Question

Describe the structure and working method of a biogas plant.

Hint:

Remember the key inputs and outputs: The input is a slurry of organic waste (like cow dung) and water. The process is anaerobic digestion. The main outputs are biogas (primarily methane) used as fuel, and a nutrient-rich slurry used as fertilizer.

Answer 1

Structure of a Biogas Plant
A typical fixed-dome type biogas plant consists of the following parts: \begin{enumerate} \item Mixing Tank: Located above ground level, this is where the raw organic material (like cow dung, called feedstock) is mixed with water to form a slurry. \item Inlet Chamber: A pipe or chamber that feeds the prepared slurry into the main tank. \item Digester Tank: A large, underground, sealed tank made of bricks and cement. It is where the anaerobic decomposition of the slurry takes place. It is built underground to maintain a relatively constant temperature. \item Gas Holder (Dome): The digester is covered by a dome-shaped roof which acts as a gas holder where the produced biogas collects. \item Outlet Chamber and Overflow Tank: The spent slurry flows into the outlet chamber and then to an overflow tank for collection. \item Gas Outlet Pipe: A pipe with a valve on top of the dome to supply the biogas for use. \end{enumerate} Working Method \begin{enumerate} \item Slurry Preparation: The feedstock (e.g., cow dung) is mixed with an equal quantity of water in the mixing tank. \item Feeding the Digester: This slurry is fed into the digester tank through the inlet chamber until the tank is filled to the required level. \item Anaerobic Digestion: Inside the airtight digester, in the absence of oxygen, anaerobic microorganisms decompose the complex organic compounds present in the slurry. This process takes 40-60 days to complete and releases a mixture of gases. \item Gas Collection: The produced gas, known as biogas, is mainly a mixture of methane (\(CH_4\)) and carbon dioxide (\(CO_2\)). Being lighter than the slurry, it collects in the dome. As more gas is produced, it exerts pressure on the slurry. \item Gas Supply: The collected biogas can be drawn through the gas outlet pipe and used as a fuel for cooking and lighting. \item Manure Removal: The pressure of the gas forces the spent slurry into the outlet chamber and then to the overflow tank. This spent slurry is a nutrient-rich manure that can be used as fertilizer for crops. \end{enumerate}