From Waste to Energy: The Science of Anaerobic Digestion Explained

---

From Waste to Energy: The Science of Anaerobic Digestion Explained

Introduction

While the oil from Jatropha curcas seeds is a powerful source for biodiesel, what happens to the leftover plant material? The concept of a true biorefinery demands that we use every part of the resource. This is where the fascinating biological process of anaerobic digestion comes in, turning waste into valuable biogas.

This excerpt from Dr. Abdul Haq’s research demystifies this complex microbial process, breaking it down into distinct, coordinated stages. It reveals the microscopic world of bacteria and archaea working in synergy and the critical environmental factors that must be perfectly balanced to transform waste into a clean, renewable energy source.

Excerpt

“Anaerobic digestion is a highly complexed process carried out by facultative and obligate anaerobic consortia. These microbial communities are synergistically dependent on one another to maintain the process stability for efficient biogas production. Basically, biogas or biomethane formation occurs through a number of steps including hydrolysis, acidogenesis, acetogenesis and methanogenesis.

A number of studies have reported hydrolysis as the rate limiting phase as various complex intermediate compounds and volatile fatty acids are produced during this phase that are inhibitory to microbial communities at the concentration higher than their optimum range.

On the other hand, some studies also suggesting methanogenesis as the rate limiting phase, especially when the feedstocks are easily degradable during anaerobic digestion (Lu et al., 2008). Broadly, the anaerobic digestion process is divided into two distinct phases including; the fermenting phase and methanogenic phase.

The microbial communities involved in these two phases are distinct from each other in terms of physiology, growth kinetics, nutritional requirements and sensitivity to environment. In most of the cases, it remains highly critical to keep balance between the microbial communities of these two phases because any perturbation in their behavior or performance may lead to reactor instability and lower biogas yield.

These two groups might be separated physically using membranes, pH or kinetic controls (Adekunle and Okolie, 2015).

Hydrolysis

Hydrolysis is the first phase in anaerobic digestion process in which highly complexed suspended/insoluble polymeric organic compounds are converted into simpler soluble forms through different enzymes.

The complexed polymeric compounds are cellulose, hemicellulose, proteins and fats and the soluble compounds are mostly monosaccharides, amino acids and fatty acids. These soluble compounds are further utilized by microorganisms as source of energy or cell carbon. This step is carried out by facultative and strict anaerobic bacteria including Bacteroides, Clostridia, Streptococci, Micrococci, Streptococci, Butyrivibrio and Fusobacteria and some others, that secrete different enzymes including cellulases, amylases, lipases, proteases and xylanases (Schnurer and Jarvis, 2010, Cirne et al., 2007a).

The hydrolysis phase is very important because the larger polymeric compounds are converted into simpler forms to increase their bioavailability to microbial communities during anaerobic digestion process. The biodegradation of all these polymeric compounds occurs through different enzymes which create cuts in these complexed structure and convert them into intermediate or simpler forms.

Some of the microbial communities produced a single specialized type of enzymes which degrade only specific substrates such as sugars or proteins. While other microorganisms secrete different enzymes that could convert a number of compounds into simpler soluble forms to be used as source of energy or nutrition.

The sugars degrading microorganisms are called saccharolytic and the proteins degraders are called proteolytic bacteria. There are a number of enzymes that are secreted by hydrolytic bacterial communities such as lipases, amylases, proteases, pectinases, cellulases and hemicellulases.

All of these enzymes are involved in different reactions catalyzing lipids, carbohydrates, sugars, proteins, cellulose and hemicellulose. The hydrolysis rate is highly dependent on the nature of substrate. Generally, the biodegradation of cellulose and hemicellulose take longer time compared to proteins (Schnurer and Jarvis, 2010).

Acidogenesis

Acidogenesis and hydrolysis are ten times faster steps in anaerobic digestion process than other steps. Acidogenesis is the fastest step in anaerobic digestion. In acidogenesis, the monomers (sugars, amino acids and long chain fatty acids) are converted into intermediate products such as H₂, CO₂, alcohols and short chain organic acids including; propionic acid, butyric acid, acetic acid, valeric acid and other short chain fatty acids.

The acidogenic microbial communities are strict or facultative anaerobes including Salmonella, Lactobacillus, Escherichia coli, Bacillus and Streptococcus species. The production of these short chain organic acids favors the lower pH ranging from 4 to 5.5 at which acidogenic and hydrolytic bacteria are active (Demirel and Yenigün, 2002).

The concentration of the hydrogen produced as an intermediate at this stage further influences the upcoming products formation. The higher the partial pressure of hydrogen, the lower the yield of reduced compounds (Gerardi, 2003).

Acetogenesis

The products of acidogenic phase are converted into other intermediate products including alcohols, acetate, hydrogen and CO₂. The optimum pH required by acetogenic bacteria is 6 and they used acetyl Coenzyme A pathway for the production of acetate, H₂ and CO₂. The enzymes involved in this pathway are highly sensitive to O₂, operational and environmental parameters.

The well-known microbial communities performing acetogenic reactions are Syntrophomonas wolfeii, Syntrophobacter wolinii that produce CO₂, H₂ and acetate (Mah, 1982). The microbial communities involved in this phase usually carried out anaerobic oxidation reactions.

The syntrophic acetogenic relations occurs in this phase, in which the product of one microbial group are converted into substrates utilized by the other microbial group (methanogenic consortia). This interaction is dependent on the partial pressure of hydrogen.

During anaerobic oxidation, protons are used as the terminal electron acceptors, which ultimately lead to the production of H₂ molecules. However, these oxidation reaction are only feasible when the partial pressure of hydrogen is low, explaining why the interaction with methanogenic consortia is important that consume hydrogen to produce methane.

Acetogens are obligate hydrogen producing microorganisms that cannot survive at higher partial pressure of hydrogen. Therefore, these hydrogen producing microorganisms are present in symbiotic relations with other hydrogen consumers, maintaining a balanced process. In conclusion, during this phase inter-species symbiotic collaboration occurs through hydrogen transfer (Gerardi, 2003).

Methanogenesis

During this phase, the intermediate products from other phases are converted into CH₄ and CO₂ by methanogenic bacteria or archaea. There are two types of methanogenic bacteria; acetoclastic methanogens and hydrogenotrophic methanogens. Acetoclastic methanogens (Methanosaeta and Methanosarcina thermophila) consume the acetate as substrate and hydrogenotrophic methanogens (Methannospirillum hungatei, Methanoculles receptaculi) produce CH₄ by reducing H₂ and CO₂ (Aslanzadeh, 2014).

Hydrogenotrophic methanogens are fast growing (doubling time 6 h) than acetoclastic (doubling time about 2.6 days) (Christy et al., 2014). In methanogenic phase, about 70% of methane is produced by acetoclastic methanogens from acetate and 30% is produced through redox reaction of hydrogen and carbon dioxide by hydrogenotrophic bacteria.

Methanogens have slow generation time from hours to days. Especially the acetoclastic methanogens are highly vulnerable to fluctuation caused by higher concentration of volatile fatty acids (VFAs) produced in acidogenic and acetogenic phases.

Therefore, this step is also considered as the limiting phase due to slow growing rate. If the intermediate products (VFAs) in the other phases are increased beyond the acceptable limit, the methanogens are inhibited resulting in reactor instability and lower biogas yield.

Therefore, the speed of all phases during anaerobic digestion must be in coordination in order to have consistently efficient and stabilized process resulting into higher biogas yield. For efficient process, these fermentative and methanogenic bacterial consortia must be in equilibrium state.

It has been reported that the methanogens are highly vulnerable to higher acidity in the reactor. A pH range from 6.5 to 8 is considered ideal for normal and efficient performance of methanogenic communities (Schmidt et al., 2009).

The acidity of the reactor is increased if the organic acids production rate is higher than the consumption by methanogens, as a result the pH goes down and creating unfavorable conditions for methanogenic bacteria (Jain et al., 2015).”

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: 49, 50, 51, 52