between %P in the plant tissue and the available P in the water column (Alaerts, Culley). As the PO4-P (bioavailable P) concentration in the water column decreases so does the %P in the tissue. While 1%-P is very common in oven dried duckweed, values have been reported from 0.3 up to 2.6%-P. Percent dry matter ranges from 5.4-8% with 69-86% being the organic (volatile) fraction. The N:P ratio is typically 5:1 (Alaerts, Edwards). Knowing the %P and %N in the duckweed tissue helps to construct a mass balance identifying the fate of phosphorus in the system.

Up to 100% phosphorus removal has been reported in bench scale tests (Chaiprapat); however, 60-75% phosphorus removal (Alaerts, Zimmo, and Kadlec) is more common. These same reports have identified duckweed biomass as contributing 13-47% of the total phosphorus removal, and one account attributes all of it to duckweed. The phosphorus concentration in the effluent coming from duckweed systems almost always falls below 1mg-TP/L and frequently less than 0.53mg-P/L down to 0.05mg-P/L (Willet, Edwards, Alaerts). Edwards observed that duckweed growth decreases when phosphorus levels fall below 0.3mg/L.

Duckweed Growth:

Most studies recommended starting and maintaining duckweed systems with enough duckweed to fully cover the surface area. Full coverage provides some of the highest growth rates (Reddy), but perhaps more importantly, it prevents algae proliferation that out competes the duckweed (Edwards, Al-Nozaily, Lemna Corp.) and leads to decreased productivity. Starting densities should be kept in the linear range between 10-120 g(dry)/m2 for Lemna minor (Reddy). Reddy and Edwards recommended starting with 10-11.9 g(dry)/m2; Culley, Chaiprapat, and Zimmo recommended 30-40 g(dry)/m2; while Willet, Lemna Corp., and Alaerts recommended 80-132 g(dry)/m2. Starting densities with fresh duckweed ranged from 500 to approx. 1500 g/m2. Typical seasonal yields ranged from 3-9.5 tons(dry)/ac·yr. Maximum yields between 17-25 tons(dry)/ac·yr have also been reported (Alaerts, Edwards). The relative growth rate (RGR, gnew/gold·day) of duckweed ranges from 0.06-0.121 for many systems (Chaiprapat, Culley, Willet) up to 0.24-0.31 for lab experiments. Al-Nozaily observed that light intensity was the single most important variable controlling RGR, and recommended providing 200-300 umol/m2·sec (ppf) for highest growth rates indoors.

Several factors limiting growth rates have been observed. Growth rate decreases as biomass accumulates to the point that fronds start overlapping each other (Al-Nozaily, Chaiprapat, Culley, Reddy). Growth rate decreases with nutrient depletion (Chaiprapat, Culley, Edwards). Duckweed prefers ammonium (NH4) to ammonia (NH3), and growth decreases when NH3>NH4 or when pH exceeds 9.25 (Al-Nozaily, Culley). Phosphorus precipitation also occurs at pH near 9.3, which also leads to nutrient deficiencies and lower growth rates. Several studies indicated that wind or movement decreased growth (Edwards, Willet). Biomass started depleting at temperatures below 17°C, and completely disappeared below 5°C (Donahue, Zimmo). Growth rate also decreased due to competition between species. Edwards noticed that Wolffia out-competed the Lemna species and yielded less biomass due to its smaller plant size. Aphids living atop duckweed mats in some instances were associated with decreased growth as well (Zimmo, Edwards).

Harvesting:

The frequency of harvesting and the amount of biomass removed per harvest varies from study to study. However, consistent observations include: 1) Maintain 100% coverage to reduce algae growth; 2) Harvest at least once every 20 days—the more frequent the better for nutrient removal; and 3) Harvest frequency and amount often depends on the available manpower and equipment available to harvest.
Continuous harvesting prevents overcrowding, biomass death, and release of nutrients back into the water column. Culley reported that up 50% of the N & P in the biomass gets released if more than 20 days go by between harvests. Alaerts harvested approx. 4.5mg(dry)/m2·day, Willet harvested 50% after the biomass had doubled the starting density, while Edwards harvested every 2-15 days depending on whether it was the dry(warm) or wet(cool) season, respectively.

Harvest rates depend not only on duckweed growth, but also on the ability to physically harvest the system. Donahue, superintendent of a Lemna Corp. duckweed covered lagoon in Boulder City, NV, reported harvesting the entire lagoon every week. This required harvesting 11acres/wk. at a rate of 37g/m2·week. This yielded approx. 71 tons (dry)-duckweed per year. Two people worked 10 hr. shifts M-Th and used mechanical harvesters with 4 ft. wide conveyors to remove the fresh duckweed that was then loaded into trucks and composted at the local landfill. Donahue reported that the duckweed system was used for approx. 10 years before being shut down because they could not keep up with the quantity of duckweed produced. Hence, careful solids management programs are necessary to guarantee sustainable and long-lasting duckweed systems.

Experimental Setup:

Background:

This study looked into the practicality of using a duckweed system to remove phosphorus from the Wellsville (UT) Municipal Sewage Lagoons. These lagoons were constructed in the 1960’s and cover 56 acres. Currently, this is a 0.5 MGD system that is expected to increase flow during the next 10 years to the point that the Utah Dept. of Environmental Quality (UDEQ) is concerned that it will not be able to meet its allowable 432kg-P/yr. discharge permit. A duckweed system for phosphorus removal seems promising in Wellsville for two principal reasons: 1) native duckweed plants (a mixed culture of Lemna minor and Wolffia) already cover the entire surface of the lagoons for at least 6 months (May through October); and 2) Wellsville has weak wastewater with approx. 4mg-P/L which results in a loading of approx. 12.2g-P/m2·yr. which is in the recommended <20g-P/m2·yr. range (Kadlec).

Material and Methods:

Phosphorus removal and duckweed growth:

A mixed culture of L. minor and Wolffia was seeded into approx. a 113 L acrylic reactor (3 ft. L x 2 ft. W x 8 in D) and divided into 3 sections simulating 3 lagoons; an identical reactor was placed next to it without duckweed as a control. The experiment took place for one year in a 25°C constant temperature room. High-pressure sodium lamps (HPSLs) were suspended 48 in. above the plants and provided 300 umol/m2/sec (ppf) 16 hrs/day. Raw wastewater from Wellsville influent was continuously fed with peristaltic pumps at a rate of approx. 1.77 Lpd. An average 66% of the influent flow evaporated per day, and so dilution tap water was continuously fed at 0.64 Lpd to provide enough effluent. The effluent was captured in 15 L buckets. Duckweed was re-seeded a few times at starting densities ranging from 15-90 g(dry)/m2 . Plant harvesting occurred every 7-14 days and removed 25-75% of the coverage. Plants were oven dried at 105°C to get dry mass of duckweed.

Measurements:

Total Phosphorus measurements were made with HACH test kit method 10127. Reactive Phosphorus (PO4-P) measurements were made with ascorbic acid APHA Standard Method 4500P-E. Total Nitrogen, Ammonia, and Nitrate measurements were made with HACH test kit methods 10071, 10031, and 10020, respectively. Alkalinity, TSS, and VSS measurements followed APHA Standard Methods. pH and DO measurements were made with Corning and Hanna probes, respectively. Duckweed tissue samples were measured by the Utah State University Analytical Lab (USUAL). Phosphorus concentrations in plant tissue and sediments were also measured for PO4-P following dry ashing at 550°C with subsequent wet aqua regia digestion; these results were verified with standard grape petiole leaves with a known 0.38%P dry weight.
References:

Alaerts, G. J., M. R. Mahbubar, and P. Kelderman. 1996. Performance analysis of a full-scale duckweed-covered sewage lagoon. Wat. Res. 30(4):843-852.

Al-Nozaily, F. G. 2001. Performance and Process Analysis of Duckweed-Covered Sewage Lagoons for High Strength Sewage. Rotterdam, NL: A. A. Balkema.

Chaiprapat S., J. J. Cheng, J. J. Classen, and S. K. Liehr. 2005. Role of internal nutrient storage in duckweed growth for swine wastewater treatment. Transaction of the ASAE. 48(6):2247-2258.

Clark, P. B., and P. F. Hillman. 1996. Enhancement of anaerobic digestion using duckweed (Lemna minor) enriched with iron. J. of the Chartered Institution of Water and Environmental Management. 10(2):92-95.

Culley Jr., D. D., E. Rejmankova, J. Kvet, and J. B. Frye. 1981. Production, chemical quality, and use of duckweed (Lemnaceae) in aquaculture, waste management, and animal feeds. J. World Maric. Soc. 12(2):27-49.

Donahue, Don. Superintendent of the Boulder City (NV) Wastewater Treatment Plant. Personal correspondence. 3/18/2009.

Edwards, P., M. S. Hassan, C. H. Chao, and C. Pacharaprakiti. 1992. Cultivation of duckweeds in septage-loaded earthen ponds. Bioresource Technology. 40:109-117.

Kadlec, R. H. 2009. Treatment Wetlands. 2nd ed. Boca Raton, FL: CRC Press.

Lemna Corporation. 1996. Operation and Maintenance Manual for Boulder City, Nevada . St. Paul, MN: Lemna Corporation.

Reddy, K. R., and W. F. De Busk. 1985. Growth characteristics of aquatic macrophytes cultured in nutrient-enriched water: II. Azolla, duckweed, and salvinia. Economic Botany 39(2): 200-208.

Willet, D. 2005. Duckweed-based Wastewater Treatment Systems: Design Aspects and Integrated Reuse Options for Queensland Conditions. Brisbane, AU: DPI&F Publications.

Zimmo, O. R. 2002. Process performance assessment of algae-based and duckweed-based wastewater treatment systems. Water Sci. and Tech. 45(1):91-101.