ANS:E - Methane.

In a sludge tank, the gas mainly produced is methane. Sludge tanks, also known as digesters, are components of wastewater treatment plants where organic matter undergoes anaerobic digestion, a biological process in the absence of oxygen. During anaerobic digestion, microorganisms break down organic matter present in sewage sludge, producing biogas as a byproduct. Biogas is primarily composed of methane (CH4) and carbon dioxide (CO2), with smaller amounts of other gases such as hydrogen sulfide (H2S), nitrogen (N2), and traces of oxygen (O2). Methane is the main component of biogas, typically accounting for around 50-70% of its composition, while carbon dioxide makes up most of the remaining portion. Therefore, in a sludge tank or digester, the gas mainly produced is methane. Methane-rich biogas generated during anaerobic digestion can be captured, collected, and utilized as a renewable energy source for various purposes, including heating, electricity generation, and fuel for vehicles, reducing greenhouse gas emissions and promoting sustainable wastewater treatment practices.