Session 1: Principles of Biogas

Session 1: Principles of Biogas#

Learning Objectives#

By the end of this session, you should be able to:

Understand how airtightness, temperature, and the Carbon/Nitrogen ratio affect biogas production.
Understand the design of the CREATIVenergie Expanding Bag Digester and how it works.
Compare different types of biodigesters and their advantages and disadvantages.

Case Study: Overview of a Biogas Digester#

Elvis introduces how a biogas digester works and its many uses.

Biogas Production Basics#

Biogas is produced when micro-organisms break down organic matter in an anaerobic (oxygen-free) environment. This is the same natural process that occurs inside a cow’s stomach, where microbes digest food and release methane gas.

There are four key principles you must understand to run a successful biogas digester.

1. Airtightness is Essential#

Biogas production only occurs without oxygen — this process is called Anaerobic Digestion (AD). If the digester is not completely sealed, microbes will not produce methane effectively.

Airtightness is achieved two ways:

Ensuring the bag is properly sealed.
Maintaining the correct balance of input and output so the slurry always covers the throat, preventing gas escaping.

Sealed vs unsealed digester — airtightness comparison — Fig. 1 A properly sealed digester retains all gas for cooking and productive use.#

Before a bag digester is filled with slurry it must be tested for leaks — even a small hole in the seam will let gas escape and waste your energy. In Session 7 (Building) you will go through the full installation process step by step; this simulation shows you the testing method so you already understand what good airtightness looks and feels like before you get there.

🫧 Interactive: Airtightness Test Simulation

A new bag has just been glued and is ready to install. Follow the steps below to test it for leaks before lowering it into the pit.

1. Close valve

2. Apply soapy water

3. Find & mark the leak

Step 1 — Click the red VALVE to close it before inflating the bag.

2. Temperature Control#

Temperature has a major impact on biogas production:

Temperature Range	Effect
Below 10°C	Production stops
10–29°C	Slow production
30–40°C (Mesophilic)	Ideal range — most common
40–70°C (Thermophilic)	Higher yield, but sensitive to fluctuation

Keeping temperature stable within 30–40°C helps maintain efficient, continuous gas production.

Temperature effect on biogas production — chart showing optimal mesophilic range — Fig. 2 Biogas production rate across the temperature spectrum — the mesophilic range (30–40°C) offers the best balance of yield and stability.#

🌡️ Temperature Explorer — try it!

Drag the slider to see how temperature affects biogas production rate.

Temperature: 35°C

✅ 30–40°C — optimal mesophilic range

3. Retention Time#

Retention time is how long organic material stays inside the digester before it is pushed out. For maximum biogas yield, material needs to stay inside for around 40 days at 30–40°C.

Things that affect retention time:

Available digester size
Temperature
Type and amount of feedstock

Note

The more you feed the digester each day, the faster you push older material out — shortening retention time.

Retention time diagram — material flow through the digester over 40 days — Fig. 3 Material moves through the digester over a 40-day retention period, emerging as nutrient-rich bio-slurry.#

4. Carbon-to-Nitrogen (C/N) Ratio#

The ideal C/N ratio for biogas production is 20–30 (20–30 parts carbon per 1 part nitrogen).

Problem	Cause	Effect
C/N too high	Too much carbon	Very slow decomposition
C/N too low	Too much nitrogen	Harms microbes, reduces gas

Common Feedstocks and Their C/N Ratios#

Feedstock	C/N Ratio	Notes
Cow Dung	20–30	✅ Ideal — best feedstock
Pig Droppings	13–20	Good, may need balancing
Chicken Manure	7–10	High in nitrogen — mix with straw
Kitchen Food Waste	13–16	Good, but avoid bones or sharp objects
Human Faeces	6–10	Usable — extend retention time to 95 days
Sawdust	200–500	Very high carbon — must mix with nitrogen-rich material

C/N ratio infographic — feedstock comparison chart — Fig. 4 C/N ratios of common feedstocks — the ideal window of 20–30 is where cow dung sits naturally.#

⚖️ C/N Ratio Explorer — click a feedstock

See where each material falls relative to the ideal 20–30 range.

0 10 20 ✓ 30 ✓ 50 100+

Select a feedstock to explore its C/N ratio.

Case Study: Feeding the Digester#

Common Types of Biodigester#

Now that you understand the principles, here are the two most common digester designs:

Feature	Bag Digester	Fixed Dome Digester
Cost	Low	Higher
Construction	Easy	Requires skilled technicians
Gas pressure	Low	High
Durability	Moderate	High
Best for	Small-scale / household	Larger-scale / long-term

The CREATIVenergie Expanding Bag Digester#

This course focuses on the CREATIVenergie Expanding Bag Digester — a design that combines the strengths of both types.

How it works:

Organic waste (e.g. cow dung + water) enters a sealed plastic/rubber bag.
Microbes break it down, producing biogas, and the bag expands.
Gas is piped for cooking and other uses.
After 30–40 days, fully digested material flows out as bio-slurry — a valuable fertiliser.

Summary#

The four key principles of biogas production are:

Airtightness — no oxygen allowed
Temperature — keep it 30–40°C
Retention time — 40 days for optimal breakdown
C/N Ratio — aim for 20–30; cow dung is ideal

In the next session we will learn how to size your digester correctly.

Next: Session 2: Sizing