agra,ahmedabad,ajmer,akola,aligarh,ambala,amravati,amritsar,aurangabad,ayodhya,bangalore,bareilly,bathinda,bhagalpur,bhilai,bhiwani,bhopal,bhubaneswar,bikaner,bilaspur,bokaro,chandigarh,chennai,coimbatore,cuttack,dehradun,delhi ncr,dhanbad,dibrugarh,durgapur,faridabad,ferozpur,gandhinagar,gaya,ghaziabad,goa,gorakhpur,greater noida,gurugram,guwahati,gwalior,haldwani,haridwar,hisar,hyderabad,indore,jabalpur,jaipur,jalandhar,jammu,jamshedpur,jhansi,jodhpur,jorhat,kaithal,kanpur,karimnagar,karnal,kashipur,khammam,kharagpur,kochi,kolhapur,kolkata,kota,kottayam,kozhikode,kurnool,kurukshetra,latur,lucknow,ludhiana,madurai,mangaluru,mathura,meerut,moradabad,mumbai,muzaffarpur,mysore,nagpur,nanded,narnaul,nashik,nellore,noida,palwal,panchkula,panipat,pathankot,patiala,patna,prayagraj,puducherry,pune,raipur,rajahmundry,ranchi,rewa,rewari,rohtak,rudrapur,saharanpur,salem,secunderabad,silchar,siliguri,sirsa,solapur,sri-ganganagar,srinagar,surat,thrissur,tinsukia,tiruchirapalli,tirupati,trivandrum,udaipur,udhampur,ujjain,vadodara,vapi,varanasi,vellore,vijayawada,visakhapatnam,warangal,yamuna-nagar

Process of wastewater Treatment, Practice problems and FAQs

Process of wastewater Treatment, Practice problems and FAQs

Did you know that the waterborne disease, cholera, was a pandemic during the 19th century? It had spread from the banks of the Ganges river in India to different parts of the world. This disease is caused by a bacteria known as Vibrio cholerae which is commonly found in water contaminated with excreta of cholera patients. You would be amazed to know that in older times, people used to defecate out in the open, near water bodies, which made it easier for them to be contaminated with harmful pathogens. In fact, this practice is quite common in many less developed villages and cities of our country, even today. This results in contamination of water bodies and increases the risks of incurring water borne diseases. Shocking, right?

Even wastes from industries, domestic and commercial settlements, hospitals, etc are often dumped into water bodies and end up polluting the water bodies. As water is one of the most important resources for living, polluted water from these water bodies not only adversely affects the lives of the aquatic organisms but it also has a massive negative impact when it comes in contact with humans. Many of these water bodies are also used as sources of drinking water. So, imagine the havoc that will be wreaked when we drink such water contaminated with pathogens, toxic chemicals, heavy metals, solid wastes, excreta, etc!

So what is the way out of this scenario? The answer to this question is wastewater treatment. Waste water carrying municipal wastes or industrial effluents should be first treated in Sewage Treatment Plants (STPs) or Effluent Treatment Plants (ETPs) before being discharged in water bodies. Come let us discuss how this is achieved.

Table of contents:

  • Municipal wastewater treatment
  • Treatment of industrial effluents
  • Integrated wastewater treatment
  • Ecological sanitation
  • Initiatives by government for controlling water pollution
  • Practice problems
  • FAQs

Municipal wastewater treatment

Sewage pollution of water bodies by municipal wastes from households or commercial settlements can be prevented by treating it in sewage treatment plants. The treatment involves three steps - primary treatment, secondary treatment and tertiary treatment.

Primary or physical treatment

Primary treatment involves the removal of particles such as grit and larger pieces of organic matter. It is achieved with the help of filtration and sedimentation. The sewage is first diluted with water, shredded to cut down large organic matter and churned and then passed through filtration and sedimentation tanks.

                                                Fig: Primary sewage treatment


In this step the wastewater or effluent is passed through a series of multiple sequentially arranged filters each of which have a different pore size. The first filter has the largest pore size and the pore size decreases gradually over the subsequent filters.

Filtration helps to get rid of all the floating solid debris which get filtered and separated out according to their size.

                                                                Fig: Filtration tank


The smaller solid particles which cannot be removed by sequential filtration are transferred to another tank called a sedimentation tank. Grit (soil and small pebbles) are heavier than water and hence when the sewage is left undisturbed in this tank, the heavier particles start settling down by the process of sedimentation. Here, grit, sand and other heavy particles settle down.

Secondary treatment

After primary treatment, the settled solid wastes and screened organic wastes together form the primary sludge and the effluent obtained after removal of primary sludge is passed on for secondary treatment.

The secondary treatment involves the biological process of microbial degradation of organic matter. It can either be achieved by a trickling filter method or activated sludge method.

Trickling filter method

In this method, the primary effluent is allowed to trickle over a bed of gravel and fungi having a thickness of 2mm or more. The decomposed organic matter is taken out at intervals.

Activated sludge method

In this method, the biological degradation of organic wastes in the primary effluent is achieved by passing the effluent through an aeration or oxidation tank, a settling tank and an activated sludge digester.

                                            Fig: Secondary Sewage Treatment

Aeration tank

The effluent consists of large amounts of organic matter and is passed into large aeration tanks where it is subjected to constant mechanical agitation and air is pumped through it.

                                                               Fig: Aeration tank

The heterotrophic microbes naturally present in the sewage are bacteria and algae. This stimulates rapid growth of the heterotrophic aerobic microbes (bacteria, protozoa, algae, filamentous fungi) that are naturally present in the effluent. These microbes form the microbial floc which is a mesh like structure formed by the mass of bacteria, algae, filamentous fungi, etc.

                                                              Fig: Microbial floc

The microbial floc devours a major part of the organic matter in the effluent and considerably reduces the biochemical oxygen demand (BOD) of the effluent. BOD is the amount of oxygen consumed by the bacteria for oxidation of all the organic matter in one litre water. As the organic matter content of the effluent reduces, the BOD also reduces. Lesser is the BOD of water, less polluted it is.

Settling or sedimentation tank

From the aeration tank, the effluent is passed through a sedimentation tank where the microbial floc are allowed to settle down and the settled material is known as activated sludge.A small part of the activated sludge is reintroduced into the aeration tanks to be used as an inoculum for the growth of the microbial floc. The rest and major part is pumped into the anaerobic sludge digesters.

Anaerobic sludge digester tank

The anaerobic sludge digester tank is free of oxygen and the anaerobic microbes grow in such an environment and feed on the bacteria and fungi present in the activated sludge. During this anaerobic digestion, gases such as methane, carbon dioxide, hydrogen sulphide are released. This gaseous mixture is highly inflammable and can be used as a fuel known as biogas.

The spent sludge from the sludge digester tank can be used for composting or as manure. The effluent is then either directly released into water bodies or is subjected to a tertiary treatment before releasing it to the water bodies.

Tertiary treatment

It is a physicochemical method for removing turbidity from the treated wastewater. It involves chemical oxidation of water using strong oxidising agents such as chlorine as, ferric chloride, lime, ozone, etc. and irradiation of the water with UV rays to kill any pathogenic microbe. Tertiary treatment is rarely used owing to its high cost.

Treatment of Industrial effluents

The treatment of industrial effluents involves different methods depending upon the nature of pollutants present in the effluent. Some of these methods are:


The effluents are tested for pH and then neutralised using an acid or an alkali.


Here, the chemicals present in the effluent are precipitated using chemicals or electrostatic precipitators.


Toxic chemicals or coloured impurities can be removed by adsorping them on the surface of activated charcoal.


Photocatalysis refers to an increase in the rate of a chemical reaction that uses light energy, with the help of a catalyst. This technique is used to convert toxic substances into harmless ones.

Integrated Wastewater treatment

Wastewater can be treated in an integrated manner and can be made reusable. In order to treat the huge amount of wastewater generated each day, an initiative was taken in the town of Arcata which is situated along the north coast of California. In collaboration with biologists from Humboldt State University, a wastewater treatment process was designed which involved treatment of wastewater using two processes: artificial processes and natural processes.

Artificial processes

It includes the conventional processes such as sedimentation, filtration, and chlorine treatment. This step involves removal of the suspended solids and majority of the organic waste from wastewater.

                       Fig: Artificial process involved in wastewater treatment

Natural processes

The treated water after artificial processes still constitute harmful elements like heavy metals. In order to solve this problem, people of Arcata and biologists developed a series of 6 marshes connected with each other over a marshland of 60 hectares. Although constructed artificially, these marshes had flora and fauna similar to the natural ones.

                                         Fig: Marshes for wastewater treatment

Plants, fungi, bacteria and algae were introduced in marshes which were capable of neutralising, absorbing and assimilating heavy metals present in treated water. Hence, treated wastewater when passed through marshes gets purified naturally.

        Fig: Natural treatment of wastewater by organisms present in the marshes

Marshes possess a variety of fishes, animals and birds and hence are sites of huge biodiversity. It thus constitutes a sanctuary. Friends of Arcata Mash (FOAM) are citizens who are the caretakers of these sanctuaries. This was such a wonderful project!!

Ecological sanitation

Ecological sanitation is one of the initiatives taken to preserve water. It is the sustainable method which uses dry composting to handle human excreta. The human excreta collected is used as fertilisers for plants. Toilets which use dry composting to dump excreta are known as EcoSan toilets and are used in Sri Lanka and Kerala. This method is a cost effective, practical, hygienic and efficient method of human waste disposal as well as water conservation.

                                       Fig: EcoSan Toilets

Initiatives by government for controlling water pollution

Considering the deteriorating quality of water in water bodies, the government of India passed the Water (Prevention and Control of Pollution) Act in 1974. Main objectives of this act are:

  • Prevention and control of water pollution.
  • Restoring or maintaining the wholesomeness of water in the country.

Practice Problems

1. During the secondary treatment of sewage, the primary effluent is constantly agitated and air is pumped into it. Select the option which gives the correct reason for this.

  1. For the vigorous growth of aerobic microbes
  2. For preventing the growth of aerobic microbes
  3. For the vigorous growth of anaerobic microbes
  4. For the vigorous growth of both aerobic and anaerobic microbes

Solution: Secondary treatment of sewage is a biological process which involves enzymatic degradation of the organic matter in sewage by the action of aerobic heterotrophic microbes. The primary effluent is poured into huge aeration tanks where it is inoculated with microbial flocs. The microbial flocs consist of a mass of heterotrophic and aerobic microbes (bacteria and filamentous fungi). Air is continuously pumped and the effluent is continuously agitated to ensure continuous supply of oxygen to the microbial flocs. This leads to the rapid (vigorous) growth of the flocs that feed on the organic wastes present in the sewage and degrade it in the presence of oxygen.

Hence, the correct option is a.

2. Select the incorrect statement from the following statements related to integrated waste water treatment at Arcata town.

  1. It is safeguarded by ‘Friends of the Arcata Marsh’
  2. In this, wastewater is treated using non-conventional methods only
  3. The wastewater is allowed to flow through a series of six connected marshes
  4. The plants, algae and other microorganisms present in the marshland neutralise and absorb the pollutants

Solution: A wastewater management system involving treatment , proper disposal and proper reuse of wastewater using its natural environment in a nonconventional way and the conventional artificial methods is called the integrated wastewater treatment.

The residents of Arcata Town in California, in collaboration with Humboldt University, have created an integrated waste water treatment process within a natural system at Arcata marsh. It is safeguarded by ‘Friends of the Arcata Marsh’ (FOAM).

In the first stage of the process the wastewater is treated with conventional sedimentation, filtration and chlorine treatments. In the second stage, the water passes through a series of six connected marshes. The plants, algae and other microorganisms present in the marshland neutralise and absorb the pollutants and purify the wastewater.

Hence the correct option is b.

3. What do you know about EcoSan toilets?
Answer: A huge quantity of water is wasted every time it is used to flush toilets. Ecological sanitation is a sustainable system of handling human excreta, using dry composting toilets that do not use water.

It is a practical, hygienic, efficient and cost effective method to dispose of human excreta. In this method, human excreta is recycled into a natural fertiliser with the help of composting toilets. This helps to reduce the need to use chemical fertilisers. These sustainable systems to handle human excreta are called ‘Ecosan toilets’. They are commonly used in many areas of Kerala and Sri Lanka.

4. The BOD test measures the rate of

  1. release of oxygen by microorganisms in a sample of water
  2. release of oxygen by bacteria in a sample of water
  3. uptake of oxygen by microorganisms in a sample of water
  4. Both a and b.

Solution: BOD stands for biochemical oxygen demand. It refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. Its unit is mg/L. The BOD test is performed to measure the rate of uptake of oxygen by microorganisms that are present in a sample of water and thus, BOD is an indirect measure of the organic matter present in the water. It is often used in wastewater-treatment plants as an index of the degree of organic pollution in water. The greater the organic matter present in a water sample, the greater would be its BOD. Greater the BOD of a water sample, more is its polluting potential.

Hence the correct option is c.


1. What is the difference between ETP and STP?
Effluent Treatment Plant (ETP) is the mechanism or process that treats wastewater from different types of industries such as pharmaceuticals, chemicals, paper, etc., before releasing it into the environment or reusing it.

The process of treating municipal wastewater and household/industrial sewage, both runoffs, for the elimination of contaminants is done in the Sewage Treatment Plants(STPs).

2. What role does pH play in water treatment?
pH, being a chemical component of wastewater, has a direct impact on the treatability of wastewater, regardless of whether it is treated physically, chemically, or biologically. As a first step to the treatment process, one has to identify all the parameters such as pH, composition, etc of wastewater. Once all the data is collected, one can determine the pH value at the beginning of the process and the desired pH value at the end. This helps to choose the treatment procedure and the chemicals to be used accordingly.

3. What is the temperature of wastewater?
Waste water temperature normally varies between 10oC to 20oC but the annual mean temperature of wastewater can vary depending on the geographic location. Wastewater is normally warmer compared to water supply due to warm water released from household kitchens and the plumbing systems. The ideal temperature of wastewater entering the treatment tanks during secondary treatment should be around 35oC as temperatures below or above this would affect the microbial activity in the treatment tanks.

4. What is the definition of turbidity in wastewater?
The cloudiness of water caused by microscopic particles or suspended solids is referred to as turbidity. These particles are so minuscule that they cannot be seen with the human eye. Turbidity is a technical term that refers to the degree of clarity in water.

Related Topics

Water pollution: Types of water pollutants, Effects of water pollution

Talk to Our Expert Request Call Back
Resend OTP Timer =
By submitting up, I agree to receive all the Whatsapp communication on my registered number and Aakash terms and conditions and privacy policy