Secondary Treatment of Wastewater

The world’s most valuable natural resource is water. Without it, life on earth would not be possible. Consequently, water treatment systems play a vital role in environmental protection and human health and safety. However, when wastewater treatment systems are improperly designed or operated, they can cause serious problems. 

One example is groundwater contamination caused by seeping sewage. The good news is that modern-day wastewater treatment plants are highly effective at preventing these types of disasters from happening. Secondary Wastewater Treatment Systems work primarily by filtering out solid waste.

However, the job of a wastewater plant does not stop there. An excellent Secondary Treatment System also has to remove as many nutrients as possible from wastewater. So let us get a brief description of the secondary wastewater treatment process. 

Secondary Treatment Definition

Secondary treatment of wastewater is a process that follows the primary treatment of sewage. 

Secondary treatment aims to remove the remaining organic matter and suspended solids from the wastewater. The secondary sewage treatment is not as efficient in removing the contaminants as the primary treatment of sewage. If the secondary treatment is insufficient to obtain the required effluent quality, tertiary treatment must be used.

Objectives of Secondary Treatment

The objectives of secondary treatment are to remove the remaining suspended solids, BOD, and COD from the wastewater. It is done to reduce the primary clarifier load and improve the quality of the effluent discharged from the treatment plant.

Secondary treatment aims to remove the dissolved solids from the water, which are responsible for most of the problems in wastewater treatment.

What are the Stages of Wastewater Treatment? 

The stages of wastewater treatment are collection, pre-treatment, primary treatment, secondary treatment, tertiary treatment, disposal, and reuse.

There are a few different stages of wastewater treatment:

Collection

The wastewater is collected from homes, businesses, and other sources, and the wastewater contains raw sewage and stormwater. 

Stormwater can carry pollutants from the ground and buildings, and roads.

Pre-Treatment 

It is where the wastewater is treated to remove large objects and debris.

Primary Treatment 

It is where the wastewater is treated to remove solids and reduce the amount of organic matter. Know more about Primary Treatment here.

Secondary Treatment

The wastewater is treated with bacteria to remove organic matter and reduce pollutants.

Tertiary Treatment

The wastewater is treated with filters or chemicals to remove any remaining pollutants.

Disposal

It is where the wastewater is disposed of. Either it’s done naturally or letting it seep into the ground or a wastewater treatment plant.

Reuse 

The treated water is reused for irrigation, cooling towers, toilet flushing, and other purposes. Reusing wastewater is a great way to conserve water and help protect our environment.

From 2000 to 2050, the global population is expected to increase by 50%. It will result in a greater demand for food, energy, and water. Cleaning wastewater is essential so that you can use it again. It makes the planet healthier for everyone.

Check out my other post on Different Types of Sewage Treatment Plants here.

What is The Secondary Treatment of Wastewater?

This article will tell you about the second stage in the wastewater treatment process.

The Primary Treatment of wastewater is incomplete removal of coarse material from sewage by settlement and sedimentation tanks. In the secondary treatment, biodegradation or destruction of organic compounds present in sewage occurs. 

It produces a substance safe for discharge into water bodies or discharges into the sewer system.

Secondary treatment aims to reduce the organic load in treated wastewater and destroy microorganisms that are not eliminated during primary treatment. 

While human activities produce many wastes, even small additions to wastewater can affect the environment. Thanks to secondary treatment, it is possible to maintain specific water quality standards, including oxygen in the water.

In some places, the chemical oxygen demand (COD) is used to monitor the quality of treated wastewater.

The cities have been better at cleaning up our waterfronts in the last few years. Still, human waste pollution in waterways remains a severe issue.

How Does Secondary Treatment Work

Secondary treatment is treating wastewater in a municipal water system that removes most contaminants from wastewater by reducing their levels to acceptably low levels. 

In other words, it helps to reduce the pollutants in water-contaminating substances such as organic matter, heavy metals, and chemical compounds. It also refers to the reduction of biodegradable material through bacterial action.

To remove contaminants, we have three secondary treatment methods: Trickling filters, activated sludge, and oxidation ponds.

Trickling Filter Method

In trickling filter plants, physical and chemical treatment occurs in one large unit. Bacteria and other biological materials adsorb or are attached to granular media that passes upwards. 

This action causes the pollutants to be broken down into harmless material. The organic matter also provides the bacterial growth media with organic matter, essential to secondary treatment.

The wastewater enters at one end of the filter and flows over the coarse media. As the water trickles downward, it becomes purified. Eventually, the water leaves through an outlet at the other end of each filter bed. The materials inside the filter, including the bacterial slime and insoluble organic materials, form sludge.

In addition, wastewater from domestic kitchens and baths is pre-treated by removing solid wastes at a preliminary treatment plant. It leaves relatively clean wastewater to be treated in the secondary settling tanks. There is less sludge by volume in this secondary treatment, which means that the filters have to be cleaned more often.

This wastewater treatment process uses bacteria to consume organic matter and convert it into gases or solids. The result is cleaner water that you can safely return to the environment.

Activated Sludge

Activated sludge is one of the most common types of wastewater treatment methods. It uses microorganisms such as bacteria to break down organic material. The pollutants in the wastewater, such as oils and fats, are consumed by the microorganisms. 

These microorganisms form a sludge broken down and converted into less harmful components such as carbon dioxide and water.

Water containing the microorganisms is passed through a tank containing an aerator. In the aeration tank, the wastewater is exposed to oxygen, which allows for the growth of microorganisms. The water is also mixed with air using an impeller, which helps the organisms to grow. 

The bacteria, which is suspended in water, becomes more concentrated near the air-water interface of the aerator. In the aeration tank, colonies of activated sludge form and grow much faster than in the other tanks.

After passing through the aeration tank, water is pumped into a clarifier area. In this processing tank, organic matter settles to the bottom as sediment, and Undissolved air bubbles collect around the solids and float them to the surface. 

The microorganisms form sludge in the aeration tank. This sludge eventually sinks to the bottom of the tank and is removed through sedimentation tanks. The sludge is ultimately removed by using scrappers. 

The wastewater purification process of activated sludge works under anaerobic conditions. If oxygen levels are low, then no emissions will occur from the biodegradation of pollutants in the wastewater. There are three types of activated sludge processes: contact stabilization, extended aeration, and oxygen aeration with high purity. 

Oxidation Ponds

An oxidation pond is a water treatment system that uses ponds to remove nitrogen from wastewater. There are many different types of oxidation ponds, but they all generally function the same way. Water in the pond is treated with natural bacteria, which converts ammonia into nitrites and nitrates from the wastewater. 

Oxidation ponds are often used when wastewater needs to be treated on-site or in remote areas. Most commonly, it is used where there is no access to a sewage system or any other water treatment facility.

Wastewater is introduced into the pond by an inlet pipe that pumps water from the ground or through roof runoff. The wastewater falls onto a moving grate at the bottom of the inlet. 

This process breaks up solids allowing them to sink to the bottom. Meanwhile, lighter waste gets carried along with flowing water. As it flows across this grate, sewage moves slowly through the pond, which allows it time to be treated.

Various structures manipulate wastewater flow in the oxidation pond, which forces water to flow at different speeds around the pond. One of these structures uses a series of baffles that are spaced evenly across the width of the pipe. 

Wastewater is forced against these baffles slowing the flow down, and it allows more time for the pollutants to react with natural bacteria in the ponds. Other structures used to slow the speed of water are called flumes which are small dams that force water through gaps between them, slowing water speeds even further.

Another structure controls how deep wastewater settles at the bottom of the oxidation pond. This structure is called the weir. Wastewater flows through an opening in the side of the pond and then over a sill that creates a shallow area in the pond. It makes another flow speed at which water can enter the oxidation pond. The section next to this sill has no water; it only contains solids that settle there due to the difference in speed.

At the end of this process, wastewater is free of most pollutants and released back into the environment. Any remaining solids are collected at the bottom and can be removed and disposed of by dredging. 

It also allows for planted areas to take up excess nitrates, making them a good choice for crops that need high fertilization levels.

Oxidation ponds are one of the oldest forms of water treatment, but it is still used today in some cases. Oxidation ponds are often more cost-effective than many other systems because they don’t require mechanical equipment to run.

However, oxidation ponds require a lot of space and are concentrated in rural areas. Also, they do not remove other pollutants such as oil or grease because of their nature.

What are The Alternative Types of Secondary Biological Processes?

There are a few different types of alternative secondary biological processes, and these include aerobic and anaerobic processes. 

Anaerobic processes are typically endergonic reactions in the absence of oxygen. 

On the other hand, aerobic processes are exergonic reactions that take place in the presence of oxygen.

Aerobic and anaerobic alternative secondary biological processes create many different complex organic molecules. These complex organic molecules can be used as food for organisms. 

Some examples of the different complex organic molecules created through alternative secondary biological processes include glucose, lactic acid, and ethanol. 

Rotating Biological Contactors

Rotating Biological Contactors (RBCs) are an alternative to the traditional activated sludge process for treating wastewater. RBCs use a disk that is filled with a mass of plastic media. 

This media provides a large surface area for bacteria to grow and consume the organic matter in the wastewater. The press is inert in removing nutrients from the wastewater, but it provides a large surface area to grow bacteria that remove the organic matter. The disks are rotated inside an aerated tank that cleans them before returning to service.

1. Aerobic

Aerobic lagoons

Aerobic lagoons are a type of lagoon where the entire volume of water is agitated and mixed by forced aeration. The most common way is to blow air into the water with an air compressor.

Anaerobic lagoon uses an aerated tank, typically circular or oval-shaped and shallow, to provide long-term storage for treated wastewater. The wastewater flows into the lagoon and is held until it’s time to process it again. It allows organic particles in the water to attract bacteria, decomposing the particles. 

The aerators added to the lagoon provide oxygen, which accelerates this biological breakdown of organic matter suspended in the water. Once broken down, you can easily remove microbes during subsequent steps in wastewater treatment.

Activated sludge

Activated sludge is a wastewater treatment process that uses aerobic microorganisms to convert organic matter into carbon dioxide and water. The treatment process generally includes three steps; primary clarifier, secondary clarifier, and an aeration basin. We have described in detail this process above.

Rotating biological contactor

A rotating biological contactor (RBC) is used in wastewater treatment. It is a cylindrical tank that contains several discs or paddles. The discs are attached to a shaft that rotates at high speed. The paddles create turbulence in the water, which helps to break down the organic matter. In a rotating physical disc reactor (RBD), the RBC is sometimes called a disc stack.

Trickling filter

A trickling filter uses a bed of media to allow microorganisms to grow and remove pollutants from the water. The wastewater is trickled over the media, where the organisms remove the contaminants. The microorganisms in the filter provide a means of biological nutrient removal. The media is generally made from gravel or sand, and other components include particular forms of activated carbon and various types of artificial foams.

Aerobic granulation

Aerobic granulation is a wastewater treatment process that uses aerobic microorganisms to form granules. These granules are then used to remove pollutants from the wastewater.

Aerobic granulation is considered an advanced wastewater treatment process. You can use it to remove ammonia, total Kjeldahl nitrogen, total phosphorus, and suspended solids in industrial wastewater.

In aerobic granular sludge, the high concentration of microorganisms involved in the process makes the treatment very effective. The ability of the organisms to form aerobic granules allows them to settle and stick together as they float in the wastewater. 

The aerobic granules formed by these tiny microorganisms can be used as a medium for growing nitrifying bacteria. These nitrifying bacteria feed on and remove ammonium and nitrite from the wastewater.

Aerobic granulation can also be used to remove nitrogen from wastewater and nitrogen gas through nitrogen stripping. This process is performed by injecting air into the granules of aerobic granulation suspended in the wastewater. The air bubbles trapped within the microorganisms and their filaments grow larger, and it results in the rise to the surface of the wastewater. 

The microorganisms and their filaments contain a large amount of nitrogen gas, which is released into the air when they float to the surface of the wastewater. 

You can use this in combination with biological nutrient removal (BNR).

2. Anaerobic

Aerated lagoons

Aerated Lagoon uses aeration of the water to speed up the decomposition of organic wastes and purification of wastewater. Aerated Lagoons can process a moderate amount of organic waste. 

Aerobic digestion is a biological treatment used to break down organic matter by aerobic bacteria, which require oxygen. Aerobic digestion is typically used for treating sewage or industrial waste. 

The types of lagoons are commonly named after the process used, such as “aerated lagoon,” “bubbling filter,” or “anaerobic lagoon.” An aerated lagoon is a lagoon where the air is injected into or drawn from the lagoon to supply oxygen for aerobic bacteria, oxidize reduced inorganic compounds, and maintain a dissolved oxygen level adequate for supporting fish life. 

Aerated Lagoons can treat wastewater from households and industries. They are also used in the treatment of sewage sludge.

Constructed Wetland

Constructed Wetland is an engineered system designed to provide water quality treatment. The wetland design is site-specific to remove BOD, TSS, and nitrogen from wastewater before discharging into the environment.

It can be used for new developments or reused for existing sites to treat wastewater before releasing it into the ocean, lake, river, stream, etc.

Constructed wetlands are one of the most cost-effective systems available for wastewater treatment. They are versatile and can be designed to treat small or large volumes of wastewater, depending on the site’s BOD loading capacity.

It has significant advantages over other treatment methods. The system consists of specific layers which are aesthetically pleasing, and it also requires minimal maintenance to keep the operating costs low. 

An indirect discharge process eliminates the need for wastewater discharging permits and amenities fees imposed by most municipalities. 

Constructed wetlands can be used in conjunction with sand filtration, UV disinfection, and bio-filtration to make it a comprehensive wastewater treatment system.

Most importantly, constructed wetlands can be used in remote areas not connected to a primary wastewater collection and treatment system.

Membrane Bioreactor

A membrane Bioreactor is a device that uses membranes to separate unwanted pollutants (nutrients or biological organisms) from wastewater. It is done by taking advantage of the physical characteristics of certain substances. 

Osmosis occurs when there is an imbalance in concentration across a membrane, causing fluids with lower concentrations to pass through it.

It has significant applications in the treatment of wastewater. This technology has been applied to treat municipal, industrial, or combined municipal/industrial wastewaters.

A membrane bioreactor(MBR) is an advanced wastewater treatment system. It is an upgraded version of the conventional activated sludge process for secondary effluent treatment, and possesses several advantages over other technologies. 

The MBR process produces very high-quality effluent, and it requires considerably less than the conventional activated sludge process. 

It makes full use of the microorganisms and the excess sludge resulting from their activity so that no final thickening is required. Since nutrients are reused in the MBR process, the end products of treatment are more stable. Neither sludge nor excess activated sludge is produced during secondary effluent treatment.

Conclusion

The secondary wastewater treatment process is more complicated than the primary wastewater treatment process. It is necessary to remove the remaining pollutants from the wastewater. The method includes various other activities that can remove the remaining impurities. We hope we successfully provide all the information about the secondary wastewater treatment process. For more information, please stay connected with us.