2.3 Overview of different digesters

A biogas digester is an airtight chamber purposefully constructed to facilitate the anaerobic degradation of organic materials (excreta, blackwater, faecal sludge, and biodegradable waste). Its primary objective is to enable the controlled breakdown of organic material without oxygen.

The biogas digesters have various forms and shapes, utilising different construction materials. The selection of construction material and the overall design of the biogas reactor are influenced by critical parameters such as:

• Technical suitability: refers to how well the construction material and design are aligned with the operational conditions.
  • For example, are the selected design and materials suitable for installing the digester below the ground?
• Cost-effectiveness: refers to the materials and design, including procurement, transport, installation, and maintenance costs, while balancing the initial investment with long-term durability and performance to ensure the economic viability of the biogas digester.
  • For example, is option A, with a higher initial investment cost, eventually more cost-effective in the long term because of more durable materials?
• Availability: refers to the materials and skilled labour.
  • For example, are the required skills and materials available to implement a reliable system?

Even if many different types of biogas digesters are available, including fixed-dome and floating-dome digesters, balloon (bag) digesters, continuously stirred tank reactors or garage-type digesters, to name a few examples, the most common types are fixed-dome, floating-dome, and balloon (bag) digesters. These wet digestion systems operate semi-continuously without heating. They are appealing by offering a cost-effective solution by utilising locally available construction materials. They are user-friendly and include a minimal number of moving parts, significantly reducing the probability of operational failures. These attributes enable widespread acceptance and implementation in diverse settings. Therefore, the following types will be introduced in the following modules.

• Fixed dome digester
• Floating dome digester
• Balloon type digester

Furthermore, the following sub-modules will introduce several types of anaerobic digesters specifically designed for sanitation to treat liquid waste.