How to Design Clarifier for STP Share design formula and Calculations?
Sewage treatment plants are crucial for ensuring a healthy and clean environment. One important component of sewage treatment plants is the clarifier, which is responsible for separating solid waste from the wastewater. Designing a clarifier for a sewage treatment plant requires careful consideration of several factors, including the required flow rate, retention time, and the settling velocity of the solids.
In this blog, we will discuss the design formula and calculations required to design a clarifier for a sewage treatment plant.
Design Formula for Clarifier:
The design formula for clarifier is given as follows:
Q = (A x v) / t
Where Q is the flow rate in m3/hr, A is the surface area of the clarifier in m2, v is the upward velocity of the wastewater in m/hr, and t is the retention time in hours.
Let us assume that we need to design a clarifier for a sewage treatment plant with a flow rate of 100 m3/hr, a retention time of 2 hours, and a settling velocity of 0.1 m/hr.
Surface Area Calculation:
To calculate the surface area of the clarifier, we can use the following formula:
A = Q x t / v
Substituting the given values, we get:
A = 100 x 2 / 0.1 = 2000 m2
Therefore, the surface area of the clarifier required is 2000 m2.
To calculate the height of the clarifier, we need to consider the settling velocity of the solids. The height of the clarifier can be calculated using the following formula:
H = Vs / (G x (ρp - ρw))
Where H is the height of the clarifier in meters, Vs is the settling velocity of the solids in m/hr, G is the acceleration due to gravity (9.81 m/s2), ρp is the density of the solids in kg/m3, and ρw is the density of the wastewater in kg/m3.
Assuming the density of the solids as 1500 kg/m3 and the density of the wastewater as 1000 kg/m3, we get:
H = 0.1 / (9.81 x (1500 - 1000)) = 0.0008 m
Therefore, the height of the clarifier required is 0.8 meters.
Retention Time Calculation:
The retention time of the clarifier can be calculated using the following formula:
t = V / Q
Where V is the volume of the clarifier in m3.
Assuming the depth of the clarifier as 3 meters, we can calculate the volume of the clarifier as:
V = A x H = 2000 x 3 = 6000 m3
Substituting the values, we get:
t = 6000 / 100 = 60 hours
Therefore, the retention time of the clarifier required is 60 hours.
Working Function of Clarifier:
The clarifier works on the principle of gravity separation. The wastewater enters the clarifier and flows slowly through it. The solid particles in the wastewater settle down due to gravity and form a sludge layer at the bottom of the clarifier. The clarified wastewater flows out of the clarifier from the top.
The sludge layer that accumulates at the bottom of the clarifier is periodically removed and sent for further treatment.
Retention Time in Sewage Treatment Plant:
Retention time is the time required for the wastewater to pass through the clarifier. The retention time in a sewage treatment plant varies depending on the design and the characteristics of the wastewater. The retention time is usually between 1 to 3 hours in a primary clarifier and can be up to 24 hours in a secondary clarifier.
The retention time is important for ensuring that the solids have enough time to settle down and form a sludge layer at the bottom of the clarifier. If the retention time is too short, the solids will not settle down completely and will carry over with the clarified wastewater. On the other hand, if the retention time is too long, the clarifier will become overloaded and the efficiency of the treatment plant will decrease.
In conclusion, designing a clarifier for a sewage treatment plant requires careful consideration of several factors including the required flow rate, retention time, and settling velocity of the solids. The design formula and calculations provided in this blog post can be used as a guide to design a clarifier for a sewage treatment plant. Additionally, the working function of a clarifier and the importance of retention time in sewage treatment plants have also been discussed. Proper design and operation of clarifiers can lead to a more efficient and effective sewage treatment process, ultimately resulting in a cleaner and healthier environment.
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