Introduction
Agricultural waste comprises a broad range of organic materials generated throughout agricultural production, including plant residues, livestock and poultry excrement, and by-products from agricultural processing (Dai et al., Reference Dai, Sun, Wang, Lu, Liu, Li, Yang, Sun, Zhang, Xu, Zheng, Hu, Yang, Gao, Chen, Zhang, Gao and Zhang2018). The global volume of agricultural waste is projected to increase by 44% between 2005 and 2025, particularly in developing economies, primarily due to rising post-harvest losses driven by population growth and changing consumption patterns (Abdelsalam et al., Reference Abdelsalam, Same, Amer and Amer2021). This surge in agricultural waste presents significant environmental and economic challenges, especially in developing countries where landfilling remains the dominant disposal method (Zhang et al., Reference Zhang, Chen, Zhang, Liu, Chen, Yang, Osman, Farghali, Liu, Hassan, Ihara, Lu, Rooney and Yap2024). However, landfills contribute to severe environmental and public health issues, as organic waste decomposes anaerobically, releasing harmful leachates, volatile fatty acids (VFAs), ammonia, and methane (CH₄), a potent greenhouse gas (Fisgativa, Tremie and Dabert, Reference Fisgativa, Tremie and Dabert2016). Landfills are now recognized as the third-largest global source of methane emissions, accounting for approximately 11% of total emissions (Oduor et al., Reference Oduor, Wandera, Murunga and Raude2022).
To mitigate these environmental concerns, biological treatment methods such as anaerobic digestion (AD) have gained prominence in managing high-organic-content agricultural waste. AD effectively converts organic waste into biogas, a renewable energy source, while simultaneously producing nutrient-rich digestates for agricultural use (Triviño-Pineda, Sanchez-Rodriguez and Peláez, Reference Triviño-Pineda, Sanchez-Rodriguez and Peláez2024). While conventional AD has been widely employed, co-digestion, which involves the simultaneous breakdown of multiple substrates, has shown superior biogas yield and process stability (Kesharwani and Bajpai, Reference Kesharwani and Bajpai2020). Several key operational parameters influence AD efficiency, including pH, temperature, organic loading rate, and hydraulic retention time (Parajuli et al., Reference Parajuli, Khadka, Sapkota and Ghimire2022). The methane yield varies depending on the chemical composition of the feedstock, with chemical oxygen demand (COD) serving as a key indicator of biogas production potential (Fisgativa et al., Reference Fisgativa, Tremie and Dabert2016). However, excessive soluble COD can lead to VFAs accumulation, causing a pH drop that inhibits methanogenesis and slows biogas production (Zhao et al., Reference Zhao, Jeanne, Hua, Huang, Droste, Chen, Wang, Yang and Yang2020). Total solids (TS) and volatile solids (VS) are also critical indicators of AD efficiency, as TS affects the degradation rate of the substrate while VS enhances biogas production(Oduor et al., Reference Oduor, Wandera, Murunga and Raude2022).
Although AD has demonstrated success in treating wastewater sludge and animal waste, its application to agricultural waste has encountered technical challenges, particularly process instability caused by excessive VFA buildup (Oduor et al., Reference Oduor, Wandera, Murunga and Raude2022). To improve process efficiency and mitigate these inhibitory effects, strategies such as two-stage AD and co-digestion have been explored (Ding et al., Reference Ding, Chen, Xu and Hu2021). While two-stage AD enhances hydrolysis and methane production, it is often costly and time-consuming, making co-digestion a more practical and economically viable alternative for treating agricultural waste (Rajendran et al., Reference Rajendran, Mahapatra, Venkatraman, Muthuswamy and Pugazhendhi2020).
The fruit and vegetable industry significantly contributes to agricultural waste, primarily due to seasonal overproduction and market quality standards (Plazzotta et al., Reference Plazzotta, Manzocco and Nicoli2017). Jackfruit (Artocarpus heterophyllus), one of the most perishable fruits, is particularly susceptible to microbial and enzymatic spoilage due to its high moisture and sugar content (Nansereko, Muyonga and Byaruhanga, Reference Nansereko, Muyonga and Byaruhanga2022). Its short shelf life often results in substantial post-harvest losses, leading to waste accumulation and associated public health concerns. While jackfruit is predominantly cultivated in Asia, with India and Bangladesh producing approximately 1.25 million metric tons annually (Balamaze, Muyonga and Byaruhanga, Reference Balamaze, Muyonga and Byaruhanga2019), Uganda also generates significant amounts of jackfruit waste. Although precise national production data are lacking, an estimated 0.3 million metric tons are produced in the central, eastern, and western regions of Uganda (Balamaze et al., Reference Balamaze, Muyonga and Byaruhanga2019). Unlike many other fruits, only 25–35% of the jackfruit is edible, with the remainder discarded as waste (Swami et al., Reference Swami, Thakor, Orpe and Kalse2016). Poor management of jackfruit waste leads to anaerobic decomposition, releasing methane and carbon dioxide, both potent greenhouse gases that contribute to global warming (El-Ramady et al., Reference El-Ramady, Brevik, Bayoumi, Shalaby, El-Mahrouk, Taha, Elbasiouny, Elbehiry, Amer, Abdalla, Prokisch, Solberg and Ling2022).
In rural Uganda, jackfruit waste is commonly repurposed as animal feed, whereas in urban areas like Kampala, it is typically disposed of in landfills alongside other agricultural waste. Its high starch (29.05–59.54%) and sugar (2.04–68.8%) content make it a promising feedstock for anaerobic digestion(Nsubuga et al., Reference Nsubuga, Banadda, Kabenge and Wydra2021), exceeding the optimal range of 20–30 required for efficient biogas production (Das and Mondal, Reference Das and Mondal2016). To optimize the C/N ratio for AD, jackfruit waste can be co-digested with nitrogen-rich substrates such as poultry droppings, cow dung, or activated sludge, which have C/N ratios below 20 (Rabii et al., Reference Rabii, Aldin, Dahman and Elbeshbishy2019). Maintaining a balanced C/N ratio is crucial, as an excessively low ratio can lead to ammonia accumulation and pH elevation, inhibiting methanogenesis (Algapani et al., Reference Algapani, Qiao, Di Pumpo, Wandera, Adani F and Dong2016), while a high ratio can result in rapid nitrogen depletion, reducing biogas yield (Oduor et al., Reference Oduor, Wandera, Murunga and Raude2022).
Jackfruit waste is also lignocellulosic in nature, containing lignin (1.79–4.03%), cellulose (5.35–26.43%), and hemicellulose (6.81–10.66%) (Nsubuga et al., Reference Nsubuga, Banadda, Kabenge and Wydra2021). This composition presents a challenge for AD, as lignin forms a structural barrier that hinders microbial hydrolysis, reducing the efficiency of biogas production (Donkor et al., Reference Donkor, Gottumukkala, Lin and Murphy2022). The presence of lignocellulose in jackfruit waste can slow down the digestion process, necessitating appropriate pretreatment methods to enhance its biodegradability and optimize anaerobic digestion efficiency (Kamperidou and Terzopoulou, Reference Kamperidou and Terzopoulou2021).
While several studies have explored the anaerobic digestion of jackfruit waste, most have focused on mono-digestion or limited co-substrate combinations, often without assessing the quality of the resulting digestate for agricultural reuse (Swami et al., Reference Swami, Thakor, Orpe and Kalse2016; Das and Mondal, Reference Das and Mondal2016; Donkor et al., Reference Donkor, Gottumukkala, Lin and Murphy2022). Furthermore, there is a lack of research integrating both energy output (biogas yield) and the agronomic value of the digestate, particularly in Sub-Saharan African contexts where resource recovery and soil fertility are critical (Nsubuga et al., Reference Nsubuga, Banadda, Kabenge and Wydra2021; El-Ramady et al., Reference El-Ramady, Brevik, Bayoumi, Shalaby, El-Mahrouk, Taha, Elbasiouny, Elbehiry, Amer, Abdalla, Prokisch, Solberg and Ling2022).
This study aims to bridge these gaps by evaluating the co-digestion of jackfruit waste with cow dung and poultry droppings, assessing both biogas production potential and the nutrient composition of the digestate. In doing so, it provides practical insights into substrate synergies and quantifies the potential of digestate to substitute synthetic fertilizers, while also estimating firewood displacement from biogas energy output. By addressing both energy and nutrient recovery, this work contributes to advancing circular bioeconomy practices in agricultural waste management and renewable energy generation.
Materials and methods
The study strictly adhered to the biochemical methane potential (BMP) protocol (Filer, Ding and Chang, Reference Filer, Ding and Chang2019) to ensure methodological accuracy and reliability. The process began with substrate preparation, ensuring that all materials complied with BMP guidelines. To enhance the efficiency of anaerobic digestion, an alkaline substrate pretreatment method was employed (Kim, Lee and Kim, Reference Kim, Lee and Kim2016), optimizing the breakdown of organic matter. Inoculum preparation was conducted with careful attention to preserving the microbial community’s integrity, a crucial factor in maintaining stable digestion conditions. The anaerobic digestion process was then executed in strict accordance with BMP standards, ensuring precise measurements and reproducibility of results. This structured and methodical approach reinforced the study’s validity, guaranteeing accurate assessment of methane production potential and digestate quality.
Substrate preparation
Jackfruits at physiological maturity were harvested from Kangulumira Subcounty, Kayunga District (0°35′54.04″N, 33°2′46.34″E, 1121 m above sea level) in Central Uganda, a region renowned for jackfruit production and processing. The fruits were initially cleaned with potable water and manually cut into smaller pieces using a panga. The edible portion was separated from the waste using traditional methods.
Five kilograms of jackfruit waste were shredded using an organic shredder to reduce particle size to the micrometer scale, thereby increasing its surface area and improving digestibility. This mechanical breakdown was essential for enhancing microbial access to the substrate during anaerobic digestion. The shredding process also helped to homogenize the waste, ensuring a more consistent composition for subsequent treatments.
Cow dung and poultry droppings (5 kg each) were sourced from Makerere University Agricultural Research Institute Kabanyoro (MUARIK) (0°28′00.38″ N, 32°36′46.01″ E, 1161 m above sea level). The poultry droppings and cow dung were mixed with deionized water at ratios of 1 kg/3 L and 1 kg/1 L, respectively, to reduce dry matter content, ensuring ease of pouring and uniform distribution. The poultry droppings mixture (PM) was then combined with jackfruit waste biochar at a concentration of 15 g/L to enhance its biogas production potential (Yu et al., Reference Yu, Sun, Liu, Yellezuome, Zhu, Bai, Liu and Sun2021).
Alkaline pretreatment
Due to its lignocellulosic composition, the crushed jackfruit waste was pretreated with 12% (w/w) NaOH, following the recommendation of Djaafri et al. (Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020). For this, 1.2 g of NaOH were precisely weighed and added to each of six 250 mL glass flasks, followed by distilled water to reach a final volume of 100 mL. The flasks were then shaken at 250 rpm for 6 hours at room temperature to ensure complete NaOH dissolution.
Next, 27.54 g of total solids (TS) from the crushed jackfruit waste was added to three of the flasks, mixed with NaOH in a solid-to-liquid ratio of 1 g TS:10 mL NaOH solution, as per Park and Kim (Reference Park and Kim2012). A control flask containing 12 g TS of jackfruit waste in 100 mL of distilled water was also prepared. All flasks were then placed in a GRIFFCHEM HH-S6 water bath and maintained at 60 °C for 24 h, following the protocol by Park and Kim (Reference Park and Kim2012).
This procedure ensured consistent and effective alkaline pretreatment of the jackfruit waste, preparing it for further analysis. The pH and COD values of both the control and pretreated samples were measured at the beginning and end of the process to assess organic matter solubilization.
Inoculum preparation
The third step focused on inoculum preparation. Five liters of bio-slurry were obtained from an operational cow-dung-fed biodigester at MUARIK. To remove residual biogas, the inoculum was pre-incubated under anaerobic digestion conditions at 37 °C for 10 days using a Bio-H2-Umwelt GmbH bioreactor (Kesharwani and Bajpai, Reference Kesharwani and Bajpai2020). This process minimized biogas production from the inoculum and ensured the complete degradation of any remaining organic matter, following the methodology of Nsubuga et al. (Reference Nsubuga, Kabenge, Zziwa, Yiga, Mpendo, Harbert, Kizza, Banadda and Wydra2023) and Djaafri et al. (Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020). The inoculum served as a vital source of microorganisms necessary to initiate anaerobic digestion.
Anaerobic digestion
The final phase focused on anaerobic digestion, conducted through a batch experiment in the laboratory following the biochemical methane potential (BMP) Assay protocol for anaerobic digestion research (Filer et al., Reference Filer, Ding and Chang2019). The alkaline-pretreated jackfruit waste was first neutralized to a pH of 6.8–7.2 using 3 M hydrochloric acid (HCl). Next, various substrate mixtures were carefully prepared in triplicate by blending jackfruit waste with poultry manure (PM) and cow dung (CW) in different proportions. These mixtures were labeled as follows: inoculum only (blank), 100% jackfruit waste (control), 75% JF:25% CW, 50% JF:50% CW, 75% JF:25% PM, and 50% JF:50% PM. The selected ratios ensured that each mixture contained at least 50% jackfruit waste, which served as the primary substrate in the study.
Following the recommendations of Al-Iraqi et al. (Reference Al-Iraqi, Gandhi, Folkard, Barker and Semple2023), an inoculum-to-substrate ratio of 1:2 was maintained throughout the experiment. Temperature was regulated at 37 °C (mesophilic conditions) using a GRIFFCHEM HH-S6 water bath, while manual shaking of the individual conical flask reactors was performed once daily to ensure proper mixing. To maintain an airtight system, the delivery tubes were securely fitted into the cocks, which were then tightly sealed into the conical flask reactors. To prevent CO2 diffusion during biogas measurement, acidified water with a pH of 3, as per ASTM D 5511 standards, was used in the liquid displacement method, enhancing measurement accuracy (Frommert, Muller and Jorg, Reference Frommert, Muller and Jorg2004). Biogas production continued throughout the 30-day hydraulic retention time (HRT) until no degradable material remained.
Analytical methods
Jackfruit waste, cow dung, and chicken droppings were characterized for total solids (TS), volatile solids (VS), and total nitrogen following the standard methods outlined by Nsubuga et al. (Reference Nsubuga, Banadda, Kabenge and Wydra2020). Biogas production was measured in triplicate using the liquid displacement method, with biogas volume recorded every third day. The methane content of the biogas was analyzed at the end of the experiment using a portable Geotech GA5000 gas analyzer (Italy), following the procedure described by Mibulo et al. (Reference Mibulo, Nsubuga, Kabenge, Kiggundu and Wydra2023). The cumulative biogas yield was determined by summing the daily biogas production over the 30-day period.
pH was measured using a Mettler-Toledo AG pH meter, while total alkalinity (TA) was determined following the procedures outlined by APHA (1999). Volatile fatty acids (VFA) were assessed through titrimetric analysis, with results expressed in acetic acid equivalents, as per the method described by Anderson and Yang (Reference Anderson and Yang1992). Chemical oxygen demand (COD) was measured using the closed reflux titrimetric Standard method 5220 C, incorporating a centrifuge device to separate the liquid from the substrate (Angelidaki et al., Reference Angelidaki, Alves, Bolzonella, Borzacconi, Campos, Guwy, Kalyuzhnyi, Jenicek and Lier2009). The COD removal rate (BD) was then calculated using Equation 1, as specified by Djaafri et al. (Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020).
$$ {COD}_{removal}\%=\left(1-\frac{COD_f}{ COD i}\right)\times 100 $$
Where
$ {COD}_f $
and
$ CODi $
are the final and initial CODs during anaerobic digestion.
Lignin, cellulose, and hemicellulose contents were determined following the methods described by Nsubuga et al. (Reference Nsubuga, Banadda, Kabenge and Wydra2021). Digestate characterization included pH, total solids (TS), total nitrogen, phosphorus, potassium, organic matter, cation exchange capacity, and heavy metals (Cu, Zn, Cd, Pb, Ni). TS was measured using the standard oven-drying method outlined by APHA (1999), while pH was determined using a portable Mettler-Toledo FG2 pH meter.
Total Kjeldahl nitrogen (TN) was analyzed using the calorimetric method specified by Nsubuga et al. (Reference Nsubuga, Banadda, Kabenge and Wydra2021). Phosphorus levels were assessed using the modified Olsen-P method (Watanabe and Olsen, Reference Watanabe and and Olsen1965). Calcium and magnesium were quantified via the EDTA titrimetric method following ammonium acetate extraction (1 N, pH 7), while sodium and potassium concentrations were measured using the flame photometric method after ammonium acetate extraction (1 N, pH 7) (Solé-Bundó et al., Reference Solé-Bundó, Cucina, Folch, Tàpias, Gigliotti, Garfí and Ferrer2017).
Heavy metal analysis was conducted by digesting samples according to the US EPA 3050B method (Massaccesi et al., Reference Massaccesi, Sordi, MicaleC, Zadra, Di Maria and Gigliotti2013), with concentrations determined using a Shimadzu AA-6800 flame atomic absorption spectrometer. Pathogenic content in the digestates was evaluated by quantifying Escherichia coli (E. coli) using a culture-based method as outlined by Le Maréchal et al. (Reference Le Maréchal, Druilhe, Repérant, Boscher, Rouxel, Le Roux, Poëzévara, Ziebal, Houdayer, Nagard, Barbut, Pourcher and Denis2019). All measurements were performed in triplicate, and the average values were used for analysis.
Statistical analysis
All experiments in this study were conducted in triplicate, and the collected data were similarly replicated. Data analysis was performed using Genstat software (14th edition) to compute the mean values for each treatment (co-digestion mixtures) and the control (jackfruit waste alone). A one-way analysis of variance (ANOVA) was applied to all evaluated parameters. Differences among treatment means were assessed using Tukey’s post hoc test following one-way ANOVA to determine statistical significance at p ≤ 0.05.
Results and discussion
Substrate characteristics
The results for the total solids (TS), volatile solids (VS), and total nitrogen (TN) content of chicken droppings were 78.66%, 79.02%, and 2.14%, respectively (Table 1). These findings are consistent with those reported by Ali, Afify and Ghanem (Reference Ali, Afify, Ghanem and Awdallah2018) for chicken droppings. For cow dung, the TS, VS, and TN content were 23.56%, 82.76%, and 1.34%, respectively (Table 1), aligning with the results obtained by Jackfruit waste, on the other hand, had TS, VS, and TN values of 24.5%, 80.15%, and 1.57%, respectively (Table 1), reflecting the findings of Nsubuga et al. (Reference Nsubuga, Banadda, Kabenge and Wydra2021). Volatile solids provide an estimate of a substrate’s potential for biogas production. Given its higher VS content, cow dung is expected to yield more biogas compared to chicken droppings.
Table 1. Characterization of poultry droppings, cow dung, and jackfruit food waste
FM, fresh matter.
z Treatments with different letters within row are significantly different (p ≤ 0.05) according to Tukey’s post hoc test.
The optimal TS content for wet anaerobic digestion of organic substrates is generally considered to fall within the range of 9.0–10.16%, as indicated by Kelly (Reference Kelly2017). However, the TS values of chicken droppings (78.66%), cow dung (23.56%), and jackfruit waste (24.5%) exceed this recommended range, which suggests that these substrates are too solid for efficient wet anaerobic digestion. To address this, the substrates were diluted with water to lower the TS content, thereby enhancing microbial activity and promoting increased biogas and digestate production, as recommended by Beevi, Jose and Madhu (Reference Beevi, Jose and Madhu2013). A recent study by Elsayed et al. (Reference Elsayed, Kakar, Abdelrahman, Ahmed, Alsayed, Zagloul, Muller, Bell, Santoro, Norton, Marcus and Elbeshbishy2024) also supports this strategy, demonstrating that dilution significantly improves substrate bioavailability and microbial performance under mesophilic conditions.
Statistical analysis revealed that the TS content of cow dung and jackfruit waste was not significantly different (p ≥ 0.05), likely due to their high-water content. Cow dung’s water content arises from both the water consumed by cows and the moisture in plant materials they ingest, which contributes to the high water content in the manure(Gupta, Aneja and Rana, Reference Gupta, Aneja and Rana2016). Similarly, jackfruit is known for its high water content, which significantly influences the size and quality of the fruit (Hou et al., Reference Hou, Li, Zhang, Yao and Wang2021). The substantial water retention in jackfruit waste leads to its lower TS content.
In contrast, chicken droppings had a significantly higher TS (p ≤ 0.05) compared to both cow dung and jackfruit waste. This can be attributed to the nature of chicken feed, which consists mainly of grains, seeds, and other solid food materials. Chickens, with their relatively short digestive systems, absorb less water, resulting in a higher solid content in their droppings (Pan and Yu, Reference Pan and Yu2014). Moreover, the protein-rich diet of chickens increases the excretion of solid waste (Hill et al., Reference Hill, Sayadi, Gendreau, Liu, Pitesky ME and Simmons2023), contributing to the higher TS. These observations are consistent with more recent evaluations by Dhungana et al. (Reference Dhungana, Lohani and Marsolek2022), who noted that high-moisture agro-wastes like jackfruit and mango peels typically require co-digestion to optimize moisture balance.
The VS as a percentage of TS showed no significant difference (p ≥ 0.05) across chicken droppings, cow dung, and jackfruit waste. Since VS represents the organic and biodegradable fraction of the substrate, this similarity suggests that all three substrates are composed of comparable organic material, making them similarly susceptible to microbial decomposition (Wang et al., Reference Wang, Hu, Wang, Wu and Zhan2023). This aligns with recent work by Prabhudessai et al. (Reference Prabhudessai, Ganguly and Mutnuri2013), who emphasized that VS content alone may not be a reliable predictor of methane yield unless it is accompanied by information on substrate biodegradability and lignin content.
Total nitrogen content in chicken droppings was significantly higher (p ≤ 0.05) compared to cow dung and jackfruit waste. This observation was in agreement with Tawfik et al. (Reference Tawfik, Eraky, Osman, Zhou, Meng and Rooney2023), who found that Chicken droppings are rich in ammonia-nitrogen. This higher nitrogen content is linked to the high-protein diet of chickens, which metabolizes into nitrogenous compounds. The rapid digestive system of chickens means food passes through quickly, leading to reduced nitrogen absorption and higher nitrogen concentration in the droppings (Pan and Yu, Reference Pan and Yu2014). The elevated ammonium nitrogen in chicken droppings may pose a challenge for anaerobic digestion, as it can be toxic to the anaerobic bacteria involved in biogas production, especially due to the low carbon-to-nitrogen (C/N) ratio (Tawfik et al., Reference Tawfik, Eraky, Osman, Zhou, Meng and Rooney2023). To mitigate this, the addition of carbon-rich materials could help balance the C/N ratio and enhance biogas production. This recommendation is supported by recent modeling efforts by Yang et al. (Reference Yang, Zhou, Hang, Chen, Liu, Su, Lv, Jia and Zhao2024), who emphasized that blending poultry manure with high-carbon agricultural waste significantly reduces ammonia inhibition and improves methane yield.
The lignin, cellulose, and hemicellulose composition of jackfruit waste was found to be 3.08%, 25.42%, and 7.31% of TS, respectively (Table 1). Cellulose represented the largest fraction of the lignocellulosic composition, followed by hemicellulose and lignin. This trend is consistent with the findings of Nsubuga et al. (Reference Nsubuga, Banadda, Kabenge and Wydra2021). The presence of lignin, cellulose, and hemicellulose in jackfruit waste highlights its lignocellulosic nature, which requires pretreatment to improve biogas production. These components act as physical barriers to microbial degradation, making the hydrolysis process more difficult and reducing the overall efficiency of anaerobic digestion. Recent advancements by Zhao et al. (Reference Zhao, Sun, Zhang, Nan, Ren, Lee and Chen2022) highlighted the use of alkaline, thermal, or biological pretreatment techniques to break down lignocellulose in fruit waste, significantly enhancing methane yields and digestion kinetics.
Alkaline pretreatment
The changes in pH and COD for untreated and pretreated jackfruit waste are summarized in Table 2. COD, which serves as a primary indicator of hydrolysis, exhibited a notable increase in the final COD of the pretreated jackfruit waste compared to that of the untreated jackfruit waste (control). This increase can be attributed to the NaOH pretreatment, which effectively facilitated the release and dissolution of large organic molecules, including cellulose and hemicellulose, from the jackfruit waste particles. This release enhanced the biodegradability of the substrate, leading to the elevated COD values (Djaafri et al., Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020). A recent study by Darmey et al. (Reference Darmey, Narra, Achaw, Stinner, Ahiekpor, Ansah, N’guessan, Angyekum and Nutakor2025) confirmed that alkaline pretreatment disrupts the lignocellulosic structure of agro-wastes, increasing soluble chemical oxygen demand (COD) and accelerating microbial access to degradable substrates. The increase in COD indicates that pretreatment improved the availability of organic material for microbial decomposition, which is crucial for enhancing biogas production.
Table 2. Alkaline pre-treatment of jackfruit food waste
JF, Jackfruit food waste.
The pH, another key parameter indicating hydrolysis, followed a different trend for the two types of jackfruit waste. The initial pH of untreated jackfruit waste was 5.36, and it slightly decreased to 5.21 by the end of the experiment. This slight decrease in pH reflects the natural microbial activity occurring during the degradation of organic matter, leading to the production of organic acids. On the other hand, the initial pH of pretreated jackfruit waste was much higher at 12.24, likely due to the alkaline nature of NaOH used during pretreatment. The final pH of pretreated jackfruit waste decreased to 9.92, indicating the reaction between NaOH and the jackfruit waste components, such as lignin and hemicellulose, which are known to be solubilized and degraded during alkali treatment (Djaafri et al., Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020). This reduction in pH further demonstrates the effectiveness of NaOH pretreatment in breaking down the lignocellulosic material. Similar pH shifts have been reported in recent alkaline pretreatment studies, where reductions from >11 to near-neutral values indicate active lignin solubilization and neutralization of base-labile compounds (Woiciechowski et al., Reference Woiciechowski, Neto, Vandenberghe, Neto, Sydney, Letti, Karp, Torres and Soccol2020).
In addition to the pH drop associated with NaOH treatment, other authors have noted that the decrease in pH during substrate hydrolysis can also result from the production of acids and alcohols, such as acetic acid and ethanol, as by-products of microbial activity (Chandra et al., Reference Chandra, Takeuch, Hasegawa and Kumar2012). The observed pH drop in the pretreated jackfruit waste is thus likely a combination of both chemical reactions and microbial activity. To further optimize conditions for anaerobic digestion (AD), the pH of the pretreated jackfruit waste was adjusted to the recommended range of 6.8–7.2 by adding hydrochloric acid. This pH adjustment is essential for creating an ideal environment for microbial activity during AD, facilitating efficient biogas production.
The results in Table 1 demonstrate that NaOH pretreatment effectively increased the COD, indicating enhanced biodegradability of the jackfruit waste, and reduced the pH, facilitating the breakdown of complex organic matter. These findings align with recent advances in pretreatment science, which highlight the effectiveness of mild-alkaline hydrolysis for enhancing the digestibility of fruit waste by altering lignin and hemicellulose fractions (Khan et al., Reference Khan, Usman, Ashraf, Dutta, Luo and Zhang2022). These changes highlight the potential of pretreatment to improve the performance of biogas production processes and other microbial applications.
Biodegradability and anaerobic digestion kinetics
The results presented in Table 3 highlight the significant differences in COD before AD, COD after AD, and COD removal for the various treatments (p ≤ 0.05). These variations can be attributed to the differences in the co-digestion mixtures used in this investigation. The initial COD values ranged from 61.48 g/L (for the 100% jackfruit waste (JF) mixture) to 91.44 g/L (for the 75JF:25 PM mixture), indicating the high solubility and decomposition potential of the substrates, especially after the alkaline pretreatment of the jackfruit waste. The decrease in COD values after anaerobic digestion (AD) suggests that soluble metabolites were utilized by the microbial community, contributing to the breakdown of organic material. The highest COD removal (47.77%) was observed in the 75JF:25 PM mixture, which also resulted in peak biogas generation. This removal efficiency is consistent with findings from Djaafri et al. (Reference Djaafri, Kalloum, Kaidi, Salem, Balla, Meslem and Iddou2020) and has recently been corroborated by Naran et al. (Reference Naran, Toor and Kim2016), who found that pretreated fruit-manure co-digestion combinations achieved over 45% COD reduction with enhanced methane yields.
Table 3. Biodegradability parameters for different treatments
z Treatments with different letters within row are significantly different (p ≤ 0.05) according to Tukey’s post hoc test.
The VFA/TA ratio before and after AD did not show significant differences (p ≥ 0.05) across treatments. This lack of distinction is likely due to the adaptability of the microbial community in the AD process. During AD, different organic compounds from various substrates can trigger shifts in microbial metabolic pathways, enabling the microorganisms to maintain a relatively stable VFA/TA ratio despite differences in substrate composition. This adaptability is well-documented in the literature (Harirchi et al., Reference Harirchi, Wainaina, Sar, Nojoumi, Parchami, Parchami, Varjani, Khanal, Wong, Awasthi and Taherzadeh2022), and it explains why the VFA/TA ratio remained constant across treatments. Recent microbial studies (Harirchi et al., Reference Harirchi, Wainaina, Sar, Nojoumi, Parchami, Parchami, Varjani, Khanal, Wong, Awasthi and Taherzadeh2022) affirm that syntrophic interactions in mixed-substrate digesters enhance buffer capacity, maintaining stability even under varying acidogenic conditions. The VFA/TA ratio is an essential parameter for assessing AD process stability. If the ratio exceeds 0.5, it can indicate digester imbalances, suggesting instability. In this study, the initial VFA/TA ratios ranged from 0.65 to 0.73, reflecting some instability in the digesters. However, after 30 days, the ratios for all treatments dropped below 0.2, indicating that the digestion processes were functioning properly and that the digester stability was achieved.
The T80 values, representing the time required to achieve 80% of the biogas production, also varied significantly (p ≤ 0.05) across the different treatments, with the 100% JF treatment showing a much longer T80 (23.75 days) compared to the co-digested mixtures (13.89 to 20.68 days). This difference can be attributed to the co-digestion of jackfruit waste with other substrates, such as cow dung and poultry manure, which likely provided additional nutrients and facilitated microbial growth, thus reducing the time required to reach 80% biogas production. The T80 for the 75% JF:25% CW and 50% JF:50% CW treatments were similar (13.89 and 14.7 days, respectively), likely due to the fact that both treatments included cow dung as a co-substrate. Likewise, the T80 for the 75% JF:25% PM and 50% JF:50% PM treatments were also similar (20.68 and 19.73 days, respectively), indicating that the inclusion of poultry manure in the mixture had a comparable effect on degradation. Kinetic models and experimental data from Petrovič et al. (Reference Petrovič, Zirngast, Predikaka, Simonič and Čuček2022) also confirm that alkaline pretreatment shortens lag phases and accelerates biogas generation when combined with nitrogen-rich co-substrates.
The technical digestion time (T80) is a crucial indicator of substrate biodegradability, with shorter T80 values typically indicating faster biodegradation and higher biogas production rates. Alkaline pretreatment of jackfruit waste enhanced its biodegradability by altering the lignin structure and making it more susceptible to microbial attack (Nsubuga et al., Reference Nsubuga, Banadda, Kabenge and Wydra2020). This pretreatment facilitated the release of soluble organic matter, which was more readily converted into biogas by the microbial community, leading to shorter T80 values across all treatments compared to untreated jackfruit waste. The combination of jackfruit waste with cow dung or poultry manure further supported microbial activity, reducing the digestion time and promoting efficient biogas production (Khatri et al., Reference Khatri, Wu, Kizito, Zhang, Li and Dong2015). This synergistic effect of co-digestion and pretreatment has been emphasized by Khan et al. (Reference Khan, Usman, Ashraf, Dutta, Luo and Zhang2022), who recommend targeted substrate combinations to optimize AD kinetics and gas yield.
Daily and cumulative biogas production
The daily and cumulative biogas production data for jackfruit waste co-digested with cow dung and poultry droppings, as presented in Figures 1 and 2, respectively, provide valuable insights into the effects of substrate combinations on biogas yields. Notably, jackfruit waste alone (100%JF) produced the lowest daily and cumulative biogas output when compared to the co-digested mixtures. However, the biogas volume generated from 100%JF surpassed the results reported by Chakravarty (Reference Chakravarty2016) for mono-digestion of jackfruit waste. This discrepancy may be attributed to the NaOH pretreatment applied in the current study, which likely enhanced the biodegradability of jackfruit waste, thereby making it more accessible to anaerobic microorganisms. Recent studies, for example, Khan et al. (Reference Khan, Usman, Ashraf, Dutta, Luo and Zhang2022) have confirmed that alkaline pretreatment of lignocellulosic substrates leads to increased solubilization of carbohydrates, improving microbial accessibility and methane potential.
Figure 1. Daily biogas production from different co-digestion mixtures and the control.
Figure 2. Cumulative biogas production from different co-digestion mixtures and the control.
Among the co-digested substrates, the combination of jackfruit waste and poultry droppings (50%JF:50%PM and 75%JF:25%PM) consistently exhibited the highest daily and cumulative biogas production, followed by the mixtures of jackfruit waste and cow dung. The increased biogas yield observed in these co-digestion combinations is consistent with the findings of Rabii et al. (Reference Rabii, Aldin, Dahman and Elbeshbishy2019), who suggested that co-digestion improves biogas production by optimizing the nutrient balance for microbial activity. This observation is further supported by Zhu et al. (Reference Zhu, Yellezuome, Liu, Wang and Liu2022), who found that co-digestion of food waste, chicken manure, and corn straw significantly increased biogas production compared to the digestion of mono substrates.
Despite poultry droppings being high in nitrogen content (as shown in Table 1), the poultry droppings mixtures outperformed cow dung in terms of gas production. This is likely due to the addition of biochar, which, as indicated by Jurgutis et al. (Reference Jurgutis, Slepetiene, Volungevicius and Amaleviciute-volunge2020), plays a significant role in mitigating ammonia toxicity by enhancing the buffering capacity of the reactor. Biochar’s strong adsorption properties help to prevent the accumulation of ammonium, a common challenge in anaerobic digestion of poultry droppings (Yu et al., Reference Yu, Sun, Liu, Yellezuome, Zhu, Bai, Liu and Sun2021). A study by Giwa et al. (Reference Giwa, Xu, Chang, Wu, Li, Ali, Ding and Wang2019) demonstrated that biochar not only adsorbs excess ammonia but also provides favorable microhabitats for methanogens, improving reactor performance and stability in systems treating high-nitrogen substrates.
This daily biogas production graph aligns with Monod’s microbial growth curve, which describes how microbial growth changes in response to substrate availability (Abubakar et al., Reference Abubakar, Silas, Aji, Taura and Undiandeye2022). An analysis of Figure 1 reveals a distinct lag phase of approximately 1 day where microorganisms adapt to their new environment and start to multiply, and during which no biogas production was observed. This period, which occurred between the setup of the experiment and the onset of biogas production, is likely attributed to the time required for anaerobic microorganisms to become active. During this phase, aerobic microorganisms likely consumed all available oxygen in the digesters (Okonkwo, Onokpite and Onokwai, Reference Okonkwo, Onokpite and Onokwai2018). Once oxygen was depleted, acid-forming bacteria began to dominate, and the system entered the exponential growth (log) phase, characterized by rapid microbial reproduction and optimal biogas production (Jameel et al., Reference Jameel, Mustafa, Ahmed, Mohammed, Jassim, Shakir, Lawas, Mohammed, Khudhur, Mahmoud, Sayadi and Kianfar2024). During this phase, the microorganisms thrive due to an ample supply of nutrients. During the log phase, biogas production increased steadily, followed by a sharp rise, with peaks observed on days 10, 14, 8, 11, and 12 for the 50%JF:50%PM, 75%JF:25%PM, 75%JF:25%CW, 50%JF:50%CW, and 100%JF mixtures, respectively (Figures 1 and 2).
As the substrates are consumed, nutrients like carbon and nitrogen become limiting, leading to a slowdown in microbial metabolism. This transition signals the stationary phase, where biogas production starts to decline. Eventually, as nutrients are completely depleted, microbial activity decreases significantly, leading to the death phase, where biogas production ceases entirely (Alavi-Borazjani, Da Cruz Tarelho and Capela, Reference Alavi-Borazjani, Da Cruz Tarelho and Capela2024). Hence, this progression follows Monod’s principles, illustrating how substrate concentration impacts microbial growth and biogas yield throughout the anaerobic digestion process. This pattern corresponds well with recent kinetic modeling studies (Zhang et al., Reference Zhang, An, Cao, Tian and He2021), which describe how substrate exhaustion and metabolic inhibition influence biogas generation curves.
The results in Figures 1 and 2 highlight the significant potential of co-digestion, particularly with poultry droppings, in enhancing biogas production from jackfruit waste. Additionally, the incorporation of biochar plays a crucial role in addressing the challenges associated with high nitrogen content, thereby improving the overall performance of anaerobic digestion systems. Recent innovations in co-digestion and biochar-enhanced digestion (Chen et al., Reference Chen, Zeng, Yang, Yang, Qiao, Zhao, Wang and Wu2024) continue to demonstrate that synergistic substrate pairing and additive strategies can significantly improve biogas system resilience and output. These findings underscore the importance of substrate selection and supplementation in optimizing biogas yields for sustainable energy production.
Biogas quality
Jackfruit waste alone exhibited the lowest methane content, measuring 25.9% (Figure 3). In contrast, co-digestion of jackfruit waste with 25% cow dung and 50% poultry droppings resulted in the highest methane content, reaching 69.9% and 72.3%, respectively (Figure 3). These values align with the findings of Awe et al. (Reference Awe, Zhao, Nzihou, Minh and Lyczko2017), who reported that raw biogas generated from the anaerobic digestion of sewage sludge, livestock manure, and agricultural waste typically contains methane concentrations ranging from 55% to 70%, with carbon dioxide comprising 30% to 45%. Notably, the methane content from the mono-digestion of jackfruit waste (25.9%) falls significantly below this expected range. Oduor et al. (Reference Oduor, Wandera, Murunga and Raude2022) also confirmed that co-digestion of Water Hyacinth (WH) with Food Waste (FW) enhances biogas production compared to mono-digestion. The co-digestion of the WH with FW improved the biogas production by 5%, 9%, 15%, 53%, and 58% for mix proportions of 85:15, 30:70, 15:85, 55:45, and 70:30 (WH: FW), respectively.
Figure 3. Methane content of the different co-digestion mixtures and control.
The reduced methane yield from mono-digestion can be attributed to the inherently high carbon-to-nitrogen (C/N) ratio of jackfruit waste, which has been reported to range from 31.60:1 to 31.97:1 (Nsubuga et al., Reference Nsubuga, Banadda, Kabenge and Wydra2021). A C/N ratio exceeding 30:1 indicates a nitrogen deficiency, which limits microbial activity and impairs optimal anaerobic digestion. Nitrogen is a crucial element for microbial protein synthesis, and its scarcity restricts microbial growth, leading to lower methane production efficiency. This observation is corroborated by Seekao et al. (Reference Seekao, Sangsri, Rakmak, Dechapanya and Siripatana2021), who emphasized the need for optimal C/N ratios (20–30) for efficient methanogenesis and warned that mono-substrates with unbalanced nutrient content often yield suboptimal methane outputs.
Conversely, the methane content from the co-digestion of jackfruit waste with cow dung and poultry droppings fell within the anticipated range. This enhancement can be ascribed to the synergistic effect of co-digestion, which improves nutrient balance and microbial activity. Recent studies Karki et al. (Reference Karki, Chuenchart, Surendra, Shresth, Raskin, Sung, Hashimoto and Khanal2021) demonstrated that co-digestion not only optimizes the C/N ratio but also diversifies microbial consortia, accelerating hydrolysis and methanogenesis. Additionally, the alkaline pretreatment of jackfruit waste likely enhanced its biodegradability, facilitating more efficient microbial breakdown and methane generation. Woźniak et al. (Reference Woźniak, Kuligowski, Świerczek and Cenian2025) affirm that pretreatment of lignocellulosic biomass improves solubilization of organic compounds, increasing substrate accessibility to methanogens. The inclusion of nitrogen-rich poultry droppings and cow dung further optimized the C/N ratio, promoting a more favorable environment for methanogenic archaea and ultimately improving methane yield. As noted by Guo et al. (Reference Guo, Xiao, Yan, Tang, Duan, Sun and Li2023), nitrogen supplementation from animal manure can buffer ammonia levels and stabilize reactor pH, further enhancing methane production.
These results clearly demonstrate that co-digestion, supported by mild pretreatment and proper nutrient balancing, holds significant potential for improving methane yield from lignocellulosic fruit wastes like jackfruit. Emerging research continues to recommend strategic co-substrate combinations for low-nitrogen agricultural residues to unlock greater renewable energy potential (Song et al., Reference Song, Pei, Chen, Mu, Yan, Li and Zhou2023).
Digestate characterization
The pH and total solids (TS) values of digestates from the various treatments, as shown in Table 4, did not exhibit significant differences (p ≥ 0.05). This indicates that co-digestion did not affect the pH and TS of the digestates. The pH values ranged from 6.32 to 6.67 across treatments, which are consistent with findings by Teglia, Tremier and Martel (Reference Teglia, Tremier and Martel2011), who reported similar results with co-digested mixtures. A neutral pH, as observed in this study, can decrease soil alkalinity, potentially improving the availability of nutrients like calcium and magnesium to plants (Mosquera-Losada, Muñoz-Ferreiro and Rigueiro-Rodríguez, Reference Mosquera-Losada, Muñoz-Ferreiro and Rigueiro-Rodríguez2010). This outcome emphasizes the potential benefits of using jackfruit waste-based digestates, as their near-neutral pH can positively influence soil properties and agricultural productivity. Recent research by Chojnacka and Moustakas (Reference Chojnacka and Moustakas2024) supports this, showing that digestates with neutral pH enhance nutrient bioavailability and soil buffering capacity, improving plant health and yield.
Table 4. Digestate characterization from different co-digestion mixtures
x, total solids.
y, organic matter of the fresh weight.
cfu, colony-forming units.
Treatments with different letters within row are significantly different (p ≤ 0.05) according to Tukey’s post hoc test.
The TS content of the digestates ranged from 9.40 to 10.48% (Table 4), reflecting a high proportion of liquid fraction in the digestate after solid–liquid separation. These values are in line with those reported by Zeshan and Visvanathan (Reference Zeshan and Visvanathan2014). Managing the liquid fraction of digestates poses a challenge due to its volume, but there are various strategies for handling it. One option is to recycle the liquid fraction back into the digester to enhance biogas production and reduce methane emissions during storage and transport (Sammy, Reference Sammy2020). Another strategy is nutrient recovery through biochar adsorption, which concentrates essential nutrients and enhances soil quality (Kizito et al., Reference Kizito, Lu, Bah, Dong and Wu2019). Aquino et al. (Reference Aquino, Santoro, Profio, La Russa, Limonti, SD’Andrea, Curcio E and Siciliano2023) have demonstrated the effectiveness of membrane separation and vermifiltration techniques in managing digestate liquids, reducing leaching risks while concentrating nutrients.
Nutrient content (N, P, K, Ca, Mg, and Na) of the digestates from the 100% jackfruit (JF) treatment showed significant differences (p ≤ 0.05) compared to digestates from co-digestion with cow dung (CW) and poultry manure (PM) in various combinations. The total N, P, and K contents ranged from 34.58 to 45.44 g/kg, 21.51 to 28.42 g/kg, and 28.57 to 32.57 g/kg, respectively, and were generally higher than those reported by Massaccesi et al. (Reference Massaccesi, Sordi, MicaleC, Zadra, Di Maria and Gigliotti2013). The disparity could be ascribed to the difference in the substrates considered. This research co-digested jackfruit waste with cow dung and chicken droppings, whereas Massaccesi et al. (Reference Massaccesi, Sordi, MicaleC, Zadra, Di Maria and Gigliotti2013) used municipal solid waste. Additionally, co-digestion with poultry droppings resulted in higher NPK content than co-digestion with cow dung. This could be due to the more nutrient-rich diet of poultry compared to cows, which mainly consume grasses. Similar results were recently confirmed by Chojnacka and Moustakas (Reference Chojnacka and Moustakas2024), who observed that poultry-manure-based digestates are richer in macronutrients than those from cattle-based digestion, leading to greater fertilization potential.
Calcium, magnesium, and sodium concentrations in the digestates ranged from 11.38 to 16.64 g/kg, 5.48 to 14.88 g/kg, and 8.54 to 9.91 g/kg, respectively. These results were slightly higher than those reported by Solé-Bundó et al. (Reference Solé-Bundó, Cucina, Folch, Tàpias, Gigliotti, Garfí and Ferrer2017), likely due to differences in the substrates used. The study by Solé-Bundó et al. (Reference Solé-Bundó, Cucina, Folch, Tàpias, Gigliotti, Garfí and Ferrer2017) co-digested microalgae with sewage sludge, while this study used jackfruit waste with cow dung and poultry droppings. Calcium and magnesium are essential for plant growth, while high sodium concentrations pose risks to soil structure, making sodium levels a critical parameter to monitor. Yang et al. (Reference Yang, Zhang, Du, Gao, Cheng, Fu and Wang2024) highlighted that maintaining optimal Ca:Mg ratios is essential to prevent nutrient antagonism in soils, reinforcing the importance of balanced digestate formulations.
Organic matter (OM) content in the digestates ranged from 21.54% to 31.57%, with the highest OM found in the digestate from co-digestion of 50% jackfruit and 50% poultry manure. These values were generally lower than those reported by Teglia et al. (Reference Teglia, Tremier and Martel2011), potentially due to differences in calculation methods (fresh versus dry weight). Organic matter is a crucial component for enhancing soil microbial activity and improving soil aeration and water retention capacity, all of which contribute to better soil health and fertility. Ragályi et al. (Reference Ragályi, Szécsy, Uzinger, Magyar, Szabó and Rékási2025) reported that digestates fundamentally affect soil chemistry by reducing acidity through their high pH and by introducing stable organic matter into the soil, thereby improving its structure. They also emphasized that digestates affect the water holding capacity, wettability, infiltration indicators, dispersibility, bulk density, porosity, aggregate stability, and the amount of aggregates.
The total copper (Cu) concentration in digestates from the 100%JF treatment did not differ significantly from those co-digested with cow dung (75%JF:25%CW and 50%JF:50%CW). However, the Cu content was significantly different in digestates co-digested with poultry manure (75%JF:25%PM and 50%JF:50%PM). This suggests that poultry manure affects the copper content in the digestates. Similarly, other heavy metals such as zinc (Zn), cadmium (Cd), lead (Pb), and nickel (Ni) were significantly different (p ≤ 0.05) between treatments. The concentrations of these metals ranged from 0.00 to 2.77 mg/kg for Cd, 6.84 to 32.49 mg/kg for Cu, 7.46 to 40.63 mg/kg for Pb, 20.79 to 98.91 mg/kg for Zn, and 80.70 to 104.40 mg/kg for Ni, all of which were below the threshold limits set by Solé-Bundó et al. (Reference Solé-Bundó, Cucina, Folch, Tàpias, Gigliotti, Garfí and Ferrer2017) for safe land application. This low concentration of heavy metals is consistent with findings by Parra-Orobio et al. (Reference Parra-Orobio, Rotavisky-Sinisterra, Pérez-Vidal, Marmolejo-Rebellón and Torres-Lozada2021), suggesting that anaerobic digestion reduces heavy metal content (Massaccesi et al., Reference Massaccesi, Sordi, MicaleC, Zadra, Di Maria and Gigliotti2013), making the digestates safer for agricultural use. Heavy metals like Pb and Cd are regarded as the ‘main threats’ since they are very harmful to living organisms (Goyal et al., Reference Goyal, Yadav, Prasad, Singh, Shrivastav, Ali, Dantu, Mishra, Naeem, Ansari and Gill2020). Hence, their concentration in the digestates should be determined before they are used as biofertilizers.
Finally, the E. coli levels in the digestates ranged from 3,157 to 69,607 cfu/100 ml, with significant differences (p ≤ 0.05) between treatments. These levels were higher than the World Health Organization’s recommended value of less than 1.0 × 103 cfu/100 ml for safe land spreading or reuse. The presence of E. coli is indicative of pathogen contamination in the digestates, which can be attributed to the mesophilic temperatures (37 °C) used in this study. At these temperatures, pathogen reduction is limited, and higher temperatures above 70 °C are necessary to eliminate E. coli effectively (McCord et al., Reference McCord, Stefanos, Tumwesige, Lsoto, Kawala, Mutebi, Nansubuga and Larson2020). Thus, additional treatments or higher temperatures may be needed to ensure the digestates are pathogen-free for agricultural use.
This study highlights the significant influence of co-digestion on the nutrient and heavy metal content of digestates and underscores the potential benefits of using these digestates as biofertilizers. However, pathogen levels and the management of the liquid fraction remain critical considerations for the safe and effective use of digestates in agriculture, as already highlighted by McCord et al. (Reference McCord, Stefanos, Tumwesige, Lsoto, Kawala, Mutebi, Nansubuga and Larson2020).
Agricultural reuse potential of the digestates and the corresponding biogas and firwood energy potential
The digestates outlined in Table 4, with their equivalent nutrient concentrations, can serve as effective biofertilizers to improve soil quality and crop productivity. Moreover, these digestates have the potential to enrich raw biochar with valuable nutrients, creating nutrient-enriched biochar. The success of digestate reuse depends on its ability to deliver essential nutrients to the soil and crops at the right time. A recent study by Karimi et al. (Reference Karimi, Sadet-Bourgeteau, Cannavacciuolo, Chauvin, Flamin, Haumon, Jean-Baptiste, Reibel, Vrignaud and Ranjard2022) confirms that co-digested organic waste-derived digestates significantly improve soil microbial biomass and nitrogen uptake in cereal crops, suggesting their suitability as substitutes for synthetic fertilizers.
In Uganda, maize is the most widely cultivated cereal crop, both in terms of acreage and production, and it ranks as the second-largest consumer of mineral fertilizers in the country (Jjagwe et al., Reference Jjagwe, Chelimo, Karungi, Komakech and Lederer2020). However, despite the large areas devoted to maize farming, yields have remained consistently low. For example, maize yields were 2.395 t/ha in 2013 and decreased to 2.353 t/ha in 2015 (Jjagwe et al., Reference Jjagwe, Chelimo, Karungi, Komakech and Lederer2020). These figures reflect concerns raised by Petrides and DeFronzo (Reference Petrides and DeFronzo1989), who highlighted that over 80% of agricultural land in East Africa suffers from nitrogen deficiency, mainly due to severe soil nutrient depletion and the lack of nutrient replacement during harvests. According to a recent Tully et al. (Reference Tully, Sullivan, Weil and Sachez2015), nutrient mining and soil degradation are key drivers of yield stagnation in Sub-Saharan Africa, further emphasizing the need for sustainable biofertilizer interventions.
The digestates listed in Table 4, with an average nitrogen content ranging from 31.5 to 45.9 g/kg, can play a crucial role in replenishing nitrogen in nitrogen-deficient soils, thereby promoting maize production in Uganda. Zimwanguyizza, Nansamba and Musinguzi (Reference Zimwanguyizza, Nansamba and Musinguzi2012) recommend applying 50 kg of nitrogen per hectare to replace lost nitrogen and improve maize yields. To provide this amount of nitrogen, the following quantities of digestates would be required: 1.089 tons for a 50%JF:50%PM mix, 1.179 tons for a 75%JF:25%PM mix, 1.585 tons for a 50%JF:50%CW mix, 1.108 tons for a 75%JF:25%CW mix, and 1.449 tons for a 100%JF mix. Field trials by Przygocka-Cyna and Grzebisz (Reference Przygocka-Cyna and Grzebisz2020) support the use of digestates at these dosages, which showed an increase in maize yield when digestates were applied at equivalent nitrogen rates.
The corresponding biogas production volumes for each digestate mixture were as follows: 43.2, 46.96, 63.12, 44.0, and 57.68 m3 for the 50%JF:50%PM, 75%JF:25%PM, 50%JF:50%CW, 75%JF:25%CW, and 100%JF treatments, respectively. As noted by Menya, Alokore and Ebangu (Reference Menya, Alokore and Ebangu2013), biogas has a calorific value of 20 MJ/m3. Therefore, the energy outputs from each treatment are 864, 939.2, 1,262.4, 880, and 1,153.6 MJ, respectively. Comparatively, a study by Solarte-Toro, Chacón-Pérez and Cardona-Alzate (Reference Solarte-Toro, Chacón-Pérez and Cardona-Alzate2018) validated biogas calorific values ranging between 19 and 26 MJ/m3 for agricultural wastes, confirming the reliability of these estimates.
In comparison, firewood from the Combretum collinum Fres. species, locally known as Mukora, has an energy potential of 0.02349 MJ/kg according to Ojelel, Otiti and Mugisha (Reference Ojelel, Otiti and Mugisha2015). To produce the same amount of energy from the different co-digestion mixtures, the firewood requirements from this tree species would be 26.78, 39.98, 53.74, 37.46, and 49.11 tons for the 50%JF:50%PM, 75%JF:25%PM, 50%JF:50%CW, 75%JF:25%CW, and 100%JF treatments, respectively. This conversion underscores the potential of biogas to substitute massive quantities of biomass fuel, thereby contributing to forest conservation and improved household air quality, as highlighted by Alao et al. (Reference Alao, Gilani, Sopian, Alao, Oyebamiji and Oladosu2024).
Conclusions
This study investigated the co-digestion of jackfruit waste with cow dung and poultry droppings under anaerobic conditions, assessing both the quantity and quality of biogas produced, as well as the nutrient composition of the resulting digestates for agricultural reuse as biofertilizers. The findings demonstrated that co-digesting jackfruit waste with cow dung and poultry droppings significantly enhances biogas production, increases methane content, and improves the nutrient profile of the digestates. The highest biogas yield was observed from the co-digestion of 75% jackfruit waste and 25% poultry droppings mixed with biochar, producing 373.0 ml/gVS of biogas. In contrast, a 50% jackfruit waste and 50% poultry droppings mixture yielded the highest methane content at 72.3%, outperforming other treatments. The digestate with the highest nutrient content was produced from the co-digestion of 50% jackfruit waste and 50% poultry droppings, with nitrogen, phosphorus, and potassium concentrations of 45.44 g/kg, 28.42 g/kg, and 32.57 g/kg, respectively. To meet the maize nutrient requirement of 50 kg N/ha, 1.089 tons of 50JF:50 PM digestate per hectare is required, which corresponds to a biogas production of 43.3 m3. However, the E. coli levels in the digestates exceeded the World Health Organization’s recommended limit of less than 1.0 × 103 CFU/100 ml for direct reuse. As a result, it is strongly recommended to pasteurize the digestates at 55 °C for 1 hour to eliminate pathogens before applying them to the soil.
Disclosure of use of AI tools
During the preparation of this manuscript, the authors used ChatGPT on March 27, 2025 tool in order to improve the language and readability of the manuscript. After using the tool, the authors reviewed and edited the content as needed and took full responsibility for the content of the publication.
Acknowledgements
Dedicated to the memory of the Late Prof. Noble Banadda, who passed away when this study was being conducted.
Funding statement
This research was supported by the Federal Ministry of Food and Agriculture (BMEL) based on a decision of the Parliament of the Federal Republic of Germany via Federal Office for Agriculture and Food (BLE) through Makerere University, Kampala (Uganda).
Competing interests
The authors declare that they have no competing interests related to this manuscript.
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