 By: Karen A. Chagwaya
Abstract: The rapid expansion of aquaculture has significantly increased the generation of fish faecal waste, creating environmental management challenges while presenting opportunities for renewable energy production and resource recovery. Although fish faecal waste has traditionally been regarded as a waste disposal problem, its high biodegradable organic matter content makes it a promising feedstock for biogas production through anaerobic digestion. This article reviews the potential of fish faecal waste as a feedstock for biogas production and its contribution to sustainable waste management and renewable energy development. The review demonstrates that the conversion of fish faecal waste into biogas can improve waste management, generate renewable energy, recover valuable nutrients, reduce greenhouse gas emissions, and promote circular resource utilization within aquaculture systems. Despite technical and economic constraints, continued advances in anaerobic digestion technologies, integrated waste management practices, and supportive policy interventions are enhancing the feasibility of fish waste-based biogas production. The article concludes that the utilization of fish faecal waste for biogas production provides a sustainable pathway for improving aquaculture productivity, advancing renewable energy development, and supporting the transition toward a circular bioeconomy.
Keywords: Fish Faecal Waste, Biogas Production, Anaerobic Digestion, Aquaculture biogas, Renewable Energy, Circular Bioeconomy.
1. Introduction
Aquaculture has become one of the fastest-growing food production sectors globally, playing an increasingly important role in food security, nutrition, employment, and economic development. As demand for fish continues to rise alongside declining wild fish stocks, aquaculture has expanded rapidly to meet global protein requirements (Food and Agriculture Organization [FAO], 2024). This growth has, however, been accompanied by a corresponding increase in the generation of organic wastes, particularly fish faecal waste and uneaten feed. When discharged untreated into aquatic environments, these wastes contribute to nutrient loading, eutrophication, oxygen depletion, deterioration of water quality, greenhouse gas emissions, and increased disease risks, thereby threatening the sustainability of aquaculture production and surrounding ecosystems (Boyd et al., 2020; FAO, 2024). Consequently, sustainable waste management has become a key priority for improving the environmental performance of aquaculture systems.
The growing emphasis on renewable energy and the circular economy has shifted attention toward recovering valuable resources from organic waste streams instead of treating them solely as environmental pollutants. Fish faecal waste contains biodegradable organic matter and nutrients that can be converted into biogas through anaerobic digestion while producing nutrient-rich digestate suitable for agricultural applications (International Energy Agency [IEA], 2023). Despite its considerable resource potential, fish faecal waste remains one of the least utilized feedstocks for biogas production compared with livestock manure, municipal organic waste, and crop residues (International Renewable Energy Agency [IRENA], 2023). Harnessing this resource offers an opportunity to simultaneously address waste management, renewable energy generation, nutrient recycling, and climate change mitigation within a circular bioeconomy framework. This article examines the potential of fish faecal waste as a sustainable feedstock for biogas production by discussing its characteristics, anaerobic digestion processes, suitable biogas technologies, environmental and economic benefits, existing challenges, and future prospects.
2. Characteristics of Fish Faecal Waste
The suitability of fish faecal waste for biogas production largely depends on its physical and chemical characteristics. Fish faeces consist primarily of undigested feed particles, microbial biomass, digestive secretions, and metabolic waste products, all of which contribute to its organic content and biodegradability (Chen et al., 2015; FAO, 2024). The composition of fish faecal waste varies considerably depending on fish species, diet, farming practices, and environmental conditions. Understanding these characteristics is essential for optimizing anaerobic digestion, improving methane yield, and selecting appropriate digester technologies
2.1 Composition of Fish Faeces
Fish faecal waste is rich in biodegradable organic matter, making it a promising substrate for anaerobic digestion. Organic matter consists mainly of undigested proteins, carbohydrates, lipids, and fibre originating from fish feed. During anaerobic digestion, these compounds are broken down by microorganisms to produce methane-rich biogas. Higher concentrations of biodegradable organic matter generally correspond to greater biogas production potential (Ward et al., 2008).
Fish faeces also contain a high proportion of moisture, typically exceeding 80 percent depending on the production system and fish species. The high moisture content facilitates microbial activity during anaerobic digestion but may also reduce the concentration of digestible solids, making co-digestion with drier organic materials beneficial for improving process efficiency (Mata-Alvarez et al., 2014).
Nitrogen is another important component because fish feeds are generally rich in protein. As proteins are metabolized, nitrogen is excreted in the faeces and other waste products. Although nitrogen supports microbial growth during anaerobic digestion, excessive nitrogen concentrations may increase ammonia levels, which can inhibit methane-producing microorganisms if not properly managed (Yenigün & Demirel, 2013).
Fish faecal waste also contains appreciable quantities of phosphorus derived from mineral supplements and feed ingredients. If released untreated into water bodies, phosphorus contributes to eutrophication and declining water quality. Recovering phosphorus through anaerobic digestion not only minimizes environmental pollution but also enables nutrient recycling through the application of digestate as an organic fertilizer (FAO, 2024).
Carbon is the principal energy source utilized by anaerobic microorganisms during biogas production. However, fish faecal waste generally has a relatively low carbon-to-nitrogen ratio because of its high protein content. This characteristic often necessitates co-digestion with carbon-rich substrates such as crop residues, sawdust, or livestock manure to improve digestion stability and maximize methane production (Mata-Alvarez et al., 2014).
Volatile solids represent the biodegradable fraction of total solids and are widely used to estimate the biogas potential of organic substrates. Fish faecal waste contains a substantial proportion of volatile solids that can be converted into methane under suitable anaerobic conditions. Consequently, volatile solids reduction is commonly used as an indicator of digester performance and substrate degradation efficiency (Chen et al., 2015).
2.2 Factors Influencing Fish Waste Characteristics
The composition and biogas potential of fish faecal waste are influenced by several biological, nutritional, and environmental factors. Fish species is one of the most important determinants because digestive physiology, feed conversion efficiency, and nutrient utilization differ among species. Carnivorous fish typically produce waste with higher protein and nitrogen concentrations than herbivorous or omnivorous species, resulting in differences in biodegradability and methane production potential (Boyd et al., 2020).
Feed composition also plays a significant role in determining the chemical characteristics of fish faeces. Diets containing high levels of protein, lipids, or indigestible fibre influence the quantity and composition of organic matter excreted. Improvements in feed formulation and digestibility can reduce waste generation while enhancing the quality of substrates available for biogas production (Naylor et al., 2021).
Water quality influences microbial activity, fish metabolism, and nutrient utilization, all of which affect waste production. Parameters such as dissolved oxygen, temperature, pH, and water exchange rates influence feed consumption, digestion efficiency, and the decomposition of organic matter within aquaculture systems. Poor water quality may increase organic waste accumulation and reduce the overall efficiency of waste recovery (FAO, 2024).
The farming system also affects the quantity and characteristics of fish faecal waste. Intensive aquaculture systems generally produce larger volumes of concentrated organic waste than extensive or semi-intensive systems because of higher stocking densities and greater feeding rates. Recirculating aquaculture systems and integrated aquaculture facilities often incorporate waste collection technologies that improve the recovery of faecal solids for subsequent treatment through anaerobic digestion (Boyd et al., 2020).
Feeding practices, including feeding frequency, ration size, feed quality, and feeding efficiency, further influence waste generation. Overfeeding increases the amount of uneaten feed and faecal waste entering the production system, reducing water quality and increasing environmental pollution. Conversely, efficient feeding management minimizes waste generation while improving feed conversion efficiency and increasing the quantity of recoverable organic material suitable for renewable energy production (Naylor et al., 2021).
3. Principles of Biogas Production
Biogas is produced through anaerobic digestion, a biological process in which microorganisms decompose biodegradable organic matter in the absence of oxygen. Fish faecal waste is a suitable substrate for this process because it contains organic compounds, including proteins, carbohydrates, lipids, and other biodegradable materials that can be converted into methane-rich biogas. During anaerobic digestion, these compounds undergo a series of microbial transformations that ultimately produce methane and carbon dioxide while generating a nutrient-rich digestate suitable for agricultural use. The efficiency of this process depends on the characteristics of the substrate, the composition of microbial communities, and operating conditions within the digester (Ward et al., 2008; Mata-Alvarez et al., 2014).
3.1 Anaerobic Digestion Process
Anaerobic digestion occurs through four interconnected biological stages, each carried out by specialized groups of microorganisms. The first stage, hydrolysis, involves the breakdown of complex organic compounds such as proteins, carbohydrates, and lipids present in fish faecal waste into simpler soluble molecules including amino acids, sugars, and fatty acids. Hydrolysis is often the rate-limiting step because microorganisms can only utilize dissolved organic compounds during subsequent stages of digestion (Appels et al., 2008).
The second stage, acidogenesis, converts the soluble products formed during hydrolysis into volatile fatty acids, alcohols, hydrogen, carbon dioxide, and ammonia through the activity of acid-forming bacteria. This stage provides the intermediate compounds required for further microbial conversion while also influencing the acidity of the digestion process. Efficient acidogenesis is important for maintaining a continuous supply of substrates for the next stage of anaerobic digestion (Chen et al., 2015).
During acetogenesis, volatile fatty acids and alcohols produced during acidogenesis are converted into acetic acid, hydrogen, and carbon dioxide by acetogenic bacteria. Since methanogenic microorganisms primarily utilize acetate and hydrogen to produce methane, acetogenesis serves as a critical link between acid-producing and methane-producing microorganisms. Any imbalance at this stage may result in the accumulation of organic acids and reduced biogas production (Mata-Alvarez et al., 2014).
The final stage, methanogenesis, is carried out by methanogenic archaea that convert acetic acid, hydrogen, and carbon dioxide into methane and carbon dioxide. This stage determines the quantity and quality of biogas produced and is highly sensitive to environmental changes such as temperature, pH, and ammonia concentration. Stable methanogenesis is therefore essential for achieving high methane yields from fish faecal waste (Ward et al., 2008; Yenigün & Demirel, 2013).
3.2 Factors Affecting Methane Production
Several operational factors influence the efficiency of anaerobic digestion and the quantity of methane produced from fish faecal waste. Among these, temperature is one of the most important because microbial activity is strongly temperature dependent. Anaerobic digestion commonly operates under mesophilic conditions (30–40°C) or thermophilic conditions (50–60°C). Mesophilic digestion is generally preferred because it provides greater process stability and requires less energy, whereas thermophilic digestion accelerates organic matter degradation but demands stricter operational control (Appels et al., 2008).
The pH of the digestion system also plays a critical role in maintaining microbial activity. Methanogenic microorganisms perform optimally within a near-neutral pH range of approximately 6.8 to 7.5. Excessive accumulation of volatile fatty acids can lower the pH and inhibit methane production, while highly alkaline conditions may increase ammonia toxicity. Maintaining a stable pH is therefore essential for efficient biogas generation (Chen et al., 2015).
The carbon-to-nitrogen (C:N) ratio influences nutrient availability for anaerobic microorganisms. Fish faecal waste generally contains high levels of nitrogen because of its protein-rich composition, resulting in a relatively low C:N ratio. Excess nitrogen may lead to ammonia inhibition, reducing methanogenic activity and lowering methane yields. Co-digestion with carbon-rich materials such as crop residues, sawdust, or cattle manure is often recommended to achieve a more balanced C:N ratio and improve digester performance (Yenigün & Demirel, 2013).
Hydraulic retention time (HRT) refers to the average period during which the substrate remains inside the digester. Sufficient retention time allows microorganisms to completely degrade organic matter and maximize methane production. Short retention periods may lead to incomplete digestion and reduced gas yields, whereas excessively long retention periods increase digester size and construction costs without substantial improvements in biogas production (Ward et al., 2008).
The organic loading rate (OLR) represents the amount of organic material introduced into the digester over a given period. Appropriate loading rates ensure that microorganisms receive a continuous supply of substrate without becoming overloaded. Excessive loading can result in the accumulation of volatile fatty acids, reduced pH, microbial inhibition, and digester failure, whereas insufficient loading underutilizes digester capacity and lowers biogas productivity. Optimizing the organic loading rate is therefore essential for achieving stable and efficient methane production from fish faecal waste (Mata-Alvarez et al., 2014).
4. Potential of Fish Faecal Waste for Biogas Production
Fish faecal waste possesses considerable potential as a feedstock for biogas production because of its high biodegradable organic matter content and the presence of nutrients that support anaerobic digestion. Through this biological process, the waste can be converted into methane-rich biogas while producing nutrient-rich digestate that can be used as an organic fertilizer. Besides generating renewable energy, the utilization of fish faecal waste contributes to improved waste management, nutrient recycling, pollution control, and the development of circular bioeconomy systems within aquaculture (Ward et al., 2008; FAO, 2024). The following sections discuss the methane production potential of fish faecal waste together with its contribution to energy recovery, nutrient recovery, waste stabilization, and greenhouse gas emission reduction (Mata-Alvarez et al., 2014; IEA, 2023).
5. Co-digestion of Fish Faecal Waste
Although fish faecal waste can be anaerobically digested on its own, its relatively high moisture content and low carbon-to-nitrogen ratio may reduce process stability and methane yield. Co-digestion with complementary organic substrates provides an effective strategy for improving nutrient balance, increasing microbial activity, enhancing methane production, and stabilizing the digestion process. Previous studies have shown that combining different organic feedstocks often results in higher biogas yields than digesting individual substrates separately because of improved substrate characteristics and synergistic microbial interactions (Mata-Alvarez et al., 2014; Astals et al., 2014). Depending on locally available resources, fish faecal waste can be co-digested with livestock manure, crop residues, or food waste to improve the efficiency and economic viability of biogas production.
6. Challenges and Limitations
Despite its considerable potential as a renewable energy feedstock, the utilization of fish faecal waste for biogas production faces several technical, operational, and economic challenges. One of the primary limitations is the high moisture content of fish faecal waste, which lowers the concentration of biodegradable solids and may reduce methane yields unless the substrate is concentrated or co-digested with drier organic materials (Mata-Alvarez et al., 2014). In addition, fish faecal waste generally exhibits a relatively low carbon-to-nitrogen ratio because of its high protein content, increasing the risk of ammonia accumulation and inhibition of methanogenic microorganisms during anaerobic digestion (Yenigün & Demirel, 2013).
Efficient collection of fish faecal waste also presents practical challenges, particularly in open pond and cage aquaculture systems where waste disperses rapidly into surrounding water. Inadequate waste recovery not only reduces the quantity of substrate available for biogas production but also contributes to nutrient pollution and declining water quality (Boyd et al., 2020). Furthermore, residues of antibiotics, disinfectants, and other aquaculture chemicals may alter microbial activity within anaerobic digesters, potentially reducing digestion efficiency and methane production if present at inhibitory concentrations (FAO, 2024).
Seasonal fluctuations in fish production, feeding intensity, and water temperature can also influence the quantity and characteristics of fish faecal waste generated throughout the year, affecting the consistency of biogas production. In addition, the establishment of biogas systems requires substantial initial investment in waste collection infrastructure, digesters, gas storage facilities, and operational management. These capital requirements may limit adoption among small-scale fish farmers unless supported through financial incentives, technical assistance, or cooperative investment models (International Renewable Energy Agency [IRENA], 2023; International Energy Agency [IEA], 2023). Addressing these challenges through improved waste collection technologies, optimized co-digestion strategies, supportive policies, and continued research will be essential for enhancing the technical and economic feasibility of fish waste-based biogas systems.
7. Conclusion
Fish faecal waste represents an underutilized organic resource with significant potential for renewable energy generation through anaerobic digestion. As aquaculture continues to expand worldwide, the effective management and utilization of this waste stream will become increasingly important for improving environmental sustainability and resource efficiency. This review has demonstrated that fish faecal waste contains sufficient biodegradable organic matter to support biogas production while simultaneously enabling nutrient recovery, waste stabilization, and greenhouse gas emission reduction. Although challenges related to substrate characteristics, waste collection, process stability, and investment costs remain, these constraints can be addressed through appropriate digester technologies, co-digestion with complementary substrates, improved farm management practices, and supportive policy frameworks. Integrating biogas production into aquaculture systems therefore offers a practical pathway for advancing the circular bioeconomy by converting waste into renewable energy and valuable biofertilizers while contributing to sustainable aquaculture, climate change mitigation, and long-term energy security.
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