The answer is massas of unicellular bacteria entangled in filamentous bacterial forms represent flocs

Biological Process of Microbial Degradation of Organic Matter in STPs

Introduction
In sewage treatment plants (STPs), the biological process of microbial degradation plays a crucial role in breaking down organic matter present in wastewater. This process involves the utilization of activated sludge as an inoculant, mainly through aerobic breakdown of organic matter, the formation of flocs, and the significant reduction of biochemical oxygen demand (BOD).

Utilization of Activated Sludge as Inoculant
- Activated sludge is a suspension of microorganisms, including bacteria, fungi, and protozoa, which are responsible for degrading organic matter in wastewater.
- This sludge is produced in the physical process of STPs, where wastewater is mixed with a large amount of air to provide oxygen for the microorganisms to carry out aerobic degradation.
- The activated sludge is then transferred to the biological reactor, where it acts as an inoculant, introducing a diverse population of microorganisms into the wastewater.

Anaerobic Breakdown of Organic Matter
- Contrary to option (b), the microbial degradation of organic matter in STPs primarily involves aerobic breakdown rather than anaerobic breakdown.
- In the presence of oxygen, microorganisms utilize organic matter as a source of energy, breaking it down into simpler compounds through a series of enzymatic reactions.
- This aerobic degradation process is more efficient and results in the complete breakdown of organic matter, reducing its potential negative impacts on the environment.

Formation of Flocs
- During the biological process, masses of unicellular bacteria present in the wastewater entangle and form aggregates known as flocs.
- These flocs consist of the microbial biomass, organic matter, and other particles present in the wastewater.
- The formation of flocs is facilitated by the production of extracellular polymeric substances (EPS) by microorganisms, which act as a glue, binding the bacteria and other particles together.
- The formation of flocs improves the settling characteristics of the wastewater, allowing the separation of solid and liquid phases in the subsequent clarification process.

Reduction of BOD
- Biochemical oxygen demand (BOD) is a measure of the amount of oxygen required by microorganisms to degrade organic matter in wastewater.
- The microbial activity in STPs significantly reduces BOD by consuming the organic matter as a source of energy.
- As the microorganisms break down the organic compounds, they metabolize them into carbon dioxide, water, and biomass, reducing the organic load in the wastewater.
- The reduction in BOD is an essential parameter in assessing the effectiveness of sewage treatment, as high BOD levels can deplete dissolved oxygen in receiving water bodies, leading to adverse ecological effects.

Conclusion
The biological process of microbial degradation of organic matter in STPs involves the utilization of activated sludge, primarily through aerobic breakdown. The formation of flocs and the subsequent reduction in BOD are crucial steps in sewage treatment, ensuring the efficient removal of organic pollutants and minimizing the environmental impacts of wastewater discharge.