In wastewater treatment, what is the required open screen area (m2) for reaching this screen velocity during treatment of the rain weather flow when dimension of the pre-treatment unit for the wastewater treatment plant is receiving a dry weather flow (dwf) of 18000 m3/day, and assuming that the flow doubles during rain, where the bar screen is used to remove the coarse waste with the rain water flow velocity in the screen channel is 0.5 m/s and the flow -through velocity of the clean screen (V) is 0.9 m/s?
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To calculate the required open screen area (m2) for reaching the desired screen velocity during treatment of the rain weather flow, you need to follow the steps below:
Step 1: Calculate the rain weather flow (RWF)
The rain weather flow can be found by doubling the dry weather flow (DWF) mentioned in the question. Therefore, RWF = 2 * DWF.
Given:
DWF = 18000 m3/day
RWF = 2 * DWF = 2 * 18000 m3/day = 36000 m3/day
Step 2: Calculate the wastewater flow rate with rain weather flow excluded (Q)
To determine the wastewater flow rate with rain weather flow excluded, subtract the RWF from the total flow rate (Q) during the rainy weather conditions.
Given:
Q = DWF + RWF
Q = 18000 m3/day + 36000 m3/day = 54000 m3/day
Step 3: Calculate the hydraulic loading rate (HLR)
The hydraulic loading rate (HLR) refers to the volume of wastewater that passes through a given area of the screen per unit of time. It can be calculated by dividing the flow rate (Q) by the open screen area (A).
Given:
Flow-through velocity of the clean screen (V) = 0.9 m/s
HLR = Q / A
Step 4: Calculate the required open screen area (A)
To find the required open screen area (A), rearrange the HLR equation from step 3 to solve for A.
A = Q / HLR
Substituting the given values:
A = Q / V
A = 54000 m3/day / 0.9 m/s
Note: Since the units of flow rate and velocity are not compatible, we need to convert the flow rate from m3/day to m3/s.
A = (54000 m3/day) * (1 / 86400 s/day)
A = 0.625 m3/s
Therefore, the required open screen area (A) for reaching the desired screen velocity during treatment of the rain weather flow is 0.625 m2.