Microbes in Sewage Treatment — Explained

Detailed Explanation

Sewage, often referred to as wastewater, is a complex mixture primarily composed of domestic, industrial, and agricultural waste products dissolved or suspended in water. Its composition includes organic matter (carbohydrates, proteins, fats), inorganic salts, nutrients (nitrogen and phosphorus), heavy metals, pathogenic microorganisms (bacteria, viruses, protozoa, helminths), and various suspended solids.

The direct discharge of untreated sewage into natural water bodies poses severe environmental and public health risks, including eutrophication, oxygen depletion leading to aquatic life mortality, spread of waterborne diseases, and contamination of drinking water sources.

Therefore, effective sewage treatment is paramount for environmental protection and public health.

Conceptual Foundation of Sewage Treatment

The fundamental principle of sewage treatment is to mimic and accelerate the natural processes of decomposition and purification that occur in ecosystems, but within a controlled environment. This is achieved through a combination of physical, chemical, and biological processes. The biological processes, driven predominantly by microbial activity, are the cornerstone of modern sewage treatment, particularly in reducing the organic load and pathogen content.

Stages of Sewage Treatment

Sewage treatment typically involves three main stages: primary, secondary, and tertiary treatment. While primary and secondary treatments are standard, tertiary treatment is employed for specific discharge requirements or water reuse.

1
Primary Treatment (Physical Process):

This initial stage focuses on the physical removal of large and settleable solids from the raw sewage. It does not significantly involve microbial activity but is crucial for preparing the wastewater for subsequent biological treatment.

* Screening: Raw sewage first passes through bar screens that remove large floating debris such as rags, sticks, plastic bottles, and other coarse materials. This prevents damage to pumps and other equipment in the treatment plant.

* Grit Removal: After screening, the sewage flows into grit chambers, where the velocity of the flow is reduced. This allows heavier inorganic solids like sand, gravel, and cinders (grit) to settle down, while lighter organic solids remain suspended.

* Sedimentation (Primary Settling Tank): The sewage then enters large, quiescent tanks called primary settling tanks or clarifiers. Here, the flow rate is further reduced, allowing suspended organic solids to settle to the bottom by gravity, forming 'primary sludge.

' Lighter materials like oil and grease may float to the surface, forming 'scum,' which is skimmed off. The effluent from this stage, known as 'primary effluent,' still contains a significant amount of dissolved and colloidal organic matter and has a high Biological Oxygen Demand (BOD).

1
Secondary Treatment (Biological Process):

This is the most critical stage where microorganisms play a central role in degrading the organic matter remaining in the primary effluent. Secondary treatment primarily targets the reduction of BOD.

* Biological Oxygen Demand (BOD): BOD is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter present in a given water sample at a certain temperature over a specific period (usually 5 days at $20^circ C$).

A high BOD indicates a high level of organic pollution, meaning more oxygen will be consumed by microbes, potentially depleting oxygen for aquatic life. The goal of secondary treatment is to significantly reduce the BOD of the wastewater.

* Aeration Tanks (Activated Sludge Process): The primary effluent is pumped into large aeration tanks. These tanks are equipped with mechanical aerators or diffusers that continuously introduce air (oxygen) into the wastewater.

This creates an aerobic environment essential for the growth and proliferation of aerobic heterotrophic microorganisms, primarily bacteria, but also fungi, protozoa, and rotifers. * Floc Formation: In this oxygen-rich environment, these aerobic microbes grow rapidly, forming dense, self-aggregating masses called 'flocs.

' Flocs are essentially a consortium of bacteria held together by slime (extracellular polymeric substances) and associated with fungal filaments. These flocs are highly efficient at consuming the dissolved and colloidal organic pollutants in the wastewater.

They metabolize the organic matter, converting it into carbon dioxide, water, and new microbial biomass. * BOD Reduction: As the flocs consume the organic matter, the BOD of the wastewater progressively decreases.

The aeration process typically lasts for several hours, during which the BOD can be reduced by 85-95%.

* Secondary Settling Tank (Clarifier): After aeration, the wastewater, now containing the microbial flocs, flows into a secondary settling tank. Here, the flocs, being heavier than water, settle down by gravity, forming 'activated sludge.' The clear water remaining above the settled sludge is called 'secondary effluent.'

* Activated Sludge Recycling: A significant portion of the settled activated sludge is continuously pumped back into the aeration tanks. This 'recycled activated sludge' serves as an inoculum, providing a rich source of active, acclimated microorganisms to treat the incoming primary effluent. This recycling ensures a high concentration of efficient microbes, accelerating the decomposition process. The remaining excess activated sludge is removed for further treatment.

1
Sludge Treatment and Biogas Production:

Both primary sludge (from primary settling) and excess activated sludge (from secondary settling) are rich in organic matter and microbes. This sludge needs further treatment to reduce its volume, stabilize it, and eliminate pathogens before disposal or reuse.

* Anaerobic Sludge Digesters: The collected sludge is transferred to large, closed tanks called anaerobic sludge digesters. These tanks are maintained under anaerobic (oxygen-free) conditions. Here, a different group of microorganisms, primarily anaerobic bacteria (e.

g., methanogens like *Methanobacterium*, *Methanococcus*), break down the complex organic polymers in the sludge through a series of anaerobic digestion steps (hydrolysis, acidogenesis, acetogenesis, methanogenesis).

* Biogas Production: As a result of anaerobic digestion, a mixture of gases, collectively known as 'biogas,' is produced. Biogas typically consists of methane ($CH_4$, 50-75%), carbon dioxide ($CO_2$, 25-45%), and trace amounts of hydrogen sulfide ($H_2S$), nitrogen ($N_2$), and hydrogen ($H_2$).

Methane is a valuable fuel and can be used to generate electricity, heat, or power the sewage treatment plant itself, making the process more energy-efficient and sustainable. The digested sludge, now significantly reduced in volume and stabilized, can be used as a fertilizer or safely disposed of.

1
Tertiary Treatment (Advanced Treatment):

While not always implemented, tertiary treatment is an advanced stage that further purifies the secondary effluent to meet specific discharge standards or for water reuse applications (e.g., irrigation, industrial cooling).

This stage may involve processes like filtration (sand filters, activated carbon), nutrient removal (biological nitrogen and phosphorus removal), and disinfection (chlorination, UV radiation, ozonation) to remove residual suspended solids, dissolved inorganic compounds, and pathogens.

Microbes Involved and Their Roles:

Aerobic Bacteria: — Dominant in aeration tanks. Examples include *Pseudomonas*, *Bacillus*, *Zoogloea*, *Flavobacterium*. They are heterotrophic, consuming organic carbon compounds as their energy source and converting them into $CO_2$, water, and new cell mass. They form the backbone of flocs.
Fungi: — Often associated with bacteria in flocs, particularly in industrial wastewater treatment or when pH is low. They contribute to floc structure and organic degradation.
Protozoa: — Ciliates, flagellates, and amoebae are common in activated sludge. They graze on free-swimming bacteria and small organic particles, helping to clarify the effluent and control bacterial populations. Their presence is often an indicator of a healthy, well-functioning activated sludge system.
Anaerobic Bacteria: — Active in anaerobic sludge digesters. These include acid-forming bacteria (e.g., *Clostridium*, *Bacteroides*) that convert complex organic matter into volatile fatty acids, and methanogenic bacteria (e.g., *Methanobacterium*, *Methanococcus*, *Methanosarcina*) that convert these acids and other simple compounds into methane and carbon dioxide.

NEET-Specific Angle:

For NEET aspirants, understanding the sequence of treatment stages, the specific microbial roles at each stage, the concept of BOD and its reduction, and the products of anaerobic digestion (biogas composition) are crucial.

Questions often focus on distinguishing primary from secondary treatment, identifying the types of microbes involved in floc formation versus biogas production, and the significance of activated sludge recycling.

The environmental impact of untreated sewage and the benefits of biogas are also frequently tested concepts.