The primar sludge is formed during primary sewage treatment, while activated sludge is a sludge during secondary sewage treatment, little decomposition occurs during primary sludge formation, while lot of decomposition occurs during activated sludge.

Primary treatment in sewage treatment involves physical removal of particles (large and small) from the sewage through filtration and sedimentation. Initially floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. 

Secondary treatment is also called biological treatment or microbial degradation. It is mainly a biological process. Biogas is a mixture of gases, containing predominantly methane (50-70%), $CO _2$ (30-40%) and traces of hydrogen, $H _2S$ and nitrogen.

Primary or physical treatment is the process of removal of small and large, floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating and suspended matter is removed through sequential filtration with progressively smaller pore filters. The sediment is called primary sludge while the supernatant is called effluent. The primary sludge traps a lot of microbes and debris. It is subjected to composting, land fill or anaerobic digestion to produce biogas and manure. 

• Primary treatment removes material that will either float or readily settle out by gravity. It includes the physical processes of screening, comminution, grit removal, and sedimentation.
  
• Human faeces, hair and plastic settle down at bottom while Oil and grease are immiscible in water and are found floating on top.

• Treatment of wastewater is done by the heterotrophic microbes naturally present in the sewage. 
• This treatment is carried out in two stages: 

1.  Primary treatment: These treatment steps basically involve physical removal of particles large and small from the sewage through filtration and sedimentation. Initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.
2. Secondary treatment or Biological treatment: The primary effluent is passed into large aeration tanks. 

• Hence, (i) In Primary treatment First, floating debris of waste is removed by sequence filters (ii) The Soil and small pebbles are removed by sedimentation in settling tanks (iii)The sediment is called primary sludge and supernatant is affluent. (iv) The Sludge is taken for Secondary treatment following statements & about wastewater treatment are correct. 
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• So, the correct answer is '(i), (ii), (iii), (iv)'.

1. The effluent from aeration tank is passed into a settling tank where the aerobic bacterial ‘flocs’ are allowed to sediment and the sediment is called activated sludge.

• Biochemical Oxygen Demand (BOD)is defined as the amount of dissolved oxygen needed (i.e. demanded) by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.
• BOD is an important water quality parameter because it provides an index to assess the effect discharged wastewater will have on the receiving environment. The higher the BOD value, the greater the amount of organic matter or “food” available for oxygen-consuming bacteria.
  
• Hence Measurement of the rate of oxygen utilization by a unit volume of water over a period of time is to measure Biochemical oxygen demand.
• So, the correct answer is 'Biochemical oxygen demand'.

Aerobic digestion of waste is the natural biological degradation and purification process in which bacteria that thrive in oxygen-rich environments break down and digest the waste. During oxidation process, pollutants are broken down into carbon dioxide ( CO$ _{2}$ ), water (H$ _{2}$O), nitrates, sulphates and biomass (microorganisms). By operating the oxygen supply with aerators, the process can be significantly accelerated. Of all the biological treatment methods, aerobic digestion is the most widespread process that is used throughout the world. Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require little or no oxygen to live. During this process, a gas that is mainly composed of methane and carbon dioxide, also referred to as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition and gas production. Wastewater pollutants are transformed into methane, carbon dioxide and smaller amount of bio-solids. The biomass growth is much lower compared to those in the aerobic processes. They are also much more compact than the aerobic bio-solids.

Sewage treatment is done in three stages: primary, secondary and tertiary treatment. Secondary treatment removes dissolved and suspended biological matter, using microorganisms in a controlled environment. Most secondary treatment systems use aerobic bacteria, which consume the organic components of the sewage (sugar, fat, and so on). Some systems use fixed film systems, where the bacteria grow on filters, and the water passes through them. Suspended growth systems use activated sludge, where decomposing bacteria are mixed directly into the sewage. Because oxygen is critical to bacterial growth, the sewage is often mixed with air to facilitate decomposition.

Methanobacterium  are anaerobic organisms.
Biogas refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen.
Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria.
Biogas can be produced by anaerobic digestion with anaerobic bacteria.
Thus, the correct answer is option (C), 'Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria'.

Proper disposal of the domestic waste before it goes in to the lakes and oceans is useful in preventing water pollution. Domestic sewage which is the waste water that goes out into the environment through several routes should be treated before being released. It contains soaps, detergents, toilet paper, household refuse all of which causes water pollution. All this sewage should be properly collected and treated in which contaminants are removed from sewage to produce sludge suitable for discharge into the environment. 

Domestic sewage primarily contains biodegradable organic matter, which readily decomposes by bacteria and other micro-organisms, which can multiply using these organic substances as substrates and hence utilise some of the components of sewage. It is possible to estimate the amount of biodegradable organic matter in sewage water by measuring biochemical oxygen demand (BOD). It can be purified treating with with micro-organisms.

The sedimentation process involves the settling down of particles suspended in water under the effect of gravity. The particles that settle down are known as sediment. In purification of water sedimentation process helps in reducing the concentration of particles in suspension. It is the first step used in the primary treatment of sewage water. 

The sewage water can be treated in the treatment plants with the use of microbes. The bacterial cells, fungal cells and algae are the common microbes which are used for the cleaning of the waste. The microbes have the ability to feed and decompose on the organic matter and convert it to simpler non-toxic forms. The organic matter of the wastewater can be reduced and the water can be purified. 

Carbon dioxide, CO$ _2$, is gaining acceptance for pH control in water treatment plant. It reduces high pH levels quickly. It is not stored as an acid solution so it is considered safer than sulfuric acid. It is non-corrosive to pipes and equipment. It requires less equipment and monitoring costs. It requires no handling costs. It can be utilized via a completely automated system. 

Primary treatment basically involves physical removal of particles large and small from the sewage through the filtration and sedimentation. These are removed in stages initially floating debris is removed by sequential filtration.

Chlorine is used for municipal drinking water purification. It is a strong oxidizing agent, hence it kills pathogenic bacteria by oxidation of organic molecules. Chlorine on hydrolysis produces hypochlorous acid that penetrates the pathogenic surface and disintegrates lipids in its cell wall. It also reacts with intracellular enzymes and proteins and deactivates them. Hence chlorination is used to prevent the spread of waterborne diseases such as cholera, dysentery and typhoid. Thus the correct answer is option B.

The polluted water can be purified using the treatment plants. Activated sludge method refers to floc of microorganisms like bacteria and fungi which are used in the process to break down organic matter into carbon dioxide, water, and other inorganic compounds. There are microbes which grow in the water forming mass or flocs and help in the clearing of the water. These flocs are used in the next round of treatment plants for efficient removal of wastes.

Sewage pollution can be prevented by treating sewage, in sewage or effluent treatment plant, before passing into water course. The treatment comprises of primary treatment, secondary treatment and tertiary treatment.
Primary treatment consists of shredding, churning, floatation, screening and sedimentation.
In secondary treatment organic matter is decomposed through sewage fungus by passing sewage through water hyacinth pond, or trickling filter method if passing sewage through thick bed of gravel, or activated sludge method i.e., passing sewage through aerated or oxidation tanks. In activated sludge method, aeration helps in oxidation of sludge or biosolid. The aerobic bacteria can be recycled. The cleared water is now chlorinated with chlorine or perchlorate salts, ozonised or irradiated with ultraviolet light to kill pathogens.
The activated sludge process is a process for treating sewage and industrial wastewaters using air and a biological floc composed of bacteria and protozoa.
The tertiary treatment is physico-chemical treatment for removing turbidity and dissolved substances. Alum, ferric chloride and lime are used for their precipitation.
Therefore, the correct answer is option B.

Sewage pollution can be prevented by treating sewage before passing into water course in sewage or effluent treatment plant.

Checking atomic blasts will reduce radioactive pollution. Sewage pollution can be prevented by treating sewage before passing into water course, in sewage treatment plant or effluent treatment plant (ETP). Electrically operated vehicles will control automobile exhaust and improve the air quality in cities.

A.Primary treatment – Removal of large and small particles from sewage through sequential filtration and sedimentation is called primary treatment.

B.Secondary treatment – It involves biological process of microbial degradation of organic matter contained in sewage effluent. In this activated sludge acts an inoculum.

C.Tertiary treatment – There is no occurrence of tertiary sewage treatment.

So, the correct option is ‘Secondary treatment’.    

Biomagnification refers to increase in concentration of the toxicant at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolised or excreted, and is thus passed on to the next higher trophic level. This phenomenon is well known for mercury and DDT. 

Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment. 

Domestic sewage primarily contains biodegradable organic matter, which readily decomposes by bacteria and other micro-organisms, which can multiply using these organic substances as substrates and hence utilise some of the components of sewage. It is possible to estimate the amount of biodegradable organic matter in sewage water by measuring biochemical oxygen demand (BOD).  

Biomagnification refers to increase in concentration of the toxicant at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolised or excreted (non degradable), and is thus passed on to the next higher trophic level. This phenomenon is well known for mercury and DDT. Biological magnification often refers to the process whereby certain substances such as pesticides or heavy metals move up the food chain, work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances become concentrated in tissues or internal organs as they move up the chain.

The activated sludge process is a process for treating sewage and industrial wastewaters using air and a biological floc composed of bacteria and protozoa. Activated sludge refers to a mass of microorganisms cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds. 

Nitrification is a biological oxidation process includes convertion of Ammonia to Nitrate increases nitrogen leaching because nitrate is more water soluble than Ammonia.