ArticlePDF Available
Energetic, economic and environmental assessment for the anaerobic
digestion of pretreated and codigested press mud
Lisbet Mailin López González
a,
, Ileana Pereda Reyes
b
, Julio Pedraza Garciga
a
, Ernesto L. Barrera
a
,
Osvaldo Romero Romero
a
a
Universidad de Sancti Spíritus ‘‘José Martí Pérez(UNISS), Centro de Estudios de Energía y Procesos Industriales (CEEPI), Avenida de los Mártires 360, CP 60100 Sancti Spíritus, Cuba
b
Universidad Tecnológica de La Habana ‘‘José Antonio Echeverría(Cujae), Centro de Estudios de Ingeniería de Procesos (CIPRO), Calle 114 No. 11901 e/ Rotonda y Ciclovía,
Marianao, CP 19390 La Habana, Cuba
article info
Article history:
Received 16 May 2019
Revised 27 October 2019
Accepted 28 October 2019
Keywords:
Environmental impact
Techno-economic analysis
Energy integration
Press mud and vinasse
abstract
This study investigates the feasibility of anaerobic digestion (AD) of press mud previously pretreated,
using two methods: Liquid Hot Water (LHW) and Thermo-Alkaline (TA), from an economic, energetic
and environmental point of view. Two scenarios, a sugar mill with and without distillery were studied,
considering monodigestion and vinasse codigestion. The results have shown that the LHW and TA pre-
treatments are self-sufficient in terms of thermal requirements since they can recover heat from the bio-
gas engine, but the maximum electric and thermal net energy (64 MWh d
1
and 95 MWh d
1
,
respectively) was obtained during co-digestion with vinasse. The results of the environmental Life
Cycle Analysis (LCA) show that the alternatives improve the environmental profiles, in both scenarios.
The endpoint impact category ‘‘Human health” had the highest contribution because of both: the burning
of fossil fuel at refinery to supply the required electricity; and the production of Ca(OH)
2
when vinasse
was fed. The AD of pretreated press mud by LHW in CSTR reactors was the most viable for the scenario
of a sugar mill without distillery, while the alternative co-digestion with the vinasse of the press mud
without pretreatment was the most viable for the scenario of a sugar mill with distillery. This research
shows that both the environmental and energetic profiles and the profitability of methane production
can improve when the pretreatment and co-digestion of these wastes from the sugar – alcohol produc-
tion process are considered.
Ó2019 Elsevier Ltd. All rights reserved.
1. Introduction
Anaerobic digestion (AD) for biogas production is one of the
most efficient technologies for providing clean and renewable
energy from organic waste (Moraes et al., 2017). The technology
can increase the renewable contribution to the energy matrix of
developing countries such as Cuba, with a great potential unex-
ploited yet. According the Ministry of Energy and Mines (MINEM),
the Cuban policy concerning renewable energy has set a goal for
supplying 24% of the energy demand from renewable energy
sources by 2020, but the focus is on solar, wind energy and bio-
mass as bagasse and marabou for its use in bioelectric (MINEM,
2019). The biogas contribution has been few, as it has been focused
mainly to treat cow and pig manure, but others biomass with a
great potential for energy generation by way of biogas would be
used, while also reducing greenhouse gas (GHG) emissions.
Among them are the vinasse and press mud, main organic
wastes from bioethanol and sugar production, respectively. Typi-
cally, press mud is used as fertilizer/soil improver, being directly
(or after composting) applied to the soil in association or not with
minerals. From an environmental point of view, the composting
process is considered one of the most suitable alternatives to man-
age and treat organic solid waste, but higher consumption values
of transport, energy, water, infrastructure, waste and emissions
of volatile organic compounds are involved when industrial com-
posting is applied (Oliveira 2017).
Vinasse is mainly used for irrigation and fertilization (ferti-
irrigation) of the sugar cane plantations, however its indiscrimi-
nate usage cause soil salinization and groundwater contamination,
as well as methane and nitrous oxide emissions during temporary
storage and transportation (Moraes et al., 2017). Environmental
impacts from vinasse and press mud disposition may be reduced
by AD treatment to produce biogas.
The AD from press mud and vinasse for biogas production has
been previously reported in full and laboratory scale studies
https://doi.org/10.1016/j.wasman.2019.10.053
0956-053X/Ó2019 Elsevier Ltd. All rights reserved.
Corresponding author.
E-mail address: lisbet@uniss.edu.cu (L.M. López González).
Waste Management 102 (2020) 249–259
Contents lists available at ScienceDirect
Waste Management
journal homepage: www.elsevier.com/locate/wasman
(Barros et al., 2016; Janke et al., 2017). Main difficulties during the
AD were improved by pretreatment and codigestion (Janke et al.,
2016; López González et al., 2014; López González et al., 2017;
López González et al., 2013), but these reports were only focused
to determine the methane yield for different severity conditions
and mixtures ratio, without considering energetic and economic
aspects. However, it is important to probe if the net energy and
economical gain attained by a pretreatment, prior to anaerobic
digestion step, can cover investment and operational costs, as well
as energetic pretreatments requirements.
A number of studies can be found in the literature analysing the
effect of substrate pretreatments on profitability indicators and net
energy production in biogas plants (Budde et al., 2016; Cano et al.,
2014; Janke et al., 2017; Monlau et al., 2013; Shafiei et al., 2013;
Zeynali et al., 2017). Solid cattle manure pretreated in thermobar-
ical pretreatment at 160 °C by 20 min delivered 202.7 kW h
th
Mg
FM
1
, but need only 169.2 kW h
th
Mg FM
1
. The pretreatment
revealed short payback periods (3.25 years) (Budde et al., 2016).
Shafiei et al. (2013) obtained an increase by 13% in the total invest-
ment costs using steam explosion pretreatment, but reducing the
manufacturing cost by 36% the economy of the biogas plant was
improved.
Anaerobic digestion can achieve GHG emission reduction when
compared to conventional fossil reference systems, being able to
improve the environmental profile of the national electric grid.
However, the environmental results obtained were strongly
dependent of the specific substrate selected, substrate transport
distances, digestate management and heat valorization (Bacenetti
et al., 2013; Lijó et al., 2017; Styles et al., 2016). Some of the criteria
needed to meet the requirements of sustainable biogas production
include secure energy supply, avoid competition with food produc-
tion, socio-economic development including creation of local
employment and reduced environmental impacts (Jacobs et al.,
2017). Therefore, the use of food and alcohol industry wastes as
substrate for biogas production is recommended, since no emis-
sions are usually assigned to the production of waste streams
and these are converted into value-added products. In this sense,
previous works were addressed to the environmental sustainabil-
ity of biogas production from sugar-alcohol production residues.
They demonstrated that the anaerobic digestion improved the
environmental profile reducing up to 77% the total score with
respect to the lagooning of Cuban vinasse (Barrera et al., 2016),
while GHG emissions were reduced by at least 48%, in comparison
with storage/transport and field application of Brazilian vinasse
(Moraes et al., 2017). It is difficult to draw conclusions from earlier
studies on the comparison between different treatment technolo-
gies and codigestion when energetic, economic and environmental
criteria have not been included. Therefore, this paper assesses the
feasibility of press mud AD previously pretreated by LHW and TA,
and untreated press mud in co-digestion with vinasse, from an eco-
nomic, energetic and environmental point of view.
2. Materials and methods
2.1. Case study and alternatives description
As case study to assess the energetic, economic and environ-
mental feasibility for the AD of pretreated and codigested press
mud, Melanio Hernandez sugar-ethanol factory located in Tuinucu,
Sancti Spiritus, Cuba, was selected. Biogas plants were modelled
according to laboratory tests and others data published in the liter-
ature (Barrera et al., 2016; López González et al., 2014; López
González et al., 2017; López González et al., 2013; Parsaee et al.,
2019). As sugar mill can be integrated or not with ethanol distil-
leries in the cuban context, the following scenarios (A and B) were
considered:
Scenario A.Sugar mill without ethanol distillery. The nominal
capacity of the sugar mill was considered as 4600 t d
1
with an
average operation time of 130 days a year, producing 161 t d
1
(3.5% of the milled sugar cane) of press mud (Data taken from Mel-
anio Hernandez sugar-ethanol factory located in Tuinicu, Sancti
Spiritus, Cuba). However, in order to reduce reactor volume while
increasing operation time for the biogas plant; it was considered
that the biogas plant operates 300 days a year, with a daily feeding
of 70 tons d
1
of press mud. The remaining period of the year
(65 days), the biogas plant is kept in standby for a proper mainte-
nance until the sugar harvest is started again. The feeding of press
mud with or without pretreatment (according to the optimal oper-
ating conditions (López González et al., 2014; López González et al.,
2013) into a continuously stirred tank reactor (CSTR) formed the
alternatives A1, A2 and A3. The alternatives for Scenario A were
grouped as follows:
A1: press mud without feeding a CSTR.
A2: pretreated press mud using LHW (150 °C, 20 min) feeding a
CSTR.
A3: pretreated press mud using TA (10 g Ca(OH)
2
100 g
1
TS,
1 h, 100 °C) feeding a CSTR.
Scenario B.Sugar mill with ethanol distillery. The ethanol dis-
tillery produces 600 hL d
1
of ethanol, and operates 300 days per
year, generating 964 m
3
d
1
of vinasse (16 L of vinasse per L of
ethanol). In addition, the 70 tons d
1
of press mud was considered
for AD according to Scenario A. The Upflow Anaerobic Sludge Blan-
ket (UASB) reactor was considered for the AD of vinasse; and the
CSTR for the AD of press mud with and without pretreatment, as
well as for the co-digestion of press mud and vinasse. The feeding
of press mud with or without pretreatment into a CSTR, while the
feeding of vinasse into UASB reactors formed the alternatives B1,
B2 and B3. In addition, alternative B4 considered the co-digestion
of press mud (without pretreatment) and vinasse in a CSTR accord-
ing to the optimal operating conditions reported (López González
et al., 2014; López González et al., 2017; López González et al.,
2013). The alternatives for Scenario B were grouped as follows:
B1: press mud without pretreatment feeding a CSTR and
vinasse feeding a UASB reactor.
B2: pretreated press mud using LHW (150 °C, 20 min) feeding a
CSTR and vinasse feeding a UASB reactor.
B3: pretreated press mud using TA (10 g Ca(OH)
2
, 100 g
1
TS,
1 h, 100 °C) feeding a CSTR and vinasse feeding a UASB reactor.
B4: co-digestion of press mud (without pretreatment) and
vinasse in a CSTR (25% press mud and 75% vinasse, based on
Chemical Oxygen Demand (COD)).
The environmental mitigation potential of the biogas produc-
tion was assessed with respect to reference scenarios (current con-
ditions): ¨Sugar mill without ethanol distillery¨(A0) (Fig. 1A); and
¨Sugar mill with ethanol distillery¨(B0) (Fig. 1B). They can be
described as follows:
A0: 70 t of press mud are treated daily in a typical windrow
composting facility (producing 2.2 t of organic carbon). The
compost produced is sold to farmers as bio-fertilizer.
B0: 70 tons of press mud are treated daily in a typical windrow
composting facility (producing 2.2 t of organic carbon). Simulta-
neously, 964 m
3
of vinasse are treated daily in lagoons (produc-
ing 6.0 t of organic carbon). After laggoning, this liquid waste
can be used for ferti-irrigation.
250 L.M. López González et al. / Waste Management 102 (2020) 249–259
Windrow composting is carried out in piles. The windrows are
turned weekly during 60 days using a compost turner. Fuel con-
sumption is that of collection and transportation of wastes,
turning-aeration and windrow management. GHG emission is from
the direct emission of the composting process during the decom-
position of organic matter. Other activities as shredding, screening
and compost spreading are out of the system boundary.
Schemes of the biogas plant for both scenarios are shown in the
Supplementary data.
The input data and assumptions used to determine the biogas
production for each alternative is shown in the Table 1. The influ-
ent composition referred to functional unit is presented in the Sup-
plementary data.
2.2. General assumptions
The calculation of the reactor volume was based on the organic
load in the feeding substrate, using the design organic loading rates
(OLR) for the CSTR. Hydrolysis provided by pretreatment was able
to accelerate the conversion of digestible solids into degradable
substrate (Janke et al., 2017). Therefore, OLR fed to the reactor
was higher. The values used were:
1 kg Volatile Solids (VS) m
R
3
d
1
when feeding press mud with-
out pretreatment.
3 kg VS m
R
3
d
1
when feeding pretreated press mud and
untreated press mud in co-digestion with vinasse (López
González et al., 2017).
For UASB reactors OLR of 10 kg COD m
3
d
1
was used when
feeding vinasse to the UASB reactor (Parsaee et al., 2019).
The water for dilution was only consumed during the
sugar cane non-harvest season, whereas during the harvest
season, 2760 m
3
d
1
of wastewater from the sugar mill was
considered available for press mud and vinasse dilution. Press
mud dilution was needed to control the TS content inside the
CSTR to 8% (w/w), while the dilution of the vinasse was
required to reduce the COD concentration from 62 to 20 kg
COD m
3
.
The produced biogas will be burned in a combined heat and
power (CHP) system, generating electrical energy for the national
electric grid and thermal energy to produce steam at low pressure
from the exhaust gases. The heat recovered from the exhaust gases
will be used as energy source to supply the thermal demand for
press mud pretreatments.
Fig. 1. System boundaries for A1–A3 (A) and B1–B4 (B) alternatives and reference scenarios (TSC: Production by the traditional supply chain).
L.M. López González et al. / Waste Management 102 (2020) 249–259 251
The specific assumptions for the energetic, economic and envi-
ronmental assessment are described in details in the next sections.
2.3. Energetic assessment
The energy balance was carried out to assess the energy
demand for pretreatments versus energy recovered from the pro-
duced biogas. The main parameters for the energetic assessment
are shown in the Supplementary data.
The thermal energy consumed during pretreatment (TE
C
,kWh
d
1
), the thermal energy recovered from the produced biogas
(TE
R
,kWhd
1
), the thermal energy required based on the efficiency
of the hydrolysis reactor (TE
RQ
, kWh d
1
), and the electrical energy
generated (EE,kWhd
1
), were calculated according o Eqs. (4.1),
(4.2),(4.3) and (4.4), respectively.
TE
c
kWh
d

¼m:c
p
:
D
T:t
p
=3600 ð4:1Þ
TE
R
kWh
d

¼y
B
:I
G
:
g
t
ð4:2Þ
TE
RQ
kWh
d

¼TE
R
=
g
H
ð4:3Þ
Fig. 1 (continued)
252 L.M. López González et al. / Waste Management 102 (2020) 249–259
EE kWh
d

¼y
B
:I
G
:
g
e
ð4:4Þ
where m is the mass of the mixture (press mud and water) to be fed
(kg h
1
), c
p
is the heat capacity of the mixture (considered as
4.18 kJ kg
1
K
1
),
D
T is the temperature increments during pre-
treatment (K), t
p
is the pretreatment time per day (h), 3600 is the
conversion factor of kJ to kWh, y
B
is the biogas production (m
3
d
1
), I
G
is the generation index from biogas (kWh m
3
),
g
t
is the
thermal efficiency of the CHP system (%),
g
e
is the electrical effi-
ciency of the CHP system (%), and
g
H
is the hydrolysis reactor effi-
ciency (%). The values for
g
t
and
g
e
are selected according to the
capacity of internal combustion engine (Supplementary data).
An index of 3.1 kWh tTS
1
was used to calculate the electricity
consumption (EE
consumed
) during pretreatments (Budde et al.,
2016), while 0.44 kWh m
3
biogas were used for the electricity
consumption in UASB reactors (Obaya et al., 2005). The electricity
consumption for CSTR was assumed as 10% of the produced energy
according to GIZ (2013).
Finally, the net electricity (EE
net
) was calculated from the differ-
ences between EE and EE
consumed
; whereas the net thermal energy
(ET
net
) was obtained from the differences between TE
R
,TE
RQ
and
TE
c
.
2.4. Economic assessment
For the economic assessment, the Total Capital Investment, the
Total Product Cost and the Cash Flow were determined for each
alternative according to the methodology recommended by
Peters and Timmerhaus (1991). The cost indexes and prices for
the calculations are also shown in the Supplementary data.
The six-tenths-factor rule updating their values to the current
prices with the Chemical Engineering Plant Cost Index was used
to calculate the purchased equipment (E) for LHW pretreatment
and equipment for biogas plant with CSTR reactors (without CHP
system). The base costs are shown in the Supplementary data.
The E value for biogas plant with UASB reactors was estimated
by an index of 2.80 USD/m
3
vinasse fed per year (Salomon et al.,
2011). The E value for TA pretreatment was calculated by
Matches (2019), based on the volume of the lime preparation tank
and the hydrolysis reactor (according to the daily feeding and the
pretreatment time) (Supplementary data).
Biogas purification unit was included in the base cost for the
equipment for biogas plants when CSTR is the type of reactor used.
The method of removing H
2
S is the in-situ desulphurization in the
biogas digester itself by dosing air to the gas phase.
For alternatives using UASB reactors the biogas purification unit
was based on biological oxidation as described before, but in this
case it is performed ex-situ in biofilters with immobilized
microorganisms.
The Evalues for the CHP system were calculated from indexes
reported in the Catalog of CHP Technologies (EPA, 2015). This is
1040 and 975 USD kW
1
for 848 and 1059 kW installed,
respectively.
A linear depreciation was considered over 10 years for installed
technical equipment. It was considered an interest rate of 10% over
the total investment cost, an inflation rate of 8%, and a tax rate of
30%. As revenues for the biogas plant, the sale of electricity, the
sales of organic fertilizers, as well as the service for ferti-
irrigation were considered.
The net present value (NPV), internal rate of return (IRR) and
payback period (PBP) were calculated for three conditions: ‘‘Pes-
simistic, Baseline and Optimist”. A price of 0.14 USD kWh
1
of elec-
tricity generated using biogas (as renewable fuel) according the
Cuban Electrical Enterprise was considered for the ‘‘Pessimistic”
and ‘‘Baseline” conditions. The sale price of the organic fertilizers
was considered as 0 USD for the ‘‘Pessimistic” condition, while
for the ¨Baseline¨condition a price of 49.54 USD per ton and
67.11 USD per ton were considered for the fertilizers produced
from CSTR and UASB reactors, respectively. ¨Optimist¨condition
was proposed according to increase by 20% of the price for the
main products (electricity and fertilizer) (Supplementary data).
2.5. Environmental Life Cycle Assessment (LCA)
The environmental assessment was carried out by using the Life
Cycle Assessment (LCA) methodology according to the ISO
Table 1
Input data and biogas production for the assessed scenarios.
A1 A2 A3 B1 B2 B3 B4
Feedstock
Press mud tons d
1
70 70 70 70 70 70 70
Vinasse m
3
d
1
964 964 964 964
Biogas yield
Press mud m
3
ton VS
1
351
1
567
2
427
3
351 567 427
Vinasse m
3
ton COD
fed
1
400
4
400 400
Press mud + Vinasse m
3
ton VS
1
562
5
Methane content
Press mud % 56 56 56 56 56 56
Vinasse % 60 60 60
Press mud + Vinasse % 60
Organic matter fed
Press mud
6
ton VS day
1
16 16 16 15.63 15.63 15.63
Vinasse
7
ton COD day
1
59.78 59.78 59.78
Press mud + Vinasse ton VS day
1
56
Biogas production m
3
d
1
5484 8856 6668 29,396 32,768 30,580 31,341
VS: Volatile Solids, COD
fed
: Chemical Oxygen Demand.
1
López González et al. (2017).
2
López González et al. (2014).
3
López González et al. (2013).
4
López González et al. (2017).
5
López González et al. (2017).
6
22.40 VS%FM.
7
20 kg COD m
3
.
L.M. López González et al. / Waste Management 102 (2020) 249–259 253
14040/44 guidelines (ISO 14040, 2006, ISO 14044, 2006) and the
ILCD handbook (European-Commission, 2010).
2.5.1. Goal and scope definition
The goal and scope of the environmental LCA was to compare
the alternatives for the use of press mud (with and without pre-
treatment) and vinasse for anaerobic digestion (biogas plants) in
sugar mill with and without ethanol distilleries (Scenarios A and
B). The functional unit, system boundaries, scenarios description,
allocations principles and main assumptions are described in detail
below.
2.5.1.1. Functional unit and system boundaries. Considering that the
main function of the studied system is the treatment of the avail-
able press mud and press mud/vinasse according the scenario A or
B, the functional unit for Scenario A was the daily treatment of 70
tons of press mud, and for Scenario B the daily treatment of 70 tons
of press mud and 964 m
3
of vinasse.
In general, Fig. 1A&B showed the gaseous emissions from
lagooning and composting, as well as the combustion gasses emis-
sion from the energy generation, to the ecosphere. The oxygen in
air and the land used were the resources taken from the ecosphere;
whereas energy, vinasse, press mud, calcium hydroxide and tap
water were the resources taken from the technosphere. The prod-
ucts of both scenarios were electricity, heat and organic carbon.
Traditional supply chains (TSC) are the common suppliers of prod-
ucts (e.g. electricity production from fuel oil in centralized power
plants); they take resources from the technosphere to produce
the required products for the market.
To make a fair comparison between the alternatives an equal
basket of benefits was constructed, which implies that TSC always
need to complete the market demand and fulfill an equal basket of
benefits. For example, when press mud is pretreated by LHW (A2),
18.0 MWh of electricity net are obtained, as this is the maximum
value among the alternatives A0, A1, A2 and A3, the difference
between the electricity net in A2 and the electricity net in A0, A1
and A3 must be supplied from the TSC (i.e. electricity production
from fuel oil in centralized power plants). The same was implicit
for all the alternatives and products, considering that electricity
is supplied from the Cuban electricity mix (fuel oil that is used in
centralized and decentralized thermal power plants (81.66%), com-
bined cycles with gas turbine using liquefied petroleum gas
(13.04%), cogeneration systems using bagasse (4.63%), and other
technologies using renewable resources (0.67%), the heat is sup-
plied as steam from fuel oil and the organic carbon is extracted
from the soil as ‘‘Carbon, in organic matter, in soil” (Barrera
et al., 2016).
2.5.1.2. Allocation principles and main assumptions. The main
assumptions for this study were as follows:
All calculations are based on 1 day of operation.
– The press mud transport to AD/composting plant is carried out
by trucks. The distance from sugar mill to AD and composting
plant are 500 m and 4 km, respectively. The compost and biofer-
tilizer transportation as well as the spreading are out of the sys-
tem boundary.
An index for diesel consumption of 1.8 L / t press mud for com-
posting management was used (data provided by the Melanio
Hernandez sugar-ethanol factory).
– For press mud composting process, the emissions were consid-
ered as 0.004 kg of CH
4
and 0.003 kg N
2
O per kg of waste (IPCC,
2006).
Digestate was separated in a liquid and solid phase by means of
a screw press. The energy consumption was estimated as
0.5 kWh/m
3
(Drosg et al., 2015).
- The gaseous emissions from the lagoons were calculated from
the multiplication of biogas productivity E
Biogas
(Lm
2
d
1
) and
occupied area in lagoons (equation (4.5)), where Tis the average
environment temperature (°C) (Picot et al., 2003). A methane
content of 65% in the biogas was considered.
E
biogas
¼4:8451 e
0:1203T
ð4:5Þ
The organic carbon content (Corg) in the sludge produced in the
UASB reactor was assumed to be 0.0313 kmol C / kg COD bio-
mass (Batstone et al., 2002), where the COD in the biomass
was 1.222 kg COD / kg biomass (Kalyuzhnyi and Fedorovich,
1998).
– Sludge production was determined from the mass balance
assuming that 10% of the removed COD is anaerobically con-
verted to biomass (Braun, 2007).
2.5.2. Life cycle inventory (LCI)
From the allocations principles and main assumptions previ-
ously described in Section 2.5.1.2, the LCI was carried out.
Others assumptions were:
– The electricity and heat produced from biogas, as well as
organic fertilizer (Corg) were variable among the alternatives,
while the amount of N, P and K, and the consumption of fuel
used for the transportation of substrates, wastewater and fertil-
izer were considered constant.
– Organic carbon was taken from the Ecoinvent Database 2.2 as
‘‘Carbon, in organic matter, in soil” and it was assumed to be
¨consumed¨in the all the alternatives, except the one with the
highest organic carbon production which does not consume/
extract organic carbon from the soil.
A 4% of biogas losses were assumed during the production pro-
cess, power generation and storage of the digested effluent
(Hrad et al., 2015).
The inputs and outputs of the studied alternatives in the LCI
were calculated by using material and energy balances, ensuring
data reliability and validity. Mass consistency was checked by
means of total and partial mass balances; whereas the energy
requirements were taken from their calculation in Section 2.3.
Infrastructure was excluded from this study. The Ecoinvent Data-
base 2.2 was used to model datasets in the background system.
2.5.3. Life cycle impact assessment (LCIA)
To account for emissions and resources the RECIPE methodol-
ogy with endpoint indicators and the hierarchic perspective was
used (Goedkoop et al., 2008). Consequently, the endpoint impact
categories ‘‘ecosystem quality”, ‘‘human health” and ‘‘natural
resources” were determined. The environmental impacts were
quantified in ‘‘Points” for the endpoint impact categories and for
the total score (sum of the endpoint scores). The open source soft-
ware for the sustainability assessment, OpenLCA version 1.3.1
(http://www.openlca.org/) was used to calculate the environmen-
tal impacts.
3. Results and discussion
3.1. Energetic assessment
The energy balances for the electrical and thermal energy con-
sumption for both scenarios (A and B) are shown in Fig. 2A–D.
3.1.1. Energetic assessment for scenario A
The biogas production of 5484, 8856 and 6668 m
3
d
1
for the
alternatives A1, A2 and A3 requires the implementation of an
254 L.M. López González et al. / Waste Management 102 (2020) 249–259
energy generation system with 848, 1059 and 848 kW installed
power capacity, respectively (Table 1).
The net, produced and consumed electrical (EE,EE
consumed
and
EE
net
) and thermal (TE
R
,TE
C
and TE
net
) energy for alternatives
A1, A2 and A3 are shown in Fig. 2A&B, respectively. For these alter-
natives the EE
consumed
represented 11% of EE; and consequently, EE
net
values were higher for the alternatives producing the higher
amount of biogas (A2 > A3 > A1) (Fig. 2A). The TE
R
obtained when
feeding the CSTR with pretreated press mud using TA (A3) and
LHW (A2) were 21 and 58% higher, respectively, than the one
obtained when feeding the CSTR with untreated press mud (A1).
Although both pretreatment consumed TE (9.4 and 12.1 MWh
d
1
for TA and LHW, respectively), they were self-sufficient
(TE
net
0). Despite the TE
C
was slightly higher (29%) for A2 with
respect to A3 (because of the higher temperature used, 150 °C). A
thermal output of 384.0 kWh t
1
FM, higher than required for pre-
treatment of 172.2 kWh t
1
FM was obtained for LHW pretreat-
ment (A2). The results were similar to those obtained by Budde
et al. (2016), where a higher recovery of thermal energy from the
pretreated solid cattle manure at 140 °C by 20 min (183.8 kWh
t
1
FM) than required for pretreatment (145.8 kWh t
1
FM) was
found.
In general, the EE
net
and TE
net
for the studied alternatives were
from 11 to 18 MWh d
1
and from 11 to 17 MWh d
1
, respectively
(Fig. 2A&B). Specifically, the alternative that considers press mud
without pretreatment (A1) rendered the highest net thermal
energy (17 MWh d
1
), whereas the one considering pretreatment
by LHW (A2) rendered the highest net electrical energy
(18 MWh d
1
).
3.1.2. Energetic assessment for scenario B
The biogas production of 29396, 32768, 30580 and 3134 m
3
d
1
for the alternatives B1, B2, B3 and B4 requires the implementation
of an energy generation system with 4.0, 4.2, 4.0 and 4.2 MW
installed power capacity, respectively (Table 1).
The EE
consumed
was approximately 17% of EE for B1, B2 and B3
(alternatives considering CSTR and UASB reactors for AD), being
10% of EE for B4 (alternatives considering only a CSTR) (Fig. 2C).
This is due to the high consumption of electricity during the oper-
ation of the UASB reactors (10 MWh d
1
). Consequently, the max-
imum EE
net
(64 MWh d
1
) was obtained for alternative B4, where
press mud without pretreatment was co-digested with vinasse in a
CSTR.
Regarding thermal energy (Fig. 2D), the alternatives considering
pretreatment (B2 and B3) had a consumption of 13 and 12 MWh
d
1
, respectively, mainly due to the requirements of the biogas
plant; whereas the maximum TE
net
(95 MWh d
1
) was obtained
also for B4.
In general, the electric and thermal net energy for these alterna-
tives (B1, B2, B3 and B4) ranged from 55 to 64 MWh d
1
and from
84 to 95 MWh d
1
, respectively (Fig. 2C&D); which would cover
100% of the daily demand of the ethanol distillery in Melanio
Hernández sugar-ethanol factory.
3.2. Economic assessment
3.2.1. Economic assessment of scenario A
The components of the Total Capital Investment (TCI) for alter-
natives A1, A2 and A3 are shown in Supplementary data. The high-
est TCI was obtained for A2 (5.07 MMUSD), due to the high
Purchased equipment cost (E) of the LHW pretreatment unit and
the fact that A2 had the highest CHP power capacity (1059 kW).
A cost index of 279 USD/m
3
of reactor was obtained for A1, being
lower than the one considered by Janke et al. (2017) for sugarcane
straw (413 USD/m
3
reactor). For the alternatives A2 and A3 the
cost index was higher (948 and 702 USD/m
3
reactor) because of
the higher Evalues when including the press mud pretreatment,
which was not included by Janke et al. (2017).
The alternatives considering the feeding of pretreated press
mud to the CSTR reactor (A2 and A3), showed the highest revenues
from the electricity sales (828 and 624 MUSD/year, respectively)
(Supplementary data). However, the revenues obtained from the
organic fertilizer in A1 were similar than the one obtained from
electricity, because of the lower biogas yield of the untreated press
mud. The sales of electricity and biofertilizer rendered between 48
and 61% of the revenues in all the alternatives.
The electricity and raw material cost represented approxi-
mately, 68 and 73% of the Direct Production Cost for the alterna-
Fig. 2. Daily energy balance for study alternatives A1–A3 (A, B) and B1–B4 (C, D).
L.M. López González et al. / Waste Management 102 (2020) 249–259 255
tives A1 and A2, respectively. However, calcium hydroxide (con-
sumption of 2 t d
1
) had the highest contribution (42%) to the
Direct Production Cost in A3, because of the calcium requirement
for TA pretreatment.
The investment feasibility was assessed for the three condi-
tions: ¨Pessimistic¨,¨Baseline¨and ¨Optimistic¨(Table 2).
For the ¨Pessimistic¨condition, the NPV was negative for all the
alternatives indicating that investment would not be recovered
during its lifespan if sales of fertilizer are not considered. However,
for the ¨Baseline¨and ¨Optimistic¨conditions these alternatives
were economically feasible showing positive NPV, IRR above 10%,
and PBP below 9 years. The alternative considering the LHW pre-
treatment was the most profitable with an NPV of 2.81 MMUSD,
IRR of 21% and PBP of 5.5 years for the ¨Optimistic¨condition
(Table 2).
According the sensibility analysis (Fig. 3), decreasing by 15% the
amount fed of press mud, the NPV was < 0, while NPV was > 0
when the biogas yield changed from 15% to 15%.
3.2.2. Economic assessment of scenario B
The highest TCI value was for B2 (14.4 MMUSD), due to the high
Evalues of the LHW pretreatment unit and the highest CHP power
capacity (1059 kW + 3177 kW) (Supplementary data).
The total revenues ranged from 4.56 to 4.90 MMUSD per year in
Scenario B (Supplementary data). The highest contribution was
obtained from the electricity sale (63–66%).
The costs of electricity consumption account for 35–52% of the
Direct Production Cost, which is the main contribution for all the
alternatives, except B4, where calcium hydroxide consumption
accounted for 34%.
The dynamic indicators of the investment showed economic
profits for the ¨Baseline¨and ¨Optimistic¨conditions in all the alter-
natives (Table 2), while for the ¨Pessimistic¨condition only the B4
alternative had a NPV positive.
The highest profitability was found for the alternative consider-
ing the co-digestion of press mud with vinasse (B4) with IRR from
15.7 to 36.0%, PBP from 3.4 to 7.2 years, and NPV from 2.84 to
15.22 MMUSD. The main economic advantages of this alternative
were: 1) the substitution of the water required for dilution by
using vinasse; 2) the lower electricity consumption than the alter-
natives considering the feeding of vinasse to an UASB reactor (B1,
B2, and B3). These aspects caused a significant reduction (from 43
to 46%) of the Direct Production Cost in B4, in comparison with the
remainder alternatives, increasing the economic profits.
In general, the economic assessment for Scenario B (sugar mill
with ethanol distillery) showed the highest benefits for the alter-
native considering the co-digestion of press mud (without pre-
treatment) and vinasse in a CSTR, in the ¨Pessimistic¨,¨Baseline¨
and ¨Optimistic¨conditions (Table 2).
According the sensibility analysis (Fig. 4), ranging from 15% to
15% the amount fed of press mud and vinasse, as well as the biogas
yield obtained from AD process, the NPV was >0 for all the alterna-
tives evaluated. Higher biogas yield implies larger biogas CHP
installation and cost increased due to electricity consumption,
which led to a slight decrease of the NPV for B4.
An economic viability for electrical power of 848 and 1059 kW
for monodigestion, and 4236 kW for codigestion were found in this
work. These values are in the optimum size of biogas plants in
mono-digestion (>740 kW) and co-digestion (>1000 kW) recom-
mended by Velásquez Piñas et al. (2019).
Payback periods between 1.8 and 11.5 years are reported in the
literature for biogas plants (Balussou et al., 2012; Budde et al.,
2016; Cano et al., 2014; Carlini et al, 2017; Dahunsi et al., 2019;
Montoro et al., 2019; Ruiz et al., 2018; Salomon et al., 2011). How-
ever, interstudy comparisons are almost impossible due to the
variability of the scenarios.
German co-digestion biogas plants, which employed biowaste
and sewage sludge for electricity production reported than diges-
tate treatment and personnel costs were responsible by 43.3% for
the operational costs, while for plants feeding energy crops, sub-
strates costs were responsible by 40% for the operational costs
(Balussou et al., 2012).
Salomon et al. (2011) studied the economic feasibility of biogas
production from vinasse in UASB reactors, showing a payback of
3.6 years and 28.4 of IRR for the use of biogas in the electricity gen-
eration using reciprocating combustion engines. Main incomes
were due the electricity sale.
Orive et al. (2016) reported payback time periods of 6.7 and
9.2 years for full scale anaerobic co-digestion and anaerobic
mono-digestion plants, respectively. In both the major revenue
was due to the sales of compost, because of the amount produced
and the high sale price (70 euros/ton). If the compost sale price was
half, revenues generated by the sale of electricity (only for the
mono-digestion plant) was uppermost.
The profitability of a biogas plant processing biowastes was pri-
marily related to the income obtained from the gate fees charged
for the management of the biowastes (80.7%) and electricity sales
(17.4%) (Ruiz et al., 2018). They estimated the revenues from elec-
tricity sale with an average price of 5.03 c
/kWh, 2.8 times lower
than that used in this work.
The results obtained suggested than it will be better to treat by
AD both residues than press mud only. Besides the vinasse – press
mud codigestion have a both less investment cost (USD 10.5 mil-
lions) and payback period (4 years).
Table 2
Results of the financial and economical assessment.
Conditions A1 A2 A3 B1 B2 B3 B4
Pessimistic
NPV (MMUSD) 2.30 1.32 1.45 1.76 0.78 0.91 2.84
PBP(years) >10 >10 >10 5.00 4.80 4.70 7.20
IRR (%) 3.96 3.15 0.27 7.05 8.78 8.43 15.65
Baseline
NPV (MMUSD) 0.44 1.42 1.29 7.10 8.08 8.60 10.89
PBP (years) 8.60 7.10 6.50 5.70 5.50 5.10 4.00
IRR (%) 12.18 15.89 17.14 20.41 21.23 22.97 29.40
Optimistic
NPV (MMUSD) 1.44 2.81 2.43 10.66 11.60 11.51 15.22
PBP (years) 6.70 5.50 5.20 4.70 4.60 4.40 3.40
IRR (%) 16.78 21.11 22.78 25.08 25.58 26.87 36.02
Pessimistic condition: Electricity Price 0.14 USD/kWh, Biofertilizer Price (CSTR reactor): 0 USD/t. Base condition: Electricity Price 0.14 USD/kWh, Biofertilizer Price (CSTR
reactor): 49.54 USD/t, Biofertilizer Price (UASB reactor): 67.11 USD/t. Optimistic condition: Electricity Price 0.168 USD/kWh, Biofertilizer Price (CSTR reactor): 59.44 USD/t,
Biofertilizer Price (UASB reactor): 80.53 USD/t.
256 L.M. López González et al. / Waste Management 102 (2020) 249–259
3.3. Environmental LCA
3.3.1. Environmental LCA for scenario A
The environmental LCA shows that the alternatives A1, A2 and
A3 improve (with respect to the reference scenario A0) the envi-
ronmental profiles (Fig. 5A), decreasing up to 93% the total score.
The best results were obtained when press mud was pretreated
by LHW to feed the CSTR reactor. This is explained by the highest
methane production in this alternative, the production of the high-
est amount of electricity and heat and consequently the lower
energy demanded from TSC to achieve the equal basket of benefits.
The worst results were for the reference scenario A0 because it
would have to supply 14.7 MWh of electrical energy and 17.0
MWh of thermal energy from fuel oil in centralized power plants,
and 2.4 t of organic carbon extracted from the soil.
The endpoint impact category ‘‘Human health” presented the
highest contribution (40–66%) to the total score for all the alterna-
tives. This result was mainly attributed to the midpoint impact cat-
egory ‘‘Climate change” (16–51%) and ‘‘Human toxicity” (37–81%),
due to the burning of fossil fuel at refinery to supply the required
electricity. The midpoint impact category ‘‘Formation of particulate
material” (9–24%) provided a lower contribution to the total
score.
3.3.2. Environmental assessment of scenario B
The environmental LCA shows that the alternatives B1, B2, B3
and B4 improve the environmental profiles with respect to the ref-
erence scenario B0 (Fig. 5B), reducing the total score from 84 to
96%.
The best results were obtained for the alternative B4, as the
highest amount of electricity and heat and consequently the lower
energy demanded from TSC to achieve the equal basket of benefits.
Similar to Scenario A, the endpoint impact category ‘‘Human
health” represented the highest contribution (44–54%) to the total
score in Scenario B, being attributed to the contribution of the mid-
point impact categories ‘‘Climate change” (43–59%) and ‘‘Human
health”, because of both: the burning of fossil fuel at refinery to
supply the required electricity; and the production of Ca(OH)
2
when vinasse was fed.
The endpoint impact category ‘‘Ecosystem quality” and ‘‘Natural
resources” contributed from 28 to 42% and 14 to 21%, respectively,
to the total score, respectively.
According the worst score found for the reference scenarios A0
and B0, it is suggested that electricity from biogas was environ-
mentally more sustainable than electricity generated from a
fossil-intensive electricity mix. Similar findings have been reported
(Fusi et al., 2016; Lijó et al., 2017; Styles et al., 2016).
Fig. 3. Sensitivity analysis for t/d of press mud fed (PM) and biogas yield (YB) for the alternatives A1 (r), A2 (j) and A3 (d).
Fig. 4. Sensitivity analysis for t/d of press mud fed (PM), m
3
/d of vinasse fed (V) and biogas yield (YB) for the alternatives B1 (r), B2 (j), B3 () and B4 (d).
L.M. López González et al. / Waste Management 102 (2020) 249–259 257
The results of the environmental Life Cycle Analysis (LCA) show
that the alternatives improve the environmental profiles, in both
scenarios. The endpoint impact category ‘‘Human health” had the
highest contribution.
The LCA results agree with environmental benefits reported for
the anaerobic digestion in the Cuban sugar factories (Barrera et al.,
2016; Contreras et al., 2009). However, direct comparison with the
results in the current study is not possible as different functional
units, types of systems, assumptions and life cycle impacts assess-
ment methodologies were used.
Four alternatives for using by-products and wastes from the
sugar mill and ethanol distilleries were compared, from a life cycle
perspective, in a study carried out by Contreras et al. (2009). The
highest benefits were obtained for the alternatives considering bio-
gas production from press mud, sugar mill wastewater, and
vinasse. In another by Barrera et al. (2016) a comparative assess-
ment of anaerobic digestion power plants as alternative to lagoons
for vinasse treatment was reported. In this study the environmen-
tal profile was improved with respect to the lagooning of Cuban
vinasse, reducing up to 77% the total score. The endpoint impact
category ‘‘ecosystem quality” contributed to more than 37% of
the total score, where the midpoint impact category ‘‘agricultural
land occupation” had the largest contribution (60%). This result
was mainly attributed to differences in the required surface area
for lagooning when 70% of the organic matter is removed at the
biogas production.
This research shows that both the environmental and energetic
profiles and the profitability of methane production can improve
when the pretreatment and co-digestion of these wastes from
the sugar – alcohol production process are considered.
4. Conclusions
LHW and TA pretreatments are self-sufficient in terms of ther-
mal requirements since they can recover heat from the biogas
engine. According to the results obtained from the energetic, eco-
nomic and environmental analysis of press mud pretreatment
alternatives for AD and co-digestion with vinasse, the digestion
of the press mud; pretreated by LHW in CSTR reactors (A2) was
the most viable for the scenario of a sugar mill without distillery,
while the alternative co-digestion with the vinasse of the press
mud without pretreatment (B4) was the most viable for the sce-
nario of a sugar mill with distillery.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This research was supported by the VLIR-UOS project entitled
‘‘Biogas production from waste from local food, wood and sugar-
cane industries for increasing self-sufficiency of energy in Sancti
Spiritus, Cuba”.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.wasman.2019.10.053.
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L.M. López González et al. / Waste Management 102 (2020) 249–259 259
... In addition to the process simulation, the financial assessment of biogas projects must be considered for final decisions. TEA has been widely used on AD, exploring different pretreatment methods for complex materials, biogas upgrading technologies, electricity, and heat generation (López et al., 2020;Rotunno et al., 2017). In the context of the AD of sugarcane vinasses, the TEA has been used to assess different alkalinization strategies , full-scale digestion plants with phase separation (i.e., acidification for biogas-H2 and methanization for biogas-CH4) , the potential impacts for replacing fossil fuel and natural gas (Silva Neto and Gallo, 2021), and the impacts of different factors on the profitability of the process and biogas application . ...
... In Cuba, Chanfón and Lorenzo (2014) evaluated different alternatives for vinasses treatment focused on waste valorization routes (e.g., biogas and yeast production); however, no biogas conversion alternatives were considered. The environmental and techno-economic performance of the anaerobic digestion of pretreated and co-digested press mud (plus vinasse) along with a combined heat and power system as biogas final used has been recently assessed (López et al., 2020). Nevertheless, biogas upgrading to biomethane as a vehicle fuels or gas grid injection was not considered. ...
... The sale of the sludge from UASB digesters, the effluent for fertirrigation, and the carbon credits are common revenues in all scenarios. The utility costs were assumed as 140 USD/MWh and 0.25 USD/m 3 for electricity and water, respectively (López et al., 2020).The cost of chemicals used in the neutralization stage (i.e., Ca(OH)2) was assumed as 150 USD per ton(López et al., 2020). In this work, Ca(OH)2 was assumed as alkalizing agent as this is the current chemical used in Heriberto Duquesne Biogas plant (the only one in Cuba) located in Villa Clara, and previously considered byLópez et al. (2020) andBarrera et al. (2016). ...
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This paper presents a process simulation model in Aspen Plus® and a techno-economic assessment for the anaerobic digestion of Cuban sugarcane vinasses considering three scenarios for biogas application: electricity production (S_1), biomethane as vehicle fuel (S_2), and biomethane for gas grid injection (S_3). From the simulation model, non-significant differences (p_value≥0.1779) between experimental and simulation results were found. S_1 showed the best economic performance among the assessed biogas applications. From the sensitivity analysis, the mean electricity price leading to a net present value of zero for S_1 was 90 USD/MWh, while for S_2 and S_3 the mean incentive required was 0.33 USD/m3biomethane and 0.67 USD/m3biomethane, respectively. The uncertainty analysis showed a chance for investment failure in S_1 less than 10%, whereas for S_2 and S_3 it ranged between 31 – 37%. The minimum scale required (milling and distillery capacities, ethanol yield) for getting profits from biomethane projects was targeted at 10 800 tcane/day, 108 m3ethanol/d at 10 Lethanol/tcane, respectively. To this end, Cuban plants should significantly increase their average capacities; otherwise, a centralized biomethane production by limiting the number of biomethane plants to one or two per province could be implemented.
... Actualmente, las instalaciones de recuperación de energía (ERF) en Cuba son a partir de residuos agroindustriales como la combustión del bagazo en la industria azucarera para su uso en plantas de energía como las bioeléctricas (Sagastume et al. 2018). Existen estudios que se han centrado en el tratamiento de algunos residuos como la vinaza y el estiércol mediante digestión anaerobia (Pagés-Díaz et al. 2011, Alfonso-Cardero et al. 2021, vinaza y cachaza mediante codigestión anaerobia (López et al. 2020) para producir biogás en zonas rurales, para generar electricidad y su consumo doméstico. ...
Thesis
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Las estrategias de manejo de residuos sólidos urbanos (RSU) en La Habana son anticuadas. El objetivo de este trabajo es proponer la alternativa más adecuada de una instalación de recuperación de materiales (MRF) integrada a tecnologías de recuperación energética para el tratamiento de los RSU de La Habana. Se consideraron siete alternativas de recuperación energética: combustión, gasificación y carbonización hidrotermal (HTC) con y sin captura de CO2 y digestión anaerobia (AD). La selección se basó en criterios ambientales, tecno-económicos y sociales utilizando un proceso de jerarquía analítica (AHP) como una herramienta de toma de decisiones multicriterio (TDMC). La TDMC-AHP consideró criterios cualitativos (basados en juicios de expertos) y cuantitativos (basados en modelos de simulación de Aspen Plus). Los resultados del AHP mostraron que los criterios ambientales tenían la máxima prioridad (61 %), mientras que los criterios sociales y tecno-económicos, tuvieron una prioridad de 22,5 % y 16,5 % respectivamente. Asimismo, los subcriterios contaminación, seguridad laboral y riesgo de inversión tuvieron la mayor prioridad. La instalación de recuperación de materiales integrada a la digestión anaerobia como alternativa de recuperación energética (MRF+AD) fue la más adecuada (21 % de preferencia) para el tratamiento de los RSU de La Habana, seguido de la combustión y la gasificación con captura de CO2, respectivamente. Este estudio confirma que la digestión anaerobia es una opción de preferencia para economías emergentes como Cuba, principalmente por la baja contaminación ambiental, alta aceptación social y estabilidad financiera a largo plazo. Palabras claves: Instalación de recuperación de materiales, Recuperación energética, Aspen Plus, Toma de decisiones multicriterio (TDMC), Proceso de jerarquía analítica (AHP).
... Hydrolysis and acidogenesis of organic matter could have been completed during dry tomb phase of landfilling as soon after recirculation of leachate, increment in pH was recorded, which shows initialization of acetogenesis and methanogenesis (González et al., 2017). Moreover, addition of sugar mill waste provided additional pH buffering capacity, which favors leachate pollutant removal and biomethane production (González et al., 2020). Srivastava and Chakma (2021) also identified positive impact of industrial organic sludge addition in MSW landfill for appropriate pH balancing. ...
Article
In this study, anaerobic co-landfilling of municipal solid waste (MSW) and sugar mill pressmud (PM) was performed in four different proportions [PM:MSW] viz. 0:1 (control: BR1), 1:3 (BR2), 1:1 (BR3) and 3:1 (BR4). Efficacy assessment of Dry tomb – Bioreactor landfill (DTLF – BRLF) operation was carried out through leachate characterization and biomethane production. Leachate recirculation as a part of bioreactor operation after 194th day onwards showed promising degradation of co-wastes. Moreover, leachate decontamination and methane production were reliant on co-disposal proportions of PM and MSW. Maximum biomethane generation of 46.355L was obtained in landfill lysimeter BR3 followed by BR4 (34.680L), BR2 (24.275L) and BR1 (12.850L). Both logistic function and Gompertz growth models showed efficient fitting (R² > 0.99) for observed methane production. This research could be a baseline study for selective operation of combined dry tomb and bioreactor landfilling at full scale in co-disposal scenarios.
... According to Fig. 5, besides the manufacture of equipment used in AD process, the production of Ca(OH) 2 also has a high environmental impact and shows a significant amount of pollution to the environment . During the production of Ca(OH) 2 , large amounts of CO 2 and CH 4 are produced, which will increase the potential of global warming (Lopez Gonzalez et al., 2020). For bio-oil, since it has low organic content, so it will generate less biogas during AD process (Torri & Fabbri, 2014). ...
Article
The integration of anaerobic digestion (AD) and pyrolysis (Py) could be a solution to economically utilize the organic fraction of municipal solid waste (OFMSW). However, it is not clear whether the environmental impact of the integrated pathway always outperforms the two single technologies. In this study, two integrated pathways (AD-Py, Py-AD) were compared with single AD and Py from the life cycle environmental impacts point of view. The results indicate that the environmental impacts of the four pathways are heavily dependent on their energy inputs and outputs. AD-Py is more environmentally friendly (-11.53 of total environmental impact /kg OFMSW) than single AD or Py. Py-AD exhibites the heaviest environmental burden (2.75 of total environmental impact /kg OFMSW) in all pathways. Therefore, AD-Py can be the top priority of treating OFMSW among the four pathways from the environmental viewpoint. This work could provide a theoretical support for the utilization of OFMSW.
... However, bio-methane can only substitute natural gas. CHP turbine generator for both electricity and heat supply can be fed to the national grid and produce steam at low pressure from the exhaust gases, respectively (López González et al., 2020). ...
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In this work, techno-economic evaluation of anaerobic digestion (AD) system (8,000 metric tons (MT)/year) with singular (dairy manure), binary (dairy manure and corn stalk), and ternary mixture (dairy manure, corn stalk, and tomato residues) under bio-methane and combined heat and power (CHP) pathways based on a plant service life of 20 years were carried out. Solid state-AD (SS-AD) of ternary mixture improved the efficiency of investment, benefited the digestate price, and was shown to be economic viability. The introduction of a CHP unit highly improved the economics of SS-AD. SS-AD of the binary mixture under CHP pathway was able to compensate the initial required investment, however was not financially attractive under bio-methane pathway. Besides, SS-AD of the ternary mixture under CHP pathway had higher net present value (NPV) ($0.60 million vs $0.40 million) and internal rate of return (IRR) (23% vs 20%) than that under bio-methane pathway.
... In terms of rural waste, long initial start-up phases biogas production, low methane yield, and reactor efficiency that are typical for anaerobic mono-digestion (AmD) of cattle manure (Adriamanohiarisoamanana et al., 2017;Matos et al., 2017;Adriamanohiarisoamanana et al., 2018;McVoitte & Clark, 2019) can lead to economic losses. In this sense, anaerobic codigestion (AcoD) process (simultaneous digestion multiple biodegradable substrate), is a promising alternative to solve problems related to AmD and to improve the economic viability of anaerobic digestion (AD) plants (Mehryar et al., 2017;Adriamanohiarisoamanana et al., 2017;Adriamanohiarisoamanana et al., 2018;Krištof & Gaduš, 2018;Clercq et al., 2019;Sarpong et al., 2019;González et al., 2020). ...
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In this study, biogas was produced from the anaerobic digestion of Sorghum bicolor stalk. Pretreatment of the biomass was carried out prior to the digestion using sulfuric acid (H 2 SO 4) and hydrogen peroxide (H 2 O 2). The physicochemical, elemental and structural analyses were carried out on the biomass before and after pretreatment. The microbial composition of the fermenting materials were also determined using standard method while the Fourier Transform Infra-red (FTIR) spectroscopy were used to quantify the structural changes that took place after pretreatments. Results showed enormous reduction of hemicellulose and partial solubilization of cellulose with the application of H 2 SO 4 for pretreatment with obvious breakdown of all important bonds in the biomass. The most suitable condition for the most efficient acidic pretreatment of the Sorghum bicolor stalk was using H 2 SO 4 concentration of 0.75% (v.v −1), autoclave temperature of 118 • C and biomass dry mass of 3.7 g for 52 min. However, the use of H 2 O 2 caused huge solubilization of lignin while partial dissolution of hemicellulose took place. The most suitable condition that gave the best result in this pretreatment procedure was H 2 O 2 concentration of 6.8% (v.v −1), shaker temperature of 28 • C, agitation at 126 rpm and 3 g of biomass for 85 min. Overall, the use of the H 2 O 2 showed reduction of lignin and hemicellulose by 73 and 42% respectively while also increasing the concentration of cellulose by 23%. The acid and alkaline pretreated biomass produced a total of 312.3 and 607.1 LNbiogas.kg VSad −1 respectively. In comparison, the biomass pretreated with H 2 O 2 produced 65% more LNbiogas.kg VSad −1 than the other and equally reduced the production time by 5 days. For the alkaline treated biomass, the 1422 kWh t −1 TS thermal energy gain exceeded the 945 kWh t −1 TS used in the pretreatment thus giving a net thermal energy of 477 kWh t −1 TS. However, the acidic pretreatment of Sorghum bicolor stalk is not profitable because the-131 kWh t −1 TS thermal energy gain was far below the 1025 kWh t −1 TS thermal energy used in pretreatment with a net thermal energy of-761 kWh t −1 TS. Till now, use of low-cost H 2 O 2 for biomass pretreatment is unpopular while the uses of other strong alkali and acids are well studies. However, hydrogen peroxide gave better product yield. Therefore, use of this alkali pose a novel biotechnological means for generating biogas.
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Ultrasonic pretreatment has been considered as an environmentally friendly process for enhancing the biodegradability of organic matter in anaerobic digestion. However the consumed energy during the pretreatment is a matter of challenge especially where energy generation is the main purpose of a biogas plant. The aim of the present work was to study the efficiency of ultrasonic pretreatment in enhancement of biogas production from fruits and vegetable wholesale market waste. Three sonication times (9, 18, 27 min) operating at 20 kHz and amplitude of 80 μm were used on the substrate. The highest methane yield was obtained at 18 min sonication (2380 kJ kg⁻¹ total solids) while longer exposure to sonication led to lower methane yield. This amount of biogas was obtained in 12 d of batch time. The energy content of the biogas obtained from this reactor was two times of the input energy for sonication.
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Energy from biomass is essential for sustainable agricultural development. The generation of biogas and bio-fertilizer from the anaerobic co-digestion is fundamental in mitigating environmental problems and in the local generation of electricity. In this context, this study examines the technical and economic feasibility of the effects of co-digestion of sweet potato with dairy cattle manure on energy and nutrient recovery using semi-continuous digesters. The use of sweet potato in co-digestion is characterized as an innovation in the technological process of anaerobic digestion, the application of which on small and medium-sized farms is still a challenge in emerging economies. The assay was conducted using different proportions of sweet potato mixed with dairy cattle manure, which produced a linear increase in methane yield with concentrations of 63–65.5% and biogas capable of generating 2376.44 kWh d−1. In addition to the generation of cleaner electrical energy, the co-digestion process produces a bio-fertilizer with a potential production of 26.53 tons/year of N, 20.45 ton/year of P and 23.6 tons/year of K. These results are superior to the mono-digestion process. Regarding the necessity of using sweet potato, this new system of production was evaluated for a model medium-size farm dedicated to dairy production in the countryside of the state of São Paulo in Brazil. The results demonstrate the economic feasibility of the investment in the five analysed scenarios, as the internal rate of return varied between 46.8% and 57% and the payback was achieved in 2–3 years. This research demonstrates that the innovation of using sweet potato as an additional input to the digestion process can increase the sustainability of small and medium-sized dairy farms in a manner consistent with a circular economy.
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Agricultural biogas plants are becoming attractive in several countries, mainly due to the possibility of obtaining additional incomes by selling energy to electrical grid. However, specific conditions under which biogas plants would be economically viable are not yet well known. This paper presents results of analysis made to find optimal sizes of biogas plants in terms of electrical capacity. They were evaluated based on economic models of centralized and decentralized technological schemes and their respective mono-digestion and co-digestion systems of cattle manure, maize silage and grass silage as feedstock, for electricity generation capacities between 100 and 1000 kWe. Results show that biogas plants using mono-substrates such as cattle manure present economic viability for electrical power higher than 740 kWe. Co-digestion system presents economic viability for electrical power higher 1000 kWe. Finally, public policies related to development of these technologies, mainly in the form of subsidies, such as those existing in developed countries, could help to make the co-digestion agricultural biogas plants economically viable for the Brazilian scenario. The novelty of the paper consists in the determination of the optimum size of mono- and co-digestion patterns for Brazilian conditions showing as results the necessary level of subsidies and the paths to economic viability.
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Vinasse is considered as the largest source of contamination in the ethanol production industry from sugarcane. Vinasse can be used to produce suitable fertilizer; however, it contains low macronutrients and micronutrients. On the other hand, it is one of the resources with a high potential for biogas production. The biogas production from vinasse has both economical and environmental benefits. Nevertheless, vinasse has a low carbon-to-nitrogen ratio; thus, complementary substances, e.g., animal manure, organic industrial waste, and lime fertilizers, should be added to improve the biogas yield. Currently, 22.4 gigalitres of vinasse are produced throughout the world, which have the potential for producing 407.68 gigalitres of biogas. This potential can be considered as a significant source of renewable energy. This paper gives a general summary of the properties of vinasse and the production of biogas from vinasse. Furthermore, a review of the optimal conditions for biogas production, the potential of biogas production, and the advantages of biogas production from vinasse are provided.
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This paper investigates the environmental and economic performance of the power production from biogas using Life Cycle Assessment, Life Cycle Costing and Cost Benefit Analysis methodologies. The analysis is based on a commercial thermophilic biogas plant located in Spain where is installed a Combined Heat and Power system that produces electricity that is sold to the grid. Power generation has been assumed as the only function of the biogas system, expanding the system boundaries to include the additional function related to the end-of-life management of the biowastes. Thus environmental burdens from the conventional management of residues were calculated separately and subtracted. The base scenario involves using agri-food waste, sewage sludge and pig/cow manure as substrates. This situation is compared against an alternative scenario where the production of synthetic fertilizer is surrogated by the digestate. The results have shown that the most impacting activities in all impacts categories of power production are primarily attributable to the operation and maintenance of the biogas plant except for water resource depletion and climate change. The avoided emissions associated with the conventional management of pig/cow manure more than offset GHG emissions of the biogas system resulting in a negative impact value of −73.9 g CO2 eq/kWh in the base case scenario. The normalized results show that local impact categories such as primarily human toxicity, fresh water ecotoxicity and particulate matter are the most significantly affected by the biogas system while global impact categories as climate change and ozone depletion are less severely affected. The operation and maintenance phase is also shown to be the largest contributor after the life cycle cost analysis, followed by the construction and dismantling of the biogas plant and the profitability of the project is primarily related to the income obtained from the management of the biowastes used as substrates.
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The conversion efficiency of high solids waste digestion as sugarcane press mud (P) may be limited due to hydrolysis step. The option of co-digestion with vinasse, main liquid waste generated from ethanol production, was investigated under batch regime at mesophilic conditions (37.5 ± 1 °C) and the best mixture was evaluated under semicontinuous regime in stirred-tank reactors. The maximum values for methane yield in batch tests were for V75/P25 and V50/P50 mixtures (on basis of the chemical oxygen demand (COD) percentage added in the mixture), with an average value of 246 N mL CH4 g⁻¹ CODfed, which was 13% higher than that of press mud alone. A highest methane production rate of 69.6 N mL CH4 g⁻¹ CODfed⁻¹ d⁻¹ was obtained for the mixtureV75/P25. During the experiment carried out in CSTR reactors, the organic loading rate (OLR) was increased from 0.5 up to 2.2 g VS L⁻¹ d⁻¹. Methane yields of 365 L CH4 kg⁻¹ VS and biogas productivities of 1.6 L L⁻¹ were obtained in co-digestion, which was 64% higher in comparison to mono-digestion. The performance of the process in mono-digestion was less stable than in co-digestion, with a significant fall of methane yield to 1.8 kg VS m⁻³ d⁻¹, and a partial inhibition of the methanogenic archaeas when the OLR was increased up to 2.2 kg VS m⁻³ d⁻¹. The co-digestion of vinasse with press mud is a good option for the treatment of streams at the alcohol-sugar industry.
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Assuring environmental sustainable bioenergy production is an international priority nowadays. The objective of this study was to identify the environmental consequences of the feedstock selection for biogas. Two real biogas plants were assessed and compared from a life cycle perspective. Plant A performs the co-digestion of energy crops (78%) and animal waste (22%) while Plant B consumes energy crops (4%), food waste (29%) and animal waste (67%). According to the results, electricity production from biogas implied lower impacts in climate change than the existing electric mix. Maize silage (650 Nm³/TVSfed) and food waste (660 Nm³/TVSfed) were identified as an interesting source of bioenergy. However, the cultivation of energy crops was identified as the main hotspot in Plant A. The use of organic substrates with lower energy potential and high nutrients concentration such as animal manure (450 Nm³/TVSfed) produces higher amounts of digestate, producing impacts in acidification and eutrophication categories. Finally, in order to improve the environmental sustainability of bioenergy, specific guideless should be established to achieve harmonised lire cycle studies. In addition, environmental policies should promote the use of waste streams and prevent the use of energy crops as well as including goals related with acidification and eutrophication impacts.
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The IWA Anaerobic Digestion Modelling Task Group was established in 1997 at the 8th World Congress on Anaerobic Digestion (Sendai, Japan) with the goal of developing a generalised anaerobic digestion model. The structured model includes multiple steps describing biochemical as well as physico-chemical processes. The biochemical steps include disintegration from homogeneous particulates to carbohydrates, proteins and lipids; extracellular hydrolysis of these particulate substrates to sugars, amino acids, and long chain fatty acids (LCFA), respectively; acidogenesis from sugars and amino acids to volatile fatty acids (VFAs) and hydrogen; acetogenesis of LCFA and VFAs to acetate; and separate methanogenesis steps from acetate and hydrogen/CO2. The physico-chemical equations describe ion association and dissociation, and gas-liquid transfer. Implemented as a differential and algebraic equation (DAE) set, there are 26 dynamic state concentration variables, and 8 implicit algebraic variables per reactor vessel or element. Implemented as differential equations (DE) only, there are 32 dynamic concentration state variables.