Introduction to Biogas Technology

What is Biogas?

Biogas is a mixture of gases produced by microorganisms during the anaerobic fermentation of biodegradable materials. This biochemical process occurs in oxygen-free environments where specific bacteria digest biomass, resulting in:

• Methane (CH₄)
• Carbon dioxide (CO₂)
• Hydrogen (H₂)
• Traces of other gases
• A decomposed organic mass (digestate)

Properties of Biogas

Biogas is composed of several gases, primarily methane and carbon dioxide. Its composition depends on the feed material characteristics and the extent of degradation.

Table 1.1: Composition of Biogas

Table 1.2: Properties of Biogas

Note: The energy content of biogas depends on its methane content, which ranges from 50–70%.

Microbiology of Biogas Production

Biogas is produced through a multi-step microbial process involving the anaerobic digestion of organic matter. The process takes place in three sequential stages, each facilitated by different types of bacteria:

Stages of Biogas Production

1. Hydrolysis (Fermentative stage)
2. Acid Formation (Acidogenesis + Acetogenesis) (Acidogenic and Acetogenic stage)
3. Methane Formation (Methanogenic stage)

1. Hydrolysis (Polymer Breakdown Stage)

• Complex organic compounds (e.g., carbohydrates, fats, proteins) are broken down into simpler soluble molecules (e.g., glucose, amino acids).
• Enzymes secreted by fermentative bacteria initiate this stage.
• End products: Glucose, fatty acids, amino acids.

Reaction: Polymers (e.g., cellulose) → Monomers (e.g., glucose)

2. Acid Formation (Acidogenesis & Acetogenesis)

• The simple molecules from hydrolysis are fermented by acid-producing bacteria to form volatile fatty acids (VFAs) and alcohols.
• Main products: Acetic acid, Propionic acid, Butyric acid, Ethanol, CO₂, H₂.

Example Reaction:  Glucose → Acetic Acid + H₂ + CO₂

3. Methane Formation (Methanogenesis)

• Specialized bacteria called Methanogens convert acetic acid and hydrogen into methane and carbon dioxide.
• These bacteria are obligate anaerobes and thrive best in pH range 6.5–8.

Major Reactions:

1. CH₃COOH → CH₄ + CO₂
2. 2CH₃CH₂OH + CO₂ → CH₄ + 2CH₃COOH
3. CO₂ + 4H₂ → CH₄ + 2H₂O

Stage I            →         Stage II           →         Stage III

Fermentative       →         Acetogenic         →         Methanogenic 

(Hydrolysis)                 (Acid Formation)              (Methane Formation)

Explanation (optional for notes or teaching use):

• Stage I – Fermentative (Hydrolysis):
  Complex organic matter (carbohydrates, proteins, fats) is broken down into simpler soluble compounds.
• Stage II – Acetogenic (Acid Formation):
  Simple compounds are converted into volatile fatty acids (like acetic acid), alcohols, hydrogen, and carbon dioxide.
• Stage III – Methanogenic (Methane Formation):
  Methanogenic archaea convert acetic acid, hydrogen, and carbon dioxide into methane (CH₄) and carbon dioxide (CO₂).

Note:

• Biogas is a renewable energy source primarily composed of methane.
• It is generated via a three-stage microbial process: Hydrolysis, Acid Formation, and Methane Formation.
• The efficiency of gas production depends on the effectiveness of microbial digestion at each stage.
• Proper conditions (temperature, pH, retention time) are critical for optimal methane yield.

Biogas Plant and Its Components

What is a Biogas Plant? A biogas plant is a physical structure designed to carry out the anaerobic digestion of organic materials, mainly cow dung and biomass, to produce biogas and organic slurry.

Main Components of a Biogas Plant

1. Mixing Tank; Cow dung is collected and mixed with water in a 1:1 ratio to form a homogeneous slurry.
2. Feed Inlet Pipe/Tank; The slurry is fed into the digester through the inlet pipe (in KVIC model) or inlet tank (in Janatha model).
3. Digester; Anaerobic fermentation of the slurry takes place here by microorganisms to produce biogas.
4. Gas Holder
   • Stores the biogas produced in the digester.
   • KVIC model: uses a floating drum.
   • Fixed dome models (Janatha and Deenbandhu): use the dome itself to store gas.
5. Slurry Outlet Pipe/Tank; The digested slurry (effluent) exits through the outlet pipe or tank, ready for use as organic fertilizer.
6. Gas Outlet Pipe; Carries the produced biogas from the gas holder to the usage point.

Classification of Biogas Plants

Based on Feeding Method

1. Batch Type
   • Feed is added once; digestion occurs over 30–50 days.
   • Gas production is intermittent.
   • Suitable for fibrous materials.
   • Requires addition of fermented slurry to initiate digestion.
2. Semi-Continuous Type
   • Feed is added at regular intervals (e.g., daily), and an equal amount of slurry is removed.
   • Most commonly used in villages.
3. Continuous Type
   • Feed is continuously added, and digested slurry is simultaneously discharged.
   • Offers continuous gas production, smaller digestion area, and low maintenance.

Types of Semi-Continuous Biogas Plants

1. Floating Drum Type – KVIC Model

Structure & Features

• Digester: Circular brick construction, typically underground.
• Gasholder: Separate steel drum that floats and rises with gas accumulation.
• Scum Breaking: By rotating the drum.
• Constant Pressure: Due to weight of the drum.
• Guiding Frame: Allows vertical movement of the drum.
• Gas Outlet Pipe: At the top of the drum.

Advantages

• High gas yield per cubic meter of digester.
• Scum can be broken easily via drum rotation.
• No gas leakage.
• Constant gas pressure.

Disadvantages

• High construction cost due to use of steel and cement.
• Heat loss from metal gasholder.
• Needs frequent painting of drum (once or twice a year).
• Flexible pipe needs regular maintenance.

2. Fixed Dome Type Models

A) Janatha Biogas Plant

• Entire structure is masonry, with no steel drum.
• Gas is stored in the dome.
• Gas pressure is variable.
• Slurry level rises in inlet and outlet tanks as gas accumulates.
• Displacement of slurry provides pressure for gas flow.

B) Deenbandhu Biogas Plant (Improved Janatha Model)

• Developed by AFPRO in 1984.
• Low-cost and efficient design using locally available materials.
• Digester constructed by joining two spheres:
  • Bottom: Segment of sphere.
  • Top: Hemisphere.
• Feed through pipe; outlet is 150 mm below gas outlet pipe to prevent gas loss.
• Gas holding capacity: ~33% of plant volume.
• 30–45% cheaper than Janatha and KVIC models.

Advantages

• Low cost due to use of only cement (no steel).
• No corrosion problems.
• Better heat insulation (fully underground).
• Maintains stable temperature.
• Can digest long fibrous materials.
• Minimal maintenance required.

Disadvantages

• Requires skilled masons (hard to find in rural areas).
• Slightly lower gas production per m³ of digester.
• Scum formation may occur (no stirring arrangement).
• Gas pressure is variable, not constant.

Comparison at a Glance

Factors Involved in Biogas Production

Biogas is generated through anaerobic digestion—a biological process that breaks down biodegradable material into methane-rich gas. Several factors affect this process:

1. C/N Ratio (Carbon to Nitrogen Ratio)

• Optimal range: 20–30:1
• Too much carbon → Nitrogen is depleted first → Slow digestion
• Too much nitrogen → Forms ammonia → Toxic to methane-producing bacteria

2. Temperature

• Optimal for methanogens: 35–38°C
• Gas production slows at <20°C, and almost stops at 10°C
• Increase in temperature boosts gas yield significantly

3. Retention Time

• Definition: Time feedstock stays in digester = (Digester Volume / Daily Input Volume)
• Longer time is needed due to slow anaerobic decomposition
• HRT = SRT in Indian digesters (where biomass gets washed out)

4. Loading Rate

• Defined as input of raw material per unit digester volume per day
• Overloading → Acid buildup → Decreased gas output
• Underloading → Wastes digester capacity

5. Toxicity

• Small amounts of minerals (Na⁺, K⁺) beneficial
• Heavy metals, antibiotics, detergents, and organic solvents inhibit microbial activity

6. pH Level

• Ideal range: 6.5–7.5 (buffered zone)
• Below 6 (acidic) or above 9 (alkaline) → Inhibits methanogenic bacteria

7. Total Solids Content

• Cow dung moisture: ~80–82%; Total solids: ~18–20%
• Ideal solid concentration: 8–10%
• Helps in faster digestion and better gas yield

8. Feed Rate

• Uniform and timely feeding is essential to maintain microbial activity
• Same quantity and quality of feedstock should be added daily

9. Diameter to Depth Ratio

• Optimal ratio: 0.66–1.00
• Affects gas yield due to temperature variation within digester

10. Nutrient Balance

• Essential nutrients: C, H₂, O₂, N, P, S
• Add phosphorus-rich materials (e.g., night soil, legumes) to balance deficiencies

11. Degree of Mixing

• Gentle mixing is beneficial; violent agitation hinders digestion

12. Type of Feedstocks

• Suitable: Animal and plant wastes
• Avoid woody/high-lignin materials unless pre-treated (e.g., chopped or pre-digested)
• Excessive plant matter may choke the digester

Applications of Biogas

Domestic Use

• Cooking: Burns with a blue flame, no smoke/soot/odor
• Lighting: Via gas mantles (2–3 cft/hr ≈ 40W bulb equivalent)

Mechanical and Electrical Energy

• Used in engines to pump water, grind grain, run threshers
• Electricity generation in decentralized rural areas using spark-ignition engines
• Engines may require H₂S removal and carburetor adjustments

Vehicular Fuel

• Raw biogas unsuitable due to CO₂
• CO₂ must be removed for storage and performance
• Requires compression and purification before use in vehicles

Uses of Biodigested Slurry

➤ As Organic Fertilizer

• Rich in N, P, K
• Improves physical, chemical, and biological soil properties
• Boosts crop yield and soil fertility

➤ Advantages of Slurry

• Odorless and insect-repelling
• Repels termites (unlike raw dung)
• Reduces weed growth by 50%
• Nitrogen in available form for easy absorption by plants
• More efficient and hygienic than traditional FYM