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Microbiology is one of the Biology Topics that involves the study of microorganisms, including bacteria, viruses, and fungi.
Microbes in Sewage Treatment – Role of Microorganisms Used in Wastewater Treatment
Sewage treatment is the process of removing contaminants from wastewater and household sewage, both runoff, domestic, commercial, and institutional. Sewage should not be allowed to pass directly to the rivers or water bodies. It contains a large number of pathogenic microbes and harmful chemicals. It endangers the lives of humans and animals with a maximum number of waterborne diseases and deaths. Therefore prior to disposal of sewage in water bodies waste water should be treated in effluent treatment plants (ETP). It involves cleaning of millions of gallons of sewage daily.
Composition of Sewage
1. Water: 99.1%.
2. Solid Particle:
- biodegradable pollutants like human faecal, paper, fiber substances, and animal wastes.
- dissolved organic compounds, e.g., carbohydrates, protein, fat, urea, etc.
- inorganic salts like calcium, chloride, and sodium.
- sand and mud dissolved in water.
- a large number of microbes like Vibrio cholerae, Salmonella typhi, Shigella dysenterica, etc., bacteria, protozoa (Entamoeba histolytica), and Hepatitis virus.
Sewage Treatment
To remove the sewage from water three processes are used.
1. Mechanical Process:
- By filtration, large grains are removed.
- By the skimming method, small and oily particles are removed.
- By precipitation method, heavy grains are precipitated.
2. Chemical Process:
Through this process, precipitation, removal of bad odour, and sterilization of chemical substances occur.
3. Special Process:
By some special process oxidation of some organic compounds occurs, and CO2, nitrogen oxide, sulphate, etc. is used in biological treatment. In 1956, Imholf et al. discovered an easy process of sewage treatment. 4 tanks are used in this process. These are the Detritus tank, Skimming tank, Sedimentation tank, and Oxidation tank.
The sewage treatment system often involves three stages: Primary treatment (Physical), Secondary treatment (Biological), and Tertiary treatment (Physio-Chemical).
Primary Treatment:
This is a physical process that involves the separation of large debris, followed by sedimentation in tanks. The physical process includes sequential Alteration through many screens. Sedimentation involves passing of liquid portion into large primary settling tanks. Grit, sand, and other heavy particles settle down. All sediments and screened organic matter constitute primary sludge. The wastewater after removing the primary sludge contains fine organic matter and pathogenic microflora. It is passed for secondary treatment.
Secondary Treatment:
This is a biological process and is carried out by microorganisms. Secondary treatment is done by several methods e.g., oxidation tanks, trickling filters, biodiesel systems, and activated sludge systems. In an activated sludge system, the primary effluent is taken to aeration tanks. Sewage is aerated by mechanical stirring. Air is pumped into the effluent. Part of the previously activated sludge is inoculated into it. A large number of aerobic heterotrophic microbes different types of bacteria, some filamentous fungi, yeasts, and protozoan grow in the aeration tank.
Due to vigorous aeration of sewage floc-formation occurs. Floes are masses of bacteria held together by slime and fungal filaments to form a mesh-like structure. The BOD (Biological Oxygen Demand) of the effluent falls to 10-15% of the raw sewage. It is taken to a secondary settling tank. In the settling tank, the bacterial flocks are allowed to undergo sedimentation. The supernatant is generally passed into natural waters like rivers and streams.
The sediment of the settling tank is called activated sludge. It is taken to an anaerobic sludge digester along with the primary sludge. Anaerobic microbes digest the organic mass as well as the killed aerobic microbes of the sludge. Anaerobic microbes are of two types, nonmethanogenic and methanogenic. Methanogens produce a mixture of gases containing methane, H2S, and CO2 The gaseous mixture is called biogas. It is inflammable and is a source of energy. The spent sludge is used as manure or part of compost.
Tertiary Treatment:
This is a physicochemical process that removes nonbiodegradable organic materials, heavy metals, and minerals. The salts of nitrogen and phosphorus must be removed because they cause eutrophication. This stage involves the chemical oxidation of wastewater by strong oxidizing agents such as chlorine gas, perchlorate salts, ozone gas, and UV radiation. By using activated carbon filters the organic pollutants can be removed, whereas by adding lime the phosphorus is precipitated as calcium phosphate. Nitrogen can be removed by stripping, and volatilization as NH3 at high pH values. Ammonia can be converted by chlorination to dichloramine which in turn is converted to N2. After tertiary treatment, the wastewater can be released into natural waters or used for irrigation.
NEERI (National Environmental Engineering Research Institute) has devised a very cheap and unconventional process of sewage treatment in specially constructed large shallow ponds. These ponds are known as oxidation or stabilization ponds. Domestic or industrial wastes with organic nutrients are stored in these pits in dilute conditions for a few days. In the presence of sunlight and organic nutrients, the medium flourishes with green algae and colonies of bacteria. The bacteria digest the organic waste and water is purified. This water is rich in nitrogen, phosphorus, and potassium and can be used as irrigation water.
Differences between Primary Sludge and Activated Sludge:
| Primary Sludge | Activated Sludge |
| 1. In this sludge, no aeration is required. | 1. In this sludge, aeration is required. |
| 2. During primary sewage treatment, it is formed. | 2. During secondary sewage treatment, it is formed. |
| 3. It does not develop floes of decomposer microbes. | 3. The decomposer microbes produce floes. |
| 4. Little decomposition occurs at this stage. | 4. A major part of the sludge gets decomposed. |
River Action Plans
Urbanization and population explosion have been very rapid in the last centuries. This has resulted in the production of several million gallons of wastewater by every country each day. But, sewage treatment plants have not been developed in sufficient numbers throughout the globe. So, the failure of proper sewage treatment along with the release of untreated waste effluents into the water sources has polluted the water bodies highly and also increased the incidences of waterborne diseases. In order to protect the rivers of India from such sewage pollution, the Ministry of Environment and Forests has taken up the development of sewage treatment plants under the National River Conservation Authority, e.g., the Ganga Action Plan (GAP), Yamuna Action Plan (YAP), Gomti Action Plan, etc.