How to design Air blowers for Sewage Treatment Plants?
In Sewage Treatment Plants(STPs), the purpose of wastewater treatment is to speed up naturally occurring processes, and bring the water back to the desired purity.
The primary and secondary stages of wastewater treatment are the fundamental ones.
· Grease and oil are suspended out of the wastewater at the top, and particles are deposited at the bottom during the primary phase, which serves as a clarifier.
· Phase two then starts with water being pumped into an aeration tank. The secondary stage further cleans wastewater using biological procedures. There is occasionally a third phase when extra chemical processing might take place.
Why do air blowers come into picture?
Wastewater and activated sludge must be processed in the sewage treatment plant, which is provided by a critical process of aeration.
Figure 1: Different steps for treating sewage in STP
The procedure of aerating sewage is done in order to promote the bio-degradation of the polluting components. Bacteria used in wastewater treatment and stabilisation are given oxygen by aeration. The bacteria require oxygen for biodegradation to take place.
Figure 1 shows the different steps in process of sewage treatment and the use of air blower.The pressure generated by the air blower system ensures a steady flow of air, thus enhancing degradation.
What is the importance of designing Air Blowers?
Aeration requires a major percentage of total electricity to be used by Sewage Treatment Plant. So, the most crucial decision is selecting a high-efficiency air aeration blower, and for that calculation and its design is important. Also, air blower's air pressure and airflow rate are its crucial components for the design purposes.
How to design air blowers for STPs?
A thorough understanding of the system requirements, including the volume of air required for aeration, the pressure needed to overcome the system's resistance, and the blower's efficiency is necessary, for designing an air blower for a sewage treatment plant.
To develop an air blower we need to follow these steps:
Step 1: Determining the air flow rate (Q):
The amount of organic matter in the wastewater determines the air flow rate needed for aeration. The following formula is typically used to determine the air flow rate:
Q = K x V x S
Where:
Q = air flow rate (m3/min)
K = oxygen transfer rate (kgO2/kWh)
V = volume of wastewater (m3)
S = oxygen demand (kgO2/ m3)
Step 2: Determining the pressure required (P):
The resistance of the aeration system determines the pressure needed for the air blower. The formula below can be used to determine the pressure:
Where:
ΔP = pressure drop in the aeration system (Pa/m)
Sf= safety factor (usually from 1.2 to 1.5)
Ps = static pressure (Pa)
Step 3: Selecting the blower:
You can choose the right blower once you know the needed air flow rate and pressure. Blowers are classified according to their capability for pressure and flow. Choose a blower that has an efficiency of at least 70% and can deliver the needed air flow rate and pressure.
Step 4: Checking the rating of motor:
To make sure the blower can deliver the necessary power, the motor rating should be examined. The following formula can be used to determine the necessary power:
P = (Q x P) / η
Where:
P = power required (kW)
Q = air flow rate (m3/min)
P = pressure required (Pa)
η = blower efficiency
Step 5: Checking the noise level:
To make sure the blower conforms with local requirements, the noise level should be assessed. By choosing a blower with a lower speed, installing a muffler or silencer, or placing the blower in a different area, the noise level can be decreased.
Shape and sizing of Air Blowers: Components of air blowers
Depending on its intended use and the treatment plant's specific design, air blowers in sewage treatment plants come in a variety of sizes and shapes. An enclosed motor and centrifugal fan often make up an air blower. The casing might have a cylindrical or rectangular shape and is often composed of metal.
Figure 2: Setup using multiple Air-blowers
In order to give the necessary amount of oxygen for the biological treatment process, air blowers for sewage treatment plants are built to handle enormous volumes of air and produce high pressure. We can also use multiple air-blowers as shown in Figure 2.
Figure 3: Vortex air blowers
Typically, the fan blades are positioned in the lower portion of the casing, and the air blower's motor is mounted on top of it. The fan blades' purpose is to move air and generate the necessary pressure. Depending on the particular needs of the treatment facility, some air blower types could also include other parts like filters, silencers, or control panels. For STPs, vortex air-blowers are used which can be seen in Figure3.
Conclusion
Overall, air blowers are a crucial part of the aeration system in sewage treatment plants, and they must function properly for the treatment process to be effective and efficient.
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