Microbes in Sewage Treatment

Sewage:

Large quantities of wastewater are generated every day in cities and towns. A major component of this wastewater is human excreta. This municipal wastewater is also called sewage. It contains large amounts of organic matter and microbes, many of which are pathogenic.

Need for Treatment:

Sewage cannot be discharged directly into natural water bodies. Before disposal, it is treated in Sewage Treatment Plants (STPs) to reduce its pollution.

Agent of Treatment:

Wastewater is treated by the heterotrophic microbes naturally present in sewage. Treatment is carried out in two stages:

1. Primary Treatment
2. Secondary Treatment (Biological Treatment)

Nature: Physical

Primary treatment involves the physical removal of particles - large and small - from the sewage through filtration and sedimentation.

Sequential steps:

1. Sewage is passed through sequential filtration - floating debris is removed
2. Passed through a grit chamber - grit (soil and small pebbles) is removed by sedimentation
3. All solids that settle form the primary sludge
4. The supernatant forms the effluent
5. This effluent from the primary settling tank is taken for secondary treatment

Nature: Biological - driven entirely by microbial action

Sequential steps:

1. The primary effluent is passed into large aeration tanks
2. It is constantly agitated mechanically, and air is pumped into it
3. This allows vigorous growth of useful aerobic microbes into flocs

Flocs = masses of bacteria associated with fungal filaments, forming mesh-like structures

• Microbes forming flocs consume the major part of the organic matter in the effluent
• This significantly reduces the BOD of the effluent

BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.

• The BOD test measures the rate of uptake of oxygen by microorganisms in a sample of water
• BOD is thus an indirect measure of the organic matter present in the water
• The greater the BOD of wastewater, more is its polluting potential

Once the BOD of the effluent is reduced significantly:

The effluent is passed into a settling tank. Bacterial flocs are allowed to sediment. This sediment is called activated sludge.

Fate of Activated Sludge:

• In the digesters, other kinds of bacteria which grow anaerobically digest the bacteria and fungi in the sludge
• During digestion, they produce a mixture of gases - methane (CH₄), hydrogen sulphide (H₂S), and carbon dioxide (CO₂) - collectively called biogas
• Biogas is used as a source of energy as it is inflammable
• Bacteria that produce methane are called methanogens - e.g., Methanobacterium - and are commonly found in anaerobic sludge during sewage treatment

Final Discharge:

The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

The Ministry of Environment and Forests has initiated the Ganga Action Plan and Yamuna Action Plan to save these major rivers from pollution.

• Under these plans, it is proposed to build a large number of sewage treatment plants so that only treated sewage may be discharged into the rivers
• Due to increasing urbanisation, sewage is being produced in much larger quantities than ever before
• The number of sewage treatment plants has not increased enough to treat such large quantities
• As a result, untreated sewage is often discharged directly into rivers, leading to pollution and an increase in water-borne diseases

• Sewage = 99.5–99.9% water + 0.1–0.5% organic/inorganic matter + microbes (bacteria, viruses, fungi, algae, etc.)
• Preliminary Treatment - Bar screens remove large solids; Grit Chamber removes sand & stones.
• Primary Treatment - Sedimentation tank settles suspended solids; reduces coliform bacteria. Removes ~50–70% solids.
• Secondary Treatment - Aerobic bacteria decompose organic matter in aeration tanks; lowers BOD by 70–80%.
• Tertiary Treatment - Anaerobic bacteria digest sludge in sludge digesters; disinfection improves water quality.
• Chlorination - Kills remaining pathogenic bacteria before water release.
• Disposal - Treated water → natural water bodies; digested sludge → proper disposal/manure.