Beyond the Lab: How Real-World Conditions Affect Biogas Production
Table of Contents
Introduction
While batch tests in a lab are essential for understanding the potential of a biogas feedstock, the real test comes from mimicking real-world conditions. Continuous operation of a biogas digester introduces dynamic variables like organic loading rates (OLRs) and hydraulic retention times (HRTs), which can dramatically impact performance. This excerpt from Dr. Abdul Haq’s research moves beyond basic potential and explores the practical application of digesting different Jatropha curcas components.
It reveals how factors like feed composition and operational parameters create a complex interplay that determines the stability, efficiency, and ultimate success of a continuous biogas system.
Excerpt
“The inhibitors/phytochemicals present in J. curcas seed and the operational parameters are the key factors that affect the process of anaerobic digestion. These factors must be addressed in a way to increase the biogas yield and economic value of J. curcas seed for the process of anaerobic digestion.
In the current study, the reactor treating MR (Jatropha pressed cake after methanolic extraction) was the most stable and efficient continuous mode reactor compared to those fed with JPC, JO and JWS (Figure 6.2).
Optimum biogas and biomethane yield for MR was obtained at OLR of 1 g VS L⁻¹ d⁻¹ and HRT of 20 days and was significantly higher (p<0.05) than that obtained at OLRs (1.5, 2, 3, 4, 5, 6, and 7) and HRT (15 and 10 days (Figure 6.2A and 6.2B).
In MR treating reactor, the biogas yield was decreased after 1 g VS L⁻¹ day⁻¹ due to its high biodegradability and increase in OLR that resulted in continuous accumulation of VFAs, subsequently not converted into methane in the same pace.
The VFAs/alkalinity ratios (Supplemental Information, Table S6) was increased at OLR of 3 g VS L⁻¹ day⁻¹ and HRT 20 days; to address the issue of high VFAs/alkalinity ratio, the reactor was transferred at two stage anaerobic digestion.
The MR treating reactor was stopped at OLR of 7 g VS L⁻¹ day⁻¹ as the small diameter pipes began to be clogged due to higher overload of MR during feeding.
The JPC treating reactor gave optimum biogas and biomethane yield at an OLR of 1.5 g VS L⁻¹ d⁻¹ and HRT of 20 days (Figure 6.2C and 6.2D).
It was sustained until OLR of 6 g VS L⁻¹ d⁻¹ (Figure 6.2C). The JO was rich in LCFAs and the reactor treating it was inhibited in early stages at OLR of 1 g VS L⁻¹ day⁻¹ at HRTs of 20, 15 and 10 days (Figure 6.2E). The LCFAs causes decrease in biogas yield and reactor failure (Xu et al., 2015).
The JWS treating reactor was fluctuated and not sustained for longer. The optimum biogas yield for JWS was obtained at 1.5 g VS L⁻¹ d⁻¹ and a HRT of 20 days and the reactor was failed at OLR of 3 g VS L⁻¹ d⁻¹.
The possible reason of this early failure of JWS might be the higher antimicrobial compounds than JPC and MR and the increase in VFAs/alkalinity ratios (Appendix 4 Table S6) beyond the acceptable limit (<0.4) leading to over acidification of the reactor.
The JWS is rich in proteins and relatively poor biodegradation of proteins during anaerobic digestion had been observed compared to the lipids and carbohydrates (Lee et al., 2016).
To sum up, the MR treating reactor was sustained for longer time and exhibited significantly higher (p<0.05) biogas yield compared to JPC, JO and JWS treating reactors at optimized OLR and HRT.
The biodegradability of MR in terms of VS reduction was higher at optimized condition (OLR and HRT) compared to that of JPC, JO and JWS (Appendix 4 Table S6). So it means using JWS for biogas production is not a better option. But if oil is extracted, can be used for biodiesel (ul ain Rana et al., 2019).
The biomethane/biogas yield of all substrates was affected by feed composition and operational parameters (HRTs and OLRs). The substrates rich in antimicrobial compounds has shown low efficiency and reactor stability in terms of biogas yield.
The main cause of reactors failure was substrate’s toxicity due to long chain fatty acids and other antimicrobials present in J. curcas seeds and oil.
Secondly, the operational parameters has also affected the reactor stability and efficiency in terms of biogas yield. At lower OLRs and longer HRTs, the reactors were more stable, and yielded higher biogas compared to that at higher OLRs and shorter HRTs.
There were no obvious differences in the observed index in all four substrates treating reactors. However, there were marked differences in richness and evenness (Shannon index) between the MR (JPC after methanolic extraction), JPC, JO and JWS treating reactors.
According to the calculated indexes, the MR treating reactor had higher microbial diversity compared to that within the JPC, JO and JWS reactors (Figure 6.3A).
The lowest Shannon index was shown by JO followed by JWS and JPC, suggesting that the presence of long chain fatty acids and other toxic compounds probably had an inhibitory effect on microbial communities.
Higher richness and evenness are signs of functional stability of a reactor and the lower richness or evenness are usually considered as a warning indicator for reactor instability (Carballa et al., 2015).
The HRT also affected the microbial diversity during anaerobic digestion. Methanogenic consortia present in anaerobic digesters are mostly slow growers and require a longer retention time (Schnurer and Jarvis, 2010).
It is assumed that the shorter HRT mostly have problems of microbial wash out leading to the reactor instability and failure (Couras et al., 2014). The observed index and Shannon index of all samples at HRT 20 was higher than at HRT of 15 and 10 days (Figure 6.3B).”
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: 144, 148
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