Evaluation of bioenergy potential from citrus effluents through anaerobic digestion

Journal Pre-proof EVALUATION OF BIOENERGY POTENTIAL FROM CITRUS EFFLUENTS THROUGH ANAEROBIC DIGESTION

E.S. Rosas-Mendoza, J.M. Méndez-Contreras, A.A. Aguilar-Laserre, N.A. VallejoCantú, A. Alvarado-Lassman PII:

S0959-6526(20)30175-X

DOI:

https://doi.org/10.1016/j.jclepro.2020.120128

Reference:

JCLP 120128

To appear in:

Journal of Cleaner Production

Received Date:

03 April 2019

Accepted Date:

11 January 2020

Please cite this article as: E.S. Rosas-Mendoza, J.M. Méndez-Contreras, A.A. Aguilar-Laserre, N. A. Vallejo-Cantú, A. Alvarado-Lassman, EVALUATION OF BIOENERGY POTENTIAL FROM CITRUS EFFLUENTS THROUGH ANAEROBIC DIGESTION, Journal of Cleaner Production (2020), https://doi.org/10.1016/j.jclepro.2020.120128

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof Graphical Abstract

Journal Pre-proof EVALUATION OF BIOENERGY POTENTIAL FROM CITRUS EFFLUENTS THROUGH ANAEROBIC DIGESTION

Rosas-Mendoza E.S.a, Méndez-Contreras J.M.b, Aguilar-Laserre A.A.b, Vallejo-Cantú N.A.b and AlvaradoLassman A.b* aCONACYT-Instituto Tecnológico de Orizaba Av. Oriente 9, 852. Col. Emiliano Zapata, C.P. 94320 Orizaba, México. bDivisión de Estudios de Posgrado e Investigación, Instituto Tecnológico de Orizaba, Av. Oriente 9, 852. Col. Emiliano Zapata, C.P. 94320 Orizaba, México. *Corresponding author. E-mail: [email protected] Tel. 01-272-72-5-70-56, Fax 72-5-70-56

Abstract: At the global level, citrus production is around 92 million tons and citrus processing is about 22 million tons generating 28 million m3 of Citrus Effluents. In the last two decades, the generation of wastewater by the citrus industry has increased, and since earlier studies have focused mostly on solid citrus waste, there is inadequate information related to the potential energy value of these effluents. The main contribution of this work is the development of a methodology for conducting an energy analysis to determine the amount of electrical energy that can be derived from the biogas produced in the anaerobic digestion of citrus effluents at the global level. This determination was made through the operation of a novel, high-rate reactor, called the Anaerobic Hybrid Reactor. The aim of this paper was to evaluate the bioenergy potential of the effluents from the citrus industry by using anaerobic digestion to generate electrical energy. As part of the results of this evaluation, it was found that one ton of processed oranges generates 1.25 m3 of citrus effluents, 0.72 m3 of methane at standard temperature and pressure, 7.16 kWh of gross electricity, and 2.15 kWh of net electricity, which represents a cost of 0.17 USD. The worldwide bioenergy potential from citrus effluents by the main citrus processing countries is approximately 16 million m3 of methane/year at standard temperature and pressure, which is equivalent to approximately 159 GWh of gross electricity per year, about 48 GWh of net electricity per year, and about 111 GWh of thermal energy per year. The biogas that is produced can replace as much as 9% of the consumption of fuel oil and natural gas used in a citrus processing plant. Anaerobic digestion is a clean, simple energy technology, and it helps to minimize the social, environmental, and economic problems caused by poor management and the lack of alternatives for the treatment of the effluents from the citrus industry. Keywords: Bioenergy potential; anaerobic digestion; citrus industry; citrus effluents; biogas; methane.

1

Journal Pre-proof 1.

Introduction

The cultivation of citrus fruits is an important activity in the agricultural sectors of many countries (Zema at al., 2018). More citrus fruits are consumed than any other fruits throughout the world (Satari and Karimi, 2018), and their global production in 2017/18 was approximately 92 million tons. Oranges accounted for 52% of this production, followed by tangerines/mandarins at 33%, lemons/limes at 8%, and grapefruits at 7% (USDA FAS, 2018). According to the official data, oranges are one of the most cultivated fruits in the world (Calabrò and Panzera, 2018), and the main producing countries are Brazil (16%), China (7.3%), the European Union (6.4%), Mexico (4.6%), and the United States (3.5%) (USDA FAS, 2018). China also is the main producer of tangerines/mandarins with 21.2 million tons and grapefruits with 4.8 million tons, while Mexico is the major producer of lemons/limes at 2.6 million tons (USDA FAS, 2018). There are at least three existing markets for citrus fruits, i.e., marketing as fresh fruits, processing to make other products, and exportation to other countries. The citrus processing industry has experienced accelerated growth, and, on an annual basis, the current amounts being processed are about 18 million tons of oranges, 2.0 million tons of lemons/limes, 1.4 million tons of tangerines/mandarins, and 0.5 million ton of grapefruits. Various products are obtained from citrus processing, e.g., processing one ton of oranges can produce 90 kg of juice at 65 °Bx (USDA FAS, 2018) and 5.35 kg of essential oil contained inside the peelings, 73.9–97% of which is Dlimonene (Satari and Karimi, 2018). Other products include marmalades, jellies, potpourris, candied peel, jams, flavoring agents for beverages, health drinks, and essences used as food-grade products (Kimball, 1999; Sharma et al., 2017; Sharma et al., 2018). Solid and liquid wastes also are generated during citrus processing. Processing oranges generates large amounts of residues, amounting to approximately 8 to 20 million tons per year worldwide (Rezzadori et al., 2012). Processing one ton of oranges can produce 0.5–0.6 ton of solid citrus waste (SCW), 0 to 9% of a ton of seeds, 60 to 75% of a ton of peels, 23 to 33% of a ton of membrane residues (Zema at al., 2018; Calabrò and Panzera, 2018), and 0.79 to 1.25 m3 of citrus effluents (CEs) (Rosas-Mendoza et al., 2018), 90–95% of which is wastewater from the processing of oranges and 5–10% of which is wastewater derived from pressing the orange peels. SCW is characterized by a water content of higher than 80%, and it is acidic (pH 3–5) due to the presence of organic acids and D-limonene (Satari and Karimi, 2018). The CE is characterized by a high content of organic matter, i.e., the total chemical oxygen demand (CODT) is in the range of 36,200–41,200 mg/L and the soluble chemical oxygen demand (CODS)is in the range of 34,100–36,700 mg/L; it has a low pH (3.78–3.98) (RosasMendoza et al., 2018). 2

Journal Pre-proof The large amounts of citrus processing residues that are generated and their peculiar characteristics create considerable constraints for their management due to both economic and environmental factors (Calabrò et al., 2016). In addition, the citrus industry creates serious social problems due to the generation of bad odors and the attraction of animal vectors associated with the residues that are produced. Consequently, this industry has incurred economic problems, including government penalties, because of its negative impacts on the soil and on bodies of water (Rosas-Mendoza et al., 2018). The impacts of the pollution resulting from solid and liquid citrus waste can be compared to the impacts caused by the use of agrochemical products. Until relatively recently, there has been no satisfactory means of disposal other than dumping the waste on the land adjacent to the production sites (Martín et al., 2010). Currently, traditional strategies for the disposal of citrus waste, e.g., incineration and landfilling, are insufficient and problematic in terms of environmental impacts and energy usage (Satari and Karimi, 2018; Wei et al., 2017). In addition, there are several current approaches for treating waste materials that are rich in lignocellulosic compounds, e.g., 1) the use of pyrolysis to convert fermentation residues into biochar, 2) water treatment, and 3) many others (Maroušek, 2014) and some others, such as the use of Nitrifier-enriched Activated Sludge (NAS) to improve the performance of wastewater treatment based on the dominance of the autotrophic microbial population in activated sludge (Sepehri and Sarrafzadeh, 2018) and also, techniques to enhance the growing of the specific biomass in biological processes have been developed, by the Nitrite-Oxidizing Bacteria (NOB) enrichment and nitratation intensification strategy through zero carbon/nitrogen ratio are able to reduce remarkably microbial metabolites 50% lower than conventional process and enhance nitrification efficiency in activated sludge involved processes (Sepehri and Sarrafzadeh, 2019). However, citrus wastes have physicochemical properties that allow them to be treated by anaerobic digestion (AD) to reduce their organic loads and obtain a product with added value, such as biogas. Further, the AD infrastructure is fundamental in mitigating agricultural emissions, and agrifood waste and by-product treatment (Selvaggi et al., 2018).

Anaerobic digestion can be defined as the biological conversion of organic matter (carbohydrates, lipids, and proteins) to a variety of products, including biogas (Olthof and Oleszkiewick, 1982; Gujer and Zehnder, 1983; Speece, 1983; Alvarado-Lassman et al., 2010; Nutiu, 2014), and it involves a series of metabolic reactions, such as hydrolysis, acidogenesis, and methanogenesis (Themelis and Ulloa, 2007). Thus, anaerobic digestion can be used to decrease environmental pollution (Khalid et al., 2011) and provide biogas as a renewable energy resource. Thus, it is a viable and attractive alternative for the treatment of citrus residues (Kaparaju and Rintala, 2006). In order to properly conduct the anaerobic digestion process, various parameters must be carefully controlled, including temperature, pH, the concentration of the substrate, the concentration of nutrients, and the concentration of toxic compounds, such as D-limonene (García-Gonzalo, 2013). 3

Journal Pre-proof

The composition of biogas depends on the types of organic matter in the waste, but, generally, biogas is composed of methane (48–75%), carbon dioxide (25–50%), nitrogen (17%), oxygen (<1%), hydrogen sulfide (32–169 ppm), and traces of other gases (Surendra et al., 2014; Ward et al., 2008). This is one of the main reasons that anaerobic digestion may be crucially important in meeting the energy challenges of future generations (Khalid et al., 2011). Sustainable management in the biogas production via anaerobic digestion process intents the use of alternative biomass sources (Bedoić et al., 2019). The development and implementation of renewable energy systems as part of the global solution to increased energy consumption have been considered as an emerging area for the exploration of alternatives to traditional energy systems, which are based on limited fossil fuel resources and cause problems, such as degradation of the environment (Balaman and Selim, 2014). In this sense, the production of bioenergy contributes to the construction of the bio-economy, playing a significant role in the development of a low-emission economy, owing to the lower carbon footprint of bio-based products (Chinnici et al., 2018). Biogas has great potential for various applications, such as heating, combined heat and electricity (Prussi et al., 2019), the improvement of the quality of transport fuel, and the replacement of natural gas for various uses. However, in developing countries, biogas is obtained mainly from digesters at the household level, and the end uses of biogas generally are limited to the kitchen and for lighting (Surendra et al., 2014). Pure methane has a heating power of 9.94 kWh/m3 at standard temperature and pressure (STP) (Muylaert et al., 2000), but the heating power of biogas ranges from about 5.2 to about 6.2 kWh/m3 at standard temperature and pressure, which is much lower than that of methane due to the dilution of the biogas with other products obtained from anaerobic digestion (Gerardi, 2003). Researchers have explored different options for obtaining biogas that is rich in methane from the anaerobic digestion of various wastes, e.g., using whey with cow dung (Rico et al., 2015) and apple waste with pig manure (Kafle and Kim, 2013). In the last few decades, several studies have been conducted with the goal of solving the problems caused by citrus residues, but these studies have focused mostly on SCW and have not considered CEs. Among the most recent researchers who have conducted investigations related to SCW are 1) Raimondo et al. (2018), who mentioned that the return from citrus waste (known in Italy as pastazzo) negatively influences the transformation of the orange by-products into pectin and feedstuff, but it positively influences the biogas way of valorization, 2) Calabrò and Panzera (2018), who reported the bio-methane production of orange peel waste ensiled for 37 days and evaluated at the laboratory scale with a value of 365 NmL CH4/gVS, 3) Zema et al. (2018), who used a pilot plant with semi-continuous feeding and found that, at thermophilic conditions, the cumulative methane production of industrial orange peels was 0.12 L/gTVS, i.e., about 25% of that at mesophilic conditions (0.46 L/gTVS), 4) Siles et al. (2016), who indicated that the biomethanisation of orange peels produced 29.2 ± 1.5 4

Journal Pre-proof LSTPCH4/kg of pre-treated waste orange peel, and 5) Calabrò et al. (2018), who reported that the highest BMP30 (Biochemical Methane Potential) values were recorded in the raw orange peel waste, resulting in methane production close to 500 NmL CH4/gVS. Even in the review papers, 1) Satari and Karimi (2018), described different values of biogas production using orange peel waste, citrus waste, orange pulp, and orange seeds as substrates, and 2) Negro et al. (2016) presented biological methane production values using citrus peelings, i.e., orange, lemon, and mandarin peelings. Conversely, there have been few investigations of citrus effluents, and, in the investigations that have been conducted, conventional reactors were used that had long hydraulic retention times (HRTs) and low organic loading rates (OLRs). In the literature review, only three manuscripts related to the anaerobic digestion of citrus effluents were found. Valdés et al. (1994), conducted the anaerobic digestion of diluted orange juice using a laboratory fixed-bed upflow reactor, that was operated at different OLRs of 0.312–4.175 gCOD/Ld for 1 to 4 days respectively and obtained about 77.6 to 97.2% COD removal with 0.3– 0.381 LCH4 at STP/gCODrem presenting a drastic increase in volatile fatty acids by increasing the organic load above 4 gCOD/Ld, which the authors attribute to the inhibitory effect of D-limonene on methanogenic bacteria; Martín et al. (2010) used a laboratory completely stirred tank as an anaerobic batch reactor operated at a mesophilic temperature of 37 °C to study the anaerobic digestion of pretreated wastewater from the pressing of orange peels with nutrients addition. The batch experiments lasted 48–72 h and the organic load was increased from 1.5 to 5 gCOD/Ld, resulting in 84% COD removal with 0.297 LCH4 at STP/gCODrem; afterwards, Koppar and Pullamimanappallil (2013) used a down-fixed, stationary film reactor fed with wastewater from the pressing of orange peels under thermophilic conditions of 55 °C, HRT of 16 days and a maximum OLR of 0.51 gCOD/Ld, with an average methane yield of 0.238 LCH4 at STP/gCODrem. In contrast to the previous works, in a recently published paper (Rosas-Mendoza et al., 2018) the use of a novel continuous biofilm Anaerobic Hybrid Reactor (AHR), allowed the use of high OLRs of 8-10 gCODT/Ld with HRT of less than one day. The COD removal was 75-85% with an average methane yield of 0.15 LCH4 at STP/gCODrem which is the basis for the calculation of the methane available for its use.

As previously mentioned, there is a knowledge gap related to the energy valorization of citrus industry effluents since past studies have focused mostly on SCW and have not considered CEs, despite the fact that it is estimated an amount close to 28 million cubic meters of citrus wastewater produced worldwide. The contributions and novelty of the present study can be summarized as follows: 1) The presentation of a viable alternative for the treatment and valorization of the effluents of the citrus industry in the form of biogas by means of anaerobic digestion, using real data at an industrial level. 5

Journal Pre-proof 2) Previous investigations have used conventional anaerobic reactors operated with low organic loading rates and long hydraulic retention times. The present study contributes to the new body of knowledge by employing a novel, high-rate anaerobic reactor, called the Anaerobic Hybrid Reactor (Rosas-Mendoza et al., 2018) to perform an even more detailed energy analysis highlighting the inhibition effects of Dlimonene, since at industrial-scale there are traces of this essential oil in the wastewater, and due to the high volumes handled its complete elimination is not economically feasible. The methane yield obtained with the inhibition of D-limonene allows a more real energy analysis compared to other investigations. 3) In the existing literature, problems related to orange residues have been addressed, but there is missing information about other important citrus fruits at the global level. This study in addition provides important information about orange effluents, adds to the state-of-the-art data on wastewater from lemons/limes, grapefruit and tangerines/mandarins as a source of biogas. 4) Facing the new challenges and demands of energy at the global level, this work shows the calculation and application of electric power generation from biogas produced in the anaerobic digestion of citrus effluents to a case study compared with fossil fuels such as fuel oil and natural gas. For these reasons, the main aim of this paper was to evaluate the bioenergy potential of the effluents from the citrus industry by using anaerobic digestion for the generation of electric energy. For this, the specific aims are: 1) to analyze the treatment and use of effluents from the citrus industry at the global level by anaerobic digestion for biogas production as a source of renewable energy through a case study, 2) to compare the energy generated from renewable fuels as biomass and biogas with the energy produced from fossil fuels, such as fuel oil and natural gas and 3) to present an alternative for the use of citrus effluents as a renewable energy source.

2.

Methodology

A three-stage methodology was established to evaluate the bioenergy potential from citrus effluents, and they are explained below. Stage 1. Case Study In order to accomplish the goal of the study, it was necessary to determine the technical details associated with the operation of the plant, such as its processing capacity, the demand for the products that will be produced, the volume of effluents and the mass the solid citrus waste generated in the plant, and the consumption of energy required to operate the plant, i.e., the amounts of electricity, fuel oil, and natural gas required.

6

Journal Pre-proof The largest citrus processing plant in Mexico, which is located in the State of Veracruz, was selected as a case study. Fig.1 shows that, in recent years, the processing of oranges has increased in Mexico, and, currently, about 1.7 million tons/year are being processed, making Mexico the third-largest orange-processing country in the world. Fig. 1. Oranges processed in Mexico from 1999 to 2018 (USDA FAS, 2018).

The citrus processing plant can process approximately 230,000 tons of oranges per year, which amounts to about 1,850 tons per day when oranges are in season, generating 26.8 L/s of citrus effluent, which consists of a combination of the wastewater from processing the oranges (90–95%) and wastewater derived from pressing the orange peels (5–10%), as is shown in detail in Fig. 2. The volume of CEs produced per ton of oranges processed is in the range of 0.79 to 1.25 m3. Fig. 2. Scheme of the orange processing plant for obtaining products and citrus effluents (SAGARPA, 2018; Juguera Allende, 2018; IQCitrus, 2018; Citrofrut, 2018a; Citrofrut, 2018b).

The wastewater generated is proportional to the production and demand of products in the market, and, for this reason, three possible scenarios can be presented for the generation of CEs, i.e., 1) processing 880 tons of oranges/day and generating 12.8 L/s of CEs; 2) processing 1,850 tons of oranges/day and generating 26.8 L/s of CEs, and 3) processing 2,760 tons of oranges/day and generating 40.0 L/s of CEs. All of these citrus effluents can be treated continuously using a novel, high-rate, anaerobic biofilm reactor by anaerobic digestion to produce methane-rich biogas with a hydraulic retention time of less than one day. Stage 2. Evaluation of bioenergy potential using biogas It was necessary to explore the scientific and engineering aspects of the process since biogas is produced in an anaerobic digestion process. The scientific aspects that were considered included the organic loading rate, concentration of the substrate, operating temperature during the AD, removal of organic matter, methane yield, and the potential presence of inhibitors. It was also considered the heating power of methane, the conversion efficiency of the electricity generator, and the price of electricity. They were obtained response variables, including the volume of methane generated by AD and the gross electricity, i.e., net electricity and thermal energy, in addition to the cost of electricity measured as savings.

7

Journal Pre-proof The CEs were characterized using analytical methods, and the results reported by Rosas-Mendoza et al. (2018) are provided in Table 1. They used a device called an Anaerobic Hybrid Reactor for the treatment of the citrus effluents because it was a viable alternative that could be implemented at the industrial level to generate and take advantage of a renewable energy source, such as biogas. Table 1 Characterization of the citrus effluents.

The bioenergy potential was calculated using Equations (1), (2), and (3), and its purpose was to perform an electrical energy and cost analysis. The methodology for the estimates is shown below. Eq. (1) was used to estimate the volume of methane generated by the AD of the citrus effluent. 𝑀𝑒𝑡ℎ𝑎𝑛𝑒 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑏𝑦 𝐴𝐷 = (𝐹𝑒𝑒𝑑𝑖𝑛𝑔 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒)(𝐶𝑂𝐷𝑇 𝑓𝑒𝑒𝑑𝑖𝑛𝑔)(𝐶𝑂𝐷𝑇 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦)(0.9)( (1) 𝑌𝑚𝑒𝑡ℎ𝑎𝑛𝑒)(8.64𝑥10 ―4) According to Rosas-Mendoza et al. (2018), the following conditions were assumed: 1. The AHR was operated in a continuous mode at a high organic loading rate between 8 and 10 gCODT/Ld. The anaerobic digestion process was conducted at mesophilic conditions of 35 ± 2 °C with the pH of the influent regulated between 6.8 and 7.2 with sodium bicarbonate and the substrate (CEs) was supplemented with macronutrients and micronutrients with ratio COD/N/P of 350/7/1. The citrus processing plant discards its effluents at approximately 40 °C, so applying a treatment under mesophilic conditions is more appropriate than using thermophilic conditions. 2. Feeding flow rate: 12.8, 26.8, and 40.0 L/s of wastewater from the citrus processing plant. 3. CODT feeding: 38,780 mg/L, according to Table 1. However, for reasons of the organic loading rate, it is recommended that 5,000 mg/L be used as the maximum concentration of organic matter. 4. COD removal efficiency: 85%. This removal efficiency was reached after 20 days of operation of the AHR with OLR = 8 gCODT/Ld, since the bacterial florae were affected by the handling of loads higher than those mentioned earlier. 5. 0.9 represents the theoretical fraction of COD removed for obtaining biogas.

8

Journal Pre-proof 6. Ymethane: methane yield 0.15 LCH4 to STP/gCODrem. An inhibitory effect was observed by the presence of D-limonene in the citrus effluent since no pretreatment was used to eliminate this essential oil, and its presence had the effect of decreasing both the removal of COD and the yields of methane. 7. 8.64x10-4 is a conversion factor, its function is to convert the units of the variables that were previously mentioned to m3CH4 at STP/d. The volume of Methane generated by AD was obtained at standard temperature and pressure, and it was expressed as m3CH4 at STP/d. Eq. (2) was used to calculate the net electricity obtained (kWh/d) from the methane that was generated. 𝑁𝑒𝑡 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 = (𝑀𝑒𝑡ℎ𝑎𝑛𝑒 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑏𝑦 𝐴𝐷)(𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟 𝑜𝑓 𝑚𝑒𝑡ℎ𝑎𝑛𝑒)(𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦) (2) where: Methane generated by AD was obtained by Eq. 1. Heating power of methane is 9.94 kWh/m3 at STP (Muylaert et al., 2000). Conversion efficiency for a commercial biogas electric generator is 30% (Rosas-Mendoza et al., 2018), while the other 70% represents the thermal energy. Eq. (3) was used to calculate the cost of the net electricity (USD/d) obtained from the methane generated in the anaerobic digestion. 𝐶𝑜𝑠𝑡 = (𝑁𝑒𝑡 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦)(𝑇𝑎𝑟𝑖𝑓𝑓 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦)

(3)

where: Net Electricity was obtained by Eq. 2. The tariff of electricity in Mexico is around 0.08 USD/kWh (CFE, 2018). In order to present an evaluation of bioenergy potential at the global level using the biogas that was generated from the citrus effluent under the considerations shown in the case study and similar to the previous stage, the main citrus processing countries according to USDA FAS (2018) were included.

9

Journal Pre-proof It was assumed that the wastewater from the citrus processing plants (orange, lemons/limes, grapefruit and tangerines/mandarins) has similar physicochemical characteristics, since citrus peel waste is rich in fermentable sugars, such as glucose, fructose, and sucrose, as well as insoluble polysaccharides, such as cellulose, hemicellulose, and 0.8–1.6% D-limonene (Choi et al., 2013). It also assumed that the contents of the total soluble and insoluble dietary fibers in the citrus fruit peels were not significantly different (p > 0.05) (Wang et al., 2015). Stage 3. Comparison of renewable energy sources with fossil fuels The citrus processing plant that was used as a case study had a section that was called the “biomass generation plant,” and this plant converted solid citrus waste to biomass (≤ 30% humidity). Thus, it was compared the use of fossil fuels with the use of renewable energy sources, which were the biogas generated by AD from the citrus effluents and the biomass produced in the biomass generation plant. The fossil fuels that considered were fuel oil and natural gas. 3.

Results and discussion

The results obtained in this research are divided into two parts, i.e., one part that shows the findings of the evaluation of the bioenergy potential of the effluents generated in the citrus industry at the global level and another part that compares renewable energy sources with fossil fuels. Evaluation of the bioenergy potential using biogas The citrus processing plant requires 67,000 kWh of electricity per day to operate, of which 49,000 kWh are used in orange processing to obtain products, such as fresh juices, concentrated juices, flavorings, oils, and dry orange peel for animal consumption. The other 18,000 kWh are used to operate the plant that generates biomass from orange peel (or Solid Citrus Waste). The use of this electricity costs 5,360 USD per day. Fig. 3. Diagram to obtain and use electrical energy from the anaerobic digestion of the citrus effluent.

According to Eq. 1, it is possible to obtain approximately 635, 1,329, and 1,983 m3 of methane at STP per day for scenario 1, 2, and 3, respectively. These values were calculated with a methane yield of 0.15 LCH4 to STP/gCODrem due to the inhibition effects caused by the presence of D-limonene in the CEs, which is lower than the theoretical value of 0.35 LCH4 to STP/gCODrem (Michaud et al., 2002). Eq. 2 indicates that 6,307, 13,206, and 19,710 kWh per day could be produced as bioenergy potential (net electricity + thermal energy), resulting in 1,892, 3,962, and 5,913 kWh of net electricity per day for each respective scenario, with the 10

Journal Pre-proof difference being thermal energy. Fig. 3 shows the complete process for the generation of electric power. In each scenario, the net electricity is available for use in the citrus processing plant, and it enough to meet 2.8, 5.9, or 8.8% of the total demand per day (67,000 kWh). Eq. 3 is used to estimate the cost of net available electricity that could be saved, and the results were approximately 151, 317, and 473 USD per day, respectively, and the energy produced by a renewable energy source, such as biogas, can be used instead of consuming electricity from the Federal Electricity Commission, which is the national electricity supply company and provides electric power to the citrus processing plant. The results obtained in scenario 1 can be compared with the findings of Koppar and Pullamimanappallil (2013) when they performed an energy analysis based on using the orange peel waste and wastewater of a citrus plant with a processing capacity of 600 tons of oranges per day. They based their analysis on a campaign period of 270 days/year, and the details of the analysis are presented in Table 2. They reported a net electricity value that was almost four times greater than obtained in this paper, which was due to the fact that their methane yield was 1.6 times higher than these results because the anaerobic digestion process was conducted at the thermophilic temperature of 55 °C with an HRT of 16 days and without having observed inhibition in methanogenesis. The experiment of Rosas-Mendoza et al. (2018) was performed at the mesophilic temperature of 35 °C, with an HRT of less than one day, and an inhibition process occurring in the methanogenic stage, and these conditions were more in line with the conditions of a full-scale plant. Table 2 Comparison of the energy analyses of citrus wastewater.

In Mexico, the orange season lasts approximately four months (124 days), and, depending on the amount of CE treated, as much as 78,681, 164,738, and 245,877 m3 of methane could be produced at STP. If this methane were used to produce electricity, 234,626, 491,248, and 733,206 kWh of net electricity could be produced, with savings of 18,770, 39,300, and 58,656 USD per year, respectively. These results represent a lower generation of electricity and lower savings than Koppar and Pullamimanappallil (2013) claimed, since, in one year, they estimated 1,989,818 kWh of net electricity produced from citrus wastewater, which meant a saving of 78,700 USD per year. In addition, their scenario assumed a campaign period of 270 days/year. It is very important to analyze the operational conditions of the anaerobic digestion process and to compare these calculations of the bioenergy potential to those of Koppar and Pullamimanappallil (2013) to understand the differences in the estimated values of electric energy. There has been extensive research, but most of it has been conducted using orange peel waste, either in continuous or semicontinuous modes. In anaerobic batch cultures, methane production rates of about 0.49 11

Journal Pre-proof m3/kgVS (added peel waste) have been reported, and, in semicontinuous anaerobic cultures, a loading of 2.8 kgVS/m3d and an HRT of 26 days generated a specific methane yield of 0.60 m3/kgVS (Siles et al., 2010). So, based on the previous evaluation of the bioenergy potential, it can be assumed that one ton of processed oranges generates 1.25 m3 of wastewater, 0.72 m3 of methane at STP, 7.16 kWh of gross electricity, and 2.15 kWh of net electricity (0.17 USD). Based on these estimates, in Mexico in 2017/18, 1.7 million tons of oranges, 0.40 million tons of lemons/limes, and 0.09 million tons of grapefruit were processed, and Figs. 4a, 4b, 4c show that this processing could generate 3.7, 0.9, and 0.19 GWh of net electricity per year, respectively. During 2016, in Mexico the National Energy Balance reported that 1.91 PJ (530.56 GWh) of energy were produced from biogas (SENER, 2016). Thus, the wastewater from the Mexican citrus industry treated by anaerobic digestion could be used to generate more than 0.06 PJ (15.66 GWh) of gross energy per year, which would be 3.0% of the national energy production. However, making an evaluation of the bioenergy potential at the global level Fig. 4 shows the energy analysis of wastewater from the processing of oranges, lemons/limes, grapefruit, and tangerines/mandarins. To complete the analysis, it is necessary to consider the energy tariffs to obtain the savings for electricity consumption, so the tariff for electricity (i.e., USD/KWh) for each country is as follows: Argentina (0.01), Brazil (0.13), China (0.08), European Union (0.25), Israel (0.15), Japan (0.22), Mexico (0.08), South Africa (0.09), and the United States (0.13) (Statista, 2018). In combination, these nine countries provide 95, 99, 99, and 90% of the global processing of oranges, lemons/limes, grapefruit, and tangerines/mandarins, respectively. Fig. 4. Bioenergy potential of the main countries processing a) oranges, b) lemons/limes, c) grapefruit, and, d) tangerines/mandarins.

Fig. 4a shows that Brazil processes more oranges than any other country, i.e., about 12 million tons per year, and it is followed by the United States, Mexico, the European Union, and China. Thus, Brazil has the greatest bioenergy potential with values of methane (about 9 million m3 at STP/year), gross electricity (about 88 GWh/year), and net electricity (about 26 GWh/year). Other researchers, such as Silva et al. (2018), evaluated the potential energy in Brazil by the utilization of biogas energy produced from the bio-digestion of seven types of organic wastes, and they determined that the potential power was between 4.5 and 6.9 GWh, which means that the CEs are a great bioenergy source for Brazil, contributing about four times more than other types of organic waste. The production rates of bioenergy found in the present work provide Brazil with annual savings of about 3.5 million USD. Observing Fig. 4a, net electricity cost is the variable that has the smallest difference between countries because it depends on the local tariff. For example, in the United States, generating about 5 12

Journal Pre-proof GWh per year saves about 0.64 million USD, whereas, in the European Union, generating about 2.8 GWh per year saves about 0.70 million USD, this means that the United States produces twice as much energy as the European Union for the same cost. Evaluating the bioenergy potential from effluents of lemons/limes, grapefruit, and tangerines/mandarins, it was obtained that: Argentina, which is the global leader in processing lemons/limes with about 1 million tons per year, can obtain 0.78 million m3 of methane at STP/year, about 7.7 GWh/year of gross electricity, and about 2.3 GWh/year of net electricity, resulting in annual savings of about 0.02 million USD, which represents similar savings as South Africa, which has a lower processing capacity than Argentina, as show in the Fig. 4b. Other important countries in the processing of lemons/limes are Mexico, the European Union, and the United States. Fig. 4c shows that, for grapefruit processing, the most important country is the United States, and it is followed by South Africa, Mexico, Israel, and the European Union. The United States is the major methane and electricity producer, with 0.14 million m3 of methane/year and 0.43 GWh/year of net electricity. The processing of tangerines/mandarins has a behavior similar to that presented by the other citrus fruits, and Fig. 4d shows the energy analysis in detail. Table 3 shows in detail the production of bioenergy from citrus effluents, classified in six regions (North America, South America, Europe, South Africa, Western Asia, and Eastern Asia) according to the geographical positions of the main citrus processing countries that were described previously. Yang et al. (2019) stated that the potential annual biogas production from restaurant food waste was estimated at 4,209 million m3 in six regions of China (Northwest, Northeast, North, Southwest, Central-South and East). In the present work, it was estimated that, in China (Eastern Asia), 0.87 million m3/year at STP can be obtained from CEs, which is a much lower value (measured as biogas) than was obtained from available food waste from restaurants. In the European Union, Scarlat et al. (2018) estimated the theoretical biogas potential of manure to be 23 billion m3 of biomethane, and the realistic biogas potential, including collectible manure, was assessed at 16 billion m3 of biomethane. In Europe, biogas plants can be to achieve a total installed capacity between 6.2 and 7.2 GW of electricity. Applying the proposed equations, the bioenergy potential from CEs in the European Union is 1.35 million m3/year at STP, about 13 GWh/year of gross electricity, and about 4 GWh/year of net electricity. The CEs have a bioenergy potential similar to the total installed capacity of the biogas plants in Europe. Mugodo et al. (2017) reported that, in South Africa, the liquor industry (clear beer and wineries) was found to have the highest potential for biogas production, i.e., about 35 million m3 per year, whereas pig farming had the least potential, i.e., about 0.02 million m3 per year. In total, an average of 86 million m3 of biogas can be produced from the sector’s industries (livestock farming, slaughterhouses, breweries, wineries, sugar mills, diary milk parlors, and fruit processing waste), which is equivalent to 148 GWh of electrical energy. It was estimated that

13

Journal Pre-proof about 0.52 GWh per year of net electricity can be obtained from the liquid residues of the citrus industry in South Africa. The production of methane from citrus effluent throughout the world is estimated to be about 16 million m3/year at STP, so it is possible to estimate a generation of about 159 GWh/year, of which about 48 GWh/year correspond to net electricity and about 111 GWh/year correspond to thermal energy. The global production of bioenergy from citrus waste can be compared with the production of other imported residues, such as coffee, and Chala et al. (2018) estimated that, in Ethiopia, the anaerobic digestion of husks, pulp, and mucilage could generate as much as 68 million m3 of methane per year, which could be converted to 238 GWh of electricity and 273 GWh of thermal energy in combined heat and power units. The generation in quantity and composition of citrus effluents can vary according to the operating conditions of the processing plant, climatic conditions, and geographical and technical considerations during the anaerobic digestion process, which would have an impact on its bioenergy potential. This is similar to what happens with citrus peel waste, since its methane yield depends on many factors, including pH, temperature, nutrient availability for microorganisms, and, secondarily, citrus cultivar (Zema et al., 2018). In this context, advocating for sustainable and affordable energy options, namely in the form of biogas, is required for continued development in these regions. Anaerobic digestion is a clean, simple energy technology, and it is less costly than comparable renewable technologies. Thus, for developing countries in particular, biogas has great potential in terms of available (and underexploited) feedstocks, the creation of jobs, the reduction of environmental impacts, and the provision of clean and reliable energy to improve the current quality of life (Surendra et al., 2014). Table 3 World production of methane, gross electricity, and net electricity from citrus effluent.

Comparison of renewable energy sources with fossil fuels In addition to the evaluation of the bioenergy potential made with biogas for the generation of electrical energy, other analyses were conducted to compare the energy generated from renewable fuels, such as biomass (from biomass generating plants) and biogas (from CEs), with the energy produced from fossil fuels, such as fuel oil and natural gas. The analyses are described below:

14

Journal Pre-proof Due to the demand for citrus products, during the high season, it is necessary to market about 200 tons of concentrated juice per day, for which 200 tons of steam is required under the conditions P = 0.01 MPa and T = 125 °C. This steam is produced by steam generators, which consume 18,000 L of fuel oil as an energy source, the calorific value of which is equivalent to 219,000 kWh; Fig. 5a shows this process. The price of fuel oil is 0.034 USD/kWh (PEMEX, 2018), so this would imply a total cost of 7,450 USD. The function of the biomass generation plant is to convert the SCW, approximately 48% w/w from citrus processing, to biomass (≤ 30% humidity). The biomass generation process consists of crushing, adding lime for dehydration, pressing to eliminate most of the water, and drying of the SCW. The biomass is used as an alternate energy source for the generation of steam, which is fed to the evaporators to obtain concentrated juice, as shown in Fig. 5b. In one season, about 1,300 tons of SCW is discarded from 2,760 tons of processed oranges (Fig. 3, scenario 3), thus obtaining approximately 600 tons of biomass, which can replace fuel oil for the generation of 200 tons of steam, as shown in Fig. 5b. This strategy of utilization the SCW converted to biomass has been used to avoid the costs of transporting the SCW and disposing it in landfills, which also eliminates the need to purchase a fossil fuel. The costs of transporting and disposing of the SCW are avoided, since these residues are handled within the citrus processing plant without having to transfer them to another site. This practice also was highlighted by Botta et al. (2003), since it is used mainly where the power stations are close to the biomass production sites to reduce transport and storage costs The previous idea is congruent, since citrus peel waste has a high calorific value (about 17 MJ/kg when dried), and it can be used directly via combustion for the production of energy (heat or electricity) (Siles et al., 2016). All this process continues within an increasing trend for energy production from crop and agro-food industry residues (Zema et al., 2018).

Fig. 5. Comparison of steam generation using different energy sources for concentrated juice production: a) fuel oil; b) solid citrus waste; c) natural gas or biogas.

Fig. 5c shows another alternative that was analyzed, i.e., the use of natural gas for the generation of steam. To obtain 219,000 kWh and 22,000 m3 of natural gas at STP would be necessary, and this would cost approximately 3,000 USD at the current price of 0.014 USD/kWh (CRE, 2018). Natural gas and biogas rich in methane have similar calorific values, so taking the 1,983 m3 of purified biogas at STP obtained in scenario 3 in Fig. 3, it can replace up to 9% the consumption of natural gas, which would save approximately 270 USD. Methane is the principal component of natural gas, and it can be produced anaerobically by methanogenic bacteria from a range of sustainable substrates. The process is well established and has been applied specifically for the generation of methane from waste orange peel (Siles et al., 2010). The increasing development of biogas production from 15

Journal Pre-proof organic waste and residues also suggests the future utilization of citrus peel waste, the methane potential of which is even higher than that of other crop residues (Zema at al., 2018); however; throughout this work, it has demonstrated that citrus effluents also have a high bioenergy potential. After completing a very thorough energy analysis, it was concluded that the citrus processing plant consumes a total energy of 286,000 kWh/day (35.4 GWh/year), of which 67,000 kWh correspond to electric power and 219,000 kWh correspond to fuel oil, natural gas, or biomass. This indicates that biogas can contribute up to 9% of the total. This total energy consumption, which is twice that reported by Koppar and Pullamimanappallil (2013), is equal to 58,650 GJ per year (16.3 GWh/year), electricity and fossil fuel, but it is a smaller plant that processes 162,000 tons of oranges per year.

4.

Conclusions

Processing one ton of oranges generates 1.25 m3 of wastewater, 0.72 m3 of methane at STP, 7.16 kWh of gross electricity, and 2.15 kWh of net electricity. These energy estimates were obtained from an experiment that was conducted is an anaerobic hybrid reactor at 35 °C, with HRT less than one day, using high OLR of 8–10 gCODT/Ld, obtaining 75–85% COD removal with 0.15 LCH4 at STP/gCODrem. No pretreatment was used for the elimination of D-limonene. Considering that the effluents from the processing citrus products, such as oranges, lemons/limes, grapefruit, and tangerines/mandarins have similar physicochemical characteristics, the world’s bioenergy potential of methane from anaerobic digestion is around 16 million m3/year at STP, which is equivalent to about 159 GWh/year of gross electricity with a conversion efficiency of 30%, obtaining about 48 GWh/year of net electricity and about 111 GWh/year of thermal energy. This bioenergy potential is contributed, in order of importance, by: Brazil, United States, Mexico, European Union, Argentina, China, South Africa, Japan and Israel; who also figure as the main citrus processing countries. Taking into account the above, it can be concluded that the bioenergy potential distribution for each continent is around 84% America, 9% Europe, 6% Asia and 1% Africa. Citrus effluents are an excellent source of bioenergy, the methane generated by anaerobic digestion can replace up to 9% of the consumption of the fossil fuels used in a citrus processing plant, i.e., such as fuel oil and natural gas. In economic terms, for a citrus processing plant this percentage represents annual savings of around 98,000 USD. The use of anaerobic digestion to generate a renewable energy source, such as biogas, helps to minimize 16

Journal Pre-proof the social, environmental, and economic problems caused by poor management and the lack of alternatives for the treatment of the effluents from the citrus industry. Through this work, it is shown that citrus effluents have an attractive bioenergy potential for scientific and industrial audiences, despite being a complex substrate due to the presence of its natural inhibitor (D-limonene), but with a proper treatment and adequate type of reactor and suitable conditions such as those established in this investigation, its potential can be maximized. In addition to all of the above, during the development of this research it was discovered that at the global level the necessary infrastructure to treat the effluents generated in the citrus industry by anaerobic digestion does not exist, which also limits the production and use of the biogas, that is one the biggest source of renewable energy. Finally, as part of future work and to improve this research, it would be necessary to carry out experiments with citrus effluents from lemons/limes, grapefruit and tangerines/mandarins, since, although it was mentioned that these effluents from citrus processing plants have similar physicochemical characteristics, the performance of the anaerobic hybrid reactor could have different values in terms of COD removals and methane yields during the consumption of the mentioned substrates. In addition, it is necessary to show a comprehensive evaluation of the bioenergy potential at the global level of the residues generated in the citrus processing plants, in which effluents and solid waste are included.

Acknowledgements Authors acknowledge the support provided by the Tecnológico Nacional de México (TecNM) and the Consejo Nacional de Ciencia y Tecnología (CONACYT) and the Secretaría de Energía (SENER).

References: Balaman, Ş., Selim, H., 2014. A network design model for biomass to energy supply chains with anaerobic digestion systems. Appl Energ. 130, 289–304. Alvarado-Lassman, A., Sandoval-Ramos, A., Flores-Altamirano, M.G., Vallejo-Cantu, N.A., MéndezContreras, J.M., 2010. Strategies for the startup of methanogenic inverse fluidized-bed reactors using colonized particles. Water Environ Res. 82, 387–391. Bedoić, R., Čuček, L., Ćosić, B., Krajnc, D., Smoljanić, G., Kravanja, Z., Ljubas, D., Pukšec, T., Duić, N., 2019. Green biomass to biogas – A study on anaerobic digestion of residue grass. J Clean Prod. 213, 700–709.

17

Journal Pre-proof Botta, G., Brignoli, V., Alberti, M., Riva, G., Scrosta, V., Toscano, G., 2003. Analisi delle iniziative per la produzione di energia elettrica da biomasse agro-industriali in Italia. In: Atti IV Convegno Nazionale ‘‘Utilizzazione Termica Dei Rifiuti” – Relazioni Tecniche. Abano Terme (PD), Italy, pp. 12–13, Giugno. Calabrò, P., Panzera, M., 2018. Anaerobic digestion of ensiled orange peel waste: Preliminary batch results. Therm Sci Eng Progr. 6, 355–360. Calabrò, P.S., Paone, E., Komilis, D., 2018. Strategies for the sustainable management of orange peel waste through anaerobic digestion. J Environ Manage. 212, 462–468. Calabrò, P.S., Pontoni, L., Porqueddu, I., Greco, R., Pirozzi, F., Malpei, F., 2016. Effect of the concentration of essential oil on orange peel waste biomethanization: Preliminary batch results. Waste Manage. 48, 440447. CFE

(Comisión

Federal

de

Electricidad).,

2018.

Tarifas

generales

en

media

tensión.

http://app.cfe.gob.mx/Aplicaciones/CCFE/Tarifas/Tarifas/Tarifas_industria.asp?Tarifa=CMAMF&Anio =2018 (Accessed 23 December 2018). Chala, B., Oechsner, H., Latif, S., Müller, J., 2018. Biogas Potential of Coffee Processing Waste in Ethiopia. Sustainability. 10, 2678 https://doi.org/10.3390/su10082678 Chinnici, G., Selvaggi, R., D’Amico, M., Pecorino, B., 2018. Assessment of the potential energy supply and biomethane from the anaerobic digestion of agro-food feedstocks in Sicily. Renew Sust Energ Rev, 82, 6–13. Choi, I.S., Kim, J.H., Wi, S.G., Kim, K.H., Bae, H.J., 2013. Bioethanol production from mandarin (Citrus unshiu) peel waste using popping pretreatment. Appl Energ. 102, 204–210. Citrofrut., 2018a. Productos cítricos. http://www.citrofrut.com.mx/producto1.html (Accessed 20 November 2018). Citrofrut., 2018b. Colaterales. http://www.citrofrut.com.mx/producto2.html (Accessed 20 November 2018). CRE (Comisión Reguladora de Energía)., 2018. Índice de Referencia Nacional de Precios de Gas Natural al Mayoreo (IPGN). https://www.gob.mx/cre/documentos/indice-de-referencia-nacional-de-precios-de-gasnatural-al-mayoreo-ipgn (Accessed 23 December 2018). García-Gonzalo, D., Espina, L., Gelaw, T., De Lamo-Castellvi, S., Pagán, R., 2013. Mechanism of bacterial inactivation by (+)-limonene and Its potential use in food preservation combined processes. Plos One. 8, 1–11. Gerardi, M., 2003. The microbiology of anaerobic digesters. John Wiley & Sons, pp. 51. Gujer, W., Zehnder, A., 1983. Conversion process in anaerobic digestion. Water Sci Technol. 15, 123–167.

18

Journal Pre-proof IQCitrus

(Internacional

Química

de

Cobre,

división

Cítricos).,

2018.

Proceso

de

extracción.

http://www.iqcitrus.com/index.php?option=com_content&view=article&id=72&Itemid=82/ (Accessed 15 October 2018). Juguera Allende., 2018. Procesos. http://www.jugueraallende.com/ (Accessed 15 October 2018). Kafle, G., Kim, S., 2013. Anaerobic treatment of apple waste with swine manure for biogas production: Batch and continuous operation. Appl Energ, 103, 61–72. Kaparaju, N., Rintala, A., 2006. Thermophilic anaerobic digestion of industrial orange waste. Environ Technol. 27, 623–33. Khalid, A., Arshad, M., Anjum, M., Mahmood, T., Dawson, L., 2011. The anaerobic digestion of solid organic waste. Waste Manage. 31, 1737–1744. Kimball, D.A., 1999. Citrus Processing: A Complete Guide. 2nd ed. Aspen Publishers Inc., Gaithersburg, Maryland (USA). Koppar, A., Pullammanappallil, P., 2013. Anaerobic digestion of peel waste and wastewater for onsite energy generation in a citrus processing facility. Energy. 60, 62–68. Maroušek, J., 2014. Significant breakthrough in biochar cost reduction. Clean Technol Envir. 8, 1821–1825. Martín, M.A., Siles, J.A., Chica, A.F., Martín, A., 2010. Biomethanization of orange peel waste. Bioresource Technol. 101, 8993–8999. Martín, M., Siles, J., Chica, A., Martín, A., 2010. Modelling the anaerobic digestion of wastewater derived from the pressing of orange peel produced in orange juice manufacturing. Bioresource Technol. 101, 3909– 3916. Michaud, S., Bernet, N., Buffière, P., Roustan, M., Moletta, R., 2002. Methane yield as a monitoring parameter for the start-up of anaerobic fixed film reactors. Water Res. 36, 1385–1391. Mugodo K., Magama, P.P., Dhavu, K., 2017. Biogas Production Potential from Agricultural and AgroProcessing Waste in South Africa. Waste Biomass Valori. (2017) 8, 2383–2392. Muylaert, M.S., Sala, J., Freitas, M.A.V., 2000. Consumo de energia e aquecimento do planeta: Análise do mecanismo de desenvolvimento limpo (MDL) do Protocolo de Quioto. Case studies. Rio de Janeiro: Postgraduate Engineering Programs Coordination (COPPE). Negro, V., Mancini, G., Ruggeri, B., Fino, D., 2016. Citrus waste as feedstock for bio-based products recovery: Review on limonene case study and energy valorization. Bioresource Technol. 214, 806–815. Nutiu, E., 2014. Anaerobic purification installation with production of biogas and liquid fertilizers. Proc Tech. 12, 632–636. Olthof, M., Oleszkiewick, J., 1982. Anaerobic treatment of industrial wastewater. Chem Eng. 15, 1321–1326.

19

Journal Pre-proof PEMEX

(Petróleos

Mexicanos).,

2018.

Precio

al

público

de

productos

http://www.pemex.com/ri/Publicaciones/Indicadores%20Petroleros/epublico_esp.pdf

petrolíferos.

(Accessed

23

December 2018). Prussi, M., Padella, M., Conton, M., Postma, E.D., Lonza, L., 2019. Review of technologies for biomethane production and assessment of Eu transport share in 2030. J Clean Prod. 222, 565–572. Raimondo, M., Caracciolo, F., Cembalo, L., Chinnici, G., Pecorino, B., D’Amico, M., 2018. Making Virtue Out of Necessity: Managing the Citrus Waste Supply Chain for Bioeconomy Applications. Sustainability, 10, 4821. Rezzadori, K., Benedetti, S., Amante, E., 2012. Proposals for the residues recovery: Orange waste as raw material for new products. Food Bioprod Process. 90, 606–614. Rico, C., Muñoz, N., Fernández, J., Rico, J., 2015. High-load anaerobic co-digestion of cheese whey and liquid fraction ofdairy manure in a one-stage UASB process: Limits in co-substrates ratio and organic loading rate. Chem Eng J. 262, 794–802. Rosas-Mendoza, E.S., Méndez-Contreras, J.M., Martínez-Sibaja, A., Vallejo-Cantú, N.A., Alvarado-Lassman, A., 2018. Anaerobic digestion of citrus industry effluents using an Anaerobic Hybrid Reactor. Clean Technol Envir. 20, 1387–1397. SAGARPA (Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación)., 2018. Estudio de Mercado para identificación de necesidades de infraestructura logística para la comercialización de jugo de

cítricos

en

Veracruz.

http://www.sagarpa.mx/agronegocios/Documents/Estudios_promercado/SISTPROD_CITRICOS.pdf (Accessed 8 July 2018). Satari, B., Karimi, K., 2018. Citrus processing wastes: Environmental impacts, recent advances, and future perspectives in total valorization. Resour Conserv Recy. 129, 153–167. Selvaggi, R., Pappalardo, G., Chinnici, G., Fabbri, C., 2018. Assessing land efficiency of biomethane industry: A case study of Sicily. Energ Policy, 119, 689–695. Scarlat, N., Fahl, F., Dallemand, J.F., Monforti, F., Motola, V., 2018. A spatial analysis of biogas potential from manure in Europe. Renew Suste Energ Rev. 94, 915–930. SENER

(Secretaría

de

Energía).,

2016.

Balance

Nacional

de

Energía

2016.

https://www.gob.mx/cms/uploads/attachment/file/288692/Balance_Nacional_de_Energ_a_2016__2_.pd f (Accessed 23 December 2018). Sharma, K., Mahato, N., Cho, M.H., 2017. Converting citrus wastes into value-added products: Economic and environmently friendly approaches. Nutrition. 34, 29–46.

20

Journal Pre-proof Sharma, K., Mahato, N., Lee, Y.R., 2018. Extraction, characterization and biological activity of citrus flavonoids. Rev Chem Eng. https://doi.org/10.1515/revce2017-0027. Siles, J., Li, Q., Thompson, I., 2010. Biorefinery of waste orange peel. Crit Rev Biotechnol. 30, 63–69. Siles, J.A., Vargas, F., Gutiérrez, M.C., Chica, A.F., Martín, M.A., 2016. Integral valorisation of waste orange peel using combustion, biomethanisation and co-composting technologies. Bioresource Technol. 211, 173–182. Silva I.F., Duarte, N., Bruni, L.G., Mambeli, R., Tiago, G.L., 2018. Assessment of potential biogas production from multiple organic wastes in Brazil: Impact on energy generation, use, and emissions abatement. Resour Conserv Recy. 131, 54–63. Sepehri, A., Sarrafzadeh, M.H., 2019. Activity enhancement of ammonia-oxidizing bacteria and nitriteoxidizing bacteria in activated sludge process: metabolite reduction and CO2 mitigation intensification process. Appl Water Sci. 9:131. https://doi.org/10.1007/s13201-019-1017-6. Sepehri, A., Sarrafzadeh, M.H., 2018. Effect of nitrifiers community on fouling mitigation and nitrification efficiency in a membrane bioreactor. Chem Eng Process-Process Intensification. 128, 10–18. Speece, R.E., 1983. Anaerobic biotechnology for industrial wastewater treatment, Environ Sci Technol. 17, 416–427. Statista.,

2018.

The

Statistics

Portal.

Global

electricity

https://www.statista.com/statistics/263492/electricity-prices-in-selected-countries/

(Accessed

prices. 23

December 2018). Surendra, K.C., Takara, D., Hashimoto, A.G., Khanal, S.K., 2014. Biogas as a sustainable energy source for developing countries: Opportunities and challenges. Renew Sust Energ Rev. 31, 846–859. Themelis, N.J., Ulloa, P.A., 2007. Methane generation in landfills. Renew Energy. 32, 1243–1257. USDA FAS (United States Department of Agriculture. Foreign Agricultural Service)., 2018. Citrus: World Markets and Trade https://apps.fas.usda.gov/psdonline/app/index.html#/app/downloads (Accessed 10 July 2018). Valdés, P., Guerrero, B., Nieves, G., De la Torre, V., 1994. Tratamiento de aguas residuales cítricas por vía anaerobia. Rev Int Contam Ambient. 10, 69–75. Wang, L., Xu, H., Yuan, F., Pan, Q., Fan, R., Gao, Y., 2015. Physicochemical characterization of five types of citrus dietary fibers. Biocatal Agr Biotechnol. 4, 250–258. Ward, A.J., Hobbs, P.J., Holliman, P.J., Jones, D.L., 2008. Optimization of the anaerobic digestion of agricultural resources. Bioresour Technol. 99, 7928–7940.

21

Journal Pre-proof Wei, Y., Li, J., Shi, D., Liu, G., Zhao, Y., Shimaoka, T., 2017. Environmental challenges impeding the composting of biodegradable municipal solid waste: A critical review. Resour Conserv Recycl. 122, 51– 65. Yang, Y., Bao, W., Xie G., 2019. Estimate of restaurant food waste and its biogas production potential in China. J Clean Prod. 211, 309–320. Zema, D., Fòlino, A., Zappia, G., Calabrò, P., Tamburino, V., Zimbone, S., 2018. Anaerobic digestion of orange peel in a semi-continuous pilot plant: An environmentally sound way of citrus waste management in agroecosystems. Sci Total Environ. 630, 401–408.

22

Journal Pre-proof AUTHOR CONTRIBUTION STATEMENT CREDIT AUTHOR STATEMENT

Rosas-Mendoza E.S.: Conceptualization, Methodology, Investigation, Writing - Original Draft Méndez-Contreras J.M.: Visualization, Formal analysis Aguilar-Laserre A.A.: Visualization, Validation Vallejo-Cantú N.A.: Resources, Project administration Alvarado-Lassman A.:Conceptualization, Writing - Review & Editing, Supervision

Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof

Figures 1.65 1.55

1.51

Millions of tons processed

1.71 1.70

1.20 1.00 1.00 0.88 0.74

0.93 0.83

0.70 0.56

0.41 0.40

0.34 0.20 0.08

Year

Fig. 1. Oranges processed in Mexico from 1999 to 2018 (USDA FAS, 2018).

Wastewater from obtaining flavorings

Oils

Gross solid waste

• •

Reception orange Storage silos

of

• • •

Extractor Pre-concentrator Centrifuge Polisher centrifuge

Damaged orange

Water-oil emulsion

Pressure washing

Scraper

Wastewater from washing orange

Wastewater from obtaining oils

Settling tank

Flavorings

Wastewater from obtaining juice

Evaporators

Concentrated juice

Filters Centrifuge

Tank of fresh juice

Fresh juice

Fresh juice

Size selection

Excess pulp as gross peel waste

Orange press

Wastewater from pressing of orange

Orange peel

Citrus Effluent Wastewater from pressing of orange peel

• • •

Dehydrator Press Dryer

• • • •

Hopper Windmill Dryer Pellet

Dry orange peel for animal consumption

Wastewater from drying orange

Biomass

Fig. 2. Scheme of the orange processing plant for obtaining products and citrus effluent (SAGARPA, 2018; Juguera Allende, 2018; IQCitrus, 2018; Citrofrut, 2018a; Citrofrut, 2018b).

Journal Pre-proof Citrus Effluent Treatment Plant Tons orange per day Scenario 1: 880 Scenario 2: 1,850 Scenario 3: 2,760

Citrus Effluent from Processing Plant

Citrus Processing Plant

Citrus Processing

Solid Citrus Waste

L/s Biomass Generating Plant

Pretreatment

Scenario 1: 12.8 Scenario 2: 26.8 Scenario 3: 40.0

Anaerobic Hybrid Reactor

Posttreatment

Effluent to final disposal

Biogas Biogas container

49,000 kWh

18,000 kWh

m3 CH4 at STP per day Biogas cleaner

67,000 kWh Electrical Energy from Federal Electricity Commission

30% Net Electricity to be Incorporated into Citrus Processing Plant kWh per day Scenario 1: 1,892 Scenario 2: 3,962 Scenario 3: 5,913

Scenario 1: 635 Scenario 2: 1,329 Scenario 3: 1,983

Clean biogas

Electric generator Biogas Utilization System

70% Thermal Energy kWh per day Scenario 1: 4,415 Scenario 2: 9,244 Scenario 3: 13,797

Fig. 3. Diagram to obtain and use electrical energy from the anaerobic digestion of the citrus effluent.

Journal Pre-proof 12.30 8.86

Brazil 3.43

United States

Mexico

European Union

China

a)

2.30 1.66

1.70 1.22

2.79 0.70

European Union

United States

9.31

0.57 0.41 4.08 1.22 0.10

South Africa

b)

Million tons of processed orange/year

Million m3 of methane at STP/year

Gross electricity (GWh/year)

Net electricity (GWh/year)

Cost (Million USD/year)

Mexico

Israel

c)

0.17

1.15

0.34 0.04 0.09 0.07

0.67 0.20 0.02

Million tons of processed lemos/limes/year

Million m3 of methane at STP/year

Gross electricity (GWh/year)

Net electricity (GWh/year)

European Union

United States

0.62

0.19

0.08 0.06

0.16 0.12

0.64 0.46

0.57

Argentina

Million tons of processed grapefruit/year Gross electricity (GWh/year) Cost (Million USD/year)

Japan

d) Million m3 of methane at STP/year Net electricity (GWh/year)

1.93

0.58

0.16 0.12 0.00

0.01 0.01 0.10 0.03 0.01

1.93

0.58

0.27 0.19 0.08

1.15

0.34

0.09 0.07 0.20 0.04

4.58

1.37

0.27 0.19 0.14

0.09 0.06

0.03

European Union

1.07

0.32

0.01

2.08

0.62 0.16

0.11

0.15 0.11 0.03

0.29 0.21

China

1.43

0.43

0.06

South Africa

2.86

0.86

Cost (Million USD/year)

0.20 0.14

United States

0.40 0.29 0.07

12.17

3.65 0.29 1.30 0.94

Mexico

7.73

2.32

0.02

16.47

4.94 0.64

1.08 0.78

Argentina

88.07

26.42

0.65

Million tons of processed tangerines/mandarins/year Gross electricity (GWh/year) Cost (Million USD/year)

Million m3 of methane at STP/year Net electricity (GWh/year)

Fig. 4. Bioenergy potential of the main countries processing a) oranges, b) lemons/limes, c) grapefruit, and, d) tangerines/mandarins.

Journal Pre-proof 18,000 L Fuel Oil a) 1,300 tons Solid Citrus Waste b)

Biomass Generating Plant

219,000 kWh

600 tons Biomass (≤ 30% humidity) 219,000 kWh

22,000 m3 at STP Natural Gas or Biogas c)

219,000 kWh

Steam Generators

Steam Generators

Steam Generators

200 tons Steam (P = 0.01 MPa and T = 125 °C)

200 tons Steam (P = 0.01 MPa and T = 125 °C)

200 tons Steam (P = 0.01 MPa and T = 125 °C)

Evaporators

Evaporators

Evaporators

200 tons Concentrated Juice

200 tons Concentrated Juice

200 tons Concentrated Juice

Fig. 5. Comparison of steam generation using different energy sources for concentrated juice production: a) fuel oil; b) solid citrus waste; c) natural gas or biogas.

Journal Pre-proof Tables

Table 1 Characterization of the citrus effluent. Parameter COD total (mg/L)

38,780

COD soluble (mg/L)

35,420

TS (mg/L)

21,662

VS (mg/L)

18,084

Ph

3.76

Table 2 Comparison of the energy analyses of citrus wastewater. Author

Electricity

Processing

Wastewater

required by the

capacity

(m3/day)

plant (kWh/day)

(Orange

Methane yield (m3

at STP/kg COD)

tons/day) Koppar and

Combined

Conversion

Net

heat and

efficiency

electricity

power energy

(%) to

(kWh)

(kWh/day)

electricity

1,200+

600

946

0.238

29,479*

25

7,370

67,000

880

1,106

0.150

6,307

30

1,892

Pullamimanappallil (2013) Present work +The

citrus processing plant requires 1,200 kWh/day of electricity, but the total energy consumed (electricity + fossil fuel)

is 60,340 kWh/day. *Citrus wastewater represents 7% of the total energy (409,334 GJ/year) obtained from the orange peel waste and wastewater.

Journal Pre-proof Table 3 World production of methane, gross electricity, and net electricity from citrus effluent.

Methane Region

Country

(million m3 at STP/year)

Gross electricity

Net electricity

(GWh/year)

(GWh/year)

Mexico

1.57

15.66

4.70

United States

2.11

20.98

6.29

Argentina

0.89

8.88

2.66

Brazil

8.86

88.07

26.42

European Union

1.35

13.42

4.03

South Africa

South Africa

0.17

1.74

0.52

Western Asia

Israel

0.06

0.57

0.17

China

0.87

8.66

2.60

Japan

0.07

0.65

0.20

15.95

158.62

47.59

North America South America Europe

Eastern Asia TOTAL