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Researchers
Abstract
Key words
Introduction
Study methodology

Influential parameters
Kinetic study

Conclusions
References

 Abstract

Every day, a large amount of fruit and vegetable waste is generated and poses serious health and environmental problems such as the production of greenhouse gases and excessive CO2 emissions.  Therefore, this research aims to produce biogas by anaerobic fermentation of fruit and vegetable waste by different authors in order to reduce the amount of waste generated. The biogas production was carried out in laboratory scale anaerobic digesters. Various parameters such as, pH, temperature, moisture, total solids, and C/N ratio are the main parameters affecting on biogas production, according to this review which provides a summary on anaerobic digestion which seems to be one of the most effective solutions due to its advantages, including the production of renewable, green energy in the form of biogas. The main objectives of this study are to carry out the process of anaerobic digestion of fruit and vegetable waste, in order to evaluate their methane yield and their biodegradability, as well as monitoring the stability of the process parameters. In addition, the majority of the studies focused on the kinetic study that was applied to the experimental data using several models in terms of methanogenic potential that can be compared to determine the most suitable method.

Key words
Anaerobic digestion, vegetable and fruit waste, anaerobic co-digestion / co-substrate, biogas,
 

 Introduction

Fruit and vegetable wastes are produced in large quantities in the world, leads different social, political and economic actors to take measures for its reduction and recovery. However, a large amount of organic waste still ends up in landfills, where it produces substances that are harmful to the environment and to living beings. Therefore, the biological treatment of waste by anaerobic digestion offers a very important advantage to valorize the putrescible potential of waste by generating biogas and digestate that can be used as organic fertilizer. Thanks to anaerobic digestion, waste becomes a source of wealth, which makes this process increasingly interesting. This work is part of the energy valorization of organic biomasses (fruits and vegetables), and thus had the objective to validate the impact of pretreatments in the anaerobic digestion of these wastes, and is to compare the digestibility of organic wastes in mesophilic conditions, to improve the production of methane from the wastes by different pretreatments and to compare the results of the methane yield with different scientific actors.
The process of anaerobic digestion is a series of metabolic processes, we can distinguish four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis and each metabolic stage is a series of microorganisms [1]. In general, this process of anaerobic digestion is infected by several physicochemical parameters (pH, conductivity, dry matter and temperature....,), and nutritional parameters (Phosphorus, total nitrogen, total organic carbon....,), and also affected by microbiological parameters to control microbiological agents. In this sense, for stability of this process, and to maintain a balance of the carbon/nitrogen ratio, it appears interesting to add co-substitutes easily biodegradable, to increase the production of biogas with a yield of high methane.
This literature review examines specific studies on the use of anaerobic digestion to valorize fruit and vegetable waste (FVW), and examines the problems of greenhouse gases and their solution through the use of this technique; thus facilitating the research for a better optimal design for the selected anaerobic digesters, as well as the pretreatment methods to degrade the organic matter.

 Study methodology

•    Feedstock
The biomass represents a very significant potential for putrescible and fermentable organic matter. Are produced in large quantities in the world, and constitute a source of nuisance in nature. All types of biomass can be used as substrates for biogas production[2], such as fruit and vegetable waste, food waste, waste from neighborhood markets, green waste, slaughterhouse waste, WWTP sludge, animal waste….etc. This waste is heterogeneous, so it is necessary to carry out a complete pre-treatment by a mixer or shredder to break down the organic matter [3]. The process economy of bio-methanisation can be significantly improved by anaerobic digestion. The majority of organic wastes can be used profitably for anaerobic digestion processes [4], For these reasons, the majority of studies have characterized the fruit and vegetable waste, each with its own compositions and properties, according on its location, origin, cooking habits, and socioeconomic conditions [5].
From the point of view of waste disposal and AD, it is important to assess the composition regarding organic components such as, content of total solids (TS) and volatile solids (VS), total Kjeldahl nitrogen (TKN), total soluble phosphorus (TP), total soluble organic carbon (TOC). A review of the characteristics and composition of different organic matter is presented in Table 1.
Various authors have studied the fermentation potential of fruit and vegetable wastes and others have focused on different wastes containing a single fruit or vegetable, the compositions of these wastes can pose effective challenges to anaerobic digestion. These compositions include pH values between 4 and 6. The organic fraction, represented by volatile solids (VS), yielded a significant percentage of about 6 to 16%. The percentages of volatile solids and total solids are high in most of the cases studied, indicating that the majority of the waste is rich in organic matter. The carbon and nitrogen concentrations, represented by the C/N ratio, are between 8 and 35 for the substrates studied (Tables 1 and 2). According to Beniche and al [6], the C/N ratio interferes with the development of micro-organisms. If the C/N ratio is high, it leads to a low rate of protein solubilisation and a reduction in buffering capacity. If it is low, it increases the risk of ammonia inhibition, leading to an increase in pH which inhibits methanogenic bacteria.
The organic content of the waste is shown in Table 02: Hemicellulose between 2 and 3 (g100 g-1VS), Cellulose between 3 and 16 (g 100 g-1VS), and Lignin between 0.1 and 3 (g 100 g-1VS). According to Astals and al [7], have shown that if lipids and proteins in substrates are dominant can inhibit the process of anaerobic digestion due to the accumulation of long chain fatty acids and ammonia. In addition, substrates with high carbohydrate content promote the accumulation of VFAs, which causes methanogen inhibition and acidification. The other parameters showed below.

 Influential parameters

The performance of the processes is closely linked to the physico-chemical parameters of the environment. The monitoring of these parameters makes it possible to evaluate the stability of the anaerobic digestion process [19], such as temperature, pH, alkalinity, VFA concentration, C/N ratio, biogas production and composition, are particularly important for process control.

Temperature
Temperature is one of the most important factors influencing anaerobic digestion, as it not only influences the biological processes but also the production of biogas and digestate [20], Micro-organisms are active in different temperature ranges and they grow and work fastest, depending on the species [21]. Overall, micro-organisms can be divided into different groups according to the temperature at which they grow best: psychrophilic, mesophilic, thermophilic and extremophilic/hyperthermophilic[22]. For example, Wang and al [23],studied the performance of a process of co-digestion of cow manure and maize straw at different temperatures (35–25 ◦C) to obtain the optimum biogas production efficiency. Similarly, Nipaney and al [24], studied the effect of different mesophilic temperatures (29.5, 33 and 37.5°C) on biogas production from P. stratiotes plants.

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pH and alkalinity
The pH plays an important role in the biochemical and physico-chemical functioning of anaerobic digestion media. Hydrolytic and acidogenic bacteria are little affected by changes in pH, which is not the case for acetogenic and methanogenic bacteria, which do not tolerate large changes in pH. Despite this, hydrolytic activity is sensitive to pH variations [25]. The pH value provides information about the stability of the medium, as it varies according to the buffer capacity of the medium itself. Variations in pH are associated with variations in the species involved in the trophic chain of the anaerobic digestion process [19].
Alkalinity is the capacity to resist changes in pH caused by the addition of acids to the medium. i.e. the capacity to neutralise the acids in a medium. It results from the presence of carbonates, hydroxides and bicarbonates of elements such as sodium, magnesium, calcium, ammonia or potassium. In the case of anaerobic digestion media, the presence of VFAs, in addition to phosphates, silicates and borates, also contributes to alkalinity [19].
Carbon/Nitrogen (C/N)
The C/N ratio is a commonly used indicator for sizing and monitoring anaerobic digestion. During anaerobic digestion, the microbial population uses about 25 to 30 times more carbon than nitrogen. Wastes rich in easily biodegradable carbon can be mixed with low nitrogen content wastes to achieve the desired carbon to nitrogen (C/N) ratio, so the effect of co-digesting the organic matter is to improve the C/N ratio of the mixture, which also results in improved biogas production [6]. There are several studies in the literature on the effect of balancing the C/N ratio by adding co-substrate, for example, ikram and al [6], studied the effect of C/N ratio on anaerobic co-digestion of cabbage, cauliflower and restaurant food waste, to see which ratio would best result in higher biogas production, Callaghan and al [26], mixed a high carbon to nitrogen (C/N) ratio and a low C/N ratio of different organic wastes to improve digester performance.

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Water content
Water is considered to be a key factor in the biochemical process of anaerobic degradation; therefore the anaerobic digestion of organic waste is strongly affected by water content. Organic matter from fruit and vegetable markets is very rich in water; this is why the average dry matter content often does not reach 10% [19], for example, Won Seo et al [27], studied the stability of dry anaerobic digestion (AD) of food waste (FW) at different organic loading rates that were applied to the system by varying the water content of the FW feed and the solids retention time (SRT), to predict the biogas production rate.

Inoculum
The inoculum is an essential element for producing quality and quantity of biogas, particularly in dry mechanization. An inoculum from previously digested waste is promising to ensure the existence of desired micro-organisms in AD systems, as it accelerates the hydrolysis step of the recalcitrant cellulose load [28]. Ma et al. have shown that adaptation of the inoculum to degrade complex substrates is essential. Another study investigated mixing equivalent amounts of cow manure, soil and waste activated sludge as inoculum with municipal solid waste, paper sludge and sewage sludge as substrates, to increase biogas production [29].
Hydraulic retention time
The hydraulic retention time is the time required to replace the entire contents of the digester. The optimal HRT of a digester depends on the nature and composition of the substrates used to feed it, the digestion temperature, and the microbial community involved in the process [30], Theoretically, a longer retention time produces a more complete degradation of the feedstock. The reaction rate, however, decreases with increasing retention time. The retention time differs for each type of substrate, and for most processes it varies from 14 to 30 days [31]. Ezekoye et al [32], studied the effect of retention time on biogas production from poultry droppings and cassava peels, found that daily gas production decreased slightly from 130 to 32 liters when the retention time of poultry droppings was increased from 10 to 40 days.
Pretreatment organic waste
Pre-treatment of organic waste is used to accelerate the hydrolysis reaction, and reduce the retention time during AD, and improve the yield of biogas production [33]. Over the last thirty years, much research has been carried out to determine the conditions for thermal, physical, chemical and/or biological pre-treatment of biomass. There are many studies on these techniques, for example, in physical pre-treatment, the structure of the biomass is modified and the particle size reduced, by the application of physical force. This directly increases the surface area available for enzymatic and microbial attack [34]. Physical pre-treatment can be carried out using microwave or ultrasonic irradiation, sonication, extrusion, mechanical beating, deflector, dispersion, refining, grinding and crushing.... [35], Chemical pre-treatments are among the most studied pre-treatment methods for the degradation of organic matter, it refers to the use of chemicals, such as ionic liquids, acids and bases, alkaline, peroxides, to modify the physical and chemical characteristics of the biomass [35]. Liu et al [36], found that thermal treatment at 175°C for 60 minutes could improve the physical and chemical properties of fruit and vegetable waste, enhance dewater ability, decrease viscidity, and increase the solubility of COD. Moreover, The biologically mediated pre-treatment process is based on the function of multiple forms of heterotrophic microbes, the significance of biological pre-treatment lies in the fact that it solubilizes the organic compounds present in the biomass with minimal energy [37], Another type of pre-treatment, a combined steam explosion pre-treatment, aims to expose the biomass to high temperature and pressure for a short period of time and then reduce the pressure rapidly. This stops the reaction and breaks down the biomass explosively. Nges and al [38] studied the anaerobic digestibility of Miscanthus lutarioriparius for biogas production, they used a steam explosion pretreatment with 0.3 M NaOH with a particle size reduced to 0.5. Another combination, the combination of thermal and chemical pre-treatments has been investigated in a number of studies in which improving the anaerobic digestibility of biomass, Yi and al [36], studied the effect of microwave irradiation with NaOH pre-treatment on anaerobic digestion in semi-continuous mesophilic digesters to improve AD. They found that a combination of alkaline pretreatment and thermal pretreatment resulted in the highest production rate, which was up to 10 times higher than the production rate without any pretreatment. Ruggeri and al [39], compared the effects of physical, chemical, thermal and ultrasonic pre-treatment methods, and a combination of these processes, to digest organic waste Market and compared the production rates with and without the pretreatment methods. It can be seen that thermal treatment is an effective measure when treatment of fruit and vegetable waste. However, optimal temperature control still needs to be studied and limited when heat treatment is performed [40].
Biogas recovery from anaerobic digestion
Among the various stabilization techniques, anaerobic digestion, or mechanization, is a natural process by which organic matter is decomposed in the absence of oxygen by the action of several micro-organisms [6]. Anaerobic digestion includes four biochemical steps (fig.1) [41]. The advantage of using anaerobic digestion is that it produces biogas with high methane (CH4). The methane in the biogas can be burned to produce heat and electricity using internal combustion engines or micro turbines in a cogeneration plant [42]. Under such circumstances, the majority of researchers are focused on anaerobic co-digestion of two or more mixtures of substrates and co-substrates to improve biogas and methane production. In addition, AcoD can improve process stabilization and synergistic effects of microorganisms, and can reduce greenhouse gas emissions and treatment costs [43]. Most often, anaerobic digestion systems use different types of digesters for biogas production [2]. The choice of an anaerobic digester type depends on the amount of substrate to be treated and valorized, the nature of the substrate, the type of digester, the fermentation conditions, the climatic conditions, the investment capital...[44]. Various new reactors have been designed and implemented in recent years[40], including batch reactors [45], Anaerobic sequencing batch reactors (ASBRs) [46], continuous stirred tank reactor (CSTR) single phase [16], up flow anaerobic sludge blanket (UASB)[47], hybrid anaerobic solid-liquid (HASL)[48], sequential batch anaerobic composting (SEBAC)[49], , semi-continuous anaerobic plug flow reactor (PFR)[50], and a solid-state stratified bed (SSB) reactor[51], all of which have been used to treat organic waste. Each of these reactors has different methods for maintaining microorganisms, as well as generating methane and compost [40].
Several scientific works dealing with digestion and co-digestion of fruit and vegetable have been published. Each study had its main objectives, for example, Francesco and a l[15], studied biogas production from fruit and vegetable waste and mixed sludge as co-substrate in a 100 liter gas tight anaerobic reactor with a removable lid, reducing the hydraulic retention time from 14 days to about 10 days. Specific biomethane production increased from about 90 NL/kg VS to a maximum of about 430 NL/kg VS when the OLR was increased from 1.46 kg VS/m3 day to 2.1 kg VSS/m3 day, in the same context, Wang and al [9], studied the co-digestion of fruit and vegetable waste (FVW) and kitchen waste (KW), in CSTRs at different percentages. Laboratory-scale experimental results showed that the 5:8 ratio of FVW to KW had higher methane productivity (0.725 L CH4/g VS), with a HRT of 10 days. Another study that deals with the co-digestion of fruit and vegetable sludge and fresh chopped artichoke waste by Ros and al [10], in a continuously stirred anaerobic reactor (8-10 rpm) with a working volume of 300 liters for (55-71 d), the increase in biogas production resulted in an average value of 354 ± 68 L kg-1 of dry matter per day, with a higher methane content (over 70%). Other studies of anaerobic digestion and co-digestion of fruit and vegetable waste with other organic waste are summarized in Table 1

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Figure 1 : Stages of decomposition of organic matter [20].

Table 3: Examples of anaerobic digestion and co-digestion of fruit and vegetable waste (FVW) with other organic waste from the literature.

Kinetic study

Kinetic modeling of methane production by AD is a practical approach to understand the ultimate technology for  methane production under optimal conditions. In the literature, several models have been developed to monitor and control physical, chemical and biological parameters using different types of substrates and reactors [56], several types of mathematical models exist for predicting biogas production. They are generally based on zero order, first order and second order decomposition kinetics.
Simulation of biogas production using the classical BUSWELL equation
The production of biogas formed during anaerobic degradation can be estimated from the knowledge of the elemental composition of the degraded organic matter, assuming that all the material is globally degraded to CO2 and CH4. For example, the theoretical methane and biogas yield potential can be calculated from the following BUSWELL equation [57]:

 CONCLUSION

The work of this review presented in this manuscript mainly provides information on the utilization of FVW waste through the anaerobic digestion process for biogas production. The main objective was to develop an efficient and simple methodology for the functionalization of this process. In this sense, we showed how the biogas yield was improved by the addition of the different substrates and thus how to use kinetic models to validate the biogas yield. In this respect, the choice of the most suitable fruit and vegetable waste based on some physical chemical characteristics such as C/N ratio, nitrogen, phosphorus, carbon, VS, TS, pH, COD..... These characteristics could constitute interesting alternatives to simulate the methane yield, and perhaps they were verified by the kinetic parameters that justify the optimal conditions for a better biogas yield.