Pick up the incorrect statement from the following:

In wastewater engineering, the incorrect statement from the options provided is Option C.

Option A: The process of decomposing the organic matter under controlled anaerobic conditions is indeed called sludge digestion. During this process, microorganisms break down organic matter in the absence of oxygen, producing biogas as a byproduct.

Option B: Sludge digestion is commonly carried out in a dedicated tank known as a sludge digester or anaerobic digester. This tank provides the controlled anaerobic conditions necessary for the decomposition of organic matter.

Option C: The statement claiming that the gases produced in the sludge digestion process contain 75% carbon dioxide is incorrect. In reality, the gases produced in sludge digestion primarily contain methane (CH4) along with some carbon dioxide (CO2) and other trace gases. Methane is the predominant component, typically constituting around 50% to 70% of the biogas produced in anaerobic digestion processes.

Option D: The statement claiming that the gases produced in the sludge digestion process contain 75% methane is correct. Methane is the main component of biogas generated during anaerobic digestion, typically accounting for around 50% to 70% of the gas composition.

Therefore, the correct option is Option C: The gases produced in sludge digestion process, contain 75% carbon dioxide, as it does not accurately reflect the composition of gases produced during sludge digestion.