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1. Enhancement of Anaerobic Digestion Using Duckweed (Lemna
minor) Enriched with Iron
P. B. Clark, PhD, BSc* and P. F. Hillman, MSc, BSc, CEng, MICE (Fellow)**
Abstract
This paper describes the enhancement of biogas
production in laboratory-scale anaerobic digesters (with
batch and semi-continuous operation) using iron-
enriched duckweed as a supplement to the digestion
of feedstock. The relationship between the level of
enhancement achieved and the retention period of the
digesters was also investigated. The trials demonstrated
that,
in batch digesters, iron-enriched duckweed
significantly improved the rate of microbial succession.
Batch digesters receiving
no duckweed took 40 days to
reach peak methane production compared to
15 days
when duckweed was added.
In the semi-continuous
digesters, an increase in gas production of about
44% was
observed.
Key words: Anaerobic digestion; biogas; duckweed;
enhancement; iron;
Lemna minor, manure.
introduction
The anaerobic process of waste organic stabilization has
always suffered in comparison with aerobic processes
because of the relatively slow rate of stabilization. This
leads to
long hydraulic retention periods and therefore
large digesters, with consequential high capital costs. In
the
UK, low ambient temperatures also add to the cost of
anaerobic digestion because of the need
to maintain
process temperatures within the mesophilic range of
30-
35O C. Most of the methane which is produced is needed
to maintain this temperature.
Maintaining the correct nutritional balance inside
an anaerobic digester is vital because it controls microbial
generation time and therefore the rate of degradation and
gas production. Speece(') concluded that the most
important nutrients to methanogenic bacteria are (in
decreasing order
of importance) nitrogen, sulphur,
phosphorus (the macronutrients); iron, cobalt, nickel,
molybdenum, selenium (the micronutrients); riboflavin
and vitamin
Bl2. Iron, due to its redox properties and the
role which it plays in energy metabolism, has been
identified as being the most important micronutrient
to
anaerobic bacteria(*).
During recent years a number of investigators have
identified important nutritional imbalances which, if
corrected, could lead to significant improvements in the
rate of dige~tion(~,~). However, the chemical environment
*Technology Development Team, North West Water PIC, Little Stanney, Cheshire,
UK.
**Lecturer in Environmental Engineering, Department of Civil and Environmental
Engineering, University
of Southampton, Hampshire, UK.
92
within an anaerobic digester is highly complex and (at
present) poorly understood, therefore attempts to
stimulate digestion with supplementary nutrients have
had varying degrees of ~uccess(~~~). The purpose of this
research was to identify whether duckweed could be
added to an anaerobic digester feedstock to improve the
nutritional balance and thus improve the rate of gas
production. Duckweed is a fast-growing aquatic plant,
capable of assimilating nutrients in high concentration.
Materials
Fresh poultry manure diluted with tap water to a total
solids (TS) concentration of
5% was used as the
feedstock. Poultry manure is relatively constant and has a
comparable constituency regardless of the geographical
location of the poultry farm. Since the aim of the
research was to investigate the value of supplements to
digestion feedstocks, the relatively universal consistency
and low variability in quality of poultry manure made
it ideal for the research purposes. Extrapolation of
the research findings to other wastes (such as sewage
sludge) can be undertaken and is planned in the next
phase of work. Poultry manure has a high volatility and
includes potentially troublesome compounds such as
ammonia. Anaerobic digestion is recognized as a
particularly suitable process for the stabilization of such
wasted').
Duckweed
(Lemnaceae) is an aquatic plant with a
number of valuable characteristics which indicate its
potential suitability as an additive
to anaerobic digestion.
For example, it possesses broad environmental growth
characteristics and can be successfully cultured in both
brackish and freshwater habitats in temperate and
tropical climates. It has a phenomenal growth rate
(38
tonnes/ha per year dry weight biomass@)) and can be
easily harvested. Unlike other aquatic plants such as
water hyacinth, duckweed does not require any pre-
treatment such as maceration before being used in a
digester; this considerably reduces operational costs.
Duckweed is also capable of removing excess nutrients
from wastewater and concentrating them in its cell
structure. It has been used in a number of integrated
waste management systems in less-developed coun-
trie~(~1'~) and has been investigated as the sole feed
substrate in anaerobic digesters(").
The source of duckweed for this research was a
series of ponds at a site at Mansbridge, Southampton.
The site is reclaimed land, and it is known that a wide
range of municipal and light industrial wastes was
deposited
on the site. The duckweed has a high iron
content
(21 000 mg/kg dry weight).
0 J CIWEM,1996, 10, April

2. Enhancement of Anaerobic Digestion Using Duckweed (lemna minor) Enriched with Iron
Constituent
Total solids (%)
Volatile solids:
as % fresh slurry
as
YO total solids
PH
COD (mg/l)
Volatile fatty acids
(mg/l)
Kjeldahl nitrogen (glkg)
Phosphorus (g/kg)
Methodology
Fresh poultry
manure
(following
dilution)
5.84
3.85
66.13
9.3
38
000
896
68.8
29.2
The research was based on the use of 1-1 vessels for batch
experiments and
25-1 digesters for semi-continuous feed
experiments. The feedstock which was used throughout
was poultry manure and the supplement was duckweed
(Lemna minor) (Table I). Batch runs lasted for 50-80 days,
and the end point was taken as the time at which gas
production ceased. Digesters were kept at
a constant
temperature
(32 k 2'C) throughout the period. Compari-
sons were drawn between digesters containing 550 g of
5% TS poultry manure alone and digesters in which 80 g
of the poultry manure had been replaced with an
equivalent wet weight of fresh duckweed
(8.5% TS). In
addition, each batch digester received 250 g of inoculum
slurry drawn from an active anaerobic digester. Gas
volumes were recorded daily and each experiment was
carried out in duplicate.
Table 1. Physical and chemical characteristics of
substrates
lnoculum
0.79
0.33
41.4
8.3
10000
287
266.3
25.3
Duckweed
8.56
7.15
83.9
6.4
8500
Nil
24.8
4.8
The semi-continuous digestion experiments took
place in six
25-1 vessels, and gas was measured continu-
ously via
a hydraulic valve gas meter. This was a
development of the design which was first proposed by
Mata-A1varez(l2).
In this series of experiments, each
digester was fed a daily volume of
1.5-3.0 1 poultry
manure
in accordance with the chosen hydraulic
retention period (which varied between
8 and 16 days).
The volatile solids concentration
of the poultry manure
feedstock was
5 f 1.25%. Unlike the batch experiments,
dry powdered duckweed was applied to the digesters and
there was no displacement of poultry manure. The effect
of different concentrations of iron-rich duckweed
on
digesters, operating at different retention periods, was
assessed.
Results and Discussion
The initial batch experiments were used to establish the
viability of iron-rich duckweed as an enhancing
supplement to the substrate of an anaerobic digester, and
then to identify the range of additions of duckweed
which would prove valuable. Fig.
1 demonstrates the
effect of the use of duckweed. The initial peak in gas
production contained mainly carbon dioxide with
methane concentrations below the limit of detection.
600
5001 h
No duckweed addition
B 14 '
0 10 20 30 40 50 60
Day of experiment
Fig. 1. Daily gas yield from batch experiments:
800 g feedstock consisting of 550 g diluted poultry
manure plus
250 g inoculum
This gas was produced by the activity of hydrolytic and
transitional bacteria (the precursors to methanogens).
These bacteria are responsible for the hydrolysis and
acetogenesis of the complex organic substrate converting
it into simpler volatile fatty acids and carbon dioxide. The
second peak in which methane concentrations of
65-80%
were measured (via gas chromatography) represents the
maximum production of methane and occurred when the
methanogens were firmly established. It can be seen that,
where the duckweed supplement has been included, the
evolution
of methane was brought forward by 14 days and
the peak methane production by over
20 days.
The amount of biogas (i.e. gas containing methane)
produced
by the non-enriched and the duckweed-
enriched batch digesters was
0.295 and 0.281 l/g of
volatile solids respectively. The biogas yield from most
types of biomass varies between
0.13 and 0.30 l/g of
volatile solids(I3), and therefore these results are within
the expected range. Despite the fact that the duckweed-
enriched digesters had a greater maximum daily biogas
yield, the similarity for each treatment indicates that the
effect of the duckweed
is to increase the rate of decompo-
sition and not increase the total amount
of biogas
produced.
The stimulatory effect caused by the duckweed was
identified as being due to its high iron content. This
conclusion was drawn from
a series of experiments not
described in detail here.
In these experiments various
tailored pond mediums were used to produce duckweed
enriched with a variety of micronutrients. Macro-
nutrients such as sulphur, nitrogen and phosphorus,
which are required in relatively large quantities by
anaerobic bacteria"), were not considered because these
were already present in non-limiting concentrations in
the manure (typical values: Kjeldahl nitrogen
= 68.8
g/kg, phosphorous = 29.2 g/kg). These different
enriched duckweeds were applied to batch digesters con-
taining poultry manure, and their stimulatory capacity
was measured. Repeated experiments indicated that iron
was the causal element.
No synergistic effect between
macronutrients was observed.
Callander and Barf~rd(~), having observed similar
responses to iron additions, drew attention to the need to
identify an economically effective iron chelator which
0 J.CIWEM,1996, 10, April 93

3. Enhancement of Anaerobic Digestion Using Duckweed (Lemna minor) Enriched with iron
Metal content
Fig. 2. Metal content of feed substrates
could be added to the digester feed without the produc-
tion of excessive precipitation. The authors believe that
the use of iron-rich duckweed fulfils this objective in the
case of the anaerobic digestion of poultry manure and
could have a more universal application.
A comparison of
the metal contents of the poultry manure, inoculum and
duckweed used in this research is given in Fig.
2.
Following the identification of a stimulatory effect
and its cause, a series of experiments was carried out to
explore the limits of iron addition. Fig. 3 shows the
impact
of enhancement by reducing the period to peak
methane production from 35 days to
14-18 days. These
1200
1000
CI
E 800
W
200
0
experiments indicate that the maximum degree of
enhancement occurs when the duckweed added to the
digester is such that the
total iron concentration is 11 000
mg/kg volatile solids. This level of addition produced
dissolved-iron concentrations of
250-280 yg/ml per kg
volatile solids.
The semi-continuous digesters were fed daily with
poultry manure over a period
40 days to establish anaero-
biosis and control conditions. For these experiments the
operating temperature was
32+2'C. Two digesters were
fed at a rate of 3
1 of slurry per day (equivalent to a
hydraulic retention period of
8.3 days) and two at a rate
of 1.5 I/d (hydraulic retention period of
16.6 days). Iron
enrichment was achieved using dried powdered duck-
weed, providing an iron equivalent of
420 mg/l within
the digester. This level of addition is similar to that
determined to be optimum
in the batch experiments
described above. The feed slurry concentration was
approximately 5% volatile solids throughout, and the
results are given in Fig.
4. Daily gas yield is smoothed
using a 3-day moving mean. Prior to enrichment, both
digesters produced
22.76 1 biogas per day (mean).
Enrichment continued over a period
of two retention
periods, and the gas production figures used in statistical
analysis are taken from the second half of this period to
avoid the period during which the digester was adjusting
to its new environment. With a hydraulic retention
period of
8.3 days, mean gas production increased by
44.2% following the addition of iron-enriched duckweed
+
No duckweed addition
Duckweed (equivalent to
150 mg Iron)
Duckweed (equivalent to 200 mg Iron)
Duckweed (equivalent to 400 mg Iron)
--e
--t
-t-
0 10 20 30 40 50 60
Day of experiment
Fig. 3. Daily gas yield from batch experiments: effect of successively larger additions of duckweed
94 0 J.CIWEM,1996, 10, April

4. Enhancement of Anaerobic Digestion Using Duckweed (Lemna minor) Enriched with Iron
Enrichment begins
15 '
0 10 20 30 40 50 60
Day of experiment
Fig. 4. Daily gas production from semi-continuous
digesters using different hydraulic retention periods.
(Each enrichment equivalent
to 420 mg iron per litre)
compared to 27.9% in an identical duckweed-enriched
digester operating on a 16.6-day retention. This supports
the conclusion drawn from the batch experiments, i.e.
that the additional concentration of iron affects the rate
of gas production, and not the overall yield.
Inevitably, some of the additional gas will have
originated from the duckweed biomass itself as it
degrades. The authors concluded from a series of batch
digester experiments (as yet unpublished) that powdered
duckweed produces approximately
150 ml biogas per
gram dry weight. The duckweed added
to the semi-
continuous digesters therefore accounts for
less than
10% of the observed increase in gas yield. Measurement
of the quality of the biogas indicated that there was no
significant effect on its methane content following the
additions of duckweed.
Conclusions
1. The enhancement of biogas production in anaerobic
digesters is both a plausible and potentially valuable
strategy to pursue. Evidence from this research has
demonstrated that, with a consistent substrate such as
poultry manure, additions of an iron-rich biomass
supplement to the digester can yield substantial
increases in the rate of biogas production and a more
rapid stabilization
of the organic waste. Since the
methane content within the biogas remained
reasonably constant throughout the experiments, it
can be concluded that an increased rate of gas
production would be directly accompanied by an
enhanced rate of energy recovery.
2. Although it was not possible to make every parameter
directly comparable, the semi-continuous experi-
ments confirmed that the results from the batch-scale
experiments
could be repeated under more realistic
operational conditions. It is believed that the results
also demonstrate the potential savings which could
accrue in design. If hydraulic retention periods were
able to be reduced significantly without a loss of
recovered biogas, construction costs and land area
taken would be reduced.
Acknowledgements
The authors wish to express their appreciation of the
financial support provided by the Science and Engin-
eering Research Council and the support and encourage-
ment given by staff in the Department of Civil and
Environmental Engineering, University of South-
ampton.
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0 J.CIWEM,1996, 10, April 95