Oxygen is very important for the microorganism to break down and degrade the organic matter which is present in the sewage or the activated sludge. If the oxygen availability of the activated sludge floc is reduced then the degradation rate will slow down.

Biogas is a mixture of gases consists primarily methane and carbon dioxide  that produced by the breakdown of organic substance into such gases  in complete lack of oxygen. This biogas production can only be done by anaerobic breakdown of biomass. As methanogens are also anaerobic bacteria. The correct answer must be 'Both (b) and (c).

The technology of biogas production from cow dung was developed in India largely due to the efforts of Indian Agricultural Research Institute (IARI) and Khadi & Village Industries Commission (KVIC) .

A part of the activated sludge is pumped back into the aeration tank to serve as an inoculum in activated sludge treatment.

Biogas is also commonly known as gobar gas. The biogas is produced due to anaerobic decomposition of the organic waste matter from the plants and animals with the help of microorganisms. The composition of biogas is methane and carbon dioxide. There are traces of hydrogen, nitrogen, hydrogen sulphide and carbon monoxide. The major component is of methane which accounts for 50-70%. Methane forms the clear and non-polluting gas and commonly used as a fuel.

Water pollution due to organic sources like pollution due to sewage can be prevented by treating sewage before passing into the water course, in sewage or effluent treatment plants (ETP). Domestic sewage primarily contains biodegradable organic matter, which readily decomposes by bacteria and other micro-organisms, which can multiply by using these organic substances as substrates and hence reduce some of the components of sewage. Thus, the correct answer is option A.

• Biogas is a mixture of mostly methane and carbon dioxide with trace amounts of other gases and is considered as a valuable renewable energy source.
• Methane comprises of 45-70% of biogas and thus biogas is inflammable. It produces a deep blue flame and can be used as an energy source.

The process of removal and digestion of inorganic and organic waste present in the sewage water is called sewage treatment.

Anaerobic digestion is a collection of a process by which microorganism break down biodegradable material in the absence of oxygen and some of these processes are used in the waste Industries to manage the waste. Primary sludge is a result of the capture of suspended solids and organics materials. In the primary process of waste management, the material is treated in a variety of ways. During the biological treatment of the waste a part of activated sludge is used as inoculum and they are passed into the last tank which is known as anaerobic sludge digester. The microorganism present in the sludge gets killed.

• Methanogens are present in the rumen of cattle that is present in dung as well. Methanogens acts on the cellulose content of the and  It helps in the production of methane gas that is a component of biogas.
  
• Hence Methanogens acts on cellulose and release biogas is a correct statement.
• So, the correct answer is 'Methanogens acts on cellulose and release biogas'.

The odour is due to the gases produced  by the bacteria Methanogens which belong to Archaebacteria.

The order of bacteria active in the conversion of organic wastes into biogas is hydrolytic, acetogenic, methanogenic. 

Hydrolytic- Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives which are used by bacteria.

Acetogenic- Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. These bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide.

Methanogenic- The methanogens convert the product of acetogenic step to methane and carbon dioxide. The produced gas consisting of methane, carbon dioxide, and traces of other ‘contaminant’ gases is known as biogas.

So, the correct answer is 'Hydrolytic $\rightarrow$ Acetogenic $\rightarrow$ Methanogenic'.

The Biogases produced during sewage treatment are

Cytophaga is a genus of Gram-negative bacillus. Cellulomonas is a genus of Gram-positive bacillus. They have the ability to digest cellulose. They are called as cellulolytic bacteria. They are involved in biogas production.