Q.Read the following passage carefully and answer the questions given below- Large quantities of waste-water is generated everyday in cities and towns. A major component of this waste water is human excreta. This waste water is also called sewage. It contains large amounts of organic matter and microbes, many of which are pathogenic. This can not be directly discharged into natural water bodies like rivers and streams. Before disposal hence sewage is treated in sewage treatment plants to make it less polluting. This treatment is carried out in two stages. In primary treatment, small and large particles are physically removed from sewage whereas in secondary treatment the primary effluent is passed into large aeration tanks and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs. (क) Why is sewage treatment necessary?
(ख) What is the difference between primary and secondary treatment of sewage?
(ग) What is 'Flocs' and what is their role?
(घ) By which method, small and large particles are physically removed from sewage in primary treatment?

In sewage treatment, primary sludge refers to the settled solid organic matter removed by physical settling in the primary treatment stage, before the wastewater proceeds to biological (secondary) treatment.

Raw sewage first undergoes primary treatment, which is essentially physical: it passes through filters to remove large floating debris, then through a grit chamber (settling of sand/grit), and finally into a primary settling/sedimentation tank. Here, the heavier, particulate suspended solids (faecal matter and other organic debris) settle down under gravity to form primary sludge, while the floating material (scum) is skimmed off separately. The clarified supernatant liquid (called effluent) is then passed on to secondary treatmen …

Primary sludge is settled solid waste from physical treatment; activated sludge is a microbial floc formed during aerobic biological treatment.

Sewage (waste water) treatment happens in stages:

• Primary treatment (physical): the sewage passes through filters and settling tanks. Heavier solid particles simply settle down under gravity, forming primary sludge, while the floating debris is removed by skimming. This is purely a physical, non-biological separation step.
• Secondary treatment (biological): the effluent from primary treatment is passed into large aeration tanks and constantly agitated and aerated. This allows aerobic micro-organisms (bacteria, protozoa, etc., collectively called "activated sludge") to grow vigorously, forming flocs, and to consume/oxidise the remaining organic matter in the effluent, reducing its BOD (biochemical oxygen demand). …

Sewage is the municipal waste water full of organic matter and microbes.

Concept. Large amounts of waste water are generated every day in cities and towns. This municipal waste water, called sewage, is carried through sewers and contains human excreta, domestic and industrial liquid waste, a high load of organic matter, and many microbes, several of which are pathogenic.

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Both aerobic floc-forming bacteria (secondary treatment) and anaerobic methanogens like Methanobacterium (sludge digestion) decompose sewage.

In secondary (biological) sewage treatment, the primary effluent is passed into large aeration tanks and continuously agitated and aerated. This allows vigorous growth of aerobic heterotrophic bacteria, which grow along with fungal filaments into masses called flocs (a mesh-like microbial community). These bacteria consume the dissolved and suspended organic matter in the sewage, significantly reducing its Biochemical Oxygen Demand (BOD).

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The anaerobic sludge in sewage treatment is digested by methanogenic bacteria (methanogens), such as Methanobacterium, which produce a mixture of methane, hydrogen sulphide and CO₂ (biogas).

Concept. Sewage treatment in a Sewage Treatment Plant (STP) has two main stages — primary (physical) and secondary (biological). In secondary treatment the effluent is passed through aeration tanks where aerobic microbes form flocs and reduce the BOD. The settled solids form the activated sludge, a part of which is passed into large anaerobic sludge digesters.

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Flocs are microbe-rich clumps that form during activated sludge treatment and are key to purifying sewage.

During secondary (biological) treatment of sewage, the effluent from primary treatment is passed into large aeration tanks and constantly agitated and aerated. This allows aerobic microbes (bacteria and fungi) to grow vigorously, forming flocs — masses of bacteria held together in a matrix by filamentous fungi, forming a slimy, mesh-like structure. These flocs actively consume the remaining organic matter in the sewage, significantly reducing its Biochemical Oxygen Demand (BOD). The floc-laden sludge is then allowe …

Mesh-like microbial masses in sewage treatment are called flocs.

In the secondary/biological treatment of sewage, the primary effluent is passed into large aeration tanks where vigorous aerobic bacteria grow. These bacteria grow along with fungal filaments to form mesh-like clumps called flocs. The microbes in the flocs consume the organic matter, reducing the BOD (bi …

Primary treatment of sewage is a purely physical process that removes solid material before the sewage undergoes microbial (secondary) treatment.

Sewage treatment plants (STPs) purify wastewater in stages. In primary treatment, sewage is first passed through screens to filter out large floating objects (rags, plastic, paper), then through grit chambers where sand and small stones settle out by gravity. The partially clarified sewage is then held in large settling (sedimentation) tanks, where the heavier suspended solids settle at the bottom as primary sludge, while the lighter material floats up as scum and is skimmed off. The clarified liquid, now called the effluent, is passed on to second …