Water Treatment "Part 17": Rapid sand filters.

River sand is frequently used as a filter material, but in order to employ granules of the right sizes for this procedure, it must first be sieved. Filtration material should also be corrosion resistant, have a consistent grain distribution, and be impurity free.
During the rapid filtration process, the filter material selection is crucial in order to prevent a number of issues that might arise. The suspended solids may concentrate at the surface layer where the smaller sand grains are situated due to inadequate sieving and uneven particle size, which will make it challenging to remove them using the reverse washing procedure. As that, due to the mud that accumulates and settles towards the bottom of the filter, where the larger and heavier sand grains reside, the nozzles might also become blocked.

The Ancient Egyptians used the water purification method to get clean water.

The rapid sand filter is made of mesh covered in a layer of gravel, followed by a layer of silica sand with a thickness of 0.5 to 0.8 mm. There is equipment for washing the filter in the opposite direction from the water running through it, as well as a cleaner for the filter's surface to get rid of the suspended items that have been deposited there.
The typical rate of filtration is 115 cubic metres per square metre of filter surface in a 24-hour period. The wash water and air are introduced into the filter during the backwashing operation in the opposite direction from the filtering direction. As long as no more than 2.5% of the water processed through the filter is used for washing, the typical washing time is 15 minutes.
Due to its efficiency, benefits on both a technical and financial level, and low cost during construction, rapid sand filters are extensively utilised.

References:

• [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]
• 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.