How to Design Cascade Aerators?
In most cases, water may be treated in treatment facilities to remove dangerous contaminants!
Pretreatment, aeration, coagulation, flocculation, sedimentation, filtration, fluoridation, conditioning, reverse osmosis and disinfection are all part of the treatment process.
Cascade aerators are effective in increasing the dissolved oxygen level of water, removing CO2, and contributing significantly to water self-purification due to an increase in DO, which speeds up the breakdown of organic materials.
At a treatment facility, aeration is frequently the first important step.
Constituents are eliminated or changed during aeration to prevent them from interfering with the treatment process. Aerators are all designed to increase the amount of contact between air and water in order to improve gas transport and oxidation. As a result, the dirty supply of water has a higher purity percentage of 60 to 70 percent.
Various types of aerators are used to carry out the aeration process.
1) Spray Aerators.
2) Air Diffuser basins or Diffused Aerators.
3) Gravity Aerators, which include,
a) Multiple Tray Aerator b) Cascade Aerator c) Slopping Tray Aerator d) Inclined Apron Aerator
What are Cascade Aerators?
A cascade aerator (one of the most frequent and oldest kind of aerator) is made up of a sequence of steps through which water flows (similar to a flowing stream). Aeration takes place in the splash zones of all cascade aerators. Blocks are placed over the slope to form splash zones. These can be used to oxidize iron and decrease dissolved gases to some extent.
Design Criteria’s for the Design of Cascade Aerators
Area required for Design of Aerator = 0.015 – 0.045 m2 /m3 /hr.
No. of cascade = 3 to 9.
Height of aerator = 1 to 3m.
Rise of each step = 20 to 50 cm.
Velocity of inlet pipe = 0.3 to 0.9 m/s
Let us understand this with the help of an example which will give you a clear designing procedure
Design a Cascade Aerator for inflow of 12 MLD. Assume other data suitably.
A. Design of Inlet pipe:-
Flow = Q = 12 MLD = (12 x 106 x 10-3)/ (24 x 60 x 60) = 0.138 m3 /s
Assuming velocity through pipe = 0.6 m/s.
Area of inlet pipe = (Q/v) = 0.138 / 0.6 = 0.23 = (Π/4)dp2,
Where, dp = Diameter of Inlet Pipe =0.6m
B. Design of Aerator:-
Assuming Area required for Design of Aerator = 0.030 m2 /m3 /hr.
Q = 12 MLD = 500 m3 /hr.
Area of Bottom Cascade aerator = 0.03 x 500 = 15 m2.
Total area of bottom cascade aerator = Area of bottom aerator + Area of inlet pipe
= 15 + {(Π/4)0.62} = 15.28 m2.
Total area of bottom cascade aerator =15.28 m2
Assuming no. of steps = 5
Diameter of bottomCascade: (Π/4)dx52 = 15.28 m2x d5 = 4.5 m
Now, assuming Height of Aerator = 2m
Rise of each step 2/5 = 0.4m
Diameter of 4th Cascade Aerator:- (4.5 / 2) = d4 / (2 - 0.4) d4 = 3.6 m
Diameter of 3rd Cascade Aerator :- (4.5 / 2) = d3 / (2 - 0.8) d3 = 2.7 m
Diameter of 2nd Cascade Aerator :- (4.5 / 2) = d4 / (2 – 1.2) d2 = 1.8 m
Diameter of 1st Cascade Aerator :- (4.5 / 2) = d3 / (2 – 1.6) d1 = 0.9 m
C. Design of Collecting Channel:-
Diameter of Collecting Channel = 4.5 + 1 + 1 =6.5m
q = Q / 2 = 1.375 b x h (3/2)
Where, q = discharge through channel in m3/s
b = width of channel in m
h = height of channel
→ q= 0.138 / 2 = 1.375 x 1 x h (3/2) h = 0.14 m
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