What are the Steps of Wastewater Treatment? A Comprehensive Guide

Water is the elixir of life, and nothing is more crucial to our existence than maintaining its purity. But in our pursuit of modern civilization, our waterways have become dumping grounds for all manner of waste, from household detergents to industrial chemicals. This pollution has a detrimental effect on our environment, damaging aquatic life and endangering public health. The world is awakening to the looming water crisis, and the need for wastewater treatment plants is more pronounced than ever.

So, what happens in a wastewater treatment plant? How do they clean up the filth and make it safe for human consumption? The process of wastewater treatment is more complicated than you would think. It involves several distinct stages and requires a combination of mechanical and biological treatment to get rid of harmful contaminants. From screening for incoming sewage to disinfection of treated water, every step is crucial in ensuring that our waterways remain clean and healthy.

If you’re thinking about touring a wastewater treatment plant, be prepared for a fascinating insight into the world of wastewater treatment. You’ll be amazed by the complex network of interconnected tanks, pipes, and machinery that work in harmony to clean up the wastewater. You’ll learn about primary and secondary treatment processes and how biological treatments work to break down organic material. The science behind wastewater treatment is impressive, and it’s exciting to know that its success depends on the tireless efforts of water treatment professionals who commit themselves to keeping our waterways safe.

Primary Treatment Processes

Wastewater treatment is the process of removing contaminants from wastewater and making it safe for discharge into the environment. Primary treatment processes are the first step in treating wastewater and involve the removal of large solids and organic matter. The objective of primary treatment is to reduce the number of pollutants in wastewater before it undergoes secondary treatment processes.

  • Screens: The first step in primary treatment is screening. Wastewater flows through a screen that removes large objects such as sticks, rocks, and debris that can cause damage to downstream equipment. The screens are usually made of metal bars or wire mesh to allow water to pass through while filtering out large solids.
  • Sedimentation: After screening, the wastewater flows to a sedimentation tank where it is held for a period of time. During this time, the heavier solids settle to the bottom, forming a sludge layer. The lighter materials such as grease and oil float to the surface, forming a scum layer. The clarified water in between the two layers is what remains and then goes to the next stage of treatment.
  • Primary Clarifiers: The sedimentation process is followed by primary clarifiers where the settled solids and scum are removed by mechanical scrapers. The scrapers skim off the solids and scum and move them to a sludge hopper where they can be further treated.

The treatment of wastewater can be complex, but the primary treatment processes are essential in removing large solids and organic matter from wastewater before it undergoes further treatment. This helps to reduce the overall cost of treating wastewater while also reducing the pollutants that can harm the environment.

Overall, the primary treatment processes work to reduce the amount of pollutants in wastewater by removing large solids and organic matter. The screening process filters out large solids while sedimentation and clarification remove heavier solids and scum. These processes ensure that the wastewater is in a suitable state for secondary treatment processes.

Primary Treatment Processes Objective Equipment
Screens To remove large solids and debris that could damage equipment downstream – Metal bars or wire mesh
Sedimentation To remove heavier solids through settling – Sedimentation tank
Primary Clarifiers To remove settled solids and scum from wastewater – Mechanical scrapers

Secondary Treatment Processes

Secondary treatment processes involve the biological breakdown of organic matter in the wastewater. This is achieved through the growth of microorganisms that break down the organic matter into simpler compounds such as carbon dioxide, water, and minerals. The process also helps remove any remaining suspended solids and nutrients from the wastewater.

  • Activated Sludge Process: In this process, wastewater is mixed with microorganisms in a tank. The microorganisms feed on the organic matter in the wastewater, converting it into carbon dioxide and water. The mixture is then settled to remove any remaining solids before the treated water is discharged.
  • Oxidation Ditch: This is a variation of the activated sludge process that involves creating a circular channel in which the wastewater and microorganisms are mixed. The channel is aerated to ensure that there is enough oxygen for the microorganisms to thrive.
  • Moving Bed Biofilm Reactor: In this process, plastic carriers that support microorganisms are added to the wastewater tank. The microorganisms attach themselves to the carriers and feed on the organic matter before settling out of the water.

An important aspect of secondary treatment is ensuring that the treated water is free of harmful microorganisms and pathogens. Disinfection is therefore carried out to kill any remaining disease-causing agents. The most common methods of disinfection include chlorination, ultraviolet (UV) light treatment, and ozonation.

Table 1 summarizes the key components of secondary treatment processes:

Treatment Process Description
Activated Sludge Wastewater mixed with microorganisms in a tank. Microorganisms break down organic matter, then mixture is settled and treated water discharged.
Oxidation Ditch Circular channel for mixing wastewater and microorganisms. Channel is aerated to ensure oxygen for microorganisms.
Moving Bed Biofilm Reactor Plastic carriers for microorganisms added to wastewater tank. Microorganisms attach to carriers and feed on organic matter before settling out of water.

Secondary treatment processes are essential to ensure that wastewater is properly treated before being discharged into the environment. They help protect public health and the environment by removing harmful substances from the water.

Tertiary Treatment Processes

Tertiary treatment processes are the final stages of wastewater treatment, where the goal is to produce clear, clean, and safe water that can be released back into the environment or reused. These processes remove the remaining traces of suspended solids, organics, and nutrients that were not fully removed in the primary and secondary treatments.

  • Filtration: The first stage of tertiary treatment involves filtration, where water passes through several layers of sand and gravel to remove any fine particles that remained in the water after secondary treatment.
  • Disinfection: One of the most important steps of tertiary treatment is disinfection. This process removes any remaining bacteria, viruses, or parasites that may still be present in the water. Chlorination, ozonation, and ultraviolet radiation are commonly used to disinfect the water.
  • Nutrient Removal: Another important aspect of tertiary treatment is nutrient removal. Phosphorus and nitrogen are two essential nutrients found in wastewater that can cause environmental problems when released into the environment. Biological nutrient removal processes and chemical precipitation are often used to remove these nutrients.

Tertiary Treatment Processes: Membrane Filtration

Membrane filtration is a form of tertiary treatment that uses a membrane to filter the wastewater. The membrane is a barrier that allows water to pass through but blocks the passage of bacteria, viruses, and other pollutants. There are different types of membrane filtration, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.

Membrane filtration systems are often used to produce high-quality water for industrial processes or for direct potable reuse. Membrane filtration is highly effective in removing suspended solids, bacteria, and other contaminants from the wastewater.

Type of Membrane Filtration Pore size (microns) Removal Efficiency
Microfiltration 0.1 – 10 Suspended solids, bacteria, and some viruses
Ultrafiltration 0.001 – 0.1 Bacteria, viruses, proteins, and some dissolved organics
Nanofiltration 0.0001 – 0.001 Dissolved organics, some minerals, and some salts
Reverse Osmosis 0.0001 Dissolved salts, minerals, and other dissolved solids

Membrane filtration is a highly effective and efficient method of wastewater treatment. It can produce high-quality water that can be reused for various purposes and reduce the amount of wastewater discharge into the environment.

Screening and Grit Removal

Wastewater treatment is a critical process that helps to clean up water before it’s released back into the environment. One of the initial stages of wastewater treatment involves screening and grit removal. Here’s a closer look at what these processes involve:

  • Screening: The first step in wastewater treatment involves screening the wastewater to remove large debris such as rocks, sticks, and other objects that can clog up the treatment process. There are many different types of screens used in this process, including rotary screens, bar screens, and vibrating screens.
  • Grit removal: Once the wastewater has been screened, it’s time to remove any grit that may be present. Grit can include things like sand, gravel, and other small particles that can damage equipment or interfere with the treatment process. To remove grit, the wastewater is moved through a grit chamber, where the flow rate is slowed down to allow the heavier grit to settle to the bottom. The grit is then removed and disposed of.

Overall, screening and grit removal are crucial components of the wastewater treatment process. Without these steps, the treatment process could be slowed down or even damaged by debris or grit that gets into the system.


Once the wastewater has been screened and grit has been removed, the next step is sedimentation. Sedimentation is a process of separating suspended solids from wastewater by allowing them to settle down at the bottom of a tank. During this process, the solids are collected and removed from the wastewater, leaving behind water that can be further treated.

  • The first step of sedimentation is to transfer the screened wastewater into the sedimentation tank.
  • Effluent from the grit chamber is transferred to the sedimentation tank and is allowed to stay for a period of time.
  • As solids settle at the bottom, a layer of sludge forms, which is continuously moved towards the center of the tank with the help of scrappers.

The amount of time the wastewater is allowed to stay in the sedimentation tank depends on the characteristics of the suspended solids and the tank’s design. In some cases, chemicals are added to speed up the settling process, which is called coagulation.

Sedimentation tanks come in various shapes, such as circular or rectangular, with a range of sizes to accommodate the amount of wastewater they need to process. A larger surface area of the tank allows more time for solids to settle, while deeper tanks can increase the settling velocity of the solids. The table below gives an idea of the sizes used for sedimentation tanks based on the wastewater flow:

Flow (m3d-1) Surface area (m2) Depth (m)
50 – 400 40 – 120 3.5 – 5.5
400 – 1200 120 – 250 5.5 – 6.5
1200 – 2400 250 – 500 6.5 – 8

After the solids have settled at the bottom and have been scraped towards the center, they are collected in a hopper and transferred to the next stage of the wastewater treatment process, which is usually digestion and dewatering.


After the physical process of wastewater treatment, the water is subjected to the filtration process. This is where solids and microorganisms that have remained are removed, purifying the water from particles that may cause harm to the environment and human health.

  • Screening: This is the first process in the filtration step. It involves removing large solid waste such as rags, screens, rocks, plastics, and debris that remained in the water despite the primary treatment. The screening process ensures that the wastewater is free from large organic solids that may disrupt the efficiency of the next treatment process.
  • Sedimentation: This is a step where smaller suspended particles such as organic and inorganic waste settle at the bottom of the tank in the presence of air bubbles. The purpose of sedimentation is to separate the solids from the water so that it’s easy to remove the sediment.
  • Membrane process: This is a process where wastewater is forced through a semi-permeable membrane that removes excess particles such as bacteria and other microorganisms. The membrane’s small pores trap the microorganisms, and only clean water is allowed to pass through. The membrane process ensures that the wastewater is 100% free from impurities and microorganisms.

Filtration also removes some undissolved impurities that may have slipped through the physical treatment process. The final result is water that is free of bacteria, viruses, and other organic and inorganic impurities.

Filtration Process Purpose
Screening To remove large solid waste such as rags, screens, rocks, plastic, and debris that remained in the water despite the primary treatment
Sedimentation To separate suspended particles such as organic and inorganic waste from the water so that it’s easy to remove the sediment
Membrane Process To filter out excess particles such as bacteria and other microorganisms using a semi-permeable membrane

The filtration step is crucial in the treatment process as it provides clean and pure water that is safe for the environment and human consumption.


Disinfection is the final step in the wastewater treatment process. Its objective is to remove any remaining harmful organisms, bacteria, viruses, and other pathogens from the treated wastewater. Disinfection is necessary to ensure that the treated wastewater that is discharged into streams, rivers, or oceans does not pose a health risk to human beings, aquatic animals or plants.

  • The most commonly used disinfectants are chlorine, ozone, ultraviolet light, and peracetic acid.
  • Chlorine: This is the most commonly used disinfectant. Chlorine gas, as well as sodium hypochlorite, are often used. Chlorine gas is a very effective disinfectant but, if not used correctly, can be a health hazard.
  • Ozone: This disinfectant is commonly used in Europe and is becoming more popular in North America. Ozone is a very strong oxidant and will oxidize any organic material that is present in the water. This is a good thing because it means that the water is being disinfected and purified at the same time.
  • Ultraviolet Light: This disinfection method is safe, easy to use, and very effective. The UV light kills bacteria and viruses by damaging their DNA. The only drawback to this method is that the water must be very clear for the UV light to be effective.
  • Peracetic Acid: This disinfectant is a powerful oxidizing agent and is extremely effective in killing bacteria and viruses. The only disadvantage is that it is very expensive and is not commonly used.

The choice of disinfectant depends on the specific requirements of the treatment plant and the discharge regulations. Generally, the goal is to achieve a certain level of disinfection which is measured in terms of the number of bacteria and virus particles that remain in the water after treatment. The level of disinfection required depends on the intended use of the treated wastewater.

A table is used to show the common disinfectants used in wastewater treatment, their advantages, and disadvantages.

Disinfectant Advantages Disadvantages
Chlorine Very effective Potential health hazard and must be handled with care
Ozone Effective in oxidizing organic material Requires clear water
Ultraviolet Light Safe, easy to use, and effective Requires clear water
Peracetic Acid Powerful oxidizer and very effective Expensive and not commonly used

It is important to note that disinfection is just one step in the wastewater treatment process. All the steps leading up to disinfection, including preliminary treatment, primary treatment, secondary treatment, and tertiary treatment, work in combination to ensure that the wastewater is properly treated and safe for discharge into the environment.

FAQs: What are the steps of wastewater treatment?

Q: What is wastewater treatment?
A: Wastewater treatment is the process of removing contaminants from used water so it can be reused or safely returned to the environment without causing harm.

Q: What are the steps of wastewater treatment?
A: The steps of wastewater treatment are generally divided into four stages: preliminary, primary, secondary, and tertiary treatment.

Q: What happens in the preliminary treatment stage?
A: In the preliminary treatment stage, larger solids and debris are removed from the water using screens, grit chambers, and sedimentation tanks.

Q: What happens in the primary treatment stage?
A: In the primary treatment stage, the water is held in large tanks and solid organic material settles to the bottom. This sludge is then removed and processed for disposal.

Q: What happens in the secondary treatment stage?
A: In the secondary treatment stage, bacteria is added to the water to break down organic matter that remaining from primary treatment. This is typically done in large aerated tanks, and the bacteria consume the contaminants and convert them into carbon dioxide and water.

Q: What happens in the tertiary treatment stage?
A: In the tertiary treatment stage, any remaining contaminants are removed from the water using advanced filtration and chemical processes. This stage ensures that the water is safe to release back into the environment or to reuse.

Q: Why is wastewater treatment important?
A: Wastewater treatment is important for protecting public health and the environment. Without proper treatment, pollutants would be released into rivers, lakes, and oceans, causing harm to wildlife and ecosystems, as well as potentially posing a risk to human health.

Closing Thoughts: Thanks for Learning About Wastewater Treatment!

We hope that this overview of the steps of wastewater treatment was helpful and informative. Remember, wastewater treatment is an essential process for protecting our natural resources and keeping our communities safe and healthy. Thank you for reading, and please visit again later for more informative articles about environmental science and sustainability.