NCERT · Class 12 · Biology · Microbes in Human WelfareDescribe the application of microbes in Sewage Treatment Plants (STPs) and as Biofertilizers in detail.

Role of Microbes in Sewage Treatment Plants (STPs)\nMunicipal wastewater, also known as sewage, contains large amounts of organic matter and pathogenic microbes. Before disposal into natural water bodies, it must be treated in Sewage Treatment Plants (STPs) to make it less polluting. This is done by utilizing heterotrophic microbes naturally present in the sewage.

• Primary Treatment: This is a physical process that involves the sequential filtration (removal of floating debris) and sedimentation (removal of grit and small pebbles). All solids that settle form the primary sludge, and the supernatant forms the effluent.
• Secondary Treatment (Biological Treatment): The primary effluent is taken to large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows rapid growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures). While growing, these microbes consume the major part of the organic matter in the effluent, significantly reducing the Biochemical Oxygen Demand (BOD).
• Sludge Digestion: Once the BOD of wastewater is reduced, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi in the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide, which constitute biogas.

Role of Microbes as Biofertilizers\nBiofertilizers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilizers are bacteria, fungi, and cyanobacteria.

• Rhizobium: These symbiotic bacteria live in the root nodules of leguminous plants and fix atmospheric nitrogen into organic forms, which is used by the plant as a nutrient.
• Free-living Nitrogen Fixers: Bacteria such as Azospirillum and Azotobacter enrich the nitrogen content of the soil while living freely in it.
• Mycorrhiza: Fungi form symbiotic associations with the roots of higher plants. The fungal symbiont absorbs phosphorus from soil and passes it to the plant, providing resistance to root-borne pathogens, tolerance to salinity and drought, and overall plant growth.
• Cyanobacteria: These are autotrophic microbes widely distributed in aquatic and terrestrial environments, many of which can fix atmospheric nitrogen, e.g., Anabaena, Nostoc, Oscillatoria. In paddy fields, cyanobacteria serve as an important biofertilizer, adding organic matter to the soil and increasing its fertility.