Water Treatment "Part 13": Cascade Aerators

One of the first aeration systems is the cascade aerator, which is an excellent method with numerous steps. Each step creates a contact surface between water and air as the water flows from above the dam into a lower basin. A mixture of gas and water is created when a lot of air lowers along with the flowing water into the receiving basin. The air bubbles are then dispersed throughout the water, creating a combination of gas and water.
Based on the height of each level in the cascading waterfall and the total number of levels, the efficiency of gas transmission through this process is estimated:

K= 1-(1-k).n

K: Proficiency per level.
n: number of levels.

The efficiency of oxygen and methane improves with higher elevations, but carbon dioxide efficiency remains constant. As the following table shows:

[created with Microsoft Word]

Since the efficiency barely rises above this height, the height of the fall of the level of the cascade aerator practically does not exceed one meter. But increasing the number of levels increases efficiency. A 4 or 5 level cascade aerator is employed for each methane removal basin.

Basin depth:

The cascade aerator's water receiving basin is designed with a depth that prevents the flowing water from filling the basin all the way to the bottom. As a result, the gas transfer period is extended as much as possible because the air bubbles are pulled to the greatest depth possible, resulting in the best contact, and longest bubble life.

[Cascade Aerator- Photo taken by me, in the water treatment plant "SEOR". Mostaganem. Algeria. (2018)]

Basin width:

The basin's breadth needs to be sufficient to hold the water that is pouring down. We use the following equation to calculate the time of water fall:

h= 1/2. g. t2

So: t= √2. h/g

References:

• Taparhudee, Wara (2002). "Applications of Paddle Wheel Aerators and Diffused-Air System in Closed Cycle Shrimp Farm System" (PDF). Witthayasan Kasetsart (Sakha Witthayasat). 36: 408–419. Retrieved 26 April 2020.
• Unsafe water kills more people than war, Ban says on World Day". UN News. 22 March 2010. Retrieved 10 May 2018
• Raymond Desjardins- Livre: Le traitement des eaux- 2éme edition- Ecole Polytechnique de Montréal- 1997- ISBN 2-553-00643-8
• Drinking Water Treatment- EDX- Delft University of Technology.
• Book- Drinking Water: Principles and Practices- by Hans J C Van Dijk (Author), Jasper Q J C Verberk (Author), Peter J De Moel.