Sicilian potential biogas production from Citrus industry by-product

Abstract

In Europe, Italy is the second nation after Spain in Citrus production and 49% ca. of national fruit production is located in Sicily. The by-product obtained from the industrial processing of Citrus fruits into juice and essential oils is called “pastazzo” or Citrus Waste (CW). This study is aimed at evaluating the Sicilian potential biogas and energy production from the above “pastazzo” and verifying the possibility of using this by-product for Anaerobic Digestion (AD) process within 30 km from processing plants.
The areas cultivated with Citrus species in Sicily and their distribution in the various municipal districts were mapped and analysed by means of QGIS software, together with the main Citrus processing plants and the three existing AD plants. The statistical data about the amounts of the by-product of Citrus processing industry were evaluated, in order to compute the Sicilian potential biogas and energy production.
In Sicily it is possible to obtain, through the use of “pastazzo”, a yearly production of 12,916,800 m3 ca. of biogas, equal to 24,250,930 kWh of electric energy and 25,463,477 kWh of thermal energy or 6,200,064 m3 of biomethane. This high potential biogas and energy production is not be currently used in a sustainable way from the energetic and economic points of view, because the existing AD plants are farer than 30 km from Citrus processing plants.
The CW produced in Sicily could be used inside a bio-reactor, together with other raw materials (e.g. pomace and wastewater from olive oil mills, cereal straw, poultry manure and Italian sainfoin or Hedysarum coronarium), for AD process.

Full text

11th International AIIA Conference: July 5-8, 2016 Bari - Italy
“Biosystems Engineering addressing the human challenges of the 21st century”
169
Sicilian potential biogas production from Citrus industry by-product
Comparetti A., Febo P., Greco C., Mammano M.M., Orlando S.
Comparetti A. (1), Febo P. (1), Greco C. (2), Mammano M.M. (2), Orlando S. (1).
(1)Dipartimento di Scienze Agrarie e Forestali, Università degli Studi di Palermo
Viale delle Scienze, Building 4, 90128, Palermo, Italy
(2)Consiglio per la Ricerca in Agricoltura e l’Analisi dell’Economia Agraria (CREA)
- Centro di Ricerca Difesa e Certificazione - S.S. 113 - km 245.500, 90011 Bagheria (Palermo), Italy
Corresponding author: Comparetti A., antonio.comparetti@unipa.it
Keywords: Citrus industry, Anaerobic Digestion, biogas, energy
Summary
In Europe, Italy is the second nation after Spain in Citrus production and 49% ca. of national
fruit production is located in Sicily. The by-product obtained from the industrial processing of
Citrus fruits into juice and essential oils is called “pastazzo” or Citrus Waste (CW). This study
is aimed at evaluating the Sicilian potential biogas and energy production from the above
“pastazzo” and verifying the possibility of using this by-product for Anaerobic Digestion (AD)
process within 30 km from processing plants.
The areas cultivated with Citrus species in Sicily and their distribution in the various municipal
districts were mapped and analysed by means of QGIS software, together with the main Citrus
processing plants and the three existing AD plants. The statistical data about the amounts of the
by-product of Citrus processing industry were evaluated, in order to compute the Sicilian
potential biogas and energy production.
In Sicily it is possible to obtain, through the use of “pastazzo”, a yearly production of
12,916,800 m3 ca. of biogas, equal to 24,250,930 kWh of electric energy and 25,463,477 kWh
of thermal energy or 6,200,064 m3 of biomethane. This high potential biogas and energy
production is not be currently used in a sustainable way from the energetic and economic points
of view, because the existing AD plants are farer than 30 km from Citrus processing plants.
The CW produced in Sicily could be used inside a bio-reactor, together with other raw materials
(e.g. pomace and wastewater from olive oil mills, cereal straw, poultry manure and Italian
sainfoin or Hedysarum coronarium), for AD process.
1. Introduction
The increase in agricultural investments, driven by the growing demand for biofuels, can
stimulate the development of a sustainable agriculture through the cultivation of dual purpose
crops, integrating food and energy production (FAO, 2013). Moreover, the increasing needs to
protect the environment require a correct management of wastes, including the by-product
deriving from Citrus industry. In Europe, Italy is the second nation after Spain in Citrus
production, with 2,924,531 tons produced per year and 49% ca. of national fruit production is
located in Sicily, i.e. 1,418,567 t (of which 40% ca. are processed), mainly orange, followed by
lemon, mandarin and clementine, above all in the provinces of Syracuse and Catania (ISTAT,
2015). Yet, the Citrus fruits produced in the provinces of Agrigento, Trapani and Caltanissetta
are marketed as fresh ones.
The industrial processing of Citrus fruits produces juice (35-45%), essential oils (0.2-0.5%) and
a by-product called “pastazzo” or Citrus Waste (CW) (55-65%), that is composed of peels,
squeezed pulp residues, seeds and residual fruits (Interlandi, 2013). It is essentially composed of
carbohydrates, acids (mainly citric and malic ones), lipids, mineral elements (mainly nitrogen,
calcium and potassium), volatile components (e.g. alcohols, aldehydes, ketones, esters and
hydrocarbons), flavonoids, essential oils (d-limonene, 95%), enzymes, pigments and vitamins.
This very acid by-product, having a pH ranging from 3.5 to 5.8 (Valenti et al., 2016), is mainly
used, both in Italy (especially Sicily) and in other countries, as food for livestock, but recently
also in Anaerobic Digestion (AD) process for producing biogas and digestate. Biogas can be
used for extracting biomethane, a fuel that can power surface transportation and agricultural
machines, or Combined Heat and Power (CHP) plants for co-generating electric and thermal

11th International AIIA Conference: July 5-8, 2016 Bari - Italy
“Biosystems Engineering addressing the human challenges of the 21st century”
170
energy (Comparetti et al. 2014, 2015). Moreover, the digestate, if treated and transformed into a
dry form (e.g. pellets), can be used as organic fertiliser.
This study is aimed at evaluating the Sicilian potential biogas and energy production from the
above “pastazzo” and verifying the possibility of using this by-product for AD process within
30 km from processing plants.
2. Materials and Methods
By means of QGIS, an open source GIS software, it was possible to use the vector map of land
use of Sicilian Region (2012). This map is coded according to the legend Corine Land Cover
(CLC) and reclassified from the Corine Biotopes map, selected from EU Corine Biotopes
classification manual (EUR 12587/3 EN), where the minimum areas classified are 1 ha.
Through the intersecting and sampling tools of QGIS software, a new point layer was obtained
for collecting the attribute data of land use, provincial and municipal boundaries, for any area of
1 ha. Thus, the areas cultivated with Citrus species in Sicily and their distribution in the various
municipal districts were mapped and analysed, together with the processing plants and the three
existing AD plants: Mussomeli (Caltanissetta), Vittoria (Ragusa) and Resuttano (Caltanissetta),
having an electric power of 999, 600 and 100 kW, respectively. Moreover, the areas cultivated
with Citrus species in the various municipal districts compared with Used Agricultural Area
(UAA), according to the 6th General Census of Agriculture, were mapped and analysed (ISTAT,
2012).
CW samples, taken at the processing plant Agrumaria Corleone (Palermo), were subjected to
laboratory analysis, in order to determine their main chemical properties.
The statistical data about the amounts of the by-product of Citrus processing industry were
evaluated, in order to compute the Sicilian potential biogas and energy production.
The potential biogas production from CW (BCW) was determined according to the following Eq.
(2.1):
   (2.1)
where: bCW is the specific biogas yield of CW mass unit, m3 t-1;
mCW is the CW mass, t
The yearly electric energy potential production from CW EeCW was determined according to the
following Eq. (2.2):
   (2.2)
where eeB is the electric value of biogas, depending on the concentration of methane in biogas,
kWh m-3
The yearly thermal energy potential production from CW EtCW was determined according to the
following Eq. (2.3):
   (2.3)
where etb is the thermal value of biogas, depending on the concentration of methane in biogas,
kWh m-3
The yearly biomethane potential production from CW MCW was determined according to the
following Eq. (2.4):
   (2.4)
where mCW is the biomethane content inside biogas volume for CW, % (Comparetti et al., 2012,
2013).
3. Results and Discussion
Citrus area, also compared with Used Agricultural Area (UAA), the production of Citrus fruits,
processed fruits and CW in Sicily are shown in Table 3.1.

11th International AIIA Conference: July 5-8, 2016 Bari - Italy
“Biosystems Engineering addressing the human challenges of the 21st century”
171
Table 3.1: Citrus area (CA), also compared with Used Agricultural Area (UAA), production of
Citrus fruits (CF), processed fruits (CPF) and CW in the nine provinces of Sicily
Provinces
CA
(ha)
CA / UAA
(%)
CF
(t)
CPF
(t)
CW
(t)
Agrigento
3394
2.24
61,092
-
-
Caltanissetta
214
0.18
3852
-
-
Catania
29,425
17.38
529,650
211,860
127,116
Enna
2981
1.63
53,658
21,463
12,878
Messina
5242
3.23
94,356
37,742
22,645
Palermo
3801
1.43
68,418
27,367
16,420
Ragusa
2518
2.77
45,324
18,130
10,878
Syracuse
21,033
18.92
378,594
151,438
90,863
Trapani
1062
0.77
19,116
-
-
Total
69,670
2.24
1,254,060
468,000
280,800
The areas cultivated with Citrus species in the various municipal districts compared with Used
Agricultural Area (%), the processing plants and the three existing AD plants are shown in
Figure 3.1.
Figure 3.1: Sicily map showing the ratio between the areas cultivated with Citrus species in the
various municipal districts and Used Agricultural Area (%), the processing plants and the three
existing AD plants.
The results of laboratory analysis of CW samples are: dry matter of 10.26%; ash/dry matter of
4.96%; biogas yield of 45.65 m3 t-1; biomethane content inside biogas volume of 48%.
By using the above formulas it was foreseen that in Sicily it is possible to obtain, through the
use of “pastazzo”, a yearly production of 12,916,800 m3 ca. of biogas, equal to 24,250,930 kWh
of electric energy and 25,463,477 kWh of thermal energy or 6,200,064 m3 of biomethane. This
high potential biogas and energy production is not currently used in a sustainable way from the
energetic and economic points of view, because the existing AD plants are farer than 30 km
from Citrus processing plants.

11th International AIIA Conference: July 5-8, 2016 Bari - Italy
“Biosystems Engineering addressing the human challenges of the 21st century”
172
4. Conclusion
Italian legislation, with the Legislative Decree no. 205/2010, clearly defines when a residue has
to be considered waste or by-product (Cerruto et al., 2016). On this basis the Regional
Department of Agricultural and Food Resources of Sicilian Region issued the document n°
14843 of 01/03/2012, that defined CW as a by-product rather than a waste and indicated the
following uses for it:
• agronomical, as soil organic fertiliser or raw material for compost production;
• energetic, for bioethanol (biofuel) production or as raw material for biogas production;
• human food, for the production of fibres;
• fresh or ensiled or dried feed for livestock (Valenti et al., 2016);
• industrial, for the extraction of pectin (used as jelly in the food industry producing, above all,
marmalade and jams).
Yet, the agronomical, human food, livestock feed and industrial uses of CW were not able to
consume the high amounts of this by-product produced in Sicily (Interlandi, 2013). Therefore,
CW can be considered as a resource rather than a by-product, as it can be used for producing
biogas and digestate through AD process. This use contributes to minimise its environmental
impact and also allows its energy valorisation. Furthermore, CW is characterised by an optimum
attitude to AD process (due to its content of sugars), the above written plentiful availability in
Sicily and a low cost of biomass unit (€ 10 t-1) and, therefore, of biogas unit that can be
produced from it (€ 0.11 m-3) (Interlandi, 2013). The CW produced in Sicily could be used
inside a bio-reactor, together with other raw materials (e.g. pomace and wastewater from olive
oil mills, cereal straw, poultry manure and Italian sainfoin or Hedysarum coronarium), for AD
process.
Yet, the disadvantages of using this by-product for AD process are its seasonal availability
(from December to June), the difficult storage (due to its high water content and high
fermentation attitude), the rapid acidification, inhibiting the activity of bacteria producing
methane, and the presence of D-limonene in the peels and water, both inhibiting the process.
Therefore, before AD process, CW must be subjected to a treatment aimed at removing
limonene (Interlandi, 2013).
The use of “pastazzo” could be a potential solution to the problem of disposal of organic
materials, caused by its high cost (€ 30 t-1) and a significant lack of suitable landfills. Due to
these problems some food processing companies illegally disposed the CW (Last Orange
operation by police).
Therefore, this work suggests the need for a territorial plan where new AD plants must be
located nearer than 30 km to Citrus processing plants, in order to reduce the transportation cost
of CW and the environmental impact of its energy valorisation process.
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“Biosystems Engineering addressing the human challenges of the 21st century”
173
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