Unit : 5 

Biological treatment systems

Biological treatment systems are methods of treating wastewater or other contaminated water using microorganisms to break down pollutants. These systems can include processes such as aeration, activated sludge, and constructed wetlands. The microorganisms consume the pollutants as a source of food and convert them into less harmful byproducts such as carbon dioxide and water. Biological treatment systems can be used to remove pollutants such as nitrogen, phosphorus, and organic matter from water.  

Aerobic Biological Oxidation : 

Aerobic biological oxidation is a type of biological treatment process that uses oxygen and microorganisms to break down pollutants in wastewater or other contaminated water. The process typically involves aeration, which is the introduction of air into the water, to provide the oxygen needed for the microorganisms to survive and perform their metabolic processes.

The microorganisms consume the pollutants as a source of food and convert them into less harmful byproducts such as carbon dioxide and water. Aerobic biological oxidation is commonly used to remove pollutants such as nitrogen, phosphorus, and organic matter from water. Examples of aerobic biological oxidation treatment systems include activated sludge and the extended aeration process. 

Aerobic biological oxidation is a biological treatment process that uses oxygen and microorganisms to break down pollutants in wastewater or other contaminated water. The process typically involves the following steps: 


Pretreatment: The contaminated water is screened to remove large debris and grit, which can damage the treatment equipment.

Aeration: Air is introduced into the water through diffusers or mechanical aerators to provide the oxygen needed for the microorganisms to survive and perform their metabolic processes.

Microbial Growth: The microorganisms consume the pollutants as a source of food and convert them into less harmful byproducts such as carbon dioxide and water.

Secondary clarification: The water is clarified to separate the microorganisms and other suspended solids from the treated water.

Discharge: The treated water is discharged into the environment.

Environmental factors that can impact the performance of aerobic biological oxidation treatment systems include temperature, pH, nutrient levels, and the presence of toxic compounds. These factors can affect the growth and activity of the microorganisms, and if not properly controlled, can lead to reduced treatment efficiency and even system failure. Additionally, the presence of high levels of heavy metals, toxic compounds, and other pollutants can inhibit the microbial activity, and a pre-treatment process is usually required before the aerobic biological oxidation process. 

Aerobic Suspended Growth Processes:

Aerobic suspended growth processes are a type of biological treatment process that uses oxygen and microorganisms to break down pollutants in wastewater or other contaminated water. In these processes, the microorganisms are suspended in the water, and are not attached to any surface.

Examples of aerobic suspended growth processes include:

Activated Sludge Process: A widely used process for the treatment of domestic and industrial wastewater. In this process, wastewater is mixed with a culture of microorganisms in an aeration tank. The microorganisms consume the pollutants in the wastewater, converting them into less harmful byproducts. The microorganisms are then separated from the treated water in a clarifier.

Oxidation Ditch: Similar to the activated sludge process, an oxidation ditch uses a circular aeration tank with a central mechanical aerator. The microorganisms are suspended in the water and consume the pollutants, which are then removed in a clarifier.

Aerated Lagoon: This process uses a large, shallow pond or lagoon that is aerated using diffusers or mechanical aerators. The microorganisms consume the pollutants in the wastewater and the treated water is discharged into the environment.

All these processes are designed to provide the microorganisms with adequate oxygen to perform their metabolic processes and break down pollutants in the wastewater. The efficiency of these processes can be affected by environmental factors such as temperature, pH, and nutrient levels, so monitoring and controlling these factors is important for maintaining optimal treatment performance. 

Complete Mix activated sludge : 

Complete Mix activated sludge is a type of aerobic suspended growth process that uses microorganisms to treat wastewater or other contaminated water. The process is commonly used for the treatment of domestic and industrial wastewater.

The process typically involves the following steps: 

Pretreatment: The contaminated water is screened to remove large debris and grit, which can damage the treatment equipment.

Mixing: The wastewater is mixed with a culture of microorganisms, usually called "activated sludge", in a tank called an aeration tank. The microorganisms consume the pollutants in the wastewater as a source of food.

Aeration: Air is introduced into the aeration tank to provide the oxygen needed for the microorganisms to survive and perform their metabolic processes.

Secondary clarification: The water is clarified to separate the microorganisms and other suspended solids from the treated water. This is done in a separate tank called clarifier.

Discharge: The treated water is discharged into the environment, and the microorganisms, now called "waste sludge" is sent to a sludge treatment process.

The Complete Mix activated sludge process is characterized by its ability to maintain a homogeneous mixture of microorganisms and wastewater throughout the aeration tank. This allows for a more consistent and efficient treatment of the pollutants. Environmental factors such as temperature, pH, and nutrient levels must be monitored and controlled to maintain optimal treatment performance.

Extended Aeration system : 

The Extended Aeration system is a type of aerobic suspended growth process that uses microorganisms to treat wastewater or other contaminated water. This process is similar to the complete mix activated sludge process, but the aeration period is longer, usually between 12 to 24 hours, allowing the microorganisms to consume the pollutants in the wastewater more effectively. 

The Extended Aeration process is characterized by its ability to provide a longer aeration period, which allows for a more efficient treatment of the pollutants. This system is well-suited for low-strength wastewaters and small treatment plants. As with other aerobic treatment systems, environmental factors such as temperature, pH, and nutrient levels must be monitored and controlled to maintain optimal treatment performance. 

Oxidation Ditch systems : 

An Oxidation Ditch system is a type of aerobic suspended growth process that uses microorganisms to treat wastewater or other contaminated water. This process is similar to the activated sludge process, but it uses a circular aeration tank with a central mechanical aerator.

The oxidation ditch systems are designed to provide a constantly mixed environment for the microorganisms to consume the pollutants in the wastewater. This allows for a more consistent and efficient treatment of the pollutants. The circular design of the aeration tank allows for a smaller footprint than traditional rectangular aeration tanks, making it more space-efficient. Environmental factors such as temperature, pH, and nutrient levels must be monitored and controlled to maintain optimal treatment performance. 

Intermittently aerated and decanted systems : 

Intermittently aerated and decanted systems refer to wastewater treatment processes where the wastewater is periodically aerated (supplied with oxygen) and then allowed to settle (decanted) to separate out the solid and liquid components. This type of system is commonly used in the treatment of domestic and industrial wastewater. The aeration step promotes the growth of bacteria that consume organic pollutants in the water, while the decanting step allows the heavier solid particles to settle to the bottom of the treatment tank, leaving clarified water at the top. 

Intermittently aerated and decanted systems refer to wastewater treatment methods that involve alternating periods of aeration and decantation.

Aeration is the process of introducing air into the wastewater to promote the growth of microorganisms that break down organic matter. Decantation is the process of separating liquids from solids by allowing the liquids to settle to the bottom. 

In an intermittently aerated and decanted system, the wastewater is first aerated for a period of time, usually several hours. This allows the microorganisms to break down the organic matter and release gases such as carbon dioxide and methane. The liquid is then decanted, allowing the solids to settle to the bottom and the clear liquid to be removed. The process is then repeated.

This method is often used in small-scale wastewater treatment systems, such as those used in residential or small commercial settings. It is relatively simple and inexpensive, and can effectively remove pollutants from the wastewater. However, it is not as efficient as continuous aeration and decantation systems and may require larger tanks or more frequent maintenance. 

Oxygen Activated Sludge : 

Oxygen activated sludge is a method of wastewater treatment that uses a combination of oxygen and microorganisms to break down organic matter in the wastewater. 

The process begins by introducing oxygen into the wastewater through aeration. This oxygen is used by microorganisms, such as bacteria and protozoa, to break down the organic matter in the wastewater. The microorganisms form a thick, dense mass called sludge, which is rich in nutrients and organic matter. 

The oxygen-activated sludge process is typically divided into two stages: the aeration stage and the clarification stage. In the aeration stage, the wastewater is mixed with oxygen and microorganisms, allowing the microorganisms to consume the organic matter and produce biomass. The clarification stage is where the microorganisms are separated from the treated water, the microorganisms are returned to the aeration stage to start the process again. 

The oxygen activated sludge process is widely used in wastewater treatment plants, particularly for treating municipal wastewater. It is a relatively efficient method of removing pollutants from the wastewater and produces high-quality treated water. However, it can be relatively costly to operate and maintain and requires a significant amount of energy to provide the oxygen needed for the process.

Oxidation ponds & stabilization ponds : 

Oxidation ponds, also known as stabilization ponds, are a type of wastewater treatment system that uses natural processes to remove pollutants from the wastewater.

The ponds are large, shallow basins that are filled with wastewater. The ponds are designed to provide the right conditions for natural processes such as photosynthesis, microbial activity, and evaporation to occur. 

In the ponds, algae and other microorganisms use the pollutants in the wastewater as a source of energy and nutrients. They use photosynthesis to convert the pollutants into biomass, while other microorganisms break down the biomass into simpler compounds. The ponds are also designed to allow for evaporation, which helps to remove water from the system, further concentrating the pollutants.

Oxidation ponds are relatively simple and inexpensive to construct and operate, making them a popular choice for small-scale wastewater treatment systems, particularly in rural or remote areas. However, the process can take a relatively long time to complete, and the treated water may not be of a high enough quality for reuse. The ponds also require large land areas and may not be suitable for densely populated areas.

Aerobic attached Growth Processes :

Introduction to attached Growth systems : 

Attached growth systems are a type of wastewater treatment method that uses a fixed bed of microorganisms to break down pollutants in the wastewater. The microorganisms are attached to a solid surface, such as a plastic media, a biofilm, or a porous ceramic material, and are constantly exposed to the wastewater. 

In attached growth systems, the wastewater is passed through the bed of microorganisms, where the microorganisms use the pollutants in the wastewater as a source of energy and nutrients. As the microorganisms consume the pollutants, they produce biomass, which is then removed from the system. 

There are several types of attached growth systems, including fixed-film systems and biofilm systems. Fixed-film systems use a solid material, such as plastic media or ceramic beads, to support the microorganisms. Biofilm systems, on the other hand, use a thin layer of microorganisms that form on the surface of a porous material, such as gravel or rocks.

Attached growth systems are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. 

Trickling Filter : 

A trickling filter is a type of attached growth system that uses a bed of rocks or plastic media to support a thin layer of microorganisms. The wastewater is distributed over the top of the media, where it trickles down through the bed of microorganisms, allowing the microorganisms to consume the pollutants in the wastewater. 

The trickling filter process begins with the distribution of wastewater over the top of the media bed. The wastewater trickles down through the bed, where it is exposed to the microorganisms that are attached to the media. The microorganisms use the pollutants in the wastewater as a source of energy and nutrients, breaking them down into simpler compounds.

The trickling filter process can be divided into three stages: the aeration stage, the clarification stage, and the recirculation stage. In the aeration stage, the wastewater is distributed over the top of the media bed, allowing the microorganisms to consume the pollutants. In the clarification stage, the microorganisms are separated from the treated water, and the microorganisms are recirculated to the aeration stage to start the process again.

Trickling filters are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition.

Oxygen Transfer and Utilization : 

Oxygen transfer and utilization refer to the process of introducing oxygen into wastewater and the subsequent use of that oxygen by microorganisms to break down pollutants in the wastewater. 

In wastewater treatment systems, oxygen is typically introduced through aeration. The aeration process pumps air into the wastewater, which increases the concentration of oxygen in the water. The microorganisms in the wastewater use the oxygen to break down the pollutants in the wastewater, converting them into simpler compounds.

The rate of oxygen transfer and utilization is a crucial factor in the efficiency of wastewater treatment systems. Factors that can affect the rate of oxygen transfer and utilization include the type of aeration system used, the temperature of the wastewater, the pH level of the wastewater, and the concentration of microorganisms in the wastewater.

To ensure efficient oxygen transfer and utilization, it is important to monitor the oxygen levels in the wastewater and make adjustments as necessary. This can be done by measuring the dissolved oxygen levels in the wastewater and adjusting the aeration rate to ensure that the microorganisms have enough oxygen to break down the pollutants.

Overall, oxygen transfer and utilization are essential for the success of a wastewater treatment system, as it is the main source of energy for microorganisms to break down pollutants. Without enough oxygen, the microorganisms will not be able to effectively remove pollutants from the wastewater, leading to poor treatment performance.

Rotating Biological Contactors (RBC) : 

Rotating biological contactors (RBC) is a type of wastewater treatment system that uses a rotating disk or drum to provide a large surface area for microorganisms to attach and break down pollutants in the wastewater. 

In an RBC system, the wastewater is introduced into a tank or basin that contains a rotating drum or disk. The drum or disk is covered with a thin layer of microorganisms that are attached to the surface. As the drum or disk rotates, it exposes the microorganisms to the wastewater, allowing them to consume the pollutants.

The RBC process can be divided into two stages: the aeration stage and the clarification stage. In the aeration stage, the wastewater is introduced into the tank and the drum or disk rotates, exposing the microorganisms to the wastewater. In the clarification stage, the microorganisms are separated from the treated water, and the microorganisms are returned to the aeration stage to start the process again.

RBC systems are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. Additionally, they are not recommended for wastewater with high amounts of solids that can clog the system.

Bio-Towers : 

Bio-Towers are a type of wastewater treatment system that uses a vertical column or tower to provide a large surface area for microorganisms to attach and break down pollutants in the wastewater. 

In a bio-tower system, the wastewater is introduced into the top of the tower and flows downward through a packing material. The packing material is typically made of plastic or ceramic beads and provides a large surface area for microorganisms to attach. As the wastewater flows through the tower, the microorganisms consume the pollutants, breaking them down into simpler compounds.

The bio-tower process can be divided into two stages: the aeration stage and the clarification stage. In the aeration stage, the wastewater is introduced into the top of the tower and flows through the packing material, exposing the microorganisms to the wastewater. In the clarification stage, the microorganisms are separated from the treated water, and the microorganisms are returned to the aeration stage to start the process again. 

Bio-towers are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. Additionally, they are not recommended for wastewater with high amounts of solids that can clog the system. 

Anaerobic Suspended and Attached Growth Processes

Mechanism of Anaerobic fermentation : 

Anaerobic fermentation is a process that uses microorganisms to break down organic matter in the absence of oxygen. The process occurs in several stages, and it is driven by the metabolism of various types of microorganisms. 

The first stage is called hydrolysis, which is the breakdown of complex organic molecules into simpler compounds, such as sugars and amino acids, by enzymes produced by the microorganisms. 

The second stage is acidogenesis, in which the simpler compounds produced in the hydrolysis stage are further broken down into simpler compounds such as acetic acid, propionic acid, and butyric acid.

The third stage is acetogenesis, in which the organic acids produced in the acidogenesis stage are converted into acetic acid, hydrogen, and carbon dioxide.

The final stage is methanogenesis, in which the acetic acid, hydrogen, and carbon dioxide are converted into methane and carbon dioxide by methanogenic microorganisms.

The overall process of anaerobic fermentation produces methane, carbon dioxide, and other by-products such as volatile fatty acids. The produced methane can be used as a source of energy and the other by-products can be used as fertilizers.

Anaerobic fermentation is typically used in wastewater treatment systems to break down organic matter and remove pollutants from the wastewater. However, it is also used in other applications such as the production of biofuels, the treatment of industrial waste, and the production of organic acids and chemicals. 

Microbiology and Biochemistry of anaerobic processes : 

The microbiology and biochemistry of anaerobic processes are complex and involve the interaction of various types of microorganisms. 

In terms of microbiology, anaerobic processes involve the use of different types of microorganisms, including bacteria, archaea, and protozoa. The specific types of microorganisms used in anaerobic processes depend on the type of substrate being treated and the conditions of the process.

In terms of biochemistry, anaerobic processes involve the conversion of complex organic compounds into simpler compounds through a series of metabolic reactions. These reactions are catalyzed by enzymes produced by the microorganisms. The specific reactions that occur depend on the type of microorganisms present and the conditions of the process.

The primary biochemical reactions that occur in anaerobic processes include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is the breakdown of complex organic compounds into simpler compounds, such as sugars and amino acids. Acidogenesis is the further breakdown of these simpler compounds into organic acids. Acetogenesis is the conversion of organic acids into acetic acid, hydrogen, and carbon dioxide. And, methanogenesis is the conversion of acetic acid, hydrogen, and carbon dioxide into methane and carbon dioxide.

Overall, the microbiology and biochemistry of anaerobic processes are complex and involve the interaction of various types of microorganisms and the conversion of complex organic compounds into simpler compounds through a series of metabolic reactions. Understanding the specific microorganisms and reactions involved in a particular anaerobic process is essential for optimizing the process and achieving optimal treatment results. 

Factors Affecting Anaerobic Treatment

There are several factors that can affect the efficiency and performance of anaerobic treatment systems, including: 

Temperature: Anaerobic microorganisms have optimal temperature ranges for growth and activity. If the temperature is too high or too low, the microorganisms may not be able to effectively break down the pollutants in the wastewater.

pH: The pH level of the wastewater can also affect the performance of anaerobic treatment systems. Most anaerobic microorganisms have an optimal pH range for growth and activity.

Organic loading rate: The rate at which organic matter is introduced into the treatment system can affect the performance of the microorganisms. If the organic loading rate is too high, the microorganisms may not be able to keep up with the demand, leading to poor treatment performance.

Hydraulic retention time: The amount of time that the wastewater spends in the treatment system can affect the performance of the microorganisms. If the hydraulic retention time is too short, the microorganisms may not have enough time to effectively break down the pollutants.

Microbial composition: The specific types of microorganisms present in the treatment system can affect the performance of the anaerobic treatment. Different microorganisms are better suited to different types of pollutants and conditions, so it is important to maintain a balance of different types of microorganisms in the system.

Nutrients and Trace elements: The presence or absence of nutrients and trace elements can affect the growth and activity of the microorganisms in the treatment system.

Environmental conditions: External conditions such as light, temperature, and humidity can affect the microorganisms and their performance in the treatment system.

Overall, it's important to monitor and control these factors to ensure optimal performance and efficiency of anaerobic treatment systems.

UASB : 

UASB stands for Upflow Anaerobic Sludge Blanket. It is a type of anaerobic treatment system that uses anaerobic microorganisms to break down pollutants in the wastewater. 

The UASB system consists of a tank or basin that is filled with a mixture of wastewater and anaerobic microorganisms. The wastewater is introduced into the bottom of the tank and flows upward through the anaerobic microorganisms, which are attached to a solid support. As the wastewater flows through the microorganisms, the microorganisms consume the pollutants, breaking them down into simpler compounds. 

The UASB process can be divided into three stages: the acidogenesis stage, the acetogenesis stage, and the methanogenesis stage. In the acidogenesis stage, the microorganisms break down complex organic compounds into simpler compounds such as organic acids. In the acetogenesis stage, the organic acids are further broken down into acetic acid, hydrogen, and carbon dioxide.

In the methanogenesis stage, the acetic acid, hydrogen, and carbon dioxide are converted into methane and carbon dioxide by methanogenic microorganisms.

UASB systems are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. Additionally, they are not recommended for wastewater with high amounts of solids that can clog the system.

Anaerobic Baffled Reactor : 

An anaerobic baffled reactor (ABR) is a type of anaerobic treatment system that uses anaerobic microorganisms to break down pollutants in the wastewater. The system is designed with a series of compartments or "baffles" that slow down the flow of wastewater and increase the contact time between the wastewater and the anaerobic microorganisms. 

The ABR system consists of a tank or basin that is filled with a mixture of wastewater and anaerobic microorganisms. The wastewater is introduced into the bottom of the tank and flows upward through the anaerobic microorganisms, which are attached to the walls of the tank or basin. The series of compartments or baffles slow down the flow of the wastewater and increase the contact time between the wastewater and the microorganisms, allowing the microorganisms to effectively consume the pollutants.

The ABR process can be divided into three stages: the acidogenesis stage, the acetogenesis stage, and the methanogenesis stage. In the acidogenesis stage, the microorganisms break down complex organic compounds into simpler compounds such as organic acids. In the acetogenesis stage, the organic acids are further broken down into acetic acid, hydrogen, and carbon dioxide. In the methanogenesis stage, the acetic acid, hydrogen, and carbon dioxide are converted into methane and carbon dioxide by methanogenic microorganisms.

ABR systems are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. Additionally, they are not recommended for wastewater with high amounts of solids that can clog the system.

The compartments or baffles in the ABR system also help to keep the microorganisms in contact with the wastewater and prevent the microorganisms from being washed out of the system.

Anaerobic Filters : 

Anaerobic filters are a type of anaerobic treatment system that uses anaerobic microorganisms to break down pollutants in the wastewater. The system uses a bed of filter media, such as gravel or plastic beads, to provide a large surface area for the microorganisms to attach and grow. 

In an anaerobic filter system, the wastewater is introduced into the top of the filter bed and flows downward through the media. The microorganisms attached to the media consume the pollutants in the wastewater, breaking them down into simpler compounds. As the wastewater flows through the filter bed, the microorganisms consume the pollutants and produce biomass, which is then removed from the system.

The anaerobic filter process can be divided into three stages: the hydrolysis stage, the acidogenesis stage, and the methanogenesis stage. In the hydrolysis stage, the microorganisms break down complex organic compounds into simpler compounds such as sugars and amino acids. In the acidogenesis stage, the simpler compounds are further broken down into organic acids. In the methanogenesis stage, the organic acids are converted into methane and carbon dioxide by methanogenic microorganisms.

Anaerobic filters are widely used in wastewater treatment plants, particularly for treating industrial and municipal wastewater. They are relatively simple and inexpensive to operate and can effectively remove pollutants from the wastewater. However, they can be relatively costly to construct and may require frequent maintenance to keep the microorganisms in optimal condition. Additionally, they are not recommended for wastewater with high amounts of solids that can clog the system. 

Standard Rate And High Rate Digester

Standard rate digesters and high rate digesters are types of anaerobic digesters used in wastewater treatment. Both types use microorganisms to break down organic matter in the absence of oxygen, but they differ in the rate at which they process the waste. 

Standard rate digesters are designed to treat wastewater with a moderate organic loading, and they operate at a slower rate. They typically require longer retention times (the amount of time the wastewater stays in the digester) of 20-40 days to allow the bacteria to fully break down the organic matter. They are often used in conjunction with other treatment systems, and are typically low-maintenance. 

High rate digesters, on the other hand, are designed to treat wastewater with a high organic loading. They have a greater biomass retention time of 3-7 days and they operate at a faster rate than standard rate digesters, meaning they can process more waste in a shorter period of time. They require more energy and maintenance than standard rate digesters, but they can achieve higher levels of treatment efficiency.

Both types of digesters produce biogas (mostly methane and carbon dioxide) as a byproduct, which can be used for energy generation.