Optimizing Biogas with Anaerobic Co-Digestion: A Jatropha Case Study

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Boosting Biogas: The Science of Co-Digestion with Jatropha and Mango Waste

Introduction

While an excellent source for biofuel, the residue from Jatropha curcas seeds presents a challenge for biogas producers: a low carbon-to-nitrogen (C/N) ratio. This nutrient imbalance can lead to reactor instability and inefficient energy production. The solution lies in a clever strategy known as anaerobic co-digestion—essentially, creating a balanced diet for the energy-producing microbes.

This excerpt from Dr. Abdul Haq’s research provides a fascinating look into a practical experiment where Jatropha seed kernel is mixed with mango peels. The results demonstrate how combining these two waste streams can dramatically enhance biogas yields, stabilizing the entire process and making waste-to-energy systems more powerful and reliable.

Excerpt

“The sustainable use of non-edible feedstocks and waste for the production of biofuels is a potential means to reduce dependency on fossil fuels and mitigate environmental pollution. In the current study, the effects of carbon to nitrogen ratios on biomethane yield during anaerobic co-digestion of Jatropha curcas de-oiled seed kernel and mango peels were evaluated in continuous reactors.

The biogas potential and effects of acid pretreatment on Jatropha curcas fruit were also evaluated during anaerobic digestion in batch setup.

The biomethane yields of co-digested mango peel and seed kernel (1:4 weight ratio based on volatile solids) were 61 and 50% higher than the biomethane yields of mono-digestion of mango peel and seed kernel, respectively.

The co-digestion of mango peel and seed kernel at 1:4 ratio resulted in the highest actual biomethane yield, followed by 1:1 ratio (25% lower yield) and 2:1 ratio (36% lower yield). The yields of 1:4, 1:1, and 2:1 ratios were 52, 39 and 32% of the theoretical yields, respectively, illustrating the importance of adjusting C/N ratio by co-digestion with the right ratios of co-substrate.

Nitrogen present in organic substrate is used for synthesis of amino acids, proteins and nucleic acids. The nitrogen is required for microbial growth and a low amount or absence of nitrogen may cause the washout of microbial communities during anaerobic digestion, ultimately resulting in lower biogas yield or reactor failure.

Organic nitrogen is converted to ammonia which is a strong base, neutralizing the volatile acids produced by fermentative bacteria, thus maintaining pH conditions essential for growth of microorganisms. However, an increase in total ammonia nitrogen (free ammonia nitrogen plus ammonium nitrogen) concentration above 3 g/L will have toxic effects on methanogens and cause reactor failure (Nielsen and Angelidaki, 2008, Yenigün and Demirel, 2013).

Thus, an appropriate concentration of nitrogen in the feedstock is needed to simultaneously avoid nutrient limitations and ammonia toxicity. Hence, an imbalance in C/N ratios would have significant effects on biogas yield and microbial activity (Kigozi et al., 2014). The optimum range of C/N ratio for biogas production has been reported to be 20:1 to 30:1 (Hessami et al., 1996, Raheman and Mondal, 2012).

Since de-oiled seed kernel of J. curcas has a low C/N ratio of 11, addition of a substrate with high C/N ratio (mango peel with a C/N ratio of 53) was investigated.

During mono-digestion, the steady state biogas and biomethane yields of seed kernel in (Table 5.2) were 21.9 and 22.7% higher than those of mango peels.

The biogas yield of seed kernels was significantly higher (p<0.05) than that of mango peels (Figure 5.1a and 5.1b). During mono-digestion of de-oiled seed kernel, the process reached pseudo-steady state on Day 23, with less than 5% variation in biogas production for the next eight days (Figure 5.1b).

The biodegradability of de-oiled seed kernel in continuous mode was higher than that of mango peels, indicated by the higher VS reduction. There were higher variations in the reactor pH of mango peels than for the de-oiled seed kernel during anaerobic digestion.

The pH range (Table 5.2) during de-oiled seed kernel digestion (6.58-7.22) was closer to the normal range of 6.5-7.6 (Labatut and Gooch, 2012) than for the digestion of mango peels (5.91-7.04), indicating the relative stability of de-oiled seed kernel treating reactor.

De-oiled seed kernel was found to have higher biomethane production than mango peels (Table 5.2). The VFA to alkalinity ratios of de-oiled seed kernel (Figure 5.1b) were closer to the normal range of 0.1-0.2 (Gerardi, 2003).

The calculated C/N ratios in co-digestion of mango peel plus de-oiled seed kernel in 1:4, 1:1, 2:1 ratios were 20, 32, and 39, respectively (Table 5.3). In co-digestion, the average biogas yield of mango peel plus de-oiled seed kernel (MP+SK) co-digested in 1:4 was 25.8 and 30.3% higher than the 1:1 and 2:1 ratios respectively.

The average biogas yield of MP+SK in 1:4 was significantly higher (p<0.05) than the co-digestion in 1:1 and 2:1 (Figure 5.2a).

Similarly, the average biomethane yield of MP+SK in 1:4 was also significantly higher (p<0.05) than the co-digestion in 1:1 and 2:1 (Figure 5.2b). The average biomethane yield of MP+SK (1:4) was 25 and 36.4% higher than MP+SK co-digested in 1:1 and 2:1, respectively (Table 5.3). The actual biogas yield of MP+SK in 1:4 (Figure 5.3a) was significantly higher p<0.05 (50% increase) than the calculated yield in same ratio (1:4).

Similarly, the actual biogas yield of MP+SK in 2:1 was significantly higher with 41.7% increase than that calculated biogas yield for the same ratio (2:1).

The VS reduction was higher during the anaerobic co-digestion of MP+SK in 1:4 compared to those at 1:1 and 2:1. This higher VS reduction results in higher biomethane production (Haider et al., 2015).

The results showed that the biogas yields of MP+SK co-digestion in 1:4 ratio were 25.8, 30.3, 51.5, and 62.1 % higher than biogas yields of 1:1, 2:1, seed kernel (mono-digestion) and mango peel (mono-digestion), respectively (Figure 5.2a).

The biogas yield during co-digestion of MP+SK in 1:4 was significantly (p<0.05) higher than all other treatments in continuous mode (Figure 5.2a). These results clearly indicate that for a stable biogas reactor with efficient biogas and biomethane yield the C/N ratio must always be in the range of 20-30.”

Source Citation

• Researcher’s full name: Abdul Haq
• Title: Biotechnological Applications of Jatropha curcas Seeds for Bioenergy Carriers and Bioactive Compounds
• Guide(s): Dr. Malik Badshah
• University: Quaid-i-Azam University, Islamabad
• Completed Date: 2020
• Excerpt Page Numbers: 140, 141, 147, 150, 151, 153