Decomposition and biogas

In a nutshell

Decay is the process of breaking down (digestion) materials by a microorganism. Decomposition is another term for decay. This process requires oxygen. However, biogas can be made through anaerobic decay of waste products.

Decay rate

The rate of decay can vary depending on many different factors. Some of these are described below.

Factor	Description
Temperature	Enzymes are involved in decomposition. Increased temperatures increase enzyme activity. However, if temperatures are too high, the enzymes will denature so decay will slow down or stop. Very cold temperatures can also slow the rate of decay.
Oxygen availability	Most organisms, including those that carry out decomposition, need oxygen for respiration. If there is no oxygen then the rate of decay will slow.
Water availability	Organisms involved in decomposition need water. Therefore, the rate of decay is increased in moist environments.
Number of organisms	The number of organisms will impact the rate of decomposition. If there are more decomposers, the rate of decomposition will be higher.

Preventing decay

Under some circumstances, like in food preparation and storage, it is important to prevent decay. The table below describes some ways that decay can be prevented.

Method	Description
Refrigeration	Refrigeration decreases the temperature so the rate of decomposition will be slowed.
Salt	Salt causes water to move out of the bacterial cells by osmosis. This means there is less water available for the decomposer to grow.
Packing in nitrogen	Packing food in nitrogen decreases the concentration of oxygen that is available for decomposers to respire. Therefore, decay will be slowed.

Biogas

Decomposers can decay plant and animal waste anaerobically. This process produces biogas. Methane is the primary component of biogas. Methane can be burned as a fuel so it is a renewable source of energy.

Example

Sludge waste from sewage plants can be used to make biogas on a large scale.

Biogas can not be stored as a liquid as the pressure required is too high. This means that the gas needs to be used straight away.

Example

Biogas can be used for cooking, heating, lighting and to generate electricity.

Biogas generators

Biogas is made in simple fermenters called generators.

Batch generators

Batch generators make biogas in small batches. The waste is put into the generator and left to decompose. The products are collected at the end and a new batch of waste is put into the generator.

Continuous generators

Continuous generators make biogas continuously. The waste is loaded into the generator at a constant rate and the products are also collected at a constant rate. Larger projects will use continuous generators.

Simple biogas generator

All biogas generators, including batch and continuous generators, must have an opening for waste material to be put into the generator (1.). The waste material (2.) decomposes to produce gas (3.). The generator must have an outlet to allow the biogas to be released (4.) and another for the digested products to be collected from (5.).

Biology; Ecosystems and material cycles; KS4 Year 10; Decomposition and biogas