CETvol87
DOI: 10.3303/CET2187081
Paper Received: 21 August 2020; Revised: 31 January 2021; Accepted: 4 April 2021
Please cite this article as: Loureiro F.G., Eusebio A., Marques I.P., 2021, Energy Recovery from Food Industry Sludge through Anaerobic
Digestion, Chemical Engineering Transactions, 87, 481-486 DOI:10.3303/CET2187081
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 87, 2021
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Laura Piazza, Mauro Moresi, Francesco Donsì
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-85-3; ISSN 2283-9216
Energy Recovery from Food Industry Sludge through
Anaerobic Digestion
Fernando G. Loureiro*, Ana Eusébio, Isabel P. Marques
Unidade de Bioenergia e Biorrefinarias, Laboratório Nacional de Energia e Geologia, Estrada do Paço do Lumiar, Lisboa,
1649-038, Portugal
fernandogonzloureiro@hotmail.com
Sludge generated in three Portuguese food processing companies - chestnut, dairy and fruit-cereal – were
anaerobically digested, over 77-day experimental time, under batch mode and mesophilic conditions of
temperature, to assess the anaerobic digestion process applicability to the energetic valorisation of these
organic material surpluses. The biogas production of 106-144 mL (STP conditions) and the methane content
of 61-72% indicate that all tested sludge reserve an energetic potential through anaerobic process.
Comparatively, the sludge digestion from the fruit-cereal processing industry provided the largest volume of
biogas, while the milk-yogurt reached the highest methane yield amount (209 L CH4 kg
-1
VSinfluent).
Anaerobic digestion is an alternative process that, according to the obtained results, can advantageously
contribute to solving the problem of excess sludge in the food industry by converting it into a digestate for
agricultural purposes and an energy carrier vector (biogas/methane).
1. Introduction
The debate about future environmental conditions is on the agenda and it is promoted by a growing concern
with climate change and the lack of sustainability. Countless events that reflect the current degradation of
Earth's environmental conditions such as pollution, the greenhouse effect and global warming are in place.
These phenomena have origin in the continuous global increase in the population, the consequent intense use
of fossil fuels as the main source of energy and the harmful management of waste (Ren et al., 2017).
In the European Union (EU), the food industry represents the most economically relevant industrial sector
whose importance still tends to grow (Zhang et al., 2011). In Portugal, over 1 million tons of products from the
dairy industry and derivatives were produced in 2017 (INE, 2019). Dairy processing sector has already been
regarded as the most significant producer of wastewater, which is often associated with the growing annual
milk production (Slavov, 2017). In addition, this industry has large water requirements for the process and
cleaning operations and, consequently, higher volumes of wastewater are produced. Industrial sectors related
to the fruit and vegetable preparation and preservation, generating in Portugal (in 2017) more than 46
thousand tons of products (INE, 2019), are being important producers of residues and by-products, such as
peels, seeds and pomace, motivated the EU to establish new goals regarding the reduction of fruit waste
amounts (Campos, Ricardo, Vilas-boas, Madureira, & Pintado, 2020).
Concerning the chestnut processing industry, in 2018, five European countries were on the list of the 10
largest chestnut producers and Portugal was the seventh largest producer, with a production of more than 34
thousand tons (FAOSTAT, 2018). Portuguese chestnut processing industry has been exporting substantial
quantities of chestnuts, namely frozen peeled chestnut (Rosa et al., 2017) and this activity generated a
positive social and economic impact (Silva et al., 2016).
Being the food industry one of the largest sectors producing effluents, it generates large volumes of sludge
from the biological treatment of the resulting wastewater (Boguniewicz-Zablocka et al., 2019). The sludge can
be discharged onto agricultural land. However, when large quantities of sludge are accumulated there is the
risk of GHG production and difficulties may arise with the lack of sludge outflow and with its consequent
improper storage (Belhadj et al., 2014).
481
Changing the global paradigm becomes urgent in order to deal with the need to develop technologies for
exploiting renewable resources and sources of energy while ensuring a clean and efficient waste management
(Batista et al., 2017; Castellano-Hinojosa et al., 2018). The EU and the United Nations (UN) have developed
policies, targets and financing structures to encourage the use of renewable energy sources such as biogas
(Scarlat et al., 2018). Effectively, anaerobic digestion (AD) is a promising and sustainable technology,
applicable to the treatment and energetic/agricultural valorisation of industrial effluents, such as the food
sector, under the circular economy concept. The present work aims to evaluate the potential of the
applicability of AD process on sludge generated in the food industry, taking as examples the dairy, chestnut
and fruit and cereal processing industries.
2. Materials and methods
2.1 Substrate and Inoculum
The potential and efficiency of energy recovery through anaerobic digestion were evaluated in samples of
liquid sludge resulting from wastewater treatment from Portuguese companies, such as a chestnut processing
plant, located in the North (“Cs”); a dairy processing industry plant (“Ms”) and a fruit and cereals processing
industry plant (“FC”) both located in the center of the country. The sludge from a Portuguese domestic effluent
anaerobic digestion plant was used for inoculum (I, 30% v/v), at a substrate/inoculum ratio of 0.75, expressed
in volatile solids.
2.2 Anaerobic digestion units: experimental set-up
The tests were carried out in batch reactors (71.5 mL of volume) in triplicate, operating under mesophilic
temperature conditions (37±1 °C). The digestion units were monitored by chemical and chromatographic
analysis and by daily recording the volume of biogas and periodically the methane content, expressed under
standard conditions of pressure and temperature (STP: 0°C and 1 atm).
2.3 Chemical and chromatographic analysis
Analytical techniques allowed to determine several parameters for the process development evaluation, such
as pH, total solids (TS), volatile solids (VS), chemical oxygen demand (COD), total nitrogen (Kjeldahl, TKN),
and ammonium (NH4-N), according to the Standard Methods manual (APHA, AWWA, 2005). Humidity was
determined by drying at 105±5ºC by a gravimetric method. Total sugars were determined by acid hydrolysis in
autoclave and determination by molecular absorption spectrometry (phenol-sulfuric reagent) in the hydrolysate
(expressed in glucose). The volume of biogas produced was determined using a pressure transducer. The
composition of the biogas, in terms of CH4 and CO2, was obtained by gas chromatography technique (Varian
430-GC, TDC; HP-5890, FID) according to the Standard Method ASTM [2000]. The characterization of the
substrates and inoculum is shown in Table 1.
Table 1: Substrate and inoculum characterization.
1a.r. – as received; 2n.d. – not determined; 3d.b. – dry base. Cs – chestnut sludge, Ms – milk-yogurt sludge, FC – fruit and
cereal sludge, I – inoculum
3. Results and Discussion
The results concerning the amount of gas accumulated during the AD process about each substrate are
shown in Figure 1 while the biogas composition, methane yield and productivity data are presented in Table 2.
All tested batch reactors displayed increasing biogas volumes over the experimental period without any
visible lag phase.
Cs Ms FC I
Humidity. a.r.
1
98.9% 83.7% 81.6% n.d.
2
VS (g/L) 10.1 40.5 188 22.9
COD (g/L) 12.8 133.6 248 103
TKN (g/L) 0.3 3.4 1.2 1.4
N-NH4 (mg/L N) <4 0.8 21 360
pH 6.3 5.9 3.8 6.9
Total sugars in the hydrolysate (in glucose). d.b.
3
3019 mg/L 2.7% 95840 mg/L n.d.
2
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Figure 1: Volumetric biogas production registered for the chestnut sludge (Cs), milk-yoghurt sludge (Ms), fruit
and cereal sludge (FC) and inoculum (I).
A more marked production progress was observed in the first testing month than in the remaining time (Figure
1). Control unit (I) exhibits a considerable biogas production due to the presence of some biodegradable
material contained in the sludge that has not undergone to previous conversion. Comparatively, the tested
samples provided higher biogas volumes than the I unit, showing that organic matter of substrates samples
was degraded over the 77 days of experiment and converted into biogas.
FC+I reactors displayed the highest biogas production and are ranked second in terms of methane yield
(143.6 mL and 190.5 L CH4 kg
-1
VS
-1
, respectively, Table 2) comparing with the other substrates. At the
beginning, FC+I reactors had a very intense biogas production, reaching high gas concentrations that required
the immediate unit’s depressurization (Figure 1).
This may be the consequence of a rapid hydrolysis stage due to the substrate composition. Effectively, the
substrate characterization prior to this experiment (Table 1) revealed a relatively high concentration of total
sugars (glucose) in the hydrolysate of FC substrate. According to Ji et al. (2017), fruit residues are considered
to be one of the most suitable substrates for AD as they are easily biodegradable with high moisture.
However, just after this initial peak, a sharp decrease in biogas production was observed and the curve
adopted a similar behaviour to a lag phase but in two steps (Figure 1). A fast hydrolysis phase could have
provided a rapid medium acidification and a consequent decrease in pH values, due to the accumulation of
intermediate VFAs that led to a biogas and methane production inhibition (Ji et al., 2017). This is represented
by the graphical steps portrayed in Figure 1 for the FC+I curve. After this stage, methane proportions started
to increase with the biogas production once the process has recovered.
Table 2: Summary of the biogas and methane production through anaerobic digestion for each substrate.
Cs+I Ms+I FC+I
Biogas production (mL) 105.8±3.6 113.6±0.1 143.6±4.2
CH4 (%) 63.3% 70.4% 52.7%
CH4 production (mL) 67.0 80.0 75.7
Methane yield (L CH4 kg
-1
VSinfluent) 152.4±5.5 209.3±0.1 190.5±6.1
Concerning the Ms+I units, they presented higher biogas production than Cs+I (Figure 1) and the highest
methane yield (Table 2). In a quick first inspection, this seems to be a surprising result because Ms+I solutions
contained lower initial concentrations, mainly for COD, compared to the Cs+I solutions (Table 3).
483
However, regarding the Cs substrate composition, it derives from a chestnut processing industry and therefore
constitutes a waste that may contain phenolic compounds/lignin. It is known that these types of compounds
are inhibitory and recalcitrant components with the ability to delay the AD process. In relation to Ms, because it
is a sludge derived from a dairy industry, it may contain relevant amounts of lipids. Lipid macromolecules have
higher energy potential than proteins and carbohydrates and, accordingly, lipids provide higher volumes of
biogas with higher fraction of methane. In fact, Ms+I displayed the highest methane yield of the whole
experiment.
Table 3: Batch reactors initial and final content characterization and their respective removal capacity.
pH COD (g/L) TS (g/L) VS (g/L) N-NH4 (mg/L) TKN (mg/L)
Cs+I Initial 6.81 95.8±9.1 13.2±0.2 10.3±0.2 87±0 423±4
Final 7.75 13.9±1.1 11.0±0.2 8.1±0.1 189±2 437±0
Removal - 85% 16 21 -118 -3
FC+I Initial 6.70 82.3±0 12.3±0.2 9.6±0.1 71±6 381±24
Final 7.67 10.1±0 8.5±0.2 5.9±0.2 167±2 378±4
Removal - 88% 31 38 -133 1
Ms+I Initial 7.01 77.6±1.7 12.0±0 9.1±0.1 105±2 484±12
Final 7.75 11.1±0 9.4±0.1 6.4±0.1 235±0 454±8
Removal - 86% 22 29 -124 6
Cs+I – chestnut sludge+inoculum, Ms+I – milk-yogurt sludge+inoculum, FC+I – fruit and cereal sludge+inculum.
The observation of Table 3 reveals that removal percentages did not vary much among the various sludge
digestion processes, although the differences seem to follow trends in biogas production. Reactors with the
slightly higher removal values were the ones with more biogas volume. This was the case of FC+I reactors,
which provided the best removal values obtained - 88, 31 and 38% in the COD, TS and VS, respectively –
evidencing the high digestibility of the substrate and its repercussion in methane production and yield (Table
2). The lowest removal capacity was observed in the Cs+I units that presented values of 85% (COD), 16%
(TS) and 21% (VS), however, considering the absolute value, it was noticed that Cs+I digestion units
managed the elimination of the greatest fraction of the initial COD content. Amounts of 82 g/L COD (Cs+I),
against 72 and 67 g/L COD (FC+I and Ms+I, respectively) were registered. The pH that initially presented
values slightly below the neutral but became basic during the process, indicating that beneficial adjustments
occurred on the environmental conditions of the microorganisms, especially regarding archaea populations.
This behaviour also enabled the development of the process. Increases in ammonium concentrations at the
end of each testing process showed that organic matter was degraded and, consequently, validated the
presence of a balanced population capable of providing this conversion.
Table 4. Methane yield and energy potential of biomass and organic waste usable as raw material in AD
Biomass Methane Yield
(L CH4 kg
-1
VSinfluent)
Energetic Value
(KWh kg
-1
VSinfluent)
Reference
Cs+I 152.4±5.5 1.52±0.06
Present work FC+I 190.5±6.1 1.90±0.06
Ms+I 209.3±0.1 2.09±0.01
Organic fraction of municipal solid waste 100-400 6.18
(Roati et al., 2012)
Dairy sludge 200 n.d.
Coffee husk 159 1.78
(Chala et al., 2018) Potato pulp 250-400 2.49-3.99
Vegetable waste 400 3.99
Corn straw 174 0.66
(FNR, 2020)
Pasture 153 0.58
Rice husk (without pre-treatment) 44 4.36
(Solarte-Toro et al., 2018)
Rice husk (with pre-treatment) 56 n.d.
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The methane yields obtained in this work (150-200 L CH4 kg
-1
VSinfluent) are of the same order of magnitude as
found in other substrates tested in other studies (Table 4), such as municipal waste, coffee husk, corn straw or
pasture. The best methane yield provided by Ms+I units, 209 L CH4 kg
-1
Sinfluent, is very close to that reported
by Roati et al. (2012), 200 L CH4 kg
-1
Sinfluent, digesting analogous dairy substrates. Similar observations can
be made comparatively regarding the energetic value provided by the sludge digestion of food industries
(Table 4).
The interest of the proposed work is reinforced by the performance of the units to digest the raw sludge in its
original state, which means, without the application of any correction of the substrate or pre-treatment. From
the obtained data, the sludge generated in dairy, chestnut and fruit-cereal processing industries, tested as
examples of food sector, hold a potential for energetic valorization by anaerobic digestion.
4. Conclusions
Sludge from Portuguese food processing companies - chestnut, dairy and fruit-cereal – has potential to be
treated/valorised by anaerobic digestion as productions of biogas of 106, 114 and 144 mL, with methane
content of 63, 70, 53%, were registered in the digestion process of each sludge, respectively. The
corresponding methane yields reveal a greater efficiency of the processes operating with dairy and fruit-cereal
sludge (209 and 191 L CH4.kg
-1
VSinfluent, respectively) than with the chestnut sludge which, comparatively,
has been reached only 152 L CH4.kg
-1
VSinfluent. The lower efficiency of the chestnut process was related to
the recalcitrant and even inhibitory characteristics of the substrate in digestion, when compared to the other
two.
These exciting data deserve further research in order to better define the operating conditions that will
optimize the process of valorising the food industry surplus sludge by anaerobic digestion.
Acknowledgments
This work was realized under the scope of CONVERTE project, co-financed by POSEUR (POSEUR-01-1001-
FC-000001), Portugal 2020 and the European Union, through the Cohesion Fund, and the Research
Infrastructure for Biomass and Bioenergy (BBRI) - LISBOA-01-0145-FEDER-022059, financed by the ERDF
through Portugal 2020, Lisbon 2020 and Norte 2020, under the partnership agreement Portugal 2020.
The authors thank the portuguese companies for making available the substrates and the biological sludge
used as inoculum. Special thanks to collaborator Natércia Santos for the laboratory technical assistance.
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