Wastewater Treatment Process Explained

Feb 14th 2022

Wastewater Treatment Process Explained

Also known as sewage, wastewater is polluted water generated from industrial processes, human activities, or rainwater runoff. Since the water has contaminants, wastewater treatment separates these pollutants from the water, leaving an effluent that leaves no negative impact on the environment.

But how does this process occur? What are the steps?

The stages broadly encompass physical, biological, and chemical processes that take place in the following order. In this article, we break down each step in the typical wastewater treatment process. If you're more of a visual learner, the video below does a great job at explaining up to the 8th stage.

Physical Processes

The water treatment process begins with these physical processes.

Stage 1: Screening

Wastewater is not all liquid and fine particles. It contains large solid objects, such as plastics, wood and metal fragments, etc. The materials can clog the pipelines or damage the pumps in the processing plant, so this step is crucial.

Screening traps these materials, which then get washed, pressed, and end up in landfills.

Stage 2: Grit Removal

Not all debris gets separated from the water using the screen. Sand, gravel, and other small particles will pass through and impede water flow downstream or damage equipment.

The only way to eliminate these fine particles is by using a grit chamber. According to the EPA, grit consists of particles larger than 0.21mm with a specific gravity higher than 2.65.

There are three main types of grit chambers: aerated, vortex, and horizontal flow. Even though the working mechanisms are different, all three remove the small materials by controlling the water flow, slowing it down to let the grit settle at the bottom.

Once this happens, wastewater flows into the primary clarifier while the grit gets physically removed from the tank, then disposed of in a landfill.

Stage 3: Primary Settling

Stage three is somewhat similar to step two because it involves settling. However, the process takes place slowly because the goal is to separate solid organic matter from wastewater. Processing plants use clarifiers to settle the sludge, and these are large tanks measuring about 75 feet in diameter.

Adjusting the wastewater flow rate into the primary clarifier determines the efficiency and settling rates, allowing them to remove between 25% and 50% of the sludge.

As an operator, you must find the perfect water flow balance because if it is too slow, the clarifier will settle more sludge but delay the stages upstream. On the other hand, if the flow is too fast, the tanks will collect a small amount of sludge, which will affect the water quality downstream.

Substances like grease might settle at the top due to the slow water flow, but skimming will remove the top layer easily.

Water leaves this stage from the top of the tank, while the sludge gets pumped out regularly from the bottom as it accumulates. The grease and solid organic matter go into digesters, usually ending up as fertilizer.

Biological Process

Next in line is this biological process.

Stage 4: Aeration

Side by side of the aeration process step at a wastewater treatment plant

Aeration is crucial in the treatment process because it converts organic material into water, nitrogen, and cell tissue. The process requires microorganisms and is similar to what happens in lake/ river bottoms, but this occurs faster.

The fast pace is due to aeration. The process occurs in aeration tanks, where they pump oxygen into the wastewater to encourage bacteria growth and the breakdown of organic matter. An alternative to pumping is aggressive agitation, which adds a sufficient amount of air to the water.

It is worth noting that oxygen levels below 2PPM will not sustain the bacteria, so monitoring the dissolved oxygen levels is crucial.

Physical Processes

After aeration, the following processes go back to physical organic waste removal, and they include:

Stage 5: Secondary Settling

Like in primary settling, this stage involves a slow water flow in large circular tanks to let the remaining organic particles settle at the bottom. Since there is no settling in the aeration stage, these tanks remove the materials created during the biological process.

The collected material at the bottom of the tank (activated sludge) contains active bacteria, so most of it gets pumped back into the aeration tank for further breakdown and to maintain healthy levels of the microorganisms. The rest gets discarded or goes into a digester.

Water exiting the secondary clarifier from the top is over 90% treated, and it almost approaches effluent specification levels.

Stage 6: Filtration

Some plants further polish the effluent by filtering it through 10-micron polyester media, which traps any residue that did not settle at the bottom of the secondary clarifier.

The collected sludge gets back-washed regularly for further treatment and to clear out of the filtration media, which leaves enough surface area for continuous filtration.

Chemical Process

Treatment is not complete without disinfection, which is usually a chemical process.

Stage 7: Disinfection

Even though high bacteria levels are excellent at breaking down sludge in the plant, they are not healthy for the environment. Disinfection helps to bring the bacteria concentration down to acceptable levels.

There are two ways of disinfecting the effluent. The cheapest and most common method is chlorination. However, since the chemical can combine rapidly with other elements to contaminate fish and water sources, modern practices avoid it.

So, what are the alternatives?

UV treatment is a viable alternative. Although expensive, it offers immediate disinfection and does not change the taste or odor of the water. There is also the option of ozone disinfection, which is particularly good at breaking through microorganism membranes compared to chlorine.

If the plant disinfects using chlorine, it must test for free chlorine to ensure the levels are lower than the acceptable limit.

Stage 8: Aeration

At this stage, the effluent is almost safe for the environment and ready to exit the plant. But first, this 2nd aeration stage is crucial because highly treated water usually has low oxygen levels. The process raises the dissolved oxygen level to acceptable levels before discharging.

Stage 9: Analysis and Testing

There is one last step before letting out the water, which is analysis and testing. After all the processes above, the water chemistry will change, so it is vital to do this step before releasing the effluent.

Testing does occur throughout the treatment process, but this stage is more comprehensive as it is the final testing.

To adhere tothe EPA-issued NPDES permit, a treatment plant must, at least, test for dissolved oxygen, residual chlorine, nitrates, pH, ammonia, and phosphates.

Any effluent released with pollutants exceeding the set limits as per the permit will lead to fines and possible imprisonment of the operator on duty.

Stage 10: Effluent Disposal

Once all the tests look good, the clean effluent can flow out into the environment.

Where Does the Sludge Go?

The sludge collected from both clarifiers plus the filter undergoes treatment to produce a usable product (fertilizer) while also reducing its volume. Sludge treatment involves the four following steps:

Stage 1: Thickening

Thickening tanks resemble clarifiers but with the addition of a stirring mechanism. The process might include using clarifying agents to help form larger aggregates that settle rapidly at the bottom or float to the surface.

Another option is to use Air Floatation Thickening (AFT), where the solids get attached and float to the surface using minute air bubbles.

The sludge is then skimmed off or pumped from the bottom of the tank while the water goes back to the wastewater treatment process.

Usually, the primary solid concentration goes from less than 1% to about 10% after this stage, but the secondary sludge thickens to about 3.5% solids.

Stage 2: Anaerobic Digestion

Anaerobic digestion is a process where microorganisms break down organic matter in the absence of oxygen. It involves heating the sludge to 98ºF in a primary digester tank, then giving it time to mix (about 35 days).

There is an option of using aerobic digestion, but most plants prefer using the anaerobic process because it produces biogas with high methane levels. The gas can heat the new batch of sludge for this anaerobic digestion or run engines to power other stages at the plant.

Stage 3: Drying

The next stage is drying, and one of the ways to do this is to treat the sludge using a polymer, then pump it onto a porous gravity belt thickener to let the water drain out.

Drying beds are cheaper alternatives, and they usually have three layers with coarse gravel, fine gravel, and sand to separate the sludge from the water.

Water from either drying process has a high ammonia content, and it goes back to treatment over a long period to lower the levels.

As for the sludge, it goes back to storage, staying for up to eight months before disposal or going into agricultural use.

Stage 4: Disposal or Use As Biosolid Fertilizer

None of the above processes eliminate sludge, so it must go somewhere after treatment. Also known as biosolid at this stage, the sludge goes to farm fields during the fall and spring, where farmers use it as fertilizer. It is rich in phosphorus, nitrogen, potassium, and ammonia.

What About Industrial, Agricultural, and Leachate Wastewater Treatment?

Most wastewater treatment plants implement the stages above, but these are not enough to tackle industrial, agricultural, and leachate waste. Industrial wastewater, in particular, tends to have heavy metals, volatile organic compounds, and other toxic contaminants that are very harmful to the environment.

So, what happens to the wastewater from these places?

The EPA has regulations for treating industrial wastewaterpesticide wasteanimal feeding operationsaquaculture wastewater, etc., and requires some of these enterprises to have specialized plants to treat their wastewater.

At least, the industries, agricultural businesses, or landfills should treat the water to remove the toxic substances before releasing it to sewage treatment plants. But, if they can handle the entire treatment process, then produce safe effluent, the better.

Other Treatment Options

Pollution is an ever-increasing problem because heavy metals, chemicals, and toxic substances get into sewer lines and place extreme pressure on existing wastewater treatment processes.

To help tackle this issue new and better treatment systems are coming online. They include reverse osmosis, distillation, carbon absorption, advanced filtration, and biological treatment. Combining two or more of these processes can remove most pollutants, making it possible to treat agricultural or industrial wastewater.


As you can see, the wastewater treatment process is not a single-stage task. Since the liquid has many contaminants, the multi-stage approach works effectively at purifying the water step by step.

However, there is room for improvement, and new technologies might change the traditional system, making the process more efficient and effective in the future. Only time will tell.