Waste TreatmentWater Pollution

Sewage Treatment With Microbes – Biological Treatment

Role of microbes in sewage treatment:


In this modern era, the population of earth is increasing day by day, and that is the reason behind most of the innovations. And one of those innovations includes the advanced technology of sewage treatment with microbes

Generally, there are many processes entangled in the treatment of wastewater or sewage, but the biological one needs lots of microorganisms. 

However, the most exciting part of sewage treatment is that those microbes we need to eradicate the pollutants from sewage water remain in itself. 

Therefore, in this lecture, we will discuss various sewage treatment technologies, and further, we will understand the role of microbes in sewage treatment. So, let’s have a closer view.

What is Sewage?

So, before understanding all the technologies, you must clear your concepts of sewage. Sewage is the undigested organic product, which is an output of the human digestive process. In other words, we can explain these as the human-excreta, which are produced by the remaining food in the human body and disposed of into the water in giant underground tanks.

Sewage is the domestic or municipal wastewater that must be treated before exposure to the environment. This statement may raise a question in your mind that, why should the sewage need treatment?

And the answer is straightforward. The sewage contains organic materials and harmful fecal pathogens, which can spread deadly diseases by the disposal of water bodies. It is also capable of making the water bodies poisonous and can cause the death of aquatic animals. Hence, sewage treatment is crucial.

Purpose of Sewage Treatment

The significant objectives or intention behind sewage treatment are as follows;

  • To remove pollutants
  • Destruction of the deadly pathogens
  • To counterbalance coarse particles.
  • Elimination of poisonous substances

Thus, we can say the principal objective of wastewater treatment is to decrease biological oxygen demand (BOD), Chemical Oxygen Demand (COD) and also eutrophication, and much more, so that it won’t spread toxicity in the environment. Moreover, microbes have an essential role in fulfilling the purpose of sewage treatment.

Process of Sewage Treatment

The process of sewage treatment requires microbes naturally. There is a significant role of microbes in sewage treatment. But before microbial or biological processing, the first physical treatment of sewage is needed.

According to the amount of sewage to be treated, the process is divisible into two types, such as large-scale sewage treatment and small-scale sewage treatment.

Typically, the small-scale sewage treatment involves several methods; however, Cesspools and Septic tanks are the most convenient ways.

And the large-scale treatment involves lots of other physical, chemical, and biological processes.


Besides these two parts, the entire sewage treatment process comprises four significant steps and various sub-steps. These four steps are:

Preliminary Treatment

The preliminary process involves removing debris, grit, oil, gross solids, and grease by the screening process. For such an eradication process, different equipment for different types of pollutants is required, such as screeners, grit chambers, and skimming tanks.

Primary Treatment

For removing various subtle organic elements, residues in the preliminary treatment, primary treatment is performed. Primary sewage treatment is required to eliminate 35% BOD, 30% COD, 60% suspended solids, 20% total nitrogen, and 10% Phosphorous.

The primary ways of sewage treatment include:


After the filtration of massive pollutants or debris, primary treatment is done, where the sedimentation process removes various fine particles. According to the sedimentation principle, the solute particles in a solvent tend to settle down due to the gravitational force.

Various factors affecting the sedimentation rate are size, density, mass, viscosity, and specific gravity. According to particles’ tendency to form solids, the settling process is of four major types such as discrete, flocculent, hindered, or zone settling and compression.

There are also many types of clarifiers or settling tanks present within which the sedimentation or settle down process occurs.

Flocculation and Coagulation

A few chemicals are used for sedimentation of some kind of colloidal particles, which takes place through three stages like coagulation, flocculation, and sedimentation.

Clariflocculator is a device used for the flocculation process. Here the destabilized particles are aggregated to form settleable solids before flocculation coagulants like alum or polyelectrolytes are given to destabilize the charged particles.

And finally, the sedimentation process occurs after such mechanical flocculation and chemical coagulant treatment. Thus, this process can be categorized into a sedimentation process and called chemical aided sedimentation.


Besides the above pollutants, sometimes the wastewater is acidic or alkaline. We take lime slurry or NaOH for acidic sewage and H2SO4 or CO² for primary sewage treatment in that condition.

The sewage water after sedimentation or settling is now called effluent, and the settled down product is known as sludge.

Secondary Treatment

So after the preliminary and primary wastewater treatment, let’s move to the actual point that is secondary treatment. Secondary sewage treatment is otherwise known as the biological process as microorganisms are utilized to treat the sewage.

The microorganisms used in such a process are heterotrophic in nature; that means they do not synthesize their own organic materials and thus consume the organic constituents of sewage.

You might think we could treat the wastewater with the utilization of chemicals. Yes, of course, we can. But the outcome is not totally pure, as chemicals are toxic, and usage of them is not a cost-effective option. Thus to replace the toxicity in the water and the high cost, we use simple biological organisms such as bacteria, fungi, and protozoa.

So after finishing the preliminary and primary treatment process, the affluent and sludge undergo secondary or biological degradation process. In the secondary treatment, two different Chambers or components are present; one is an addition for supplying Air, and another is a secondary clarifier.

The secondary clarifier is just like the primary clarifier in the immediate treatment process, but the functionality is different. This is like a big culture plate where nutrients are given to which microbes utilize for their growth.

So, after passing through the aeration chamber, the effluent passes to the secondary clarifier. Now you may have a question, “why aeration is needed?” Well, addition is the crucial step as the Air combines with the wastewater and assists the aerobic microorganisms in doing their job.

Now, let’s discuss the classification of biological secondary wastewater treatment. It is divided into two types :

Suspended and attached treatment

In the suspended culture, the secondary clarifier, a giant vessel where only water is present, shows the microorganisms present in the water reduce the solid organic particles. The Residue settles down at the bottom by the gravity of that watery tank.

The most common suspended sewage treatment processes include activated sludge process, aerated lagoons, sequencing batch reactor, and aerobic digestion.

However, the attached treatment process involves a large vessel filled with solid rock-like beads above which bacteria, fungi, and protozoa like microbes gives rise to a biofilm and water sprinkler, drizzle the wastewater on the top of the biofilm; so that the biofilm can receive water and those microbes can take the required nutrients. After that, the remaining water moves downward and is transferred to another vessel through tubes.

Mostly four types of attached growth processes are present, such as trickling filters, roughing filters, rotating biological contactors, and packed bed reactors.

Both the suspended and attached growth have their pros and cons, so both are used alternatively.

Aerobic and anaerobic

Here we can also use aerobic as well as anaerobic microorganisms. Both aerobic and anaerobic microbes use the nutrients from the affluent and then stabilize it to produce an excretory product that settled down at the bottom.

Mostly aerobic biological treatment is used as the tanks are facilitated for aerobic bacteria, and anaerobic organisms limit their growth in the presence of oxygen. But, sometimes, anaerobic microbes preferred rather than the aerobic one as it fails to break certain compounds.

The suspended growth works with aerobic microorganisms, whereas the attached growth uses both aerobic and anaerobic microbes in a proportion.

Also, the biodegradation of sludge materials comprises three steps like hydrolysis, acidogenesis, and methanogenesis.

Microorganisms use:

Per ml, about 100,000 to 1,000,000 microorganisms are present in typical domestic wastewater before entering the processing plant. Mostly these microbes perform the wastewater process, but some other microorganisms are also used according to the organic materials present.

But how could we know what was present in the sewage? For that reason, the analysis of wastewater is first performed to identify the types of components present with their quantity. Then required microbes are added from the outside. Generally, three types of microbes, bacteria, fungi, and protozoa, are utilized for different functions.

Bacteria decompose the sewage and convert them into settleable solids; fungi can degrade some constituents that cannot decompose by bacteria, and protozoa are the predators that help control the bacterial population.

From this, you can determine the role of microbes in sewage treatment.

Tertiary wastewater treatment 

Tertiary or the final step is concerned with subtracting non-biodegradable organic materials, heavy metals, and minerals. After processing in the secondary treatment plant, the effluent undergoes into the tertiary wastewater treatment, where mostly chemicals are used.

The secondary treatment process involved microbes that consume the dissolved organic materials in the wastewater, and after this process, we don’t need the microorganisms anymore. We need to kill those harmful, deadly pathogens because they can contaminate the water and releasing it into the water bodies, ultimately results in the spread of dangerous diseases.

Thus, before releasing that treated water, we need to destroy the microbes through tertiary or advanced treatment, where mostly gaseous chemicals like chlorine and ozone are used.

The tertiary treatment or advanced treatment takes place through three steps such as,

Solids removal

For the removal of dissolved solid first activated carbons are used to absorb the constituents such as organic herbicide, pesticide, tannin, lignin’s color and odor, and inorganic toxic metal components. We perform this by granular activated carbon contactors.

In the ion-exchange process (one ion is replaced by another), we practice the removal of the remaining solids.

Biological nutrients removal

The biological nutrients like phosphorus and nitrogen are eliminated through this process right after solids removal.

Biological nitrogen removal is done by assimilation, nitrification, and denitrification steps, whereas an anaerobic phosphorus stripper is required for biological phosphorus removal. 


The last step of advanced treatment is disinfection, i.e., the inactivation of disease-causing microorganisms. This can be done by using some chemicals or some physical processes like mechanical destruction or radiation application.

The most widely used disinfectant is chlorine as it satisfies the criteria of an ideal disinfectant as it is soluble and stable in water, non-poisonous to higher organisms or humans, quickly and cheaply available, and toxic to the pathogen at low concentration.

Also Read About: Effects of Acid Rain


In today’s world, microbes became essential for any kind of industrial process, and thus, various techniques are discovered where microorganisms are utilized for solving multiple problems. In that way, the role of microbes in sewage treatment is critical. Such utilization of these small creatures benefits both the environment and human society. 

Moreover, here we discussed several sewage treatment techniques and the biological or the process of applying microbes in sewage treatment in detail. We hope you clear all of your core concepts regarding this topic. If you have doubts, comment down below and get an instant answer.


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