ANS:C - The gases produced in sludge digestion process, contain 75% carbon dioxide

1. The process of decomposing the organic matter under controlled anaerobic conditions, is called sludge digestion: This statement is correct. Sludge digestion is indeed the process of breaking down organic matter present in sewage sludge or wastewater treatment sludge under controlled anaerobic conditions, typically in a digester tank. Anaerobic digestion is used to stabilize the organic matter, reduce the volume of sludge, and produce biogas.
2. Sludge digestion is carried out in sludge tank: This statement is also correct. Sludge digestion is carried out in a dedicated tank called a digester or sludge tank. This tank provides the controlled anaerobic environment necessary for the digestion process to occur. The sludge is mixed and heated to facilitate the breakdown of organic matter by anaerobic bacteria.
3. The gases produced in the sludge digestion process contain 75% carbon dioxide: This statement is incorrect. In the sludge digestion process, the biogas produced typically contains a significant amount of methane (CH4) along with carbon dioxide (CO2) and other gases such as hydrogen sulfide (H2S) and traces of nitrogen and hydrogen. The exact composition of biogas can vary depending on factors such as the feedstock (type of sludge), temperature, and retention time in the digester. While carbon dioxide is indeed produced during anaerobic digestion, it does not typically account for 75% of the biogas composition.
4. The gases produced in the sludge digestion process contain 75% methane: This statement is also incorrect. While methane is a major component of the biogas produced in the sludge digestion process, the exact percentage of methane can vary depending on the factors mentioned earlier. A typical biogas composition might contain around 50-70% methane, with the remainder consisting of carbon dioxide and other gases.