The Advantage of Citrus Residues as Feedstock for Biogas Production: A Two-Stage Anaerobic Digestion System

Abstract

Anaerobic digestion (AD) is an important step in waste recovery. In Colombia, the production of citrus food significantly contributes to environmental impact via waste generation. In 2021, the waste produced, specifically citrus rind, amounted to 725,035 tons/year. During degradation, wastes generate leachate and greenhouse gases (GHGs), which negatively impact water sources (leachate), soil, and human and animal health. This article describes the design of a two-phase biodigestion system for the degradation of organic matter and biogas production. The system uses citrus waste to produce biogas with neutral emissions. The biodigestion process begins with the stabilization of the methanogenesis reactor (UASB), which takes approximately 19 days. During this period, the biogas produced contains approximately 60% methane by volume. Subsequently, the packed bed reactor operates for 7 days, where hydrolytic and acetogenic bacteria decompose the citrus waste, leading to the production and accumulation of volatile fatty acids. The final step involves combining the two phases for 5 days, resulting in a daily biogas production ranging from 700 to 1100 mL. Of this biogas, 54.90% is methane ($\mathrm{C}{\mathrm{H}}_4$) with a yield of $0.51 \mathrm{L}\mathrm{C}{\mathrm{H}}_4{\left(\mathrm{g}\mathrm{S}\mathrm{V}\right)}^{-1}$. This study assesses the methane production capacity of citrus waste, with the process benefiting from the pH value of the leachate, enhancing its degradability. Consequently, this approach leads to a notable 27.30% reduction in solids within the digestion system. The two-phase anaerobic biodigestion system described in this article demonstrates a promising method to mitigate the environmental impact of citrus waste while concurrently producing a renewable source of energy.

1. Introduction

The production and consumption of citrus in Colombia play leading roles in the different economic sectors of the country. In 2021, according to the Ministry of Agriculture, a net production of 1,450,071 tons of citrus fruits (year)⁻¹ was reached. Of these, oranges accounted for 47%, mandarins 27%, and lemons/limes 26%. Currently, citrus fruits make up an important part of the Colombian economy, using a large percentage of the areas destined for their cultivation. In 2021, there was a total planted area of 87.638 hectares (Ha) and a yield $\mathrm{o}\mathrm{f} 15 \mathrm{T}\mathrm{o}\mathrm{n} {\left(\mathrm{H}\mathrm{a}\mathrm{s} \mathrm{c}\mathrm{u}\mathrm{l}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\right)}^{-1}$. This increase in production is proportional to the increase in hectares planted with respect to previous years [1,2,3,4,5].

According to a study from the Central University of Venezuela, which characterized citrus peel, specifically orange, the percentage of peel in the fruit is approximately 45 to 60% of the total weight [6]. Based on this and the citrus production in Colombia, an amount of waste of around $725.036 \mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}{\left(\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}\right)}^{-1}$ can be estimated in 2021. This waste is mixed with inorganic waste and reaches landfills, causing contamination, bad odors, and diseases.

Degradation processes with microorganisms have been successfully implemented in the production of organic fertilizers [7], wastewater treatment [8], greenhouse gas emissions control [9], and greenhouse emissions reduction [10], among others. Anaerobic digestion, also known as biomethanization, is a biological process that degrades and stabilizes various agro-industrial, domestic, and agricultural residues. In an anoxic environment found in nature, complex substrates are converted into biogas via microbial action. This results in high efficiency in organic load removal, greater flexibility compared to other processes, low space requirements, energy consumption, chemicals, and nutrients. This anaerobic treatment can be applied in the laboratory and on the industrial scale [11].

Anaerobic digestion is a process in which organic matter is converted into methane ($\mathrm{C}{\mathrm{H}}_4$), carbon dioxide ($\mathrm{C}{\mathrm{O}}_2$), and hydrogen (${\mathrm{H}}_2$) via the action of a bacterial consortium [12]. The process undergoes three phases: hydrolysis, acidogenesis, and methanogenesis [13]. In the first phase, organic matter is broken down into smaller molecules. In the second phase, acidogenesis, volatile fatty acids (VFAs) are produced. And finally, $\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{O}}_2$ are produced by methanogenic bacteria [14]. It has been shown that when anaerobic digestion is carried out in a single phase, high levels of VFAs can build up, which can lead to a decrease in pH and in the inhibition of methanogenic bacteria [15]. The physical separation of the formation of organic acids, $\mathrm{C}{\mathrm{H}}_4$, and $\mathrm{C}{\mathrm{O}}_2$ can significantly improve the metabolic activity of microorganisms in these processes. Employing a cascade of two individual biodigesters, where the output of one feeds the other, can effectively achieve this. The first biodigester produces organic acids and traces of $\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{O}}_2$, while the second biodigester primarily produces $\mathrm{C}{\mathrm{H}}_4$ and $\mathrm{C}{\mathrm{O}}_2$ [16].

The two-phase system has a shorter total retention time than the single-phase system. This allows for greater conversion of organic waste, as cellulose and hemicellulose can be broken down into monosaccharides (a vital pretreatment for lignocellulosic biomass) [16]. Some disadvantages of anaerobic digestion include bad odors, which require careful control of the environment and conditions. The implementation of anaerobic digestion can also be expensive and requires a long initial time to stabilize the system if it starts without an inoculum treatment.

Anaerobic digestion has several limitations, especially with solid waste. These limitations are more evident when anaerobic digestion is carried out in a single phase. This can lead to structural and performance problems, which can affect the productivity of metabolic activity and generate clogging in the recirculation system. Due to these limitations, a two-phase system is proposed. This system has a hydrolysis reactor, in which the organic matter is degraded. The hydrolysis reactor has a sprinkler at the top to distribute the inoculum throughout the citrus residues. The second reactor is a UASB reactor, in which the methanogenesis stage is carried out. The leachates generated in the packed bed reactor are transferred to the UASB reactor. The two-phase system also has a shorter retention time, which can help reduce the risk of clogging [17] and is more efficient in terms of biogas production. The two-phase system also has a shorter retention time and is less likely to experience structural and performance problems.

The objective of this project was to study the performance of anaerobic digestion in two phases using citrus peels as a substrate, specifically orange and lemon, taking into account different aspects of the operation.

2. Materials and Methods

2.1. Collection of Citrus Waste

The citrus residues used were orange peel and lemon peel, which were obtained from points where there is a considerable consumption of these raw materials; more specifically, the orange peels were obtained from approximately 2 kg of oranges from an orange juice stand, and an equal amount of 2 kg of lemon peels were obtained from a seafood restaurant.

2.2. Characterization of Citrus Residues

After collecting the organic matter, tests were carried out at the research center of Universidad de América, where moisture and ash content analyses were carried out in accordance with the ASTM D 2216-10 [18] volatile solids, according to the EPA Method 160.4 [19]. Total solids were analyzed according to the EPA Method 160.2 [20] standard before and after the anaerobic treatment to which it was submitted. The physicochemical analysis of carbon/nitrogen (C/N) was carried out by means of the acid digestion–open pathway: N according to Kjeldahl [21].

2.3. Chemical Analysis

Daily analysis of different gas concentration such as ${\mathrm{H}}_2\mathrm{S}$ (hydrogen sulfide), ${\mathrm{H}}_2$ (hydrogen), $\mathrm{C}{\mathrm{H}}_4$ (methane), and $\mathrm{C}{\mathrm{O}}_2$ (carbon dioxide) were carried out using a Power Electronics MP400 (mPower Electronics, Santa Clara, CA, USA) portable infrared biogas concentration meter and the corresponding volume displaced into the gasometers; on the other hand, for the liquid phase of the system, the analysis of parameters such as pH was carried out, using an Apera PH700 Benchtop (Apera Instruments, Columbus, OH, USA) pH Meter Kit potentiometer; COD, using the traditional “photometric” method (preparing the digestion, acid and sample solutions) and with the reagent test in Merck Millipore cuvettes of 500–10,000 $\mathrm{m}\mathrm{g}{\left(\mathrm{L}\right)}^{-1}$ using the TR320 thermoreactor (Spectroquant® Thermoreactor, Darmstadt, Germany) and NOVA 60 spectroquant (Spectroquant® Thermoreactor, Darmstadt, Germany), volatile fatty acids (VFA) and alkalinity, carrying out the latter two according to the tests made based on literary reviews where it is carried out by means of titration decreasing the nominal pH of the analyzed sample to a value of 5.0 and 4.4, respectively, by means of 0.1 N sulfuric acid solution [22].

2.4. Hydrolysis Reactor (Packed Bed Reactor)

This reactor was built with acrylic and PVC with the following dimensions: diameter 15.24 cm, height 30 cm, total volume 3 L, and useful volume 2 L; for this, a false bottom was implemented in the internal part of the equipment in order to obtain the leached. A mixture of 500 g of citrus waste (lemon and orange), cut into pieces of 1.30 $\mathrm{c}{\mathrm{m}}^2$, and 0.7 L of inoculum was added to the equipment together with PVC plastic rings measuring a diameter of 1.27 cm and a width of 1 cm to increase porosity and ease leachate collection. To define this ratio of shell/inoculum, it was based on the work carried out by Alzate-Gaviria [23], where he defined a volumetric ratio of 1:1 to 1:2 to give a good yield and a greater amount of VFA. In the upper part, there is a gas outlet used for the daily volume measurement that was carried out by means of a gasometer for the recirculation process. An Iwaki magnet pump (centrifugal magnetic drive pump) (Lenntech Water treatment & purification) [24] and a sprinkler with a temperature of 20 °C were used. The research work carried out by [25] was taken as a guide for the construction of the reactors, where it specifies a wide and short tubular structure, where it is clear that solid waste will be used, and it can be distributed evenly inside the reactor, optimizing space and allowing for more contact of the organic matter with the inoculum entered, as shown in Figure 1.

2.5. Methanogenic Reactor (UASB)

The methanogenic reactor was built with PVC with the following dimensions: diameter 7.62 cm, height 60 cm, total volume 2.5 L, and useful volume 2 L (Figure 2). It was inoculated with a mixture of activated sludge from a WWTP (wastewater treatment plant) active in Bogotá and kept at a temperature of 30–35 °C by means of a water bath. Synthetic wastewater (SW) was used, as mentioned [26]. The biogas obtained in this unit passed to a 2 L gasometer through which the determination of the daily volume was carried out. An Iwaki magnet pump (centrifugal magnetic drive pump) was used to maintain a rate of 11 to 12 $\mathrm{L}{\left(\mathrm{m}\mathrm{i}\mathrm{n}\right)}^{-1}$ [18]. The operation of the UASB reactor had an adaptation time of four weeks with an initial OLR (organic load rate) of 4.0 kg COD (${\mathrm{m}}^3{\mathrm{d}}^{-1}$) and a final OLR of 5.5 kg COD (${\mathrm{m}}^{-3}{\mathrm{d}}^{-1}$) after 10 days. This parameter was kept constant for coupling packed bed and UASB reactors (Figure 3). To maintain a constant organic loading rate in the UASB reactor, the system was coupled by taking an aliquot with known COD and calculating the corresponding volume. The system was then manually measured and fed daily. The research work carried out by Pererva [27] was taken as a guide for the construction of the reactors.

3. Results and Discussion

3.1. Characterization of Citrus Residues

The tests carried out on citrus residues resulted in

Table 1. Data collection of physical and chemical characteristics of citrus waste.

Sample	TS	VS	%H	%Cz	%SV	Reference
Orange	5.02 ± 0.04	4.99 ± 0.03	81.51 ± 0.75	0.59 ± 0.01	17.89 ± 0.77	This studio
Lemon	4.99 ± 0.05	4.96 ± 0.05	85.15 ± 1.06	0.62 ± 0.04	14.22 ± 1.07
Citrus waste	5.01 ± 0.05	4.98 ± 0.04	83.33 ± 0.91	0.60 ± 0.03	16.05 ± 0.92
Citrus waste	7.10 ± 1.20	6.87 ± 1.20	85.90 ± 1.60	3.29 ± 0.19	10.86 ± 0.19	[28]
Orange waste	20.17 ± 0.08	19.31 ± 0.11	79.83 ± 0.08	4.27 ± 0.11	15.90 ± 0.11	[29]

ST, total solids; SV, volatile solids; %H, moisture; %Cz, ash; %SV volatile solids.

.

According to [28], orange and lemon peels generate 3.29% ash and 85.90% humidity. However, after testing the material, 0.60% of ash is obtained, a difference attributed to the fact that the volatile solids of the analyzed samples provide higher data; it burns more material in the furnace and produces very little ash, less than the expected amount of 2.57%. Bearing in mind that the moisture content, ash, and volatile solids will affect the digestion process of microorganisms, it was concluded that organic waste has a high performance as substrates since, having a high moisture and volatile solids content, the microorganisms will possess a way to mobilize and efficiently made their digestion process, also with little interference from the ashes.

Table 2. Comparison of the C/N ratio of different authors with the test carried out in the citrus laboratory.

Sample	C/N	Reference
Citrus waste (lemon and orange)	33.69	This study
Citrus waste	28.00	[31]
Citrus waste	18.12	[29]
Citrus waste	27.50	[32]
Fruit waste	33.00	[30]

shows different values of the carbon/nitrogen ratio of the citrus residues, demonstrating close values in the carbon content with respect to the percentage of nitrogen in the sample, thus demonstrating that a comparison with the registered value and those obtained in the bibliography have values similar to those reported by [30] and closer than [31].

3.2. Stabilization of Methanogenic Microorganisms (UASB Reactor)

Daily data collection was carried out, and the variations in the VFA, alkalinity, temperature, pH, biogas, and percentage of methane produced were evident. These variations were taken into account, as well as the quantity and concentration of food entered into the biodigester. A methane yield of $0.044 {\mathrm{m}}^3{\mathrm{C}\mathrm{H}}_4{\mathrm{k}\mathrm{g}\left(\mathrm{C}\mathrm{O}\mathrm{D} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{d}\right)}^{-1}$ was obtained, which is similar to that reported by [33,34].

Analyzing the data for each evaluated aspect, it was observed that volatile fatty acids (VFA) observed an increasing behavior, alkalinity gradually decreased, pH decreased, and methane concentration stabilized within a range of 60% to 70% (Figure 3). These behaviors were similar to those reported by [26,35] in the stabilization of the UASB reactor, which allows for establishing the starting point to start joining the process of hydrolysis and methanogenesis.

Figure 4a provides evidence that in the acclimatization period, the pH remained in the range of 6.5 to 8, which is similar to that reported by [36], without exceeding the pH limits where it is reported that at pH higher than 8.0 and less than 6.0, there is a sudden drop in the concentration of methane produced.

In Figure 4b, the behavior of alkalinity against the amount of VFAs agrees with the premise reported by [37] that the relationship is inversely proportional between these two variables. While the concentration of VFAs in the culture medium decreased, the alkalinity increased, establishing an active behavior of the methanogenic bacteria that used the VFAs for their metabolic processes.

In Figure 4c, biogas concentration values have been reported, divided into two of the main compounds produced by metabolic processes, which are $\mathrm{C}{\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{H}}_4$, denoted as a percentage (% v/v). The stabilization parameter was the concentration of methane superior to 60% [26,38,39].

In Figure 4d, COD values obtained throughout the stabilization process of the UASB can be observed, evidencing a significant decrease in value due to microbial activity, which is related to the increase in the removal percentage, reaching a value of 93%. This value is superior to that reported by [40], managing to establish the indicated starting point to establish the two phases of the process (hydrolysis and methanogenesis) and ensuring that the bacteria of the activated inoculum carry out their metabolic process in an ideal way.

3.3. Stabilization of Hydrolytic and Acetogenic Reactor (Packed Bed Reactor)

Daily data collection was carried out, from which the variations in the liquid and gaseous phases of the reactor are evident. Figure 5a shows the pH profile for the packed bed reactor, where a downward trend is identified with an average of 4.97, which is close to the range established by [41], where it was between 5.0 and 6.0, optimal for the degradation of organic matter. It is mentioned that there is a need to carry out a pretreatment of citrus peels to increase the yield of methane production because the lignocellulose content inhibits the microbial activity of the inoculum, limiting the bacteria to not fully take advantage of the carbon content of the shells, according to [42]. The objective of the packed bed reactor is to maintain a low pH to increase the degradation of inhibitory compounds.

In Figure 4b, the VFA concentration obtained an average of 2813 mg${\left(\mathrm{L}\right)}^{-1}$, with the maximum VFA obtained being 4241 mg${\left(\mathrm{L}\right)}^{-1}$ on day 5, ranging from 4000 mg${\left(\mathrm{L}\right)}^{-1}$ to 7000 mg${\left(\mathrm{L}\right)}^{-1}$ as reported by [43], which indicates that it reached the acidogenesis phase, thus defining the union of the reactors. This is corroborated by the alkalinity profiles where the reduction in this variable is evidenced due to the considerable increase in VFA in the reactor according to what was reported by [37]; under reactor overload conditions and the presence of inhibitors, the growth rate of methanogenic microorganisms is lower than the production of VFA, leading to the consumption of alkalinity, as well as the increase in the concentration of CO₂.

On the other hand, in Figure 5c, it is evident that as the COD in reactor outflow increases, a significant increase in the organic load and the concentration of various substances in the effluent is observed, as reported by [44], which considers the increase in COD the citrus waste degrades.

3.4. Coupled System

Following the stabilization process of the UASB reactor, the packed bed reactor was put into operation, where the degradation of the organic material was carried out, generating the hydrolysis and acidogenesis stage of the system. The UASB reactor was operated with increasing OLR variations, visualizing an increase in the amount of methane produced when increasing the organic loading rate.

The behavior of the data at the beginning of the hydrolysis and acidogenesis stage, until the union of the two reactors, was favorable to what was reported in the literature according to [35], who managed to define homogeneous and reproducible behavior, with variables such as pH, VFA, biogas concentration and COD for both reactors. In Figure 6a, the specific behavior of the pH before and after the union of the reactors is shown, evidencing a semi-constant behavior before starting the union of the two systems, where it can be evidenced that the coupling of the systems did not affect the pH of the UASB reactor and remained in the same range. As the VFA of the coupled system was achieved, its acidity decreased, and its pH increased until a similar final value in both reactors of approximately 9.35, as reported by [37].

In Figure 6b, the VFA concentration profiles for the reactors in their pooling period are presented, where an accelerated decrease in the VFA concentration in the packed bed reactor is observed immediately after reaching its maximum value and was more noticeable after the system was engaged, due to the constant consumption in the UASB reactor of the leachate removed from the packed bed reactor, to the point of needing an excessive amount of leachate to maintain the organic loading rate at 5.5 kg (m3d)⁻¹. In turn, the UASB reactor increased its VFA concentration due to the feed variation; however, it did not affect metabolic activity.

As shown in Figure 6, the behavior of the variables mentioned above can be identified days before the coupling of the biodigesters (packed bed and UASB) and after their coupling, thus giving the results of the different variables in the degradation stage of the citrus waste, indicated by the vertically drawn line. On the other hand, Figure 5c shows the behavior of the alkalinity of the coupled system, showing an equalization of the values of both reactors at the time of finishing the digestion process completely; it shows how the value of alkalinity in the packed bed reactor increases at the end due to the reduction in the VFA produced as analyzed in Figure 6b, and the same behavior reported by [37] is observed.

At the same time, in Figure 6d, a considerable decrease in COD is evident after having the system coupled due to the fact that a large part of the leachate generated in the hydrolysis reactor was transferred to the UASB reactor for its subsequent methanogenic process. This leads to the need for a greater amount of leachate to maintain the organic loading rate according to [45], until the transferred volume value exceeds 50% of the total volume of the 2 L UASB reactor and determines the end of the shell digestion process. As a consequence, a less aggressive reduction in the UASB was also noted, due to the fact that as the COD in the packed bed reactor was reduced, the UASB was fed. However, withdrawing more and more inoculum to be able to enter the leachates, which allows both reactors to be equalized in the mentioned control variables, is similar to that reported by [35]. Another trace compound could be present in the biogas mixture, according to [3], but was not measured.

On the other hand, in Figure 6e, the behavior of the acetogenesis and methanogenesis stage is shown, visualizing a high concentration of methane produced, with an average concentration of 54.92% in its union period, achieving a yield of $0.51 \mathrm{L}\mathrm{C}{\mathrm{H}}_4{\left(\mathrm{g}\mathrm{S}\mathrm{V}\right)}^{-1}$. These variables were compared with a bibliographic review, establishing similar behavior parameters, as demonstrated by [26,38,39], with a production range around 60% methane in the two-phase anaerobic digestion process. The other authors named in Table 3 report similar values.

In Figure 6f, the behavior of the concentration values in the packed bed reactor is shown, exhibiting low $\mathrm{C}{\mathrm{H}}_4$ data because it is in the degradation stage, and the generated biogas releases greater amounts of $\mathrm{C}{\mathrm{O}}_2$ due to the microbial respiration. At this stage, the production of VFAs is prioritized due to the hydrolysis of the carbon chains of organic residues (citrus) without the fermentation process.

On the other hand, the COD removal from the coupled system gave a value of 62.08%, a slightly lower value compared with [46], which was a value of 80% COD removal, with differences mainly in the rate of organic load used and the installed system. In Table 3, there is a comparison of the yields of different substrates, based on the volatile solids they contain, evidencing that the substrate (citrus waste) has a yield of approximately 0.51$\mathrm{L}\mathrm{C}{\mathrm{H}}_4{\left(\mathrm{g}\mathrm{S}\mathrm{V}\right)}^{-1}$, which is a value close to onion residues $0.43 \mathrm{L}\mathrm{C}{\mathrm{H}}_4 {\left(\mathrm{g}\mathrm{S}\mathrm{V}\right)}^{-1}$ and thus determining the superior capacity of citrus to produce methane as evidenced (Table 3).

Table 3. Comparison of the performance of various substrates.

Sample	$\mathbf{Performance} \mathbf{L}\mathbf{C}{\mathbf{H}}_4 {\left(\mathbf{g}\mathbf{S}\mathbf{V}\right)}^{-1}$	Reference
Citrus waste	0.51	This study
Onion residues	0.43	[26]
Coffee by product	0.48	[47]
Dairy cattle waste	0.22	[48]

Table 3. Comparison of the performance of various substrates.

Sample	$\mathbf{Performance} \mathbf{L}\mathbf{C}{\mathbf{H}}_4 {\left(\mathbf{g}\mathbf{S}\mathbf{V}\right)}^{-1}$	Reference
Citrus waste	0.51	This study
Onion residues	0.43	[26]
Coffee by product	0.48	[47]
Dairy cattle waste	0.22	[48]

Finally,

Table 4. Solids reduction comparison.

Sample	%ROM	Reference
Citrus waste	27.30%	This study
Citrus waste	20.00%	[49]
Stillage	22.40%	[50]
Pig effluent (slurry)	15.00%	[51]

%ROM removal of organic matter.

shows a comparison of the organic matter removal percentage, determining that the citrus waste obtained a degradation of 27.30%, with an initial weight of 500 g of citrus waste. Once the process was finished, it reached a weight of 363.50 g, with a degraded amount of 136.50 g, and in the range of citrus residues and stillage.

As for the use of digestate as a fertilizer, it has become an increasingly popular practice in sustainable agriculture due to its numerous environmental and agronomic benefits. Several studies support the efficacy of digestate (with a ratio similar to that posited in this study of 363.5 g per 500 g of original residue) as a source of plant nutrients and its ability to improve soil health.

For example, according to Smith et al. (2018) [52], digestate application can increase soil nutrient availability and promote higher crop yields. Likewise, Garcia et al. (2016) [10] point out that the use of digestate can improve soil structure and increase its capacity to retain water, which is especially beneficial in regions with variable climatic conditions.

On the other hand, Zhang et al. (2020) [8] found that digestate can reduce the need for synthetic chemical fertilizers, which contributes to more sustainable and environmentally friendly agriculture.

Furthermore, highlighting the crucial role of digestate in the circular economy underscores the transformation of organic waste into valuable resources for agricultural production, promoting sustainability and resource efficiency. Digestate therefore represents a valuable source of nutrients and organic matter for plants. Its use as a fertilizer can improve soil fertility, increase crop yields, and promote more sustainable and environmentally friendly agricultural practices.

4. Conclusions

In conclusion, this study provided a comprehensive analysis of the anaerobic digestion of citrus residues, with a particular focus on orange and lemon peels. The economic importance of citrus in Colombia is undeniable, but at the same time, the considerable waste production generated presents significant environmental challenges.

The implementation of the two-phase anaerobic digestion system, consisting of a hydrolysis reactor and a methanogenic UASB reactor, has been shown to be an effective strategy to address this issue. The meticulous collection of physicochemical data during the process allowed for an in-depth understanding of the transformations in the residues throughout the hydrolysis and methanogenesis phases. The results obtained indicate not only a substantial reduction in waste but also a considerable production of methane. Daily analysis of the gases generated, such as methane, carbon dioxide, hydrogen, and hydrogen sulfide, offered valuable insights into the system’s performance.

Importantly, despite the inherent limitations of anaerobic digestion, this study provides a solid basis for future improvements. Specifically, it is recommended to implement a physicochemical pretreatment of orange peels to remove essential oils that can inhibit microbial activity, potentially significantly increasing methane production. Ultimately, this research not only contributes to the understanding of sustainable citrus waste management but also highlights promising opportunities for biogas production. Continuing these efforts, including consideration of pretreatment improvements and process optimization, will certainly open doors to more effective and environmentally sustainable solutions in the agro-industrial landscape.

Anaerobic digestion, although an efficient method for the conversion of organic waste into biogas, has notable disadvantages, among which are bad odors, requiring meticulous environmental control and operating conditions. However, the proposal to implement two-phase anaerobic digestion, as addressed in the article, emerges as a strategy to mitigate this drawback. This modality offers a greater control capacity over the process variables, which makes it possible to reduce undesired emissions, thus contributing to improving the environmental and operational acceptability of the technology.