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Maria Teresa Varnero and Ian Homer
Department of Engineering and Soils, Faculty of Agricultural Sciences,
University of Chile
Biogas production
16
ORGANIC
TRASH
AND
TOILET
ANIMAL
MANURE
NOPAL CLADODES
INLET
BAG
OUTLET
STOVE
STORAGE
POOL
BIOGAS
TV
PRESSURE
GAUGE
CROP
LAMP
DESULFRIZER
ORGANIC
TRASH
AND
TOILET
ANIMAL
MANURE
NOPAL CLADODES
INLET
BAG
OUTLET
STOVE
STORAGE
POOL
BIOGAS
TV
PRESSURE
GAUGE
GENERATOR
CROP
LAMP
DESULFRIZER
Family size biogas system
Figure 1
Family size
biogas system
INTRODUCTION
Non-conventional renewable energy (NCRE) is
increasingly prominent, providing an inexhaust-
ible energy source compatible with human and
environmental sustainability. The various forms
of NCRE include wind, solar, small hydro, tidal,
geothermal and biomass. Biomass uses biological,
chemical and physical processes to generate liquid
or gaseous biofuels, such as biodiesel, bioethanol
and biogas.
Biogas is a viable and essential form of energy in
agricultural and rural areas, obtained from the
processing of organic waste through anaerobic
digestion. In addition to biogas (comprising mainly
methane and carbon dioxide, plus other trace gas-
es), the process also produces a stabilized organic
waste, digestate (also known as biol or bioferti-
lizer), which can be used as a soil conditioner or
biofertilizer (Varnero, 1991, 2001).
The biodegradation rate of organic residues is
related to the microbial activity in the anaerobic
system. This activity depends on the type of raw
material, the pH of the medium, the total level of
solids, the temperature of the process and other
parameters that determine the digestion period
for the production of biogas and biofertilizer.
USING CACTUS WASTE IN
BIOGAS PRODUCTION
Dry climate areas have reduced availability of or-
ganic waste − an obvious disadvantage for biogas
production. This obstacle can be overcome by de-
veloping energy crops well adapted to arid areas.
In this context, opuntias − among them, Opuntia
cus-indica (L.) Mill. − characterized by crassu-
lacean acid metabolism (CAM), are recommended
as an alternative energy source as they have a
high potential for biomass production (García de
Cortázar and Nobel, 1992; García de Cortázar
and Varnero, 1995). Farmers can thus reduce
their electricity and gas bills (liqueed petroleum
gas, LPG) by producing their own energy, while
improving the quality and conditions of the soil by
applying the digestate to the elds.
In the Faculty of Agricultural Sciences of the
University of Chile, experiments with Opuntia -
cus-indica (Uribe et al., 1992; Varnero et al., 1992;
Varnero and López, 1996; Varnero and García de
Cortázar, 1998) indicate that the cladodes are not
a good methanogenic material. The quality of
the starting material in the digesters, particularly
when batch-loaded (Hilbert, 2009), is vital for
the process. It is, therefore, necessary to include
16 Biogas production
189
Biogas production
Gases
(%)
CH4
CO2
pH of manure-cladodes mixture
7,5
0
20
40
60
80
7,4
6,7
6,1
5,3
4,8
Figure 2
Biogas composition
as a function of pH
of manure-cladode
mixture (Varnero and
Arellano, 1991)
a particular material derived from another digest-
er and enriched with methanogenic bacteria, or
to incorporate a percentage of animal manure.
Such adjustments advance the starting time of the
methanogenic phase in the digester and increase
the production of biogas. Moreover, the pH of the
pulp is very low, and this too affects the production
of biogas; for this reason it is preferable to mix with
other raw materials, mostly animal manure.
The fermentation efciency of mixtures containing
different proportions of cladodes and animal ma-
nure showed that it is crucial to maintain the pH of
the mixture close to pH = 6 in order to obtain biogas
with a methane content of > 60%. The composition
of the biogas produced by methanogenic fermenta-
tion is closely related to the pH of the raw materials
biodigested. At pH < 5.5, biogas is predominantly
carbon dioxide, with reduced combustibility and
energy content; conversely, with a neutral or basic
pH, the biogas is methane-enriched. It is, therefore,
important to increase the proportion of animal ma-
nure in the mixture and use cladodes older than 1
year. The particle size of the chopped material has
no signicant inuence on the efciency of the
fermentation process (Varnero and López, 1996;
Varnero and García de Cortázar, 1998).
During the anaerobic digestion of animal manure,
the addition of cactus cladodes promotes the meth-
anogenic fermentation, provided that the pH of the
mixtures of these raw materials remains within a
neutral or slightly acidic range. Furthermore, adding
an appropriate percentage of cladodes to the animal
manure helps the fermentation process start earlier
(Uribe et al., 1992; Varnero et al., 1992): the energy
and carbon content of the cladodes favours the
development of acidogenic bacteria, which generate
the substrate required by methanobacteria, thereby
accelerating the methanogenic process and reduc-
ing the time required for this activity (Varnero and
García de Cortázar, 2013).
OPUNTIA
SPP. PLANTATIONS
FOR BIOGAS PRODUCTION
Opuntias can grow successfully in areas with a
range of climates and soils; therefore formal plan-
tations can be established to optimize biomass
production. Its economic evaluation is still pending.
Studies have shown that 1 ha of Opuntia over 5
years old can produce up to 100 tonnes of fresh
cladodes per year in areas with little rainfall (≤
300 mm) (García de Cortázar and Nobel, 1992).
In some semi-arid parts of Mexico, cladodes are
traditionally collected from wild cactus plants as a
source of forage; regular pruning boosts yield and
improves fruit or nopalitos quality.
Pruning can yield approximately 10 tonnes of dry
matter (DM) ha−1 year−1, and the prunings can be
used for biogas, compost or animal feed (García de
Cortázar and Varnero, 1995). Pruning can also pro-
vide the raw material to feed digesters, combined
with animal manure. Mature cladodes (1 year old)
can be cut, chopped and fed directly into the digest-
ers. It is important to use them as soon as they have
been chopped, in order to reduce biodegradation
and improve the efciency of biogas and biofertiliz-
er production. If the capacity of the digester is not
sufcient for immediate use, the cladodes can be
stored in a shaded, cool, dry place for several days
(Varnero and García de Cortázar, 2013).
As the plantation matures, the growth of the
cladodes slows, because the net photosynthetic
190
rate decreases due to the shading effect of the
upper cladodes (Acevedo and Doussoulin, 1984).
The dry matter content is not affected, howev-
er, as growth continues throughout the year. In
Chile, the maximum commercial fresh fruit yield
is estimated at 16 tonnes ha−1 for plants 16-20
years old under good management. This begins to
decline between 21 and 35 years of age, reaching
8 tonnes ha−1 (Acevedo and Doussoulin, 1984;
Pimienta Barrios, 1990). During the January-April
harvest season, yield is 5-16 tonnes ha−1, while
and in June-September, it is just 0.5 tonnes ha−1
(Sudzuki et al., 1993).
Tohá (1999) indicates that 3 kg of dried cladodes
produce 1 m3 of biogas, which is equivalent to an
output of 10 kWh. Moreover, Baeza (1995) indi-
cates that the caloric value of biogas from cactus
is 7 058 kcal m−3 (range of 6 800-7 200 kcal m−3)
and the biogas potential of Opuntia is equivalent
to 0.360 m3 kg−1 DM.
• Scenario 1: low production. With a yield of
10 tonnes DM ha−1 year−1, the potential biogas
production is equivalent to 9.86 m3 biogas day−1
(27.40 kg DM day−1, with a potential estimated
0.36 m3 biogas kg−1 (27.40 × 0.360 = 9.86 m3
biogas day−1).
• Scenario 2: intermediate conditions. Prun-
ing waste production of 18 tonnes ha−1 year−1
generates 17.75 m3 biogas day−1 and 49.3 kg
DM day−1.
• Scenario 3: optimum production. A commer-
cial plantation with irrigation and fertilization
can produce ≤ 30-40 tonnes DM ha−1 (García
de Cortázar and Nobel, 1992; Franck, 2006).
Production of 30 tonnes year−1 is equivalent to
82.2 kg day−1, which can be used as raw mate-
rial for biogas production with a potential of 29
m3 day−1 (82.2 × 0.360 = 29 m3 biogas day−1),
or 10 885 m3 ha−1 year−1 biogas − comparable to
6.4 tonnes of oil (Varnero, 1991).
There is a signicant difference in biogas produc-
tion between the best and worst scenario as a
result of the constraints involved.
Based on the family biogas system for small oper-
ations illustrated in Figure 1, organic waste can
be collected by connecting the bathroom to the
digester, and/or by accumulating kitchen waste.
Furthermore, if there are animals (e.g. one cow
and two pigs), they too provide organic matter
(Table 2). This would give a biogas potential of
1.05 m3, leaving a further 2.6 m3 to achieve the
3.61 m3 needed. In order to determine the min-
imum cactus plantation area required to supply
enough raw materials to achieve this amount, it
should be considered that a maximum of 3 kg of
cactus is necessary to produce 1 m3 of biogas.
The digester must, therefore, be provided with 7.1
kg day−1; this is obtained in an area of 0.28 ha,
assuming an availability of 10 tonnes of cladodes
ha−1 year−1, equivalent to 27.47 kg ha−1 day−1.
Crop ecology, cultivation and uses of cactus pear
Practical example
TABLE 1 Average consumption of biogas energy in a family of 5 people
Average biogas consumption Cactus biogas consumption
Caloric value 5 000 kcal m−3 Caloric value 7 058 kcal m−3 (75% of CH4)
Kitchen (5 hours) 0.30 m3 h−1 × 5 h = 1.50 m3 day−1 0.21 m3 h1 × 5 h = 1.05 m3 day1
3 lamps (3 hours) 0 .15 m 3 h1 × 3 h × 3 = 1.35 m3 day−1 0 .11 m3 h1 × 3 h × 3 = 0.99 m3 day1
Cooling medium 2.20 m3 h1 × 1 = 2.20 m3 day11.57 m3 h−1 × 1 = 1.57 m3 day−1
Total 5.05 m3 day−1 3.61 m3 day−1
Source: Baeza, 1995.
TABLE 2 Summary of the calculations
Quantity (units) kg unit−1 kg Potential biogas
(m3 biogas kg−1)Biogas (m3)
Kitchen waste 50.56 2.8 0.092 0.26
Human faeces 5 0.13 0.65 0.092 0.06
Cow manure 1 10 10 0.04 0.40
Pig manure 2 2.8 5.6 0.06 0.336
subtotal 1.053
Cladodes 0.28 27.47a7.7 0.3 2.60
total 26.64 3.62
a 10 tonnes ha−1 year−1 (364 days).
191
Biogas production
On the basis of the biogas production described
above, 0.45 m3 of gas is obtained per m3 of digest-
er; the minimum size of the digester is, therefore,
8 m3. In addition, a daily load of 26.24 kg must
be incorporated, combined with sufcient water
for a solid concentration of 7%: equivalent of 221
litres (also equivalent to the digester’s volume, di-
vided by 35 days, i.e. the time required to degrade
organic matter). When loading the digester with
221 litres, the same amount of biofertilizer is pro-
duced, which can be used for irrigation, fertiliza-
tion and organic matter input (5.20 g N kg−1, 3.90
g P kg−1, 3.60 g K kg−1 and 561 g Mo kg−1 DM).
BIODIGESTERS DESIGN
AND OPERATION
The biodigester must have certain characteristics:
• Airtight − to prevent both output of undesirable
gas and intake of unwanted air.
• Thermally insulated − to avoid major tempera-
ture changes.
• Fitted with safety valve.
• Easily accessible − for loading and unloading
of the system with raw material, removal of
digester scum and maintenance of the digester
(Varnero, 1991, 2001).
There is extensive information available in various
countries, including India, China and Germany,
on the design of biodigesters (García de Cortázar
and Varnero, 1995). While most production and
use of biogas is obtained from family biodigesters
(Figure 1), community digesters may also be fea-
sible in some situations, in particular when large
amounts of raw material and technological exper-
tise are available.
There are two types of digester: continuous and
batch (discontinuous).
• Continuous. Material loading is frequent (daily
or weekly), each load replacing approximately
5-15% of the total volume. The solids concentra-
tion is low (6-8% of the volume), and once the
digestion process starts, the biogas production
rate is relatively constant (this is mainly depend-
ent on temperature). Continuous digesters are
best suited to situations where there is a constant
production of material for biodigestion, i.e. if
cladodes are collected throughout the year. They
are also suited to small properties where house-
hold waste can be added as a raw material − for
example, incorporating faeces produced by the
farm animals or through a connection between
the bathroom and digester (Varnero and García
de Cortázar, 2006; FAO, 2011). Three models of
continuous digester are available:
− Taiwan type, made of plastic sleeves (polyeth-
ylene) (Figure 4a);
− Indian type (Figure 3a) − gasometer included
in the digester in the form of a oating bell;
and
− Chinese type (Figure 3b) − closed, with gas
accumulation at the top, while the Indian
digester.
• Batch. Discontinuous digesters (Figure 3c)
comprise a sealed battery of tanks or deposits,
with a gas outlet connected to a oating gas-
ometer, where the biogas is stored. With multi-
ple digesters, one is always loading or unloading
while the others are in biogas production.
Feeding or charging the digester with the raw
material, which has a higher concentration of
solids (40-60%), is done only once, since there is
no recharging during the fermentation process.
The stabilized organic material is discharged
once the biogas production is complete. Biogas
production has an initial waiting period, during
which fermentative hydrolysis, organic acid
formation and methane formation take place.
Most of the biogas production then occurs, be-
Biogas
a)
Biogas
b)
Figure 3
a) Indian digester;
b) Chinese digester;
c) Batch digester
c)
192 Crop ecology, cultivation and uses of cactus pear
Figure 4
Different materials used
for the construction of
biodigesters:
a) plastic sleeves;
b) brick;
c) concrete;
d) recycled plastic
drums;
e) recycled metal
drums;
f) prefabricated
fore slowing down and eventually decreasing to
almost zero, as the batch-loaded materials run
out. The total duration of the process depends
on the temperature. The discontinuous system
is suited to certain situations, for example,
when: raw materials exhibit handling problems
in continuous systems; materials are difcult to
digest by methanogenic fermentation; or raw
materials are available intermittently. Raw ma-
terial from the cladode harvest is available once
or twice a year (Varnero and García de Cortázar,
2006; FAO, 2011).
Under optimal conditions and for the same vol-
ume of dry matter, the two types of digester
produce the same amount of biogas. Therefore,
the choice should be based on the frequency of
waste production (in this case, cladodes) and the
availability of water.
For small and medium-sized producers, a wide
range of materials may be used for constructing
a biogas digester. The most economical continu-
ous types are made from low-cost polyethylene
tube (or EPDM, PVC, HDPE) as shown in Figure
4a. Known as the Taiwan type, it is widespread
in Asia and some Latin American countries. The
material costs for this type are US$7 m−3. The Indi-
an or Chinese models can be made with different
materials (Figures 4b-f).
ECONOMIC ASPECTS
The initial cost of biogas production in rural
households is around US$50 per biodigester (Bui
Xuan An et al., 1999). This cost is recovered within
9-18 months through savings in fuel costs. In rural
areas where the main fuel is wood, the use of bio-
gas reduces ecosystem damage (less deforestation
and contamination) and leads to time savings of
up to 5 hours a day per household − time which
can be used for other more productive tasks
(Rutamu, 1999). To calculate the economics of
using biogas, it is assumed that one pound (0.45
kg) equals 1 m3 of biogas; therefore a theoretical
calculus of 3.61 m3 day−1 would correspond to
3.61 pounds (1.63 kg) of gas daily, with a value of
approximately US$2.98 day−1, or US$1 078 year−1.
The residue obtained from digestion processes
(Figure 5b) has a high nutrient content; it is,
therefore, a valuable fertilizer and allows to save
on the expense of commercial fertilizers. Accord-
ing to Varnero (1991), 1 tonne of biofertilizer is
equivalent to 40 kg of urea, 50 kg of potassium
nitrate and 94 kg of triple superphosphate. In-
ternational fertilizer prices vary from US$255 to
US$380 tonne−1 (Indexmundi, 2015). Assuming
an average price of US$0.32 kg−1 of fertilizer, each
tonne of biofertilizer saves US$58.8 on fertilizers
costs; this saving is in addition to the important
a
d
b c
e f
193
Biogas production
Figure 5
a) Solid digestate;
b) biofertilizer (biol
or liquid digestate)
contribution in terms of microorganisms and or-
ganic material, as well as the possibility of obtain-
ing solid materials when emptying the digester
(Figure 5a).
OTHER BIOENERGY USES
Cactus cladodes have other bioenergy uses, such
as biodiesel or ethanol production. With an an-
nual production of 40 tonnes ha−1 in crops grown
specically for energy use or 10 tonnes ha−1 in
pruning waste from fruit plantations, energy can
be obtained through direct burning. The cladodes
are harvested, sun-dried and crushed, then used
in direct burning or cogeneration mix coal-red;
the caloric value is 3 850-4 200 kcal kg−1.
The technology for ethanol production is more
complex than that for biogas production; it there-
fore adapts better to a larger scale, given the high
investment costs, and produces concentrations of
> 98% ethanol. At fermentation, specic yeast is
required to maximize the alcohol production. The
ethanol concentration at fermentation is 8-12%
(García de Cortázar and Varnero, 1995), attain-
able only by distillation to achieve the required
concentration of ethanol as fuel.
Estimates indicate that cactus mucilage can be used
to produce small amounts of ethanol: about 20 ml
kg−1 of mucilage. On the other hand, 8.6 litres were
produced from 100 kg of dried cladodes, and 24.7
litres from 100 kg of dried fruits, and so it is not
considered competitive compared with production
from fermented fruits. With a density of 635-5 000
plants ha−1, if only the cladodes are used (Retamal
et al., 1987), an average of 300-3 000 litres of
ethanol can be obtained from non-irrigated and
irrigated plantations, respectively.
a b
