What Is Sand Filter

17^th February, 2024.

In this post, we will see the concept of sand filtration.

When you are drinking water or using water for domestic purposes, it is not possible that it has reached to you without any filtration. Raw water cannot come to you directly for use; it is filtered. In water treatment, there are so many filtration techniques that you will virtually feel tired of reading it. It is such vast and important to understand. One of the most used techniques is sand filtration. Sand filters are extensively used in pure water treatment; means they will mostly be used where drinking water is required. So, you can imagine how important it is. In this post, we will see the concept of sand filtration.

·       What is sand filtration?

As the name suggests, sand filtration uses sand for filtering water. It sounds simple, but it is sophisticated in operation. This is because the material used is sand and as you know, it is very small in size. If impure type or bigger size of sand is used, water will not be pure. So, the complexity comes in terms of choosing the material, and maintaining it. But apart from that, a sand filter is one of the finest technologies developed till date for water purification.

The sand used is of a very fine dimension, typically between 0.75 mm to 1.5 mm. The sand is distributed on a bed medium. The sand type will be either graded or coarse monograde. It is to be noted that the bed depth and height varies depending on manufacturer to manufacturer. And, it is not only comprised of fine sand. The bed is made of fine sand, coarse sand, fine gravel and coarse gravel. Coarse is used to remove bigger impurities and fine will remove smaller impurities. So, as the water flows from fine to coarse, impurities will be removed from smaller to bigger sequence. The particles will be trapped between sand layers, allowing only pure water to be passed ahead. You must be wondering why sand does not drain out. If you see the earlier discussion, you will find that gravels (stones) too are used in the bottom layers. This prevents sand from draining out to the pores. Sand filters can easily remove suspended solids and other fine form of pathogens, but not dissolved salts. It must be aided with sedimentation, flocculation and other chemical dosing for efficient removal of salt and other impurities.

·       How does a sand filter work?

Refer the below image. There are two processes generally – filter (service) and backwash. During filter mode, water flows from top to the sand media. The media will capture dirt on it’s layers, as it will not be able to pass through it. Smaller particles get trapped easily at the top, and as the sand dimension increases, bigger impurities get caught below between the sand layers. Finally, pure water will be able to pass through the small size pores to the outlet. The pores are so small that they allow only water to pass, neither sand nor impurities.

As sand is capturing dirt, it needs to be cleaned frequently, otherwise it will get choked. So, after filtration time, backwash or rinse process is started. Water flows in a reverse direction from bottom to top. Due to the reverse force applied, the dirt will be thrown out to the top layer pipelines. Backwashing needs to be done after every frequency of time, otherwise sand will get choked and it will no longer function as a filter. Also, water pressure will increase in the pipeline as water will not find any path to pass due to blockage. It is to be noted that compressed air flow also needs to be allowed through the filter during backwash for aiding the process successfully, as sticky dirt will be removed through the force of the air.

·       Applications of sand filter:

1.      Drinking water requirement

2.     Swimming pools

3.     Groundwater treatment

4.     Car washes

5.     Food and beverages industry

6.     Metal industries

I have covered some general principles related to sand filtration technology. I have also not attempted to cover all the methods, as it can vary from case to case and there is no end to it. Once you are familiar with this filter technique, you can easily tackle all types of problems in it.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.