The activated sludge have to settle quickly so that it can be pumped back to the aeration tank. So, the correct option is 'be rapidly pumped back from sedimentation tank to aeration tank'

The primary treatment involves physical removal of particles from the sewage through filtration and sedimentation. These are removed initially by sequential filtration which removes the floating debris and then grit is removed by sedimentation. 

Wastewater treatment generates a large quantity of activated sludge during secondary treatment which is used as a inoculum and a small part of it is pumped back to the large aeration tanks whereas the remaining part of that sludge is treated by anaerobic sludge digesters anaerobically. During this digestion, bacteria produces a mixture of gases called biogas. 

Activated sludge should have the ability to settle quickly so that it can be rapidly pumped back from sedimentation tank to the aeration tank, where it can start the process of decomposition of the organic matter again. Activated sludge has filamentous bacteria that are capable of performing the aerobic digestion of organic waste is the sewage water.

• Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen.
•  The process is used for industrial or domestic purposes to manage waste and to produce fuels. 
• Hence, Wastewater treatment generates a large amount of sludge, which can be treated by anaerobic digesters.
• So, the correct answer is 'anaerobic digesters'.

Chlorine is a powerful disinfectant that can kill the bacteria and viruses by destabilizing the chemical bonds of the pathogens. So, it is used in purification of water.

Answer is option C i.e. "Conversion of milk to curd"

Domestic sewage consists biodegradable organic matter and easily decomposes by the micro-organisms. These microorganisms can multiply using these organic substances as substrates and hence, utilize some of the components of sewage. Micro-organisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result, there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. This causes mortality of fish and other aquatic creatures. 

Decay is what happens when dead animals and plants break down. The key factors involved in decay are: Temperature, Amount of oxygen, Amount of water and the presence of microorganisms. Microorganisms are important in decay. For example, they are used to break down human waste (sewage) and plant waste (compost). Decay recycles important nutrients for plant growth.

Sewage is composed of organic matter and domestic waste. The pollutants are removed by the sewage treatment process to mitigate it's adverse impact on the environment when it is released. The sewage pass through primary,  secondary and tertiary treatment. The microorganisms by aerobic and anaerobic metabolism degrade the organic matter in secondary and tertiary treatment respectively making it disinfected.

The sewage treatment unit employs the primary, secondary and tertiary treatment for the complete process. In the primary treatment, filtration of the large and solid waste is done. Then in secondary treatment, the decomposing microbes are used for the breakdown of the organic sewage. So, the secondary treatment is the biological treatment of the waste water. In the tertiary treatment, the chemical treatment is provided to the water.

Sewage sludge is a product of wastewater treatment. Once treated, sewage sludge is then dried and added to a landfill, applied to agricultural cropland as fertilizer, or bagged with other materials and marketed as “bio-solid compost” for use in agriculture and landscaping.

Chlorination is the process of treating the water with chlorine to kill the water-borne microbes that can possibly cause diarrhoea, typhoid, jaundice, amoebiasis etc. It is performed during the tertiary treatment step. Chlorine compounds like sodium hypochlorite are added to water to give effect to the process.

The sewage treatment unit employs the primary, secondary and tertiary treatment for the complete process. In the primary treatment, filtration of the large and solid waste is done. Then in secondary treatment, the decomposing microbes are used for the breakdown of the organic sewage. So, the secondary treatment is the biological treatment of the waste water. In the tertiary treatment, the chemical treatment is provided to the water.

Activated sludge treatment is the step involved in the treatment of sewage water. The sewage is treated in a large aerated tank and bacteria. Once, the process of decomposition is over, the filtrate obtained is called the activated sludge because it contains a high number of bacteria. This is then recycled in the next slot of treatment when the polluted sewage is available for the treatment. The activated sludge speeds up the process of decomposition by acting as an inoculum in the aeration tank. Primary treatment is the removal of large solid particles from the sewage. Tertiary treatment involves chemical treatment of the sewage water.