Treatment of wastewater is done in a sewage treatment plant to make it less polluting. Explain the following with reference to this treatment process :

(a) Primary sludge

(b) Activated sludge

(c) Anaerobic sludge digesters

The correct option is:
(a) Primary Sludge: All the solids that settle down, during the primary treatment of sewage water.
(b) Activated Sludge: Produced during the secondary treatment or biological treatment of sewage, primary effluent + aerobic microbes flocs (bacteria and fungus) – get converted to a sediment whose BOD has reduced significantly.
(c) Anaerobic sludge digesters: Large tanks where activated sludge is treated with anaerobic bacteria which digest the bacteria and fungi, and produce a mixture of CH₄, H₂S and CO₂/ Biogas

Explanation:

1. Primary Sludge:
Primary treatment is a physical process that involves the removal of large and small particles from sewage through filtration and sedimentation. First, floating debris is removed by sequential filtration. Then, the grit (soil and small pebbles) is allowed to settle. The solid material that settles at the bottom of the settling tank during this initial physical stage is called the primary sludge, while the supernatant liquid above it is called the primary effluent.

2. Activated Sludge:
Secondary treatment is a biological process. The primary effluent is passed into large aeration tanks where it is constantly agitated and air is pumped into it, allowing aerobic microbes (bacteria and fungi) to grow rapidly into masses called flocs. These microbes digest the organic matter present in the effluent, which substantially reduces the Biochemical Oxygen Demand (BOD) of the wastewater. The mixture is then passed into a settling tank where the bacterial flocs are allowed to sediment. This settled mass of active biological sediment is called the activated sludge.

3. Anaerobic Sludge Digesters:
The majority of the activated sludge is pumped into large, closed tanks called anaerobic sludge digesters. In these tanks, anaerobic bacteria grow and digest the organic matter along with the aerobic bacteria and fungi present in the activated sludge. During this digestion process, a mixture of gases is produced, including methane (CH₄), hydrogen sulfide (H₂S), and carbon dioxide (CO₂). This gas mixture is known as biogas and can be used as an energy source because it is highly inflammable.