How to Calculate diffusers numbers in MBBR and Equalization tank?
Moving Bed Biofilm Reactor (MBBR) is a popular wastewater treatment technology that utilizes biofilm on small plastic carriers, called media, to remove organic pollutants from sewage water. One of the critical components of an MBBR-based sewage treatment plant is the diffusers, which provide oxygen to the biofilm and maintain an aerobic environment for the microorganisms to thrive.
In this blog, we will discuss how to calculate the diffuser numbers for MBBR tanks and equalization tanks in an MBBR-based sewage treatment plant.
Diffusers in MBBR Tanks:
The number of diffusers required in an MBBR tank depends on the design parameters of the plant, such as the volume of the tank, the desired hydraulic retention time (HRT), and the expected organic load. The HRT is the average time that wastewater stays in the reactor, and it is calculated by dividing the volume of the tank by the flow rate of the influent wastewater. A longer HRT allows for more contact time between the biofilm and the wastewater, which leads to better treatment efficiency.
The diffuser density in an MBBR tank is typically expressed in terms of the specific air demand (SAD), which is the amount of air required to maintain the dissolved oxygen (DO) concentration at a desired level. The DO level is usually maintained between 2-4 mg/L to ensure sufficient oxygen supply for the microorganisms. The SAD is expressed in terms of cubic meters of air per hour per square meter of the tank surface area (m3/h/m2).
The SAD can be calculated using the following formula:
SAD = (Q × SOTE × F) / (A × ΔDO)
Where, Q = Flow rate of influent wastewater (m3/h) SOTE = Standard oxygen transfer efficiency (0.75-0.8) F = Safety factor (1.2-1.5) A = Total surface area of the tank (m2) ΔDO = Desired change in DO concentration (mg/L)
The standard oxygen transfer efficiency (SOTE) is a measure of how efficiently the diffusers transfer oxygen from the air to the water. The SOTE varies depending on the type of diffusers used and their operating conditions. A value of 0.75-0.8 is generally used for fine bubble diffusers.
The safety factor (F) is used to account for any uncertainties in the design parameters and ensure that the diffusers can supply sufficient oxygen even under unfavorable conditions. A value of 1.2-1.5 is commonly used for MBBR tanks.
Once the SAD is calculated, the number of diffusers required can be determined by dividing the total airflow required by the airflow per diffuser. The airflow per diffuser is calculated by dividing the SAD by the average diffuser density (ADD), which is the number of diffusers per square meter of tank surface area. The ADD is typically in the range of 0.5-1.5 diffusers/m2 for fine bubble diffusers.
The total airflow required can be calculated using the following formula:
Total airflow = SAD × A
Where, A = Total surface area of the tank (m2)
For example, let's assume that we have an MBBR tank with a volume of 500 m3, a desired HRT of 4 hours, and an expected influent flow rate of 100 m3/h. The total surface area of the tank is 175 m2 (assuming a cylindrical tank with a height of 5 m and a diameter of 10 m). We want to maintain a DO concentration of 3 mg/L, and we are using fine bubble diffusers with a standard oxygen transfer efficiency of 0.
SOTE = 0.75 and a safety factor of 1.2. Using these values, we can calculate the SAD as follows:
SAD = (100 × 0.75 × 1.2) / (175 × 1) / 1 = 0.51 m3/h/m2
Assuming an ADD of 1 diffuser/m2, the airflow per diffuser would be:
Airflow per diffuser = 0.51 / 1 = 0.51 m3/h
Therefore, the total number of diffusers required would be:
Total diffusers = Total airflow required / airflow per diffuser
Total diffusers = (0.51 × 175) / 0.51 = 175 diffusers
Diffusers in Equalization Tanks:
Equalization tanks are used to store incoming wastewater and balance out any fluctuations in flow rate and organic load. The diffusers in an equalization tank are used to provide mixing and prevent settling of solids. The number of diffusers required in an equalization tank depends on the volume of the tank and the expected organic load.
The diffuser density in an equalization tank is typically expressed in terms of the specific mixing energy (SME), which is the amount of energy required to provide adequate mixing in the tank. The SME is expressed in terms of watt per cubic meter (W/m3).
The SME can be calculated using the following formula:
SME = (P × t) / V
Where, P = Power required to provide mixing (W) t = Mixing time (seconds) V = Volume of the tank (m3)
The power required to provide mixing depends on the type and number of diffusers used, the diffuser spacing, and the flow rate of the wastewater. A value of 0.5-1.5 W/m3 is commonly used for fine bubble diffusers.
The mixing time depends on the size and shape of the tank and the desired level of mixing. A value of 30-60 seconds is commonly used for equalization tanks.
Once the SME is calculated, the number of diffusers required can be determined by dividing the total power required by the power per diffuser. The power per diffuser is calculated by dividing the SME by the diffuser density (DD), which is the number of diffusers per cubic meter of tank volume. The DD is typically in the range of 0.5-1.5 diffusers/m3 for fine bubble diffusers.
The total power required can be calculated using the following formula:
Total power = SME × V
Where, V = Volume of the tank (m3)
For example, let's assume that we have an equalization tank with a volume of 200 m3 and a desired mixing time of 45 seconds. We want to use fine bubble diffusers with a power density of 1 W/m3. Using these values, we can calculate the SME as follows:
SME = (1 × 45) / 200 = 0.225 W/m3
Assuming a DD of 1 diffuser/m3, the power per diffuser would be:
Power per diffuser = 0.225 / 1 = 0.225 W
Therefore, the total number of diffusers required would be:
Total diffusers = Total power required / power per diffuser
Total diffusers = (0.225 × 200) / 0.225 = 200 diffusers
In conclusion, calculating diffuser numbers for MBBR tanks and equalization tanks in an MBBR-based sewage treatment plant requires attention to detail and a comprehensive understanding of the principles of aeration and mixing. By carefully evaluating the design parameters and operating conditions and working with experienced professionals, plant operators can ensure that the diffuser system is appropriately sized and optimized for optimal performance, energy efficiency, and cost-effectiveness.