Question

Give an account of the effects of digester in four or five sentences?

Answer 1

Carrying out anaerobic digestion in a closed reactor, with sufficient volume for the biological reactions to occur without stress, comprises the primary technical requirement. Based on external limitations, such as capital outlay, treatment efficiency, net energy yields and operational skill, the technology available ranges from very rudimentary to quite sophisticated systems for both family scale and full commercial scale. The fact that anaerobic digestion has been used in practical situations for over 100 years demonstrates that it is a viable technology. Problems can arise, however, when there are external constraints, such as limited capital and low operational skills.

Many biogas officers in Developing Countries and commercial companies in Developed Countries have the optimum technology to tailor a plant to specific situation. The following is a summary of the main types of digester in common use.

Batch and Dry Fermentation: This is the simplest of all the processes. The operation involves merely charging an airtight reactor with the substrate, a seed inoculum, and in some cases a chemical (regularly a base) to maintain almost neutral pH. The reactor is then sealed, and fermentation is allowed to proceed for 30 - 180 days, depending on ambient temperature. During this period, the daily gas production builds up to a maximum, and then declines. This fermentation can be conducted at "normal" solids content (6 - 10%) or at high concentrations (>20%), which is then known as "dry" fermentation. Its main components are shown in Figure 7.1.

Figure 7.1: Batch Digester.

Answer 2

Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity".This is the source of marsh gas methane as discovered by Alessandro Volta in 1776.

The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.

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