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Filtration Process in Water Treatment

Sedimentation, along with coagulation and flocculation, removes a large percentage of suspended impurities from raw water. However, the water coming from the sedimentation tanks may still contain very fine suspended particles, turbidity, odour, and bacteria. To remove these remaining impurities, the filtration process in water treatment is used.

In this process, water from the sedimentation tank passes through beds of fine granular material called sand. Filtration helps remove fine suspended particles, turbidity, odour, and some harmful bacteria, thereby improving the quality of drinking water.

Types of Filtration

There are mainly two types of filtration:

1. Surface Filtration - Membrane Filtration

2. Depth Filtration - Sand Filtration

Sand filtration is further divided into:

1. Slow Sand Filter

2. Rapid Sand Filter

Slow Sand Filter

Slow sand filters remove a very high percentage of suspended impurities and bacteria from water compared to rapid sand filters. However, the filtration rate is relatively slow, requires a larger surface area, and involves higher construction costs.

Slow sand filters are mainly used in small water treatment plants and generally receive water from plain sedimentation tanks.

Sand Layer

  • Upper layer
  • Depth of about 90–110 cm
  • Made of fine sand

2. Rapid Sand Filter

  • Lower supporting layer
  • Consists of gravel of different sizes (30–75 mm)
  • Supports the sand layer

The thickness of these layers depends on the plant capacity and the quality of raw water.

Working of Slow Sand Filter

Water from the sedimentation tank enters the inlet chamber and is uniformly distributed over the filter bed. It slowly percolates through the sand layer, where impurities are removed. The filtered water then passes through the gravel layer and is collected in a storage tank or sump.

Cleaning of Slow Sand Filter

After a certain period, the filter bed becomes clogged and must be cleaned.

Cleaning is carried out by:

  • Scraping and removing the top 1.5–3 cm layer of sand
  • Washing the removed sand with clean water
  • Replacing it with fresh sand, if required

Backwashing is not required for slow sand filters.

Rapid Sand Filter

Rapid sand filters are widely used in modern water treatment plants. Water from the sedimentation tank is directly fed into the rapid sand filter.

These filters are used to:

  • Increase the filtration rate
  • Reduce space requirements
  • Handle large quantities of water efficiently

Types of Rapid Sand Filters
Rapid Gravity Filter

Rapid gravity filters use comparatively larger sand particles, allowing a much higher filtration rate than slow sand filters.

They are mainly used in:

  • Large municipal water treatment plants
  • Public water supply systems

Pressure Sand Filter

Pressure sand filters operate under pressure, which further increases the filtration rate.

They are commonly used in:

  • Small treatment plants
  • Industrial water treatment plants
  • Swimming pools

Backwashing Process

Rapid sand filters are cleaned using a process called backwashing. During backwashing, water and compressed air are passed upward through the filter media. This upward flow agitates the sand particles and removes the trapped suspended impurities.

Backwashing Procedure

1. Close the inlet and filtered water outlet valves.

2. Start the wash water supply and compressed air.

3. After some time, stop the compressed air supply.

4. Open the wastewater drain valve.

5. Continue washing for about 5 minutes.

6. Close the wash water supply valve.

7. Open the inlet valve.

8. Close the wastewater drain valve.

9. Open the filtered water outlet valve.

The complete backwashing process generally takes 10–30 minutes.

Conclusion

Filtration is one of the most important stages in the water treatment process, ensuring the removal of fine suspended particles, turbidity, odour, and harmful microorganisms. Depending on the plant capacity and treatment requirements, either a slow sand filter or a rapid sand filter is selected to produce safe and clean water for various applications.

In the next topic, we will discuss the Disinfection Process in Water Treatment.

 

Mr. Prashant Walke
Trainer & Coordinator
Water & Wastewater Management Course
AISSMS College of Engineering, Pune