How to calculate anoxic tank in MBBR STP with retention time calculation?
MBBR (Moving Bed Biofilm Reactor) based Sewage Treatment Plants (STP) have become popular due to their efficiency and low operational and maintenance costs. One of the critical components of an MBBR based STP is the anoxic tank.
Let’s have an understanding on how to calculate the size of anoxic tank in MBBR based STP.
What is an Anoxic Tank?
Anoxic tanks are an essential component of MBBR based STP. These tanks provide an environment where bacteria can break down the nitrogenous compounds in sewage by converting nitrate to nitrogen gas under anaerobic conditions.
Factors Affecting Anoxic Tank Size:
The size of the anoxic tank in MBBR based STP depends on various factors such as:
· Sewage flow rate
· Nitrogenous load
· Required effluent quality
· Hydraulic Retention Time (HRT)
· Mixed Liquor Suspended Solids (MLSS)
Calculating Anoxic Tank Size:
The anoxic tank size can be calculated using the following formula:
Anoxic Tank Volume = (F × V × θ) / (R × (SNOx-SNit))
Where,
F = Nitrogenous Load (kg N/day)
V = Sewage Flow Rate (m3/day)
θ = Anoxic Tank Retention Time (days)
R = Nitrate Removal Efficiency
SNOx = Nitrate Concentration in influent (mg/L)
SNit = Nitrate Concentration in effluent (mg/L)
Step-by-Step Calculation Process:
Let's consider an example to calculate the anoxic tank size for an MBBR based STP with the following parameters:
· Sewage flow rate = 100 m3/day
· Nitrogenous Load = 100 kg N/day
· Anoxic Tank Retention Time = 4 hours
· Nitrate Removal Efficiency = 90%
· Nitrate Concentration in influent = 50 mg/L
· Nitrate Concentration in effluent = 5 mg/L
Step 1:
Calculate the Required Anoxic Tank Volume.
Anoxic Tank Volume = (F × V × θ) / (R × (SNOx-SNit))
Anoxic Tank Volume = (100 × 100 × 4) / (0.9 × (50-5)) = 988.89 m3
Step 2:
Determine the Dimensions of the Anoxic Tank.
Once the anoxic tank volume is calculated, the dimensions of the tank can be determined. The tank should have a depth of at least 3 meters to ensure proper mixing and to avoid dead zones.
Step 3:
Calculate the Anoxic Tank Retention Time.
The retention time of the anoxic tank is the time taken for the sewage to pass through the tank.
It can be calculated using the formula:
Anoxic Tank Retention Time = Anoxic Tank Volume / Sewage Flow Rate Anoxic Tank
Retention Time = 988.89 / 100 = 9.89 hours
Step 4:
Verify Nitrate Concentration in Effluent.
The nitrate concentration in the effluent should be verified to ensure that it meets the required effluent quality. If the concentration is higher than the required limit, the anoxic tank size should be increased.
Conclusion.
The calculation of the anoxic tank size is an essential aspect of designing an MBBR based STP. The anoxic tank size depends on various factors such as sewage flow rate, nitrogenous load, required effluent quality, and design parameters such as HRT and MLSS. The formula to calculate the anoxic tank size involves nitrogenous load, sewage flow rate, anoxic tank retention
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