ANS:C - digestor

The underground well of a biogas plant is called the digestor. The digestor, also known as the digester or anaerobic digester, is a key component of a biogas plant where the anaerobic digestion process occurs. In the digestor, organic materials such as animal manure, agricultural residues, food waste, or sewage sludge are broken down by anaerobic bacteria in the absence of oxygen. This process produces biogas, which is primarily methane and carbon dioxide, along with small amounts of other gases. The biogas produced in the digestor can be captured and used as a renewable energy source for cooking, heating, electricity generation, or as a vehicle fuel. Additionally, the byproduct of the digestion process, called digestate, can be used as a nutrient-rich fertilizer. The other options mentioned are different components or processes related to wastewater treatment or sewage systems:

• A septic tank is a component of a septic system used for wastewater treatment, where anaerobic bacteria decompose solid waste.
• An oxidation well is not a commonly recognized term in the context of biogas plants or wastewater treatment. Oxidation ponds or oxidation ditches are components of some wastewater treatment systems where aerobic bacteria consume organic pollutants by oxidizing them.
• A lagoon is a shallow body of water used for the treatment of wastewater or sewage. It allows the natural processes of settling, aeration, and microbial action to purify the water.