experiments, they found they could increase
duckweed starch content from 9.5 to 40%
through 11 days of growth on clean water, and
that a hydraulic residence time (HRT) of 6 days
achieved their treatment standards and optimized
the Landoltia punctata starch yield above maize
and wheat to 13.9 t/ha yr. Considering that these
are experimental water treatment plants, their
duckweed yield is expected to rise with further
optimization, or in more intensive cultivation.
For their size, length of study, and abundance of
publicized information, these two facilities stand
as prime examples to study duckweed’s water
treatment capabilities, yield, and applications in
practice.
If a wastewater stream comes from an industrial point source or a large municipality, it likely
has persistent chemical compounds, such as textile dyes and metalworking fluids, or bioaccumulating heavy metals in it. There is a large body
of academic evidence illustrating the potential of
duckweed and other plants to treat wastewater
from cities, tanneries, mines, metalworking
shops, and textile mills by degrading compounds
like pharmaceuticals and antibiotics, and accumulating phenols along with heavy metals (van
der Spiegel et al. 2013). Rezania et al. reviewed
the heavy metal absorption of 5 different plant
species and described 19 studies evaluating
Lemna minor and gibba as moderate or hyperaccumulators, often concentrating metals over
400-fold, depending on the metal and circumstance; even when used as a dried powder
(Rezania et al. 2016). A table of 10 studies illustrated removal efficiencies of Cu, Cd, Pb, Zn, and
9 other metals, with the lowest being 29% and the
majority being over 70%. In these cases, duckweed and its microbial communities can treat a
variety of harmful wastewater streams and then be
utilized outside of the food supply for biofuel
applications to further concentrate the metals.
1.2.3 Bioenergy
While these applications have been researched
academically, few have been practiced in large
scale. The simplest bioenergy application would
be direct combustion of dried duckweed, possibly as a drop-in fuel for a trash incinerator or
coal-fired power plant. This would concentrate
heavy metals in the smoke, which could be
scrubbed, and ash for proper disposal, or even
encapsulated reuse in concrete or gypsum as coal
ash is in the USA. A second relatively simple
option would be anaerobic digestion to produce
methane. Conveniently, many municipal
wastewater treatment plants already have anaerobic digesters to treat sludge, and the liquid
digestate has been well studied as a fertilizer for
duckweed ponds. A duckweed and pig manure
mixture increased gas production 41% in comparison with pig manure, while the increased
production from cow manure tapered after a 2%
inclusion of duckweed (Cui and Cheng 2015).
Another possibility is pyrolysis of dried biomass or hydrothermal liquefaction (HTL) of wet
biomass. Both processes are similar, yet we will
focus on HTL since it conveniently avoids drying
the *90% water content duckweed biomass.
In HTL, biomass and water processed at 200–
400 °C and 50–200 times atmospheric pressure
for 10–90 min to create aqueous solutes, H 2 ,
CO 2 , and CH 4 gasses, high molecular weight
bio-char, and bio-crude oil with 95% of the
energy content of diesel (Zhang et al. 2014a).
A wide variety of feedstocks from algae to wood
and to sewage sludge can be used, separately, or
mixed, and each requires significant testing to
optimize, which is likely why there are no
large-scale HTL operations at the present day.
The algae can be converted to crude oil with a 26–
68% yield depending on the conditions, yet all the
crude oil tends to have a high water content and
require hydro-deoxygenation to dewater it
thereby matching the stability and viscosity of
petroleum crude oil. A wide range of molecules
can be created and isolated so there is petrochemical potential as well. This option is interesting for its theoretical ability to match the wide
variety of the crude oil applications in a carbon
neutral manner and the ability to produce in hours
what naturally takes *150 million years.
The most versatile and best studied application of potentially harmful duckweed is fermentation of the starch, which can be accumulated at
6
P. Fourounjian et al.
duckweed starch content from 9.5 to 40%
through 11 days of growth on clean water, and
that a hydraulic residence time (HRT) of 6 days
achieved their treatment standards and optimized
the Landoltia punctata starch yield above maize
and wheat to 13.9 t/ha yr. Considering that these
are experimental water treatment plants, their
duckweed yield is expected to rise with further
optimization, or in more intensive cultivation.
For their size, length of study, and abundance of
publicized information, these two facilities stand
as prime examples to study duckweed’s water
treatment capabilities, yield, and applications in
practice.
If a wastewater stream comes from an industrial point source or a large municipality, it likely
has persistent chemical compounds, such as textile dyes and metalworking fluids, or bioaccumulating heavy metals in it. There is a large body
of academic evidence illustrating the potential of
duckweed and other plants to treat wastewater
from cities, tanneries, mines, metalworking
shops, and textile mills by degrading compounds
like pharmaceuticals and antibiotics, and accumulating phenols along with heavy metals (van
der Spiegel et al. 2013). Rezania et al. reviewed
the heavy metal absorption of 5 different plant
species and described 19 studies evaluating
Lemna minor and gibba as moderate or hyperaccumulators, often concentrating metals over
400-fold, depending on the metal and circumstance; even when used as a dried powder
(Rezania et al. 2016). A table of 10 studies illustrated removal efficiencies of Cu, Cd, Pb, Zn, and
9 other metals, with the lowest being 29% and the
majority being over 70%. In these cases, duckweed and its microbial communities can treat a
variety of harmful wastewater streams and then be
utilized outside of the food supply for biofuel
applications to further concentrate the metals.
1.2.3 Bioenergy
While these applications have been researched
academically, few have been practiced in large
scale. The simplest bioenergy application would
be direct combustion of dried duckweed, possibly as a drop-in fuel for a trash incinerator or
coal-fired power plant. This would concentrate
heavy metals in the smoke, which could be
scrubbed, and ash for proper disposal, or even
encapsulated reuse in concrete or gypsum as coal
ash is in the USA. A second relatively simple
option would be anaerobic digestion to produce
methane. Conveniently, many municipal
wastewater treatment plants already have anaerobic digesters to treat sludge, and the liquid
digestate has been well studied as a fertilizer for
duckweed ponds. A duckweed and pig manure
mixture increased gas production 41% in comparison with pig manure, while the increased
production from cow manure tapered after a 2%
inclusion of duckweed (Cui and Cheng 2015).
Another possibility is pyrolysis of dried biomass or hydrothermal liquefaction (HTL) of wet
biomass. Both processes are similar, yet we will
focus on HTL since it conveniently avoids drying
the *90% water content duckweed biomass.
In HTL, biomass and water processed at 200–
400 °C and 50–200 times atmospheric pressure
for 10–90 min to create aqueous solutes, H 2 ,
CO 2 , and CH 4 gasses, high molecular weight
bio-char, and bio-crude oil with 95% of the
energy content of diesel (Zhang et al. 2014a).
A wide variety of feedstocks from algae to wood
and to sewage sludge can be used, separately, or
mixed, and each requires significant testing to
optimize, which is likely why there are no
large-scale HTL operations at the present day.
The algae can be converted to crude oil with a 26–
68% yield depending on the conditions, yet all the
crude oil tends to have a high water content and
require hydro-deoxygenation to dewater it
thereby matching the stability and viscosity of
petroleum crude oil. A wide range of molecules
can be created and isolated so there is petrochemical potential as well. This option is interesting for its theoretical ability to match the wide
variety of the crude oil applications in a carbon
neutral manner and the ability to produce in hours
what naturally takes *150 million years.
The most versatile and best studied application of potentially harmful duckweed is fermentation of the starch, which can be accumulated at
6
P. Fourounjian et al.