Harvesting and storage

\This week I will dive deeper into harvesting and storing of duckweed and focus in my country of choice (the Netherlands). Large scale production of duckweed requires sewage lagoons that have mechanical sewage systems with mechanized harvesting, drying and possessing(FAO, n.d.)  In the Netherlands, many different methods have been used to harvest duckweed such as:

Figure 1: Wheeled excavator https://edepot.wur.nl/374052

1. Wheeled excavators

Back in 2007, the possibility of using duckweed in order to feed cows was explored, in order to meet the demand of feed excavators as harvesters for duckweed were introduced. The excavators had a flat sieve pan that could easily extract the plant from ponds. This method is time-consuming but ends up bringing a clean product. (See figure 1)

2. Automatic solar-powered conveyors

The machine works without manpower and the plant is harvested by a paddle wheel which is powered only by solar power. This method is very sustainable since the machine is only powered by renewable energy.

3. Duckweed guzzler with belt

The guzzler machine is meant to be used in shallow waters. It takes advantage of the water current and removes duckweed from the water. The belt brings all the harvest to the land.

Figure 2: Colubris harvester https://edepot.wur.nl/374052

4. Colubris

In contrast to the others, colubris sucks the duckweed to a special basket and then the harvest will be pumped away. (See figure 2)

(Timmerman & Hoving, 2016)

Harvest regime

Research shows that if duckweed is harvested in a shorter time interval there is a higher biomass density, this occurs because more space is freed and allows the plant to keep growing and not get stuck. It was estimated that in order to have 100 g/m² biomass and a 5-6% dry matter, duckweed should be harvested with a two-day interval. (Timmerman & Hoving, 2016)

If the plant has had optimal growth conditions it can yield 10-30 tom DM/ha/year. (Leng R A, 1995) In table1the composition of duckweed after harvest can be found.

 Table 1: Duckweed composition after harvesting

Fiber	5-15%
Crude protein	35-43%
Polyunsaturated fat	5%
Water content	92-94%

There are many different ways in which duckweed can be used. Each use requires different harvest and storage conditions. Some of the uses are listed below.

• Animal feed
• In areas that lack feed for animals, duckweed can be a good replacement for vegetable protein
• The plant counts with a protein content of 15-45%
• If  grown for chicken feed, duckweed needs to be dried in order to achieve a dry matter content of 97,83%, a crude protein content of 29,05% and fat content of 5%, later on, duckweed will be mixed with other feed supplements
• Nevertheless, since the plant is able to absorb heavy metals it can also transfer certain pathogens. (Timmerman, M., & Hoving, 2016)

• Fuel production 
• After harvesting duckweed needs to have a pretreatment and be fermented in order to be used for fuel production 
• The fermentation lasts about 7 days and it can produce up to 75 ml H₂/ gr dry biomass. (Timmerman, M., & Hoving, 2016)

References: 

FAO. (n.d.). DUCKWEED: A tiny aquatic plant with  enormous potential for agiculture and environment. Retrieved September 20, 2019, from http://www.fao.org/ag/againfo/resources/documents/DW/Dw2.htm

Leng R A, S. J. H. and B. R. (1995). Duckweed - a potential high-protein feed resource for domestic animals and fish. Retrieved October 29, 2019, from http://www.lrrd.org/lrrd7/1/3.htm

Timmerman, M., & Hoving, I. E. (2016). Purifying manure effluents with duckweed pdf. Retrieved October 29, 2019, from https://edepot.wur.nl/374052

Timmerman, M., & Hoving, I. E. (2016). Purifying manure effluents with duckweed. Retrieved from www.wageningenUR.nl/livestockresearch