What is Wastewater and Why is it Important to Treat It?

Typical aeration tank in a wastewater treatment plant

Introduction

Water scarcity and pollution are two of the most pressing global challenges today. While water sustains life, its misuse and contamination create serious risks for both humans and ecosystems. This is where wastewater treatment becomes crucial.

According to Metcalf & Eddy’s Wastewater Engineering: Treatment and Reuse, wastewater is not just “dirty water.” It is a complex mixture of contaminants that require scientific and systematic treatment before being released or reused. In this article, we will explore what wastewater is, its sources, and why treating wastewater is so important for a sustainable future.

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What is Wastewater?

Wastewater refers to the water that has been used in households, industries, agriculture, or urban runoff and has become contaminated. It contains physical, chemical, and biological pollutants that make it unsafe for direct discharge.

Major Sources of Wastewater

1. Domestic Wastewater (Sewage):
   From kitchens, bathrooms, toilets, and laundries. Contains organic matter, soaps, detergents, and pathogens.

2. Industrial Wastewater:
   Discharges from industries like textiles, food processing, pharmaceuticals, and chemicals. Often contains heavy metals, toxic substances, and oils.

3. Stormwater Runoff:
   Rainwater flowing over roads and urban areas picks up oil, grease, plastics, and sediments.

4. Agricultural Wastewater:
   Runoff from fields carries fertilizers, pesticides, and animal waste.

Composition of Wastewater

Wastewater typically contains:

• Organic Matter (proteins, fats, carbohydrates)

• Nutrients (nitrogen and phosphorus)

• Suspended Solids (silt, grit, plastics)

• Pathogens (bacteria, viruses, parasites)

• Toxic Chemicals (pesticides, solvents, pharmaceuticals, heavy metals)

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Importance of Wastewater Treatment

Treating wastewater is essential for multiple reasons:

1. Environmental Protection

Untreated wastewater pollutes rivers, lakes, and oceans. Organic matter depletes oxygen, killing fish and aquatic organisms. Excess nutrients cause eutrophication and algal blooms, destroying ecosystems.

2. Safeguarding Public Health

Pathogens in wastewater spread diseases like cholera, typhoid, dysentery, and hepatitis. Wastewater treatment eliminates harmful microorganisms and ensures safe water discharge.

3. Water Reuse

In water-scarce regions, treated wastewater becomes a valuable resource. It can be reused for:

• Agricultural irrigation

• Industrial cooling and processes

• Construction activities

• Groundwater recharge

• Potable reuse (with advanced treatment)

4. Legal Compliance

Regulations require industries and municipalities to meet specific discharge standards. Effluent Treatment Plants (ETP) and Sewage Treatment Plants (STP) are necessary to avoid fines and legal issues.

5. Resource Recovery

Modern wastewater treatment focuses on resource recovery, including:

• Biogas from sludge (renewable energy)

• Nutrient recovery for fertilizers

• Safe water reuse

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Stages of Wastewater Treatment

Based on Metcalf & Eddy, wastewater treatment typically involves:

1. Preliminary Treatment: Screening and grit removal.

2. Primary Treatment: Settling of suspended solids.

3. Secondary Treatment: Biological treatment using microorganisms (activated sludge, trickling filters, etc.).

4. Tertiary Treatment: Advanced treatment for nutrient removal and polishing.

5. Disinfection: Chlorination, UV, or ozone to kill pathogens.

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Global Significance of Wastewater Management

Globally, more than 80% of wastewater is released untreated into the environment. This practice threatens ecosystems, drinking water supplies, and public health. Countries facing water scarcity—such as India, South Africa, and the Middle East—are turning to wastewater reuse as a sustainable solution.

Wastewater should not be viewed as a liability but as a resource. Proper treatment and reuse can reduce water stress, protect rivers, and support economic growth.

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Conclusion

Wastewater is more than just “used water.” It is a mixture of contaminants that, if untreated, can harm people and the planet. At the same time, it is also an opportunity for resource recovery and sustainable water management.

As Metcalf & Eddy’s Wastewater Engineering highlights, wastewater treatment is not only about compliance but also about protecting public health, conserving natural resources, and ensuring water security for the future.

The choice is ours: do we treat wastewater as a burden, or do we embrace it as a valuable resource?

Earth Day 2022

World Earth Day- 2022

World Earth Day serves as a reminder to humanity to protect and preserve the mother earth and her species in

order to make the planet a better place for future generations. Earth Day will be celebrated for the 52nd time on April 22, 2022.

It is celebrated to create awareness among the residents of this beautiful home of ours to take care of this.

Because we don’t have an alternative to this. 

We all are aware of the reason for its destruction.

The followings are the contribution we can add to protect and preserve our mother earth:

1.      3 R’s- Reduce, Reuse & Recycle as much as possible. (we can reuse the paper from the other side which is not used still.)

2.      Volunteer- participate in community cleanups as a volunteer. (can be a part of your local groups like Himalaya Putra, Parivartan Sena, Youth for Seva, Joy etc.)

3.      Educate- Be educated yourself and also educate others, especially the children studying in elementary schools.

4.      Water conservation- Use only the water you required. Do not keep the tap open whenever you using water.

5.      (In the canteen for washing hands or utensils, or in the home for cleaning or other purposes.)

6.      Plant Green- Plant more trees. Try to plant a tree on your birthday.

7.      Save Electricity- Turn off the light and electrical equipment when not in use at home as well as in offices. Use LED bulbs to reduce the energy consumption also the heat produced by the light bulb.

8.      Your Positive Attitude- Your positive attitude can change the world. Do not throw the garbage here and there even though no one is seeing you.

Always keep in mind that “We Don’t have another option other than planet Earth.”

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Biochemical oxygen demand (BOD)

Biochemical oxygen demand (BOD)

The biological oxygen demand (BOD) is also called biochemical oxygen demand. The BOD refers to the amount of oxygen required for the biotic degradation of organic matter in bodies of water.

The BOD is a pollution parameter mainly to assess the quality of effluent or wastewater. Untreated wastewater has usually a high oxygen demand. Industrial wastewater tends to a higher chemical oxygen demand. COD caused by dissolved chemicals or pollutants from washing processes which adds more pollutants in the water. Both BOD or COD requires a lot of dissolved oxygen from the water. The significance of BOD is very clear. If oxygen is used from organic impurities, it can’t be used by other creatures living in the water.

Drinking water is tested as well to check, if there is any organic matter present. To evaluate drinking water, the TOC - Total Organic Carbon is measured, instead of the biochemical oxygen demand. This TOC method is faster and can be done online, whereas the method to measure the biochemical oxygen demand takes 5 days to get results.

The biochemical decomposition of organic substrates is carried out by microorganism. For this work the bacteria need energy. Aerobic bacteria, that do this work, need dissolved oxygen to produce energy. This oxygen is consumed in this process and the amount of dissolved oxygen in the water gets less. If there is a lot of organic material present in the water, the oxygen demand is correspondingly high to carry out the decomposition. This oxygen is then lacking the plants and animals that also live in this water.

A low biochemical oxygen demand ensures that the dissolved oxygen is not only consumed by organisms from the wastewater. But that there is enough oxygen left for fish and plants.

To perform BOD analysis there are some standards are set.

like to store sample for 5 days at 20*C.

There are two methods to measure the BOD level. Both methods are empirical tests.

Method I: A sample of the water is kept at a constant temperature of 20°C in the dark. After a period of five days, the oxygen content is measured. In comparison to the original value, the oxygen consumption during the measurement period indicates the oxygen demand in the water.

Method II: If a very high BOD is to be expected or if other toxic or inhibitory substances are present in the water, the sample can be diluted at the beginning. In this way it can be prevented that too little oxygen is present to break down the organic substances.

After the 5th day the remaining dissolved oxygen in the water sample is measured. With this oxygen the BOD level can be calculated.

Drinking water should have after 5 days a BOD of well below 1 mg/l. Acceptable wastewater from a sewage treatment plant should have a BOD of around 20 mg/l.

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Effluent/Wastewater Treatment Plant | ETP Process

What is Wastewater Treatment Plant?

Effluent or wastewater treatment is the process of converting Effluent – water that is no longer needed or is no longer suitable for use – into bilge water that can be discharged back into the environment. It’s formed by a number of activities performed in factories including manufacturing, washing and using the toilet. Effluent is full of contaminants including bacteria, chemicals and other toxins. Its treatment aims at reducing the contaminants to acceptable levels to make the water safe for discharge back into the environment.

Effluent treatment plant

When in a factory, we are converting the raw materials into good finished products, exactly at the same time we are converting the good input water into wastewater. Public welcomes the product but who will welcome to the waste? However, if the wastewater is once again subjected to a conversion of water, then such effort will be again welcome. This responsibility is on the operator of ETP. Water is a universal solvent and many solids get dissolved into it. Purification is nothing but an attempt to remove solids from liquid. The solids are divided in four classes as per their size such as Coarse, suspended, colloidal and dissolved solids. These four types of solids are required to be removed in descending order which is a train of treatment.

The effluent treated in many stages namely preliminary, primary, secondary, tertiary and sludge management. All the treatment unit mentioned are based on physical, chemical or biological principles or a combination thereof. The units based on physical are relatively easy, those employing chemistry are also easy if proper dose is administrated, and the biological are difficult to manage but less costly if once we are accustomed to it. The physical and biological units are subtractive in nature while the chemical unit process of treatment is additive and more energy consuming. When the water becomes wastewater, it means that some solids, liquids or gasses are mixed into it. Thus when we asked to make water out of wastewater, what we have to do is to take away the solids liquids or gaseous pollutants.

Wastewater Treatment Process

The following is a step by step process of how wastewater is treated:

1. Wastewater Collection

Waste water is required to be taken from the factory to the ETP site. This can be done by closed pipe lines or by open gutters. The shorter is the distance or near is the ETP, the better it is. If the travel distance is too high, then the properties or nature of the effluent (call it characteristics) gets changed. The oil-grease gets mixed due to travel turbulence the solids become smaller in size. Due to grinding action or attrition while flowing, thereby both becoming difficult for separation. One more point during this collection transport. The following waste water should not leak and go outside.

2. Screening

The screen fitted in ETP is like a watchman. If a watchman is alert, further mishap can be averted. Likewise if the screen is OK, it will not allow the gunny bags, plastics, branches, rubbers, packing materials, gaskets to pass further and damage pump, agitator, mixture, aerators. Thus screen is an important unit.

bar-screen

3. Grit Chamber

Wastewater brings sand, ash, stone particles, road sweeping, grit, building materials, packing nails, earth, coconut shells pieces etc. This too is required to be removed. The peculiarity of this pollutant is that it is largely inorganic and inert in nature, heavier in weight and easily, quickly settleable. If these are not removed here itself, it creates problem further. By friction it erodes many parts of ETP, which is a slow death of efficiency.

4. Oil-Grease Trap

Oil and grease should be removed from the effluent at least up to a certain state value. Oil-grease removing unit is therefore necessary. Oil is of two types, edible and mineral. Generally in an industrial effluent, we come across the mineral one, which again maybe either used for machines lubrication or as coolant to save machine or tools or the job from heating up. Oil being lighter than water, floats. This property is used to separate it out.

5. Primary Treatment

In primary treatment the effluent is collected in a tank called equalization tank or neutralization tank. Where pH is adjusted by sodium hydroxide and HCL. The effluent then treated with chemicals called Aluminium sulfate and polyelectrolyte. The settled sludge than transferred with help of pump to the sludge drying beds (SDB)

6. Secondary Treatment

In Secondary Treatment the bacteria who are capable of purifying the effluent are classified in three groups. Aerobic, anaerobic and facultative. Bacteria need oxygen for the slow combustion. If this oxygen is lifted from water (having it in dissolved oxygen) those bacteria are aerobic. The anaerobic bacteria pickup oxygen from the bound formed by decomposing carbonates, sulfates, nitrate etc.; and the facultative class can use either of the source of oxygen i.e. work both in presence or absence of oxygen.

6.1 Anaerobic Lagoons

The anaerobic bacteria too needs oxygen but there is no need of fixing aerator. Their need of urea and superphosphate is limited, they get less disturbed by shock load and normally there is no arrangement needed for returning the sludge. The anaerobic bacteria are slow workers, the hydraulic retention time is large (HRT) and the lagoons are very wide and long.

Primary clarifier is used to store the culture temporary. In the duration of few days the culture is recycled in the tank again.

6.2 Activated sludge process

In anaerobic lagoon bacteria work. They are slow workers. In their place if fast working aerobic bacteria are employed, the size of ETP and time of treatment can be attempted to be reduced. As the availability of oxygen is assured, the efficiency is also expected to be higher. Various techniques like trickling filter, oxidation pond, aerated lagoons, activated sludge etc. Where aerobes will work can be designated and operated. Activated sludge is one such process, where lakhs of tiny microorganism stay, grow, live, multiply and work. They eat constantly and increase the population as much as possible!! For the growth they use the dissolved or even un-dissolved organic solids as food. This food is transformed in the growth of microbial community. When these growing bacteria floc together the lump becomes bigger in size and heavier in weight and as a result will show a tendency of setting out from the remaining water. This settled bacteria could now be termed as “activated sludge” because it is active to consume the Biochemical oxygen demand (BOD) as food and looks collectively as sludge.

 Same as the primary clarifier, secondary clarifier is also used to store the activated sludge. Where the clear effluent called supernatant is forwarded and the activated sludge recycled back to the aeration tank to maintain the MLSS.

7. Tertiary treatment

This stage is similar to the one used by drinking water treatment plants which clean raw water for drinking purposes. The tertiary treatment stage has the ability to remove up to 99 percent of the impurities from the wastewater. This produces effluent water that is close to drinking water quality. Unfortunately, this process tends to be a bit expensive as it requires special equipment, well trained and highly skilled equipment operators, chemicals and a steady energy supply. All these are not readily available.

8. Sludge Treatment

Sludge settles in the clarifier. We remove daily. Some portion of it is sent back into the aeration tank as recycled activated sludge (RAS) as per requirement to maintain the MLSS or F:M ratio therein. This leaves the remaining portion of which we have to take care of. This is a wasted activated sludge (WAS). Before this wasted sludge enters into the environment for further disposal, it has to be made amenable for handling. Dewatering is to be made atleast to an extent that a spadable cake is formed. A number of modern gadgets are available for this purpose. For a small or medium size ETP, however the dewatering is done by placing the sludge on sand. Water bound with solids percolated through the layered sand drying bed SDB. The separated water is called as sludge liquor and is high in BOD and sometimes high in the nutrient Nitrogen and Phosphorus. It therefore cannot be sent outside. The sludge liquor should be sent back for treatment in ETP.

In the final, the treated effluent send to the common effluent treatment plant (CETP) or used for irrigation within factory premises. But the pollution parameters as assigned by pollution control board should be within limits.

 Wastewater treatment has a number of benefits. For example, wastewater treatment ensures that the environment is kept clean, there is no water pollution, makes use of the most important natural resource; water, the treated water can be used for cooling machines in factories and industries, prevents the outbreak of waterborne diseases and most importantly, it ensures that there is adequate water for other purposes like irrigation.

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