Microbes in Sewage Treatment: Overview, Process, Odour Control

Microbes in Sewage Treatment: Indeed, microbes cause numerous diseases to animals and even plants but these tiny creatures are a lot more important to us in several ways. One such important use is in sewage treatment. Microbes play an important role in sewage treatment.

Wastewater generated from households, industries and other sources is treated using a well-designed technique to make it reusable. Sewage treatment is performed in two stages namely, primary and secondary treatment. In this article, we will provide detailed information on microbes in sewage treatment. Scroll down to learn more!

Microbes in Sewage Treatment: Overview

What is sewage? Sewage refers to the municipal wastewater that is generated in cities and towns on daily basis.
Researchers estimate the indicator species, such as coliform bacteria or Pseudomonas aeruginosa in the sewage water. Although most of the coliform bacterias do not cause diseases, these are commonly found in the human gut and sewage. Their presence indicates that human waste has reached the water supply.
Human excreta is the major component of it, along with large amounts of organic matter and microbes, out of which many are pathogenic. So, it cannot be directly discharged into natural water bodies like rivers, streams, etc. Hence, sewage has to be treated in Sewage Treatment Plants (STPs) in order to make it less polluting before disposal. The treatment of wastewater is done by the heterotrophic microbes, naturally present in the sewage.
Thus, Microbes Sewage treatment, also known as wastewater treatment, is the removal of impurities from sewage before it enters natural water bodies.

Fig: Microbes and Sewage Water Treatment

What is Microbes Treatment Method?

Residential and industrial establishments generate sewage. It involves household waste liquid from bathrooms, toilets, kitchens, sinks, etc., disposed of through sewers. In certain regions, sewage also carries wet waste from industry and commerce.

1. Greywater is water produced from domestic activities like laundry, dishwashing, and bathing and can be reused more readily.
2. Blackwater comes from toilets and contains human waste. The parting and draining of household waste into greywater and blackwater have become more common in today’s world.
Since sewage contains toxic materials, microorganisms play an important role by removing the toxic using sewage treatment.

Sewage treatment is performed in two stages:
1. Primary treatment
2. Secondary treatment

Primary Sewage Treatment: This process is also known as a physical process. In primary treatment, sewage is collected in a basin where sludge can settle to the base, whereas oil and lighter substances float on the top. Substances in these layers are then removed, and the remaining liquid is sent to secondary treatment. Sewage sludge is treated in a separate process called sludge digestion.

Fig: Simplified flow chart of Sewage Treatment

Secondary Sewage Treatment: This process is also called the biological process. In secondary treatment, dissolved and suspended biological matter are removed, often with the help of microorganisms in a controlled environment.

1. Most secondary treatment systems use aerobic bacteria that consume the organic components of the sewage. Some systems use fixed-film techniques, where the bacteria grow on filters, and the water passes through them.
2. The sewage is often mixed with air to facilitate decomposition as oxygen is critical for the growth of bacteria. This air helps in the growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filament to form mesh-like structures).
3. During their growth period, these microbes consume a major part of the organic matter transforming it into microbial biomass, a lot of minerals are also released during this. This significantly decreases the Biochemical Oxygen Demand (BOD). Hence, the sewage water is treated till the BOD is reduced.
4. When the BOD of effluent is reduced significantly, it is then passed into a settling tank, where the bacterial ‘flocs’ are left to settle down as sediment. This sediment is called activated sludge. A small part of the activated sludge is then pumped back into the aeration tank to serve as the inoculum.
5. The remaining part of the sludge is pumped back into large tanks called anaerobic sludge digesters. Here, other anaerobic bacteria like methanogens are also present. Along with organic mass, these microbes also digest aerobic microbes (bacteria and fungi) of the sludge.
6. A mixture of gases like methane \(\left( {{\rm{C}}{{\rm{H}}_4}} \right),\) hydrogen sulphide \(\left( {{{\rm{H}}_2}{\rm{S}}} \right),\) carbon dioxide \(\left( {{\rm{C}}{{\rm{O}}_{\rm{2}}}} \right),\) etc., are produced during this digestive process. These gases form biogas that is used as an alternate source of energy.
7. The effluent from the secondary treatment plant is discharged into natural water bodies like rivers and streams.

Fig: Diagrammatic Presentation of Sewage Treatment Process

Which Microbes are Used in Sewage Treatment?

There are various microorganisms used in sewage treatment, but among these three types of bacteria play a key role in keeping sewage clean. Each of these bacteria contributes to the treatment process in a different way, ensuring minimal influence on the environment.

1. Aerobic Bacteria: Aerobic bacteria are most commonly used in aerated environments in modern treatment plants. These bacteria degrade the contaminants in the wastewater using oxygen in the water, then turn into the energy that can be used to grow and reproduce. This helps the bacteria to complete their tasks, continue to grow and reproduce.
2. Anaerobic Bacteria: Anaerobic microorganisms are commonly employed in wastewater treatment. Primary function of these bacterias in sewage treatment is to reduce sludge volume and create methane gas from it. This gas can be used as an alternative energy source when properly cleaned and managed. This type of bacterias can utilize enough oxygen from its food supply and does not require additional supply of oxygen. Another advantage of anaerobic microorganisms in sewage treatment is that they remove phosphorus from wastewater. Most common anaerobic forms belong to Actinomyces, Bifidobacterium, Clostridium, Propionibacterium and Peptostreptococcus genera.
3. Facultative Bacteria: In sewage treatment, facultative microorganisms are bacteria that can switch between aerobic and anaerobic states depending on their surroundings. These bacteria like to reside in an aerobic environment.

Odour Control in Sewage Treatment

Anaerobic or septic conditions are usually indicated through smells produced by sewage treatment. Early stages of processing will tend to supply foul-smelling gases, with sulfide being the commonest in generating complaints.

1. Carbon reactors, a contact medium with bio slime, low doses of chlorine, are some examples of processes used to reduce the foul odour.
2. The addition of iron salts, peroxide, nitrate, etc., are some other ways of odour control and managing hydrogen sulfide levels.
3. High-density solids pumps are used for reducing the odour by moving sludge through sealed closed pipework.

Microorganisms in Energy Generation

Microbial fuel cells are microorganisms involved in the production of energy. These are used to generate energy-producing sources such as biogas and electricity.
Biogas Plant: Biogas is produced using agricultural waste, manure, and household garbage as basic materials. The biogas is produced in a massive concrete tank known as a biogas plant.
Biomasses (Biowastes), rich in organic matter are collected at the biogas plant to feed the slurry. They are subjected to the action of methanogens and biogas is formed. Biogas is removed from the biogas plant through a separate outlet.
Electricity: The organic matter from the wastewater treatment plant is used in microbial fuel cells. Organic matter is transformed into a simple molecule during digestion, releasing carbon dioxide and electrons. The electrode absorbs the electrons, which are then employed as a source of electricity. In addition to generating electricity from wastewater, microbial fuel cells are also utilized to create heat.

Summary

Though \(70\%\) of the earth surface is surrounded by water only \(2.5\%\) is freshwater. Most of this freshwater is present as ice in Antarctica and Greenland. Therefore, only a small amount of water is available for our usage. We need water for industrial purposes, in our daily life for almost everything such as cooking, washing, bathing, toilet, etc., and also for numerous industrial purposes. Water, after being used for these purposes, is released into natural bodies making it polluted. Therefore, it is important to treat sewage and reuse this treated effluent in our day to day activities.

FAQs on Microbes in Sewage Treatment

Q.1. What is a sewage treatment plant and how does it work?
Ans: A semi-solid waste or slurry byproduct of sewage treatment is called sewage sludge. Different processes like physical, chemical and biological methods are used to eliminate contaminants from wastewater and produce treated wastewater or effluent which is safe to be released in water bodies or the environment.

Q.2. What are the main steps in sewage treatment?
Ans: a. Primary treatment or Physical process
b. Secondary treatment or Biological process

Q.3. What is the major function of Microbes in Sewage Treatment?
Ans: Sewage is treated in sewage treatment plants (STPs) by the heterotrophic microbes present in the sewage before being disposed of in water bodies. Microbes are used in the secondary treatment or biological processes of sewage treatment. Most secondary treatment systems use aerobic bacteria that consume the organic components of the sewage, like sugar, fat, etc. Anaerobic bacterias in sewage treatment also reduce sludge volume and create methane gas from it which is used as an alternative energy source.

Q.4. Explain the types of microbes used in sewage treatment?
Ans: Aerobic Bacteria: These bacteria degrade the contaminants in the wastewater using oxygen in the water, then turn into the energy that can be used to grow and reproduce.
Anaerobic Bacteria: Primary function of these bacteria in sewage treatment is to reduce sludge volume and create methane gas from it.
Facultative Bacteria: In sewage treatment, facultative microorganisms are bacteria that can switch between aerobic and anaerobic states depending on their surroundings.

Q.5. Why is sewage treatment important?
Ans: Sewage treatment helps in reducing the rate of harmful contaminants that cause pollution of water and soil. Wastewater that is treated in these STPs can be reused for several purposes. Thus, sewage treatment helps in the conservation of water as well as the environment.

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