Q.The release of municipal wastewater and industrial waste into our natural water bodies is causing disastrous effect to the aquatic life. Explain the biological treatment that should be given to it before releasing into the natural water bodies.

The key idea behind biological wastewater treatment is the use of aerobic heterotrophic microorganisms to consume organic matter present in the wastewater, thereby significantly reducing its Biological Oxygen Demand (BOD).

1. Primary Treatment: Wastewater first undergoes physical processes like filtration and sedimentation to remove large and small suspended solids, producing primary effluent.
2. Secondary Treatment (Aeration): The primary effluent is then transferred to large aeration tanks. Here, air is continuously pumped in, and the effluent is agitated, promoting the vigorous growth of aerobic microbes (flocs) that consume the organic matter, drastically reducing the BOD. …

The release of untreated or inadequately treated wastewater into natural water bodies poses a severe threat to aquatic ecosystems. This is primarily because wastewater contains a high concentration of organic matter, nutrients (like nitrogen and phosphorus), and often pathogenic microorganisms. The decomposition of organic matter by microbes in natural waters consumes dissolved oxygen, leading to oxygen depletion (eutrophication) which is lethal to fish and other aquatic life. Excess nutrients also fuel algal blooms, further exacerbating oxygen depletion when these algae die and decompose.

Biological treatment is a crucial secondary stage in wastewater treatment that harnesses the power of microorganisms to remove these harmful components. The core idea is to create an environment where beneficial bacteria and other microbes can thrive and efficiently consume the organic pollutants and transform nutrients into less harmful forms. This process mimics natural decomposition but in a controlled and accelerated manner.

Here is a step-by-step explanation of the biological treatment process:

1. Preliminary and Primary Treatment (Setting the Stage):

   Before biological treatment, wastewater undergoes preliminary treatment (screening to remove large debris) and primary treatment (sedimentation to remove settleable solids and floating materials). These physical processes reduce the load on the subsequent biological treatment units, making them more efficient. The water leaving primary treatment is still rich in dissolved and colloidal organic matter, which is the target for biological treatment.

2. Secondary Treatment: Aerobic Biological Processes (The Main Event):

   This is the heart of biological treatment, primarily focusing on the removal of dissolved and colloidal organic matter.

   • Aeration Tanks (Activated Sludge Process):

     The primary effluent is introduced into large aeration tanks. Here, air is continuously pumped into the wastewater, providing dissolved oxygen. This oxygen is vital for the growth and activity of aerobic microorganisms (bacteria, protozoa, fungi) that are suspended in the water. These microbes consume the organic pollutants (carbohydrates, proteins, fats) as their food source, converting them into carbon dioxide, water, and new microbial cells. As they grow, these microbes clump together to form dense, settleable aggregates called 'flocs' or 'activated sludge'. The amount of organic matter is often measured by Biochemical Oxygen Demand (BOD), which is the amount of oxygen consumed by microorganisms to break down organic matter. Biological treatment significantly reduces BOD.

     Important

     The goal of aerobic biological treatment is to reduce the Biochemical Oxygen Demand (BOD) of the wastewater by allowing aerobic microorganisms to metabolize organic pollutants.

   • Secondary Clarifiers (Sedimentation Tanks):

     From the aeration tanks, the mixture of treated water and activated sludge flocs flows into secondary clarifiers. These are large, quiescent tanks where the heavy microbial flocs settle to the bottom due to gravity, separating from the now much cleaner treated water (secondary effluent). A significant portion of this settled activated sludge is recycled back to the aeration tanks to maintain a high concentration of active microorganisms, ensuring continuous and efficient treatment. The excess sludge, which consists of dead microbes and accumulated inorganic matter, is removed for further treatment.

   • Other Aerobic Methods:

     While the activated sludge process is most common, other aerobic biological methods include trickling filters (where wastewater trickles over a bed of media coated with microbial film) and rotating biological contactors (RBCs, where rotating discs covered with microbial film are partially submerged in wastewater). These methods also rely on aerobic microorganisms to degrade organic matter.

3. Sludge Treatment (Anaerobic Biological Process):

   The excess sludge generated from primary and secondary clarifiers is a concentrated waste product that still contains a high amount of organic matter and pathogens. It requires further treatment before disposal.

   • Anaerobic Digestion: …