Design of each elements for the construction of sedimentation tank is necessary for effective treatment of wastewater for removal the suspended particles. Various components of sedimentation tank and their design basis is discussed.
Design Elements for Construction of Sedimentation TankThe sedimentation tank is constructed based on many factors. Some important design elements of sedimentation tank are:
- Detention period
- Displacement efficiency
- Surface loading
- Basin dimensions
- Inlet and outlet arrangements
- Sludge removal devices
Detention Period of Sedimentation TankDetention period is a time taken by a water particle to travel from inlet to outlet of settling tanks. It is the ratio of volume of basin to discharge rate.
Detention period (t) = V/Q
Where V = volume of basin
Q = discharge rateSo, if the volume of basin is increases, detention period also increases. It also depends on the quantity of suspended particles in water. In general, detention period is in between 4 hours to 8 hours.
Displacement Efficiency of Sedimentation TankDisplacement efficiency is the ratio of flowing through period to detention period. Flowing through period is the time taken by a batch of water to travel from the entry to exit of settling tank. Because of short circuits produced in water current, flowing through period is less than the detention period. Flowing through period can be calculated easily. For this, sodium chloride is added to influent and after some time collected effluent is subjected to chloride content test which decides the flowing through time. In general, flowing through period should be at least 0.3 times the detention period and displacement efficiency should be 0.25 to 0.5.
Surface LoadingSurface loading is nothing but over flow rate which is equal to the amount of water passing per day through one unit of horizontal area. Here, the amount of water passing per day is nothing but discharge rate.
Hence, surface loading = Q/A = liters per day per square meter.According to above equation surface loading is also equal to velocity of settlement which is defined as ration of discharge rate to area.
Basin Dimensions of Sedimentation TankBasin dimensions of sedimentation tank are derived from its area. So, first step is to calculate the area of basin. The basin area is derived from the volume of water in liters per hour and surface loading rate. Hence,
From the derived area, the three dimensions of tank (length, width, depth) are calculated based on the following conditions:
Area of basin = volume of water / surface loading rate
= liters per hour / liters per hour per m2
- The depth of basin should be 3 to 4.5 meters.
- The width should be greater than or equal to 0.25 times of its length.
- The width should not be more than 12 meters.
- The cross-sectional area provided should allow horizontal velocity of 0.3 meters per minute.