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# Study on anaerobic co-digestion of cow dung and fruit waste for biogas production

Akash Phillip, Sheelendra Bhatt, Navneet Sharma

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Akash Phillip, Sheelendra Bhatt, Navneet Sharma. Study on anaerobic co-digestion of cow dung and fruit waste for biogas production. Biological Reviews, 2025, 2 (2), pp.121-129. 10.5455/jbrr.20250316121948. hal-05057349

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Submitted on 15 May 2025

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# Study on anaerobic co-digestion of cow dung and fruit waste for biogas production

Akash Phillip ID, Sheelendra Mangal Bhatt ID, Navneet Sharma ID

Department of Biotechnology, IIMT University, Meerut, India

## Abstract

**Aim/Background:** The declining availability of fossil fuels cannot meet the increasing global energy demands and contributes significantly to greenhouse gas emissions, worsening global warming. Additionally, poor management of organic waste, including livestock and food scraps, poses environmental pollution risks. Anaerobic co-digestion (AnCoD) of livestock waste and food scraps offers a sustainable solution, producing renewable biogas while minimizing ecological impact. This study aims to evaluate whether co-digesting cow manure (CM) with fruit waste (FW) in varying ratios can enhance biogas production and methane quality.

**Methods:** This experimental study was conducted using five batch anaerobic digesters (each with a 5 kg capacity) under mesophilic conditions (37°C) over a 14-day retention period. The digesters, labeled Case-1 through Case-5, contained different CM-to-FW ratios: 1:1, 4:1, 1:4, 3:2, and 2:3. Pretreatment of substrates was performed prior to digestion, and critical process parameters such as temperature, pH, and biogas volume were monitored throughout the study.

**Results:** Among all tested ratios, Case-2 (4:1 CM to FW) demonstrated the highest biogas yield, achieving a methane concentration of 49%. The results indicated that proper substrate proportioning significantly influences the efficiency of anaerobic digestion. Additionally, the process remained stable under mesophilic conditions, with favorable pH levels throughout the experiment.

**Conclusion:** This study confirms that the AnCoD of CM and FW under mesophilic conditions is a promising strategy for enhancing biogas production. The findings highlight that substrate composition plays a critical role in determining biogas yield and methane quality.

## ARTICLE HISTORY

Received March 16, 2025
Accepted April 11, 2025
Published April 27, 2025

## KEYWORDS

Anaerobic digestion;
anaerobic co-digestion;
bioenergy; biogas;
renewable energy;
Synergistic effect

## Introduction

The escalating global demand for energy, in conjunction with the dwindling reserves of fossil fuels and their detrimental environmental consequences, has catalyzed considerable scholarly interest in renewable energy technologies [1,2]. Fossil fuels, which currently fulfill a significant fraction of the global energy requirements, are primary contributors to greenhouse gas emissions and the phenomenon of climate change [3,4]. Conversely, renewable alternatives such as biogas, derived from the anaerobic digestion (AD) of organic wastes, provide a sustainable solution by not only generating clean energy but also alleviating environmental degradation [5-7].

AD is a well-characterized process that transforms complex organic substrates into biogas through a succession of microbial-mediated biochemical reactions [8,9]. This process not only facilitates the generation of methane-rich biogas but also produces nutrient-rich digestate, which can function as an organic fertilizer [10], thereby fostering circular economy practices within the agricultural sector [11-14]. Nevertheless, the efficacy of AD, particularly when utilizing single substrates, may be constrained by nutrient imbalances and suboptimal carbon-to-nitrogen (C/N) ratios [15,16]. In order to address these limitations, anaerobic co-digestion (AnCoD) has emerged as a promising methodological approach [17-19].
By concurrently processing multiple substrates, such as livestock manure and food scraps, AnCoD enhances biochemical equilibrium, improves process stability, and augments methane yield through synergistic interactions [20-23]. In numerous regions, including India, where significant quantities of livestock manure and considerable volumes of food waste are produced daily [24], this methodology not only addresses waste management issues but also contributes to energy security and environmental sustainability [24-27].

This research investigates the AnCoD of cow manure (CM) and fruit waste (FW) under mesophilic conditions. Through a series of batch experiments with varying substrate ratios, the study examines the influence of feedstock composition, operational parameters, and pretreatment methods on biogas production and quality. By optimizing these variables, the study aspires to establish a comprehensive framework for enhancing biogas yield and process efficiency, thereby promoting the applicability of AnCoD in decentralized energy production and waste valorization.

# Materials and Methods

## Materials

In the present investigation, a variety of FW materials such as orange peels, mosambi peels, pineapple skins, pomegranate skins, and banana peels, which were obtained from a local fruit juice establishment situated in Meerut city, have been systematically collected and combined with CM that was sourced from a nearby dairy farm located within the same vicinity of Meerut city. Within the context of India, two prevalent examples of organic solid wastes include CM and FW.

These materials have been utilized as substrates for biogas generation within five cylindrical plastic anaerobic digesters, each with a capacity of 5 l, in the form of plastic bottles. Within the aforementioned biogas digesters or plastic bottles to be more specific, the two aforementioned substrates, namely FW and CM, are subjected to a process of AnCoD in accordance with varying weight-based ratios (W/W). This experiment also involved the measurement of some key parameters such as solar intensity, pH, effect of pretreatment, and surrounding temperature.

## Anaerobic digester

A biogas chamber, in the form of a plastic bottle with a slurry capacity of 5 kg, has been implemented

for indoor simulation above the ground. This arrangement allows indirect solar energy to reach the digester and dome [28]. The dimensions of the digester vessels are recorded as 0.135 m in diameter and 0.23 m in height. Likewise, the biogas chamber exhibits measurements of 0.18 m in depth, and 90 g in weight each (see Fig. 1). The dimensions of an anaerobic digester play a crucial role in determining biogas yield, as they influence factors such as hydraulic retention time (HRT), organic loading rate, and microbial activity [29]. In the present study, the selected digester dimensions were designed to accommodate the expected organic load while ensuring optimal HRT for efficient AD.

## Construction of anaerobic digester for batch system

FW and CM were combined to produce anaerobic biogas using a digester, as shown in Figure 2. To aid the release of the created gas, a rubber tube was connected from the bottle's mouth to the digester to facilitate gas collection, with an attached balloon serving as biogas storage at the end, to prevent leaks, sealing was done using an M-Seal. Maintaining air tightness in the perforations during the digesting process was critical to achieving maximum gas generation [10].

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Photograph of plastic bottle used as biogas chamber.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Photograph anaerobic digester for batch system.</figcaption></figure>

## Substrate collection

### Fruits wastes (FWs)

For this study, FWs such as orange peels, mosambi peels, pineapple skins, pomegranate skins, and banana peels were collected from juice outlets in Meerut. The selection involved manual sorting to ensure diverse fruit types prevalent in Meerut's culinary practices were included [30] (Fig. 3).

### Cow manure

The current study investigates the use of CM as a co-substrate, a specific sample of which is visually depicted in Figure 4, that was acquired from a dairy farm located in close proximity within the same geographical region of Meerut city. The CM utilized in this study was freshly collected, underscoring the sample's relevance. Traditionally, cow and pig manure have been integral to biogas production, illustrating established practices. This research concentrated on extracting methanogenic bacteria, or Methane-forming Archaea, underscoring their crucial role in methane gas production [31].

## Methodology

### Sample preparation

FW was collected from juice vendors in Meerut City, a densely populated urban area. The organic material (peels/skins of mosambi, oranges,

<figure><img src="image_5.png" /><figcaption>**Figure 3.** FWs used for the present study (The diversity in FW ensures a balanced nutrient profile, contributing to an optimal C/N ratio for efficient AD).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Sample of CM used for the present study (Cow dung contains a high concentration of organic matter as well as methanogens, which are microorganisms that aid in the AD of organic waste and the production of biogas).</figcaption></figure>

pomegranates, pineapples, and bananas) was carefully sorted to remove plastics, bones, and other inorganic contaminants [32]. The biodegradable components were then pretreated using a home grinder (MAHARAJA WHITELINE MX-154 750 W Mixer Grinder) to produce finely chopped feedstock for biogas production, as illustrated in
Figure 5. FW were mechanically processed to minimize their size and increase their surface area available to microorganisms [33].

## Preparation of mixed wastes

Various mixtures of cow dung, fruit leftovers, and tap water collectively known as slurry, were carefully added based on weight into a total of five digesters as detailed in Table 1. These digesters were specifically designed to act as controlled settings for the investigation of varying proportions of the mentioned organic substances with the aim of studying their impacts on the entirety of the digestion procedure [34,35]. The purpose of this setup was to assess how different combinations of CM, FW, and tap water influenced the overall efficiency and effectiveness of the AnCoD process.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Pretreated (finely grind) FW.</figcaption></figure>

<table><caption>**Table 1.** Comparison among various ratios of FW, CM, and tap water for AnCoD study.</caption><thead><tr><th>S.no</th><th>Characters</th><th>Case 1</th><th>Case 2</th><th>Case 3</th><th>Case 4</th><th>Case 5</th></tr></thead><tbody><tr><td>1.</td><td>Cow manure(w/w)</td><td>500 g</td><td>800 g</td><td>200 g</td><td>600 g</td><td>400 g</td></tr><tr><td>2.</td><td>Fruit waste(w/w)</td><td>500 g</td><td>200 g</td><td>800 g</td><td>400 g</td><td>600 g</td></tr><tr><td></td><td>Total weight of FW+CW</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td></tr><tr><td>3.</td><td>Water(w/w)</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td></tr><tr><td>4.</td><td>The ratio of CM to FW (w/w)</td><td>1:1</td><td>4:1</td><td>1:4</td><td>3:2</td><td>2:3</td></tr><tr><td>5.</td><td>The ratio of CM and FW to water (w/w)</td><td>1:1</td><td>1:1</td><td>1:1</td><td>1:1</td><td>1:1</td></tr></tbody></table>

# Experimental

## Start-up

The slurry was prepared in 5 different ratios/cases as indicated in Table 1, to begin the AD process over the course of 7 days consisting of a mix of FW, fresh CM, and tap water in specific proportions based on weight. In this study, these precise substrate ratios were chosen to optimize the C/N ratio in order to increase microbial activity and biogas production efficiency [36]. The chosen ratios were intended to balance the high carbon content of FW with the nitrogen-rich cow dung, resulting in a steady AD process [37].

## Feeding and operating the digesters

Before being introduced into the digester, the slurry underwent manual mixing by hand within a distinct container over the course of a few minutes, as part of a daily routine lasting for seven consecutive days. Subsequent to the introduction of the mixed waste into the digester, the openings of the digesters were securely sealed and wrapped in black polythene bags for better heat absorption as well as to maintain a temperature between 35°C and 37°C [10] (Fig. 6).

# Result

## Comparison among various ratios of FW, CM, and tap water in biogas plants

This comprehensive investigation spans a duration of 2 weeks (14 days) dedicated to the meticulous examination of a biogas facility. Throughout this systematic inquiry, a varied array of proportions involving FW, CM, and water have been examined carefully for their potential synergistic effects within the biogas production process. The complex interplay between FW, CM, and water is documented and analyzed, with the specific ratios elaborated upon in detail within the confines of Table 2, providing a comprehensive overview of the experimental setup.

# Discussion

After 2 weeks of observation, we discovered that the best results were obtained in Case 2 (Fig. 7), with a methane concentration of 49%. In this case (Case 2), the pH was 7, the slurry temperature was 35°C–37°C, and the presence of more CM than FW resulted in a more stable pH and improved methanogenic activity due to the digestion medium's buffering effect.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Photograph of the experimental setup.</figcaption></figure>

<table><caption>**Table 2.** Observation after 14 days of retention time.</caption><thead><tr><th>S.no</th><th>Characters</th><th>Case 1</th><th>Case 2</th><th>Case 3</th><th>Case 4</th><th>Case 5</th></tr></thead><tbody><tr><td>1.</td><td>Cow manure(w/w)</td><td>500 g</td><td>800 g</td><td>200 g</td><td>600 g</td><td>400 g</td></tr><tr><td>2.</td><td>Fruit waste(w/w)</td><td>500 g</td><td>200 g</td><td>800 g</td><td>400 g</td><td>600 g</td></tr><tr><td>3.</td><td>Total weight of CW+FW</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td></tr><tr><td>4.</td><td>Water (w/w)</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td><td>1000 g</td></tr><tr><td>5.</td><td>The ratio of CM to FW (w/w)</td><td>1:1</td><td>4:1</td><td>1:4</td><td>3:2</td><td>2:3</td></tr><tr><td>6.</td><td>The ratio of CM and FM to water (w/w)</td><td>1:1</td><td>1:1</td><td>1:1</td><td>1:1</td><td>1:1</td></tr><tr><td>7.</td><td>Total biogas production (m³)</td><td>0.01845</td><td>0.04785</td><td>0.04810</td><td>0.01167</td><td>0.00123</td></tr><tr><td>8.</td><td>The highest percentage of methane</td><td>40%</td><td>49%</td><td>20%</td><td>35%</td><td>No</td></tr><tr><td>9.</td><td>pH</td><td>7.5</td><td>7</td><td>5.5</td><td>6.5</td><td>6</td></tr></tbody></table>

In the analysis of Case 1 (as illustrated in Figure 8), it was found that positive outcomes were indeed achieved, with a methane concentration of 40%, however, the biogas yield in Case 1 was lower than in Case 2. Despite the fact that Case 1 and Case 2 both resulted in favorable results, it is imperative to highlight that the overall findings for Case 4 (refer to Figure 9) and Case 5 (as shown in Figure 10) were rather unsatisfactory, with a methane concentration of 35% and 0%, respectively, with a minimal quantity of biogas generated in these cases.

Furthermore, within Case 3 (illustrated in Figure 11) with a methane concentration of 40%, as a result of the acidic nature of the pH levels encountered in this scenario. This occurrence can be attributed to the fact that there is a larger quantity of FW relative to CM, leading to an inadequate buffering capacity that is essential for maintaining an alkaline pH level. Consequently, this deficiency inhibits the proliferation and metabolic activity of anaerobic bacteria which are crucial for the effective functioning

of the overall process. The temperature observed in all digesters varied between 35°C and 37°C, falling within the mesophilic range. This temperature range of 25°C-45°C is considered suitable for the generation of biogas.

Flammability of the produced biogas within the digesters was checked using a Bunsen burner connected with rubber tubing to the gas outlet. The incombustibility of the biogas generated in other digesters may possibly arise from various factors:

1. A deficiency in the population of methanogenic bacteria responsible for transforming acetic acid and CO₂ into methane.

2. Scum may form at the surface of the slurry from solid particles that settle at the bottom of the digester or from digested solids accumulated at the bottom.

3. Limited interaction between microorganisms and the required substrates for biogas production.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Photograph of case 2 ratio.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Photograph of case 1 ratio.</figcaption></figure>

4. The initial period during which the digester was constructed led to a decrease in environmental temperature and a shift away from the mesophilic temperature range. Such a decrease in temperature can lead to the reduction of gas produced, which eventually inhibits the process.

The data provided by the Indian government's esteemed policy body, NITI Aayog, highlights a significant finding indicating that the daily quantity

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Photograph of case 4 ratio.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 10.** Photograph of case 5 ratio.</figcaption></figure>

of CM generated by Indian livestock stands at an estimated three million tonnes. Furthermore, the United Nations Environment Programme's comprehensive Food Waste Index Report underscores
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Photograph of case 3 ratio.</figcaption></figure>

a concerning statistic revealing that on a yearly basis, a staggering 68.7 million tonnes of food are squandered within the confines of Indian households. The utilization of FW for AnCoD presents a valuable opportunity within the context of community-level biogas initiatives, contributing to the conservation of limited Liquefied Petroleum Gas resources typically reserved for culinary applications.

## Conclusion

In the comprehensive analysis conducted, it was observed that the bio-digester constructed from a plastic bottle covered with black polythene has the capacity to effectively absorb a greater amount of sunlight, thereby facilitating an elevation in the internal temperature of the digester. FW were mechanically processed to minimize their size and increase their surface area available to microorganisms. Biogas yield increased significantly when FW was co-digested with CM. These findings highlight the potential of co-digesting CW and FW as a sustainable waste management and renewable energy solution. Future research should focus on scaling up the experiment to pilot- and industrial-scale digesters to assess the feasibility of large-scale implementation. Additionally, investigating different pretreatment methods, such as enzymatic hydrolysis or thermal pretreatment, could further enhance biodegradability and methane yield.

## Acknowledgment

The author expresses profound gratitude to the entire unit of the Biotechnology Department for their unwavering support throughout the manuscript composition process.

## Author's contribution

This scholarly investigation was undertaken as a joint endeavor among all the authors, with each individual making substantial contributions to diverse facets of the study, encompassing the conceptual framework, methodology, analysis, and interpretation of the findings. Both authors were diligently engaged in the review and enhancement of the manuscript to ascertain the precision and caliber of the work exhibited.

## Conflict of interest

The authors report there are no competing interests to declare.

## Ethical approval

Ethical approval not required.

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