Unit: 2
Overview of Biological Wastewater Treatment
There are several different types of Biological wastewater treatment systems, including:
Activated sludge process: This is a common method of biological wastewater treatment. In this process, microorganisms are added to the wastewater in an aeration tank, where they consume the organic matter and convert it into biomass. The resulting mixture of microorganisms and wastewater is then sent to a clarifier, where the microorganisms settle out and are recycled back to the aeration tank.
Trickling filters: In this method, wastewater is distributed over a bed of rocks or plastic media, where microorganisms grow and consume the pollutants. The treated water is collected at the bottom of the filter and is then sent to a secondary treatment process.
Rotating Biological Contactors: This process involves the use of a rotating drum covered with plastic media, where microorganisms grow and consume pollutants. The microorganisms are scraped off the drum and recycled back to the treatment process.
Aerated lagoons: These are large, shallow ponds that are aerated to provide oxygen for the microorganisms. Wastewater is sent to the pond, where the microorganisms consume the pollutants and convert them into biomass.
All these methods use microorganisms to break down and remove pollutants from the water. They can remove dissolved and suspended organic matter, nitrogen and phosphorus compounds, and pathogens. The treated water can be safely discharged into the environment or reused for irrigation or other purposes.
The choice of the method will depend on the characteristics of the wastewater to be treated, the space available, the desired quality of the effluent, and the budget.
Objectives of Biological Wastewater Treatment :
The main objectives of biological wastewater treatment are to remove or reduce the amount of organic matter and pollutants present in the wastewater, making it safe for discharge into the environment. This is typically done through the use of microorganisms, such as bacteria and fungi, which break down the organic matter and consume the dissolved oxygen in the process.
The goal of this treatment is to reduce the levels of pollutants such as BOD (biochemical oxygen demand), nitrogen, and phosphorus to levels that are safe for discharge into natural bodies of water. Additionally, some biological wastewater treatment processes also help to remove other pollutants such as heavy metals and pathogens. Another important objective of biological wastewater treatment is to recover valuable resources like energy, nutrients, and water from the waste stream.
Role of Microorganisms in Biological Wastewater Treatment :
Microorganisms play a vital role in biological wastewater treatment by breaking down the organic matter present in the wastewater. The process of breaking down the organic matter is known as biological oxidation. The microorganisms that are commonly used in biological wastewater treatment include bacteria and fungi. These microorganisms consume the organic matter and other pollutants in the wastewater, converting them into biomass, carbon dioxide, and water.
Bacteria are the most commonly used microorganisms in biological wastewater treatment. They can be divided into two groups: heterotrophic and autotrophic. Heterotrophic bacteria consume organic matter and convert it into biomass, carbon dioxide, and water. Autotrophic bacteria use inorganic matter as a source of energy and convert it into biomass. Fungi can also play a role in the treatment of wastewater by breaking down complex organic matter.
The microorganisms used in the treatment process are typically cultured in large numbers in aeration tanks and clarifiers to ensure that enough microorganisms are present to effectively break down the pollutants in the wastewater. Additionally, the process of biological wastewater treatment can also be enhanced by adding nutrients, such as phosphorous and nitrogen, to the aeration tanks to promote the growth of microorganisms.
In summary, microorganisms play a crucial role in breaking down the organic matter and pollutants present in wastewater, making it safe for discharge into the environment. They convert the pollutants into biomass, carbon dioxide and water, which can be further treated to meet discharge standards.
Types of Biological processes for wastewater Treatment :
There are several types of biological processes that are commonly used for the treatment of wastewater, including:
Activated Sludge Process: This is one of the most widely used biological treatment processes. It involves the use of microorganisms in an aeration tank to break down the pollutants in the wastewater. The microorganisms are then separated from the treated water in a secondary clarifier.
Trickling Filters: In this process, wastewater is distributed over a bed of rocks or plastic media and is passed through a layer of microorganisms. The microorganisms consume the pollutants in the wastewater and convert them into biomass, carbon dioxide, and water.
Rotating Biological Contactor (RBC): This process involves the use of a rotating drum with a surface that is covered with microorganisms. The drum rotates through the wastewater, allowing the microorganisms to consume the pollutants.
Oxidation Ditch: This is a variation of the activated sludge process. It involves the use of a large, open tank where wastewater is aerated and mixed with microorganisms. This process is typically used for the treatment of large flows of wastewater.
Membrane Bioreactor (MBR): This process combines biological treatment with membrane filtration. Microorganisms are used to break down the pollutants in the wastewater, and the treated water is then passed through a membrane to remove any remaining solids or microorganisms.
Anaerobic Treatment: This process uses microorganisms that thrive in the absence of oxygen to treat wastewater. They can convert organic matter into methane, carbon dioxide and water. This process is typically used to treat wastewater that has high levels of organic matter such as industrial wastewater or sewage.
Each process has its own set of advantages and disadvantages and is chosen based on the characteristics of the wastewater, the size and capacity of the plant and the end-use of the treated water.
Suspended and attached growth systems in Biological processes :
In biological wastewater treatment, there are two main types of systems: suspended growth systems and attached growth systems.
Suspended Growth Systems: In a suspended growth system, microorganisms are suspended in the liquid and are not attached to any solid surface. Activated sludge process and Oxidation Ditch are the examples of suspended growth systems.
In these systems, microorganisms are constantly mixed with the wastewater in an aeration tank, allowing them to consume the pollutants. Once the microorganisms have consumed the pollutants, they are separated from the treated water in a secondary clarifier.
Attached Growth systems: In an attached growth system, the microorganisms are attached to a solid surface, such as rocks or plastic media. Trickling filters and Rotating Biological Contactor (RBC) are the examples of attached growth systems.
In these systems, wastewater is distributed over the surface where the microorganisms are attached, and the microorganisms consume the pollutants as the water flows over them.
Both suspended and attached growth systems have their own advantages and disadvantages, and the choice between them depends on the characteristics of the wastewater, the size and capacity of the plant, and the end-use of the treated water.
Suspended growth systems are typically better for treating high-strength wastewater and for removing a wide range of pollutants, while attached growth systems are more efficient for treating low-strength wastewater and for removing specific pollutants.
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